Reactivity Consequences of Steric Reduction in Cyclopentadienyl

Nov 6, 2009 - Reactivity Consequences of Steric Reduction in Cyclopentadienyl Chromium β-Diketiminate Complexes. K. Cory MacLeod†, Julia L. Conwayâ...
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Organometallics 2009, 28, 6798–6806 DOI: 10.1021/om900788c

Reactivity Consequences of Steric Reduction in Cyclopentadienyl Chromium β-Diketiminate Complexes K. Cory MacLeod,† Julia L. Conway,† Liming Tang,‡ Joshua J. Smith,‡ Liam D. Corcoran,‡ Katherine H. D. Ballem,‡ Brian O. Patrick,§ and Kevin M. Smith*,† †

Department of Chemistry, University of British Columbia Okanagan, 3333 University Way, Kelowna, BC, Canada V1V 1V7, ‡Department of Chemistry, University of Prince Edward Island, 550 University Avenue, Charlottetown, PEI, Canada C1A 4P3, and §Department of Chemistry, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1 Received September 9, 2009

A series of Cr(III) half-sandwich β-diketiminate complexes, CpCr[(ArNCMe)2CH]X, X = I (2), CH3 (3), or Cl (4), were prepared. Compared to previously communicated complexes with Ar = 2,6-iPr2C6H3 (Dpp, a), Cr(III) complexes with less sterically demanding ligands such as Ar = 2,6Me2C6H3 (Xyl, b), 2,4,6-Me3C6H2 (Mes, c), or 2,6-Et2C6H3 (Dep, d) were more readily synthesized via salt metathesis reactions. Iodide compounds 2b-d were prepared by oxidation of the corresponding Cr(II) species CpCr[(ArNCMe)2CH] 1b-d with half an equivalent of iodine. The Cr(III) chloride complexes CpCr[(ArNCMe)2CH]Cl 4c-d were prepared in a two-step, one-pot reaction from anhydrous CrCl3. The reaction of MeMgI with either the Cr(III) chloride or iodide precursors yielded the Cr(III) methyl complexes CpCr[(ArNCMe)2CH](CH3) 4b-d. All of the paramagnetic Cr(III) complexes were characterized by UV-visible spectroscopy and elemental analysis, and the structures of 2b, 2c, 3b, 3c, 3d, 4c, and 4d were determined by X-ray crystallography. Reaction of CpCr[(ArNCMe)2CH] with iodomethane generates the Cr(III) iodide and Cr(III) methyl complexes. The rate of this single-electron oxidative addition reaction was shown to remain relatively invariant upon changing the β-diketiminate N-aryl substituent.

Introduction The renaissance of the β-diketiminate ligand may be attributed to its appealing tunability of electronic and steric properties.1 At one extreme in the range of sterics is the readily prepared (DppNCMe)2CH [Dpp=2,6-(Me2CH)2C6H3]2 and the even more demanding (DppNCtBu)2CH3 or metaterphenyl-substituted4 variants. These ligands have been demonstrated to stabilize first-row transition metal complexes with unusually low coordination numbers and interesting new modes of reactivity.5 Readily modifiable ancillary ligands are critical for understanding structure-activity relationships in organometallic reactivity. For paramagnetic organochromium complexes, a variety of carbon-carbon bond-forming applications are of current research interest. Ethylene polymerization

catalysts continue to be a major focus, for both systems involving alkyl aluminum cocatalysts6 and well-defined, single-component species.7 More recent studies have also examined oligomerization reactions8,9 and the selective trimerization10 and tetramerization11 of ethylene. For each of these cases, systematic variation of the ancillary ligands has provided crucial information regarding the mechanisms of these reactions and the nature of the catalytically active species. A distinct class of chromium-catalyzed C-C bond formation is the coupling of organic halides and aldehydes.12 The activation of organic halides, R-X, with Cr(II) is generally accepted to proceed via two single-electron oxidative addition steps, as shown in Scheme 1.13 The key step of this reaction is the rapid trapping of organic radicals by Cr(II) to form inert Cr(III) organochromium species.14 In fact,

*Corresponding author. E-mail: [email protected]. (1) (a) Bourget-Merle, L.; Lappert, M. F.; Severn, J. R. Chem. Rev. 2002, 102, 3031–3065. (b) Mindiola, D. J. Angew. Chem., Int. Ed. 2009, 48, 6198–6200. (2) Feldman, J.; McLain, S. J.; Parthasarathy, A.; Marshall, W. J.; Calabrese, J. C.; Arthur, S. D. Organometallics 1997, 16, 1514–1516. (3) Budzelaar, P. H. M.; van Oort, A. B.; Orpen, A. G. Eur. J. Inorg. Chem. 1998, 1485–1494. (4) Kenward, A. L.; Ross, J. A.; Piers, W. E.; Parvez, M. Organometallics 2009, 28, 3625–3628. (5) (a) Holland, P. L. Acc. Chem. Res. 2008, 41, 905–914. (b) Fan, H.; Adhikari, D.; Saleh, A. A.; Clark, R. L.; Zuno-Cruz, F. J.; Sanchez Cabrera, G.; Huffman, J. C.; Pink, M.; Mindiola, D. J.; Baik, M.-H. J. Am. Chem. Soc. 2008, 130, 17351–17361.

(6) (a) Gibson, V. C.; Spitzmesser, S. K. Chem. Rev. 2003, 103, 283– 315. (b) Smith, K. M. Curr. Org. Chem. 2006, 10, 955–963. (7) Theopold, K. H. Eur. J. Inorg. Chem. 1998, 15–24. (8) Small, B. L.; Carney, M. J.; Holman, D. M.; O’Rourke, C. E.; Halfen, J. A. Macromolecules 2004, 37, 4375–4386. (9) Albahily, K.; Al-Baldawi, D.; Gambarotta, S.; Duchateau, R.; Koc, E.; Burchell, T. J. Organometallics 2008, 27, 5708–5711. (10) Dixon, J. T.; Green, M. J.; Hess, F. M.; Morgan, D. H. J. Organomet. Chem. 2004, 689, 3641–3668. (11) Wass, D. Dalton Trans. 2007, 816–819. (12) F€ urstner, A. Chem. Rev. 1999, 99, 991–1045. (13) Smith, K. M. Coord. Chem. Rev. 2006, 250, 1023–1031. (14) van Eldik, R.; Gaede, W.; Cohen, H.; Meyerstein, D. Inorg. Chem. 1992, 31, 3695–3696.

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r 2009 American Chemical Society

Article

Organometallics, Vol. 28, No. 23, 2009 Scheme 1

catalytic amounts of cobalt or nickel salts are routinely added to generate R 3 from less reactive alkyl15 or aryl16 R-X substrates, respectively, which then react with Cr(II) to give the functional-group-tolerant organochromium(III) complex. Since the development of Nozaki-Hiyama-Kishi reactions that are catalytic in chromium,17 several different classes of chiral ligands have been used to perform the catalytic reaction asymmetrically.18,19 We previously reported the synthesis of CpCr[(DppNCMe)2CH], its reaction with iodomethane, and the independent synthesis of the corresponding Cr(III) methyl and iodo complexes.20 We were interested in determining how reducing the steric bulk of the β-diketiminate ligand would influence the reactivity of these complexes, in terms of both the synthesis and relative stability of the Cr(II) and Cr(III) complexes, as well as for the rate of iodomethane oxidative addition. Theopold and co-workers pioneered the recent revival of β-diketiminate ligand structure-activity studies with the synthesis of well-defined Cr(II) and Cr(III) complexes for catalytic olefin polymerization.21 Related structure-activity studies involving systematic variation of β-diketiminate ligands have been reported for applications ranging from C-H bond22 and dioxygen activation23 to catalytic copolymerization of CO2 and epoxides24 and olefin metathesis.25

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The chemistry in Scheme 1 has acquired unanticipated new urgency since our initial communication. Radical intermediates pervade C-C bond-forming reactions catalyzed by firstrow metals.26 These radicals are integral to shuttling between discrete two-electron redox cycles (Fe),27 intermolecular addition of radicals to olefins (Co),28 and enantioselective synthesis from racemic substrates (Ni).29 The relationships between oxidative addition by single-electron transfer,30 atom transfer radical polymerization (ATRP),31 and organometallic-mediated radical polymerization (OMRP)32 have been elucidated.33 If rendered reversible, the two equations in Scheme 1 are the equilibria responsible for ATRP and OMRP, respectively. Improvement of all of these catalytic reactions, as well as the development of new ones, will be achieved through synthetic paramagnetic organometallic chemistry. In order to establish structure-activity relationships for the critical bond-dissociation energies (BDEs) in Scheme 1, improved general routes to well-defined Cr(II), Cr(III)-X, and Cr(III)-R complexes will be required. We previously reported the use of CpCr[(DppNCMe)2CH] as a radical trap for the OMRP of vinyl acetate initiated with V-70.34 With the correct match of alkyl and ancillary ligand steric and electronic effects to attenuate the Cr(III)-R BDE, a singlecomponent OMRP reagent can be synthesized.35 The Cr(III)-CH3 compounds reported in this paper provide an excellent baseline for these ongoing studies, due to both the minimal steric pressure of the methyl ligand and the relative instability of the 3 CH3 radical.

Results and Discussion (15) Takai, K.; Nitta, K.; Fujimura, O.; Utimoto, K. J. Org. Chem. 1989, 54, 4732–4734. (16) (a) Jin, H.; Uenishi, J.-I.; Christ, W. J.; Kishi, Y. J. Am. Chem. Soc. 1986, 108, 5644–5646. (b) Takai, K.; Tagashira, M.; Kuroda, T.; Oshima, K.; Utimoto, K.; Nozaki, H. J. Am. Chem. Soc. 1986, 108, 6048–6050. (17) F€ urstner, A.; Shi, N. J. Am. Chem. Soc. 1996, 118, 12349–12357. (18) Hargaden, G. C.; Guiry, P. J. Adv. Synth. Catal. 2007, 349, 2407– 2424. (19) Guo, H.; Dong, C.-G.; Kim, D.-S.; Urabe, D.; Wang, J.; Kim, J. T.; Liu, X.; Sasaki, T.; Kishi, Y. J. Am. Chem. Soc. 2009, 131, 15387– 15393. (20) Doherty, J. C.; Ballem, K. H. D.; Patrick, B. O.; Smith, K. M. Organometallics 2004, 23, 1487–1489. (21) (a) Kim, W.-K.; Fevola, M. J.; Liable-Sands, L. M.; Rheingold, A. L.; Theopold, K. H. Organometallics 1998, 17, 4541–4543. (b) MacAdams, L. A.; Kim, W.-K.; Liable-Sands, L. M.; Guzei, I. A.; Rheingold, A. L.; Theopold, K. H. Organometallics 2002, 21, 952–960. (c) Theopold, K. H.; MacAdams, L. A.; Puttnual, C.; Buffone, G. P.; Rheingold, A. L. Polym. Mater. Sci. Eng. 2002, 86, 310. (d) MacAdams, L. A.; Buffone, G. P.; Incarvito, C. D.; Golen, J. A.; Rheingold, A. L.; Theopold, K. H. Chem. Commun. 2003, 1164–1165. (e) MacAdams, L. A.; Buffone, G. P.; Incarvito, C. D.; Rheingold, A. L.; Theopold, K. H. J. Am. Chem. Soc. 2005, 127, 1082–1083. (f) Monillas, W. H.; Yap, G. P. A.; MacAdams, L. A.; Theopold, K. H. J. Am. Chem. Soc. 2007, 129, 8090– 8091. (g) Monillas, W. H.; Yap, G. P. A.; Theopold, K. H. Angew. Chem., Int. Ed. 2007, 46, 6692–6694. (22) (a) Fekl, U.; Kaminsky, W.; Goldberg, K. I. J. Am. Chem. Soc. 2003, 125, 15286–15287. (b) Bernskoetter, W. H.; Lobkovsky, E.; Chirik, P. J. Organometallics 2005, 24, 6250–6259. (c) West, N. M.; Templeton, J. L. Can. J. Chem. 2009, 87, 288–296. (d) Lin, B.-L.; Bhattacharyya, K. X.; Labinger, J. A.; Bercaw, J. E. Organometallics 2009, 28, 4400–4405. (23) (a) Spencer, D. J. E.; Reynolds, A. M.; Holland, P. L.; Jazdzewski, B. A.; Duboc-Toia, C.; Le Pape, L.; Yokota, S.; Tachi, Y.; Itoh, S.; Tolman, W. B. Inorg. Chem. 2002, 41, 6307–6321. (b) Hong, S.; Hill, L. M. R.; Gupta, A. K.; Naab, B. D.; Gilroy, J. B.; Hicks, R. G.; Cramer, C. J.; Tolman, W. B. Inorg. Chem. 2009, 48, 4514–4523. (24) Cheng, M.; Moore, D. R.; Reczek, J. J.; Chamberlain, B. M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2001, 123, 8738– 8749. (25) Tonzetich, Z. J.; Jiang, A. J.; Schrock, R. R.; M€ uller, P. Organometallics 2007, 26, 3771–3783.

Synthesis of Chromium(II) Complexes. In our initial communication,20 the Cr(II) complex CpCr[(DppNCMe)2CH], 1a, served as the starting material for the Cr(III) CpCr[(DppNCMe)2CH]X complexes via single-electron oxidation reactions.30 Unlike [Cp*Cr( μ-Cl)]2,36 isolable monocyclopentadienyl Cr(II) complexes are not accessible from the direct reaction of NaC5H5 and CrCl2.37 Initially, CpCr[(DppNCMe)2CH] was prepared by reacting NaCp with the Cr(II) bridging-chloro dimer [Cr[(DppNCMe)2CH]( μ-Cl)]2 reported by Gibson and co-workers.38 (26) Rudolph, A.; Lautens, M. Angew. Chem., Int. Ed. 2009, 48, 2656–2670. (27) F€ urstner, A.; Martin, R.; Krause, H.; Seidel, G.; Goddard, R.; Lehmann, C. W. J. Am. Chem. Soc. 2008, 130, 8773–8787. (28) Affo, W.; Ohmiya, H.; Fujioka, T.; Ikeda, Y.; Nakamura, T.; Yorimitsu, H.; Oshima, K.; Imamura, Y.; Mizuta, T.; Miyoshi, K. J. Am. Chem. Soc. 2006, 128, 8068–8077. (29) Saito, B.; Fu, G. C. J. Am. Chem. Soc. 2008, 130, 6694–6695. (30) Smith, K. M. Organometallics 2005, 24, 778–784. (31) Ouchi, M.; Terashima, T.; Sawamoto, M. Chem. Rev., in press (doi: 10.1021/cr900234b). (32) Smith, K. M.; McNeil, W. S.; Abd-El-Aziz, A. Macromol. Chem. Phys., in press (macp.200900581). (33) Poli, R. Angew. Chem., Int. Ed. 2006, 45, 5058–5070. (34) Champouret, Y.; Baisch, U.; Poli, R.; Tang, L.; Conway, J. L.; Smith, K. M. Angew. Chem., Int. Ed. 2008, 47, 6069–6072. (35) Champouret, Y.; MacLeod, K. C.; Baisch, U.; Patrick, B. O.; Smith, K. M.; Poli, R. submitted for publication. (36) Heintz, R. A.; Ostrander, R. L.; Rheingold, A. L.; Theopold, K. H. J. Am. Chem. Soc. 1994, 116, 11387–11396. (37) Hermans, P. M. J. A.; Scholten, A. B.; van den Beuken, E. K.; Bussaard, H. C.; Roeloffsen, A.; Metz, B.; Reijerse, E. J.; Beurskens, P. T.; Bosman, W. P.; Smits, J. M. M.; Heck, J. Chem. Ber. 1993, 126, 553–563. (38) Gibson, V. C.; Newton, C.; Redshaw, C.; Solan, G. A.; White, A. J. P.; Williams, D. J. Eur. J. Inorg. Chem. 2001, 1895–1903.

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Subsequently, a more direct route was developed to prepare CpCr[(ArNCMe)2CH] 1a-d (Ar = Dpp 1a; Ar = Xyl (2,6-Me2C6H3) 1b; Ar=Mes (2,4,6-Me3C6H2) 1c; Ar=Dep (2,6-Et2C6H3) 1d), as shown in eq 1. As reported for the synthesis of 1b in 2008,34 the reaction involves sequential addition of NaCp, then Li[(ArNCMe)2CH] to CrCl2 suspended in THF. The reaction can be applied to all four β-diketiminate derivatives 1a-d and gives similar results using CrCl2(tmeda) or commercial CrCl2, as well as with isolated NaCp(THF)x or commercial 2.0 M NaCp in THF.

Adding NaCp to CrCl2 generates a mixture containing Cp2Cr as well as unreacted CrCl2, and yet subsequent addition of Li[(ArNCMe)2CH] gives the mono-Cp Cr(II) complexes in good yields. The apparent cyclopentadienylide transfer from Cp2Cr involved in this synthesis is presumably due in part to the steric bulk imparted by the 2,6-R2 substituents of the β-diketiminate ligands, which would disfavor the formation of Cr[(ArNCMe)2CH]2 complexes. The parent Cr[(PhNCMe)2CH]2 was prepared by Theopold and co-workers by salt metathesis of CrCl2 and the phenyl-substituted β-diketiminate ligand,21a while M[(XylNCMe)2CH]2 complexes can be generated for other first-row transition metals.5a In contrast, the very bulky (DppNCtBu)2CH ligand can be used to prepare threecoordinate Cr[(DppNCtBu)2CH]X complexes.5b Related cyclopentadienylide displacement reactions from chromocene have previously been reported. Jonas described the reaction of LiC6H4CH2NMe2 (prepared by directed ortho-metalation of N,N-dimethylbenzylamine) with Cp2Cr to give CpCr(C6H4CH2NMe2),39a which was shown to be a monomeric complex by X-ray diffraction.39b Other 14-electron, high-spin, monocyclopentadienyl Cr(II) d4 complexes have also been prepared from Cp2Cr.40 Synthesis of Chromium(III) Iodo Complexes. We previously communicated the use of PbX2 (X = Cl, Br, I)41 to oxidize CpCr[(DppNCMe)2CH] to the corresponding Cr(III) halides.20 Synthesis of the CpCr[(ArNCMe)2CH]I complexes (2b-d) by single-electron oxidation is more conveniently achieved using 0.5 equiv of I2 in Et2O (eq 2). The isolated Cr(III) iodide complexes are air stable as crystalline solids.

MacLeod et al.

isolated and structurally characterized from the reaction of Cp2Cr and NdI2 or DyI2.42 Reaction of Li[(DppNCMe)2CH] with CrI2 also gives a bridging iodide dimer, as demonstrated by X-ray crystallography.21f The Cr(II) [Cr[(DppNCMe)2CH]( μ-I)]2 dimer can be reduced to a β-diketiminate Cr(I) species capable of a range of small molecule activation reactions.21f,44 Synthesis of Chromium(III) Methyl Complexes. Chromium(III) methyl compounds are frequently prepared as catalyst precursors for olefin polymerization.7,8,21,38 As shown in eq 3, CpCr[(ArNCMe)2CH](CH3) complexes (3b-d) can be synthesized by alkylation of the corresponding Cr(III) iodide (2b-d) or chloride (4b-d) compounds with MeMgI in Et2O. Addition of 1,4-dioxane prior to workup aids in the removal of the MgX2 byproducts of the salt metathesis reactions. All of these methyl compounds are highly soluble in nonpolar solvents, including pentane and hexanes.

The ease of synthesis of CpCr[(ArNCMe)2CH](CH2R) complexes with Grignard reagents appears to depend on the degree of steric congestion in the Cr(III) alkyl compounds. For Ar = Dpp, the Cr(III) methyl complex was most readily prepared from a Cr(III) triflate precursor. For the smaller β-diketiminate ligands where Ar = Xyl, Mes, or Dep, complexes 3b-d can be obtained from the corresponding Cr(III) halides. Synthesis of Chromium(III) Chloride Complexes. As in the synthesis of 3b-d, the salt metathesis reaction to prepare the Cr(III) chloride complexes 4b-d also proceeds more readily with the smaller β-diketiminate ligands.45 A similar trend was recently reported by Jin and co-workers, who prepared CpCr[(PhNCMe)2CH]Cl by reaction of Li[(PhNCMe)2CH] with CpCrCl2(THF),46 while attempts to prepare CpCr[(DppNCMe)2CH]Cl by this procedure were unsuccessful.20,46 Interestingly, CpCr[(PhNCMe)2CH]Cl46 and related CpCr(LX)Cl and Cp*Cr(LX)Cl complexes47 generate efficient ethylene polymerization catalysts when activated with relatively small amounts of AlR3 cocatalysts.

Related iodide species have been recently reported for several Cr(II) complexes.21f,42,43 Dimeric [CpCr( μ-I)]2 was (39) (a) Jonas, K. Angew. Chem., Int. Ed. Engl. 1985, 24, 295–311. (b) Angermund, K.; Claus, K. H.; Goddard, R.; Kr€uger, C. Angew. Chem., Int. Ed. Engl. 1985, 24, 237–247. (40) (a) Voges, M. H.; Roemming, C.; Tilset, M. Organometallics 1999, 18, 529–533. (b) Abernethy, C. D.; Clyburne, J. A. C.; Cowley, A. H.; Jones, R. A. J. Am. Chem. Soc. 1999, 121, 2329–2330. (41) Luinstra, G. A.; Teuben, J. H. J. Chem. Soc., Chem. Commun. 1990, 1470–1471. (42) Burin, M. E.; Smirnova, M. V.; Fukin, G. K.; Baranov, E. V.; Bocharev, M. N. Eur. J. Inorg. Chem. 2006, 351–356. (43) (a) Monillas, W. H.; Yap, G. P. A.; Theopold, K. H. J. Chem. Crystallogr. 2009, 39, 73–77. (b) Monillas, W. H.; Yap, G. P. A.; Theopold, K. H. J. Chem. Crystallogr. 2009, 39, 377–379. (c) Monillas, W. H.; Yap, G. P. A.; Theopold, K. H. J. Chem. Crystallogr. 2009, 39, 842–845.

As shown in eq 4, Cr(III) chloride complexes 4b-d are prepared in a two-step, one-pot reaction starting from (44) Tsai, Y.-C.; Wang, P.-Y.; Chen, S.-A.; Chen, J.-M. J. Am. Chem. Soc. 2007, 129, 8066–8067. (45) A related Cr(III) chloride complex with the smallest β-diketiminate ligand, Cp*Cr[(HNCMe)2CH]Cl, was prepared 20 years ago by Theopold and co-workers from the reaction of [Cp*Cr(CH3)( μ-Cl)]2 with NCMe: Richeson, D. S.; Mitchell, J. F.; Theopold, K. H. J. Am. Chem. Soc. 1989, 8, 2570–2577. (46) Huang, Y.-B.; Jin, G.-X. Dalton Trans. 2009, 767–769. (47) (a) Xu, T.; Mu, Y.; Gao, W.; Ni, J.; Ye, L.; Tao, Y. J. Am. Chem. Soc. 2007, 129, 2236–2237. (b) Huang, Y.-B.; Yu, W.-B.; Jin, G.-X. Organometallics 2009, 28, 4170–4174.

Article

anhydrous CrCl3 suspended in THF, as described previously for complex 4b.35 Treatment with Li[(ArNCMe)2CH] generates red-brown solutions, presumably containing Cr[(ArNCMe)2CH]Cl2(THF)n complexes analogous to those previously reported by Gibson38 and Theopold.21 Reaction of these solutions in situ with NaCp leads to the desired CpCr[(ArNCMe)2CH]Cl complexes 4b-d, which can be recrystallized from hexanes/CH2Cl2. While the intermediate species are air-sensitive, the anhydrous CrCl3 precursor and complexes 4b-d are air-stable as solids. Like the Cr(III) iodide compounds 2b-d, the Cr(III) chloride complexes 4b-d are useful precursors for the synthesis of the Cr(III) methyl species 3b-d (eq 3). X-ray Crystal Structures of Cr(III) Methyl and Halide Complexes. Single-crystal X-ray diffraction was an invaluable technique for confirming the identities of the new Cr(III) complexes. Crystallographic data for complexes 2b, c, 3b-d, and 4c,d are shown in Tables 1 and 2. The Cr-X bond lengths for CpCr[(ArNCMe)2CH]X (X = I (2a-c), CH3 (3a-d), or I (4a-d)) are shown in Table 3. The molecular structures of the complexes are shown in Figures 1-7 (CNT denotes the centroid of the cyclopentadienyl ring). The reactivity of the Cr(III) methyl and halide complexes was expected to be related to their Cr-X bond dissociation energies.33 Comparing the structural data for the current CpCr[(ArNCMe)2CH]X complexes and those previously reported20 was therefore of interest to obtain structural evidence for the influence of the β-diketiminate aryl substituent on the Cr-X bonds. As shown in Table 3, no general trend in Cr-X bond lengths with varying Ar substituents was evident. The Cr-CH3 distances are close to that reported for [CpCr( μ-Cl)(CH3)]2 (2.073(3) A˚),48a Cp*Cr(PMe3)(CH3)2 (2.067(5) A˚),48b and other neutral Cr(III) methyl half-sandwich complexes.48c As expected, the Cr-CH3 bond lengths in 3a-d were longer than in cationic Cr(III) half-sandwich complexes49 or in neutral38 or cationic21c β-diketiminate Cr(III) methyl compounds with coordination numbers of 5 or less. The overall geometries of the CpCr[(ArNCMe)2CH]X complexes were relatively constant, as shown by the selected bond lengths and angles listed in the captions of Figures 1-7. The only notable difference was the distance from the Cr to the centroid of the cyclopentadienyl ligand, Cr-CNT, which was longer for the methyl complexes (1.952 to 1.967 A˚ for 3b-d) than for the iodide or chloride compounds (1.901 to 1.911 A˚ for 2b,c and 4c,d), presumably resulting from the increased electron-donating ability of the alkyl ligand over that of the halides. The anomalously long Cr-CH3 distance in 3d needs to be accounted for. The thermal parameters and electron density distribution of the methyl carbon atom in 3d do not appear to be unusual. One possible explanation is that 3d, which was synthesized from 2d and MeMgI, contains a small amount of CpCr[(DepNCMe)2CH]I as a cocrystallized impurity. Parkin and co-workers have demonstrated that in such cases the (48) (a) Richeson, D. S.; Hsu, S.-W.; Fredd, N. H.; Van Duyne, G.; Theopold, K. H. J. Am. Chem. Soc. 1986, 108, 8273–8274. (b) Grohmann, A.; K€ ohler, F. H.; M€ uller, G.; Zeh, H. Chem. Ber. 1989, 122, 897–899. (c) D€ ohring, A.; G€ ohre, J.; Jolly, P. W.; Kryger, B.; Rust, J.; Verhovnik, G. P. J. Organometallics 2000, 19, 388–402. (49) Thomas, B. J.; Noh, S. K.; Schulte, G. K.; Sendlinger, S. C.; Theopold, K. H. J. Am. Chem. Soc. 1991, 113, 893–902. (50) (a) Yoon, K.; Parkin, G. J. Am. Chem. Soc. 1991, 113, 8414– 8418. (b) Parkin, G. Acc. Chem. Res. 1992, 25, 455–460.

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Table 1. Crystal Data and Refinement Parameters for X-ray Structures of 2b, 2c, and 3b 2b formula fw cryst color, habit cryst dimens, mm

C26H30N2CrI 549.42 black, prism 0.50  0.40  0.20 cryst syst monoclinic space group P21/n a, A˚ 8.2236(4) b, A˚ 22.710(1) c, A˚ 12.8442(7) R, deg 90.0 β, deg 97.115(4) γ, deg 90.0 2380.3(2) V, A˚3 Z 4 3 1.533 Dcalc, g/cm 1108.00 F000 1.793 μ(Mo KR), cm-1 data images (no., t/s) 620, 8 55.1 2θmax, deg reflns measd 24 822 4706, 0.016 unique reflns, Rint 0.463, 0.699 absorp, Tmin, Tmax obsd data (I > 2.00σ(I)) 4150 no. params 297 R1, wR2 (F2, all data) 0.037, 0.075 R1, wR2 (F, I > 0.031, 0.073 2.00σ(I)) goodness of fit 1.19 0.64, -0.42 max., min. peak3, e-/A˚3

2c

3b

C28H34N2CrI 577.47 black, block 0.35  0.25  0.20 orthorhombic Pnma 14.758(1) 21.794(2) 7.9989(7) 90.0 90.0 90.0 2572.7(4) 4 1.491 1172.00 16.62 580, 8 55.0 25 524 2924, 0.013 0.371, 0.717 2618 154 0.028, 0.059 0.023, 0.057

C27H33N2Cr 437.55 black, irregular 0.40  0.12  0.08 triclinic P1 8.055(1) 11.195(2) 14.967(2) 102.143(8) 103.662(7) 110.500(6) 1163.0(3) 2 1.249 466.00 5.07 620,16 55.1 12 272 4406, 0.028 0.805, 0.960 3548 294 0.056, 0.096 0.039, 0.090

1.06 0.39, -0.50

1.04 0.32, -0.27

observed bond lengths are actually more sensitive than either the electron-density distribution or the thermal parameters.50 If present, the amount of the putative 2d cocrystallized impurity in 3d would have to be very small, since the observed bond length would be weighted by the vastly greater scattering power of the iodide ligand over that of the methyl group, as shown by Parkin for tris(3-tert-butylpyrazolyl)hydroboratozinc complexes with cocrystallized I and CH3 ligands.50a Indeed, our attempts to model a residual electron density peak beyond the methyl group with a very low occupancy (around 1%) of an iodide atom did result in the contraction of the Cr-CH3 distance to 2.078 A˚. Although we were unable to quantitatively model such a small amount of the presumed impurity with confidence, we currently consider this to be the most plausible explanation of the anomalous Cr-CH3 bond length in 3d. In any event, being cognizant of the potential for impurity cocrystallization is especially critical for paramagnetic organometallic complexes. While the remarkable crystallinity engendered by the cyclopentadienyl and ortho-disubstituted N-aryl β-diketiminate ligands indisputably aids in the synthesis and structural characterization of CpCr[(ArNCMe)2CH]X complexes, these same properties also promote insidious cocrystallization problems. Reactivity with Iodomethane. The ability of Cr(II) to undergo single-electron oxidative addition reactions with organic halides has been known since the reaction of [Cr(H2O)6]2þ with benzyl chloride was found to generate [Cr(H2O)5Cl]2þ and [Cr(H2O)5(CH2Ph)]2þ in aqueous solution.51 The mechanism shown in Scheme 1 was initially (51) Anet, F. A. L.; Leblanc, E. J. Am. Chem. Soc. 1957, 79, 2649– 2560.

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Table 2. Crystal Data and Refinement Parameters for X-ray Structures of 3c, 3d, 4c, and 4d

formula fw cryst color, habit cryst dimens, mm cryst syst space group a, A˚ b, A˚ c, A˚ R, deg β, deg γ, deg V, A˚3 Z Dcalc, g/cm3 F000 μ(Mo KR), cm-1 data images (no., t/s) 2θmax reflns measd unique reflns, Rint absorp, Tmin, Tmax obsd data (I > 2.00σ(I)) no. params R1, wR2 (F2, all data) R1, wR2 (F, I > 2.00σ(I)) goodness of fit max., min. peak3, e-/A˚3

3c

3d

4c

4d

C29H37N2Cr 465.61 black, plate 0.05  0.30  0.50 triclinic h P1 9.4802(10) 11.9897(14) 12.2729(14) 70.025(5) 79.696(5) 77.347(5) 1271.0(2) 2 1.217 498.00 4.68 1694, 7 55.0 21 914 5785, 0.026 0.894, 0.977 5022 296 0.043, 0.094 0.035, 0.088 1.06 0.37, -0.30

C31H41N2Cr 493.66 black, prism 0.30  0.50  0.50 triclinic P1 10.0044(7) 12.3899(7) 13.0646(9) 62.733(1) 81.202(2) 70.457(2) 1356.5(2) 2 1.209 530.00 4.42 2354, 5 55.7 28 265 5712, 0.024 0.804, 0.876 4613 326 0.064, 0.135 0.049, 0.126 1.04 0.99, -0.46

C28H34N2CrCl 486.02 black, tablet 0.20  0.40  0.44 orthorhombic Pca21 14.3818(10) 15.2520(10) 25.8696(19) 90.0 90.0 90.0 5674.5(7) 8 1.138 2056.00 5.13 1157, 5 56.0 57 907 13 222, 0.042 0.809, 0.902 10 409 593 0.054, 0.090 0.038, 0.084 1.00 0.29, -0.32

C30H38N2CrCl 514.07 green, needle 0.04  0.15  0.30 triclinic P1 8.0490(18) 11.214(3) 15.462(4) 81.291(7) 86.346(8) 74.509(8) 1329.0(6) 2 1.285 546.00 5.52 941, 20 45.6 10 278 3437, 0.060 0.771, 0.978 7267 317 0.090, 0.144 0.058, 0.130 1.04 0.49, -0.48

Figure 1. Thermal ellipsoid diagram (50%) of 2b. Selected bond lengths (A˚): Cr(1)-N(1), 2.017(2); Cr(1)-N(2), 2.021(2); Cr(1)-CNT, 1.908; Cr(1)-I(1), 2.6933(5). Selected bond angles (deg): N(1)-Cr(1)-N(2), 90.09(9); N(1)-Cr(1)-I(12), 95.65(7); N(2)-Cr(1)-I(1), 95.53(7); CNT-Cr(1)-I(1), 117.40.

Figure 2. Thermal ellipsoid diagram (50%) of 2c. Selected bond lengths (A˚): Cr(1)-N(1), 2.0277(15); Cr(1)-CNT, 1.905; Cr(1)-I(1), 2.6941(5). Selected bond angles (deg): N(1)-Cr(1)-N(1*), 91.54(9); N(1)-Cr(1)-I(1), 96.51(5); CNT-Cr(1)-I(1), 117.06.

established with the Cr(II) aqueous system, where rate constants for the trapping of carbon-based radicals with [Cr(H2O)6]2þ were shown to be remarkably rapid despite the need to displace a bound water molecule.14 The overall rate of the reaction depended on the initial halogen atom

Figure 3. Thermal ellipsoid diagram (50%) of 3b. Selected bond lengths (A˚): Cr(1)-N(1), 2.0246(17); Cr(1)-N(2), 2.0250(17); Cr(1)-CNT, 1.952; Cr(1)-C(27), 2.076(2). Selected bond angles (deg): N(1)-Cr(1)-N(2), 89.96(7); N(1)-Cr(1)-C(27), 94.25(9); N(2)-Cr(1)-C(27), 94.35(9); CNT-Cr(1)-C(27), 118.10.

Figure 4. Thermal ellipsoid diagram (50%) of 3c. Selected bond lengths (A˚): Cr(1)-N(1), 2.0245(13); Cr(1)-N(2), 2.0231(13); Cr(1)-CNT, 1.967; Cr(1)-C(29), 2.0645(17). Selected bond angles (deg): N(1)-Cr(1)-N(2), 90.73(5); N(1)-Cr(1)-C(29), 93.98(6); N(2)-Cr(1)-C(29), 93.98(6); CNT-Cr(1)-C(29), 117.72.

abstraction step and showed the usual trends in terms of halide (increasing rate with Cl < Br < I) and alkyl (increasing rate with primary < secondary < tertiary) upon

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Table 3. Comparison of Cr-X Bond Lengths (A˚) in CpCr[(ArNCMe)2CH]X Complexes (X = I (2a-c), CH3 (3a-d), or Cl (4a-d) Cr-X Cr-I Cr-CH3 Cr-Cl a

Figure 5. Thermal ellipsoid diagram (50%) of 3d. Selected bond lengths (A˚): Cr(1)-N(1), 2.0287(19); Cr(1)-N(2), 2.017(2); Cr(1)-CNT, 1.965; Cr(1)-C(31), 2.112(3). Selected bond angles (deg): N(1)-Cr(1)-N(2), 90.20(8); N(1)-Cr(1)-C(31), 93.65(9); N(2)-Cr(1)-C(31), 94.73(9); CNT-Cr(1)-C(31), 118.64.

Figure 6. Thermal ellipsoid diagram (50%) of 4c (one of two independent molecules in unit cell shown). Selected bond lengths (A˚): Cr(1)-N(1), 2.023(2); Cr(1)-N(2), 2.015(2); Cr(1)-CNT, 1.911; Cr(1)-Cl(1), 2.3090(7). Selected bond angles (deg): N(1)-Cr(1)-N(2), 90.65(8); N(1)-Cr(1)-Cl(1), 92.97(6); N(2)-Cr(1)-Cl(1), 94.46(6); CNT-Cr(1)-Cl(1), 102.72.

Figure 7. Thermal ellipsoid diagram (50%) of 4d. Selected bond lengths (A˚): Cr(1)-N(1), 2.016(3); Cr(1)-N(2), 2.023(2); Cr(1)CNT, 1.901; Cr(1)-Cl(1), 2.2972(11). Selected bond angles (deg): N(1)-Cr(1)-N(2), 90.29(12); N(1)-Cr(1)-Cl(1), 93.54(9); N(2)-Cr(1)-Cl(1), 95.03(9); CNT-Cr(1)-Cl(1), 119.90.

varying the R-X substrate.52 Addition of more electrondonating ligands such as ethylenediamine52 or tetraazamacrocycles53 increased the measured rates of the oxidative addition reaction. The chromium-mediated coupling of alkyl halides and aldehydes, the Takai-Utimoto reaction,15 is typically con(52) Kochi, J. K.; Powers, J. W. J. Am. Chem. Soc. 1970, 92, 137–146. (53) (a) Samuels, G. J.; Espenson, J. H. Inorg. Chem. 1979, 18, 2587– 2592. (b) Espenson, J. H. Acc. Chem. Res. 1992, 25, 222–227.

Ar = Dpp a

2a 2.6813(5) 3a 2.071(2)a 4a 2.292(2)a

Ar = Xyl

Ar = Mes

Ar = Dep

2b 2.6933(5) 3b 2.076(2) 4b 2.308(2)b

2c 2.6941(5) 3c 2.0645(17) 4c 2.3090(7)

3d 2.112(3) 4d 2.2792(11)

Ref 20. b Ref 34.

ducted in coordinating solvents such as DMF and requires a cobalt catalyst to activate the organic halide.54 These requirements complicate the process of screening ligand reactivity effects, particularly since any added ligands could potentially bind to either transition metal present.55 Well-defined organometallic chromium(II) complexes capable of single-electron oxidative addition of organic halides typically lead to the Cr(III) halide products but not the more reactive Cr(III) alkyl species.17,21b,40a,56

The reaction shown in eq 5 addresses several of theses issues. As discussed above, both the reactant and the products are stable and can be independently synthesized. Complexes 1a-d do not have any additional ligands that require displacement prior to reaction, and comparison of the X-ray crystal structures of reactants and products suggests that only minimal reorganization at the Cr center would be required for the reaction.53b The UV-vis spectra of the Cr(II), the Cr(III) iodide, and Cr(III) methyl species are all readily distinguishable, with the peak around 530 nm for the methyl complexes 3a-d being particularly distinctive. As previously reported for 1a,20 complexes 1b-d react with iodomethane in noncoordinating solvents without any additional cobalt reagent being required to give UV-vis spectra identical to 1:1 mixtures of the corresponding Cr(III) iodide and Cr(III) methyl complexes. The ability of complexes 1a-d to successfully trap organic radicals is likely at least partially due to the stability of Cr(III) CpCrLX2 half-sandwich complexes.57,58 The rates of iodomethane (54) Wessjohann, L. A.; Schmidt, G.; Schrekker, H. S. Tetrahedron 2008, 64, 2134–2142. (55) The potential for ligand scrambling between Cr and Mn or Ni has been discussed by Cozzi and Kishi, respectively: (a) Bandini, M.; Cozzi, P. G.; Umani-Ronchi, A. Chem. Commun. 2002, 919–927. (b) Namba, K.; Cui, S.; Wang, J.; Kishi, Y. Org. Lett. 2005, 7, 5417–5419. (c) Namba, K.; Wang, J.; Cui, S.; Kishi, Y. Org. Lett. 2005, 7, 5421–5424. (56) (a) Fischer, E. O.; Ulm, K.; Kuzel, P. Z. Anorg. Allg. Chem. 1963, 319, 253–265. (b) Hermes, A. R.; Morris, R. J.; Girolami, G. S. Organometallics 1988, 7, 2372–2379. (c) Nefedov, S. E.; Pasynskii, A. A.; Eremenko, I. L.; Orazsakhatov, B.; Ellert, O. G.; Novotortsev, V. M.; Katser, S. B.; Antsyshkina, A. S.; Porai-Koshits, M. A. J. Organomet. Chem. 1988, 345, 97–104. (d) Fryzuk, M. D.; Leznoff, D. B.; Rettig, S. J.; Young, V. G. Jr. J. Chem. Soc., Dalton Trans. 1999, 147–154. (57) (a) Poli, R. Chem. Rev. 1996, 96, 2135–2204. (b) Heintz, R. A.; Leelasubcharoen, S.; Liable-Sands, L. M.; Rheingold, A. L.; Theopold, K. H. Organometallics 1998, 17, 5477–5485, and references therein. (c) Gallant, A. J.; Smith, K. M.; Patrick, B. O. Chem. Commun. 2002, 2914–2915. (58) The stabilizing effect provided by the Cp ancillary ligand was demonstrated by Fryzuk and co-workers in ref 56d with the reactivity of CrR[N(SiMe2CH2PPh2)2] with benzyl chloride. The square-planar Cr(II) alkyl with R = CH2SiMe3 reacted with PhCH2Cl to give the Cr(R)(Cl)[N(SiMe2CH2PPh2)2] complex only, while the compound with R=Cp gave a mixture of both CpCr(III) chloride and CpCr(III) benzyl complexes.

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activation were gauged by reacting 1b-d with at least 10 equiv of MeI and monitoring the growth of the absorption at 530 nm. The second-order rate constants were calculated from the slope of plots of kobs vs [MeI] under the pseudofirst-order conditions. The rate constants obtained were k = (2.4 ( 0.3)  10-2 M-1 s-1 for 1b, k = (2.8 ( 0.2)  10-2 M-1 s-1 for 1c, and k = (1.9 ( 0.2)  10-2 M-1 s-1 for 1d, compared to the value of k = (2.8 ( 0.3)  10-2 M-1 s-1 previously obtained for 1a.20 The similarity of the rate constants for the CpCr[(ArNCMe)2CH] complexes was unexpected. It had been anticipated that varying the ortho substituents of the β-diketiminate aryl groups would influence the approach of the MeI to the Cr center in the inner-sphere iodide atom abstraction, which constitutes the rate-determining step of the reaction (Scheme 1). It is possible that the Cr 3 3 3 I interaction in the transition state is too long for the differences in Ar groups to have a significant impact on the rate of the reaction. As noted previously for 1a,20 the rate constants for 1b-d are comparable to that obtained for the activation of MeI in aqueous solution for the Cr(II) complex with the macrocyclic [15]aneN4 ligand, where a value of k = (4.6 ( 1)  10-2 M-1 s-1 was obtained.53a The rate-enhancing effect of chelating monoanionic N-donor ligands on the oxidative addition of MeI was also observed in Rh(I) carbonyl complexes.59

Conclusions The CpCr[(ArNCMe)2CH] framework appears well suited to the investigation of single-electron transfer oxidative addition of organic halides. Well-defined Cr(II) complexes with tunable ancillary ligands that effectively trap organic radicals are required for the development of structure-activity relationships for this reactivity mode, which has applications for organic synthesis as well as controlled radical polymerization. Modifying the β-diketiminate N-aryl groups from the popular 2,6-diisopropylphenyl group to less sterically demanding substituents did not adversely effect the relative stability or crystallinity of the Cr(III) products. In fact, the steric reduction aided in the synthesis of the Cr(III) chloride and Cr(III) methyl complexes via salt metathesis reactions. The slight decrease in hexanes solubility also aids in the recrystallization of these complexes, particularly for the Cr(III) methyl compounds due to their high solubility in nonpolar solvents. The CpCr[(ArNCMe)2CH] complexes all reacted with iodomethane with rate constants ranging from (1.9 ( 0.2)  10-2 to (2.8 ( 0.2)  10-2 M-1 s-1.

Experimental Section General Considerations. All reactions were carried out under nitrogen using standard Schlenk and glovebox techniques. Solvents were dried by using the method of Grubbs.60 Celite (Aldrich) was dried overnight at 120 °C before being evacuated and then stored under nitrogen. Iodine was purified by sublimation before use. n-BuLi (1.6 M in hexanes), p-toluenesulfonic acid monohydrate, NaCp (2.0 M in THF), CrCl3 (anhydrous), and methylmagnesium iodide (3.0 M in Et2O) were purchased from Aldrich and used as received. The β-diketiminate ligands H[(2,6-Me2C6H3NCMe)2CH], H[(2,4,6-Me3C6H2NCMe)2CH], (59) Gaunt, J. A.; Gibson, V. C.; Haynes, A.; Spitzmesser, S. K.; White, A. J. P.; Williams, D. J. Organometallics 2004, 23, 1015–1023. (60) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J. Organometallics 1996, 15, 1518–1520.

MacLeod et al. and H[(2,6-Et2C6H3NCMe)2CH] were prepared according to the literature procedure.24 The Cr(II) β-diketiminate compounds 1a,20 1b,34 1c,d,35 and 4b35 were prepared as described in the literature. UV/vis spectroscopic data were collected on a Varian Cary 100 Bio UV-visible spectrophotometer in pentane or hexanes solution in a specially constructed cell for air-sensitive samples: a Kontes Hi-Vac Valve with PTFE plug was attached by a professional glassblower to a Hellma 10 mm path length quartz absorption cell with a quartz-to-glass graded seal. Elemental analyses were performed by Guelph Chemical Laboratories, Guelph, ON, Canada. Synthesis of CpCr[(2,6-Me2C6H3NCMe)2CH]I (2b). To a solution of 1b (273 mg, 0.646 mmol) in Et2O was added I2 (81.9 mg, 0.323 mmol) as a solution in 4 mL of Et2O. After stirring overnight, the solvent was removed in vacuo, the residue was extracted with Et2O and filtered through Celite, and the green solution (red to transmitted) was cooled to -30 °C. 2b (198 mg, 55.9%) was isolated in four crops over several days. Anal. Calcd for C26H30N2CrI: C, 56.84; H, 5.50; N, 5.01. Found: C, 56.67; H, 5.75; N, 5.12. Mp: 223-225 °C. UV/vis (pentane; λmax, nm (ε, M-1 cm-1)): 431 (7690), 586 (1520). Synthesis of CpCr[(2,4,6-Me3C6H2NCMe)2CH]I (2c). To a solution of 1c (176 mg, 0.391 mmol) in Et2O was added I2 (49.5 mg, 0.195 mmol) as a solution in 4 mL of Et2O. After stirring overnight, the solvent was removed in vacuo, the residue was extracted with Et2O and filtered through Celite, and the green solution (red to transmitted) was cooled to -30 °C. 2c (156 mg, 69.0%) was isolated in two crops over several days. Anal. Calcd for C28H34N2CrI: C, 58.24; H, 5.93; N, 4.85. Found: C, 58.01; H, 6.23; N, 5.00. Mp: 232-234 °C. UV/vis (pentane; λmax, nm (ε, M-1 cm-1)): 422 (6710), 581 (1550). Synthesis of CpCr[(2,6-Et2C6H3NCMe)2CH]I (2d). To a solution of 1d (104 mg, 0.218 mmol) in 10 mL of Et2O was added I2 (32.4 mg, 0.128 mmol) as a solid. After stirring overnight, the green solution (red to transmitted) was cooled to -30 °C to yield 2d (77.5 mg, 58.8%). Anal. Calcd for C30H38N2CrI: C, 59.50; H, 6.33; N, 4.63. Found: C, 59.14; H, 6.51; N, 4.75. Mp: 209-211 °C. UV/vis (pentane; λmax, nm (ε, M-1 cm-1)): 423 (7310), 580 (1690). Synthesis of CpCr[(2,6-Me2C6H3NCMe)2CH]Me (3b). To a solution of 4b (108 mg, 0.235 mmol) in THF was added MeMgI (0.15 mL of a 3.0 M solution in Et2O, 0.45 mmol), and the mixture was left to stir at room temperature overnight. The solvent was removed in vacuo, the residue was extracted with pentane and filtered through Celite, and the solvent was again removed in vacuo. The purple solid was extracted with a minimum amount of pentane, filtered, and cooled to -30 °C overnight to yield black crystals of 3b (52 mg, 51%). Anal. Calcd for C27H33N2Cr: C, 74.11; H, 7.60; N, 6.40. Found: C, 73.82; H, 7.89; N, 6.15. Mp: 170-172 °C. UV/vis (pentane; λmax, nm (ε, M-1 cm-1)): 415 (6070), 543 (2160). Synthesis of CpCr[(2,4,6-Me3C6H2NCMe)2CH]Me (3c). To a solution of 4c (105 mg, 0.216 mmol) in 20 mL of Et2O was added MeMgI (0.080 mL of a 3.0 M solution in Et2O, 0.24 mmol), and the mixture was left to stir at room temperature overnight. To the resulting purple solution was added an excess of 1,4-dioxane (0.20 mL) and allowed to stir for 1 h, at which time the reaction mixture was filtered through Celite. The solvent was removed in vacuo, and the residue was extracted with 1 mL of hexanes, filtered, rinsed with 1 mL of hexanes, and cooled to -35 °C to yield black crystals of 3c (77.5 mg, 77%) isolated in two crops over several days. Anal. Calcd for C29H37N2Cr: C, 74.81; H, 8.01; N, 6.02. Found: C, 75.14; H, 8.33; N, 6.18. Mp: 181-183 °C. UV/vis (pentane; λmax, nm (ε, M-1 cm-1)): 406 (6940), 533 (2360). Synthesis of CpCr[(2,6-Et2C6H3NCMe)2CH]Me (3d). To a solution of 1d (113 mg, 0.237 mmol) in Et2O was added I2 (35.8 mg, 0.141 mmol) as a solid. After stirring for 2 h, MeMgI (0.090 mL of a 3.0 M solution in Et2O, 0.27 mmol) was added, and the solution was left to stir for 1.5 h, at which time an excess

Article of 1,4-dioxane (0.10 mL) was added. After stirring for 15 min, the solvent was removed in vacuo, and the residue was extracted with 15 mL of pentane, filtered through Celite, rinsed with 2 mL of pentane, and cooled to -30 °C overnight to yield 3d (46.0 mg, 39%). Anal. Calcd for C31H41N2Cr: C, 75.42; H, 8.37; N, 5.67. Found: C, 75.09; H, 8.76; N, 5.69. Mp: 156-158 °C. UV/vis (pentane; λmax, nm (ε, M-1 cm-1)): 392 (5000), 521 (1790). Synthesis of CpCr[(2,4,6-Me3C6H2NCMe)2CH]Cl (4c). A solution of H[(2,4,6-Me3C6H2NCMe)2CH] (2.35 g, 7.03 mmol) in 20 mL of THF in a Schlenk flask was cooled to 0 °C in an ice-water bath. n-BuLi (4.90 mL of 1.6 M solution in hexanes, 7.84 mmol) was added dropwise with stirring. The resulting yellow solution was allowed to react for 40 min while warming to room temperature, at which time it was cannulated into a Schlenk flask containing a suspension of CrCl3 (1.13 g, 7.15 mmol) in THF (10 mL) and was left to stir at room temperature overnight. NaCp (3.90 mL of a 2.0 M solution in THF, 7.80 mmol) was added, and the solution was again left to stir overnight at room temperature. The solvent was removed in vacuo, and the residue was extracted with 65 mL of hexanes and 18 mL of dichloromethane, filtered through Celite, and rinsed with 10 mL of hexanes. The resulting green solution (orange to transmitted) was concentrated and cooled to -20 °C to yield dark green crystals of 4c (2.55 g, 74.7%) isolated in four crops over several days. Anal. Calcd for C28H34N2CrCl: C, 69.19; H, 7.05; N, 5.76. Found: C, 69.17; H, 7.43; N, 5.43. Mp: 221-223 °C. UV/vis (hexanes; λmax, nm (ε, M-1 cm-1)): 419 (8060), 583 (798). Synthesis of CpCr[(2,6-Et2C6H3NCMe)2CH]Cl (4d). A solution of H[(2,6-Et2C6H3NCMe)2CH] (3.65 g, 10.1 mmol) in 60 mL of THF in a Schlenk flask was cooled to 0 °C in an ice-water bath. n-BuLi (6.90 mL of a 1.6 M solution in hexanes, 11.0 mmol) was added dropwise with stirring. The resulting yellow solution was allowed to react for 40 min while warming to room temperature, at which time it was cannulated into a Schlenk flask containing a suspension of CrCl3 (1.60 g, 10.1 mmol) in THF (30 mL) and was left to stir at room temperature overnight. NaCp (5.50 mL of a 2.0 M solution in THF, 11.0 mmol) was added, and the solution was again left to stir overnight at room temperature. The solvent was removed in vacuo, and the residue was extracted with 65 mL of hexanes, 30 mL of Et2O, and 25 mL of dichloromethane, filtered through Celite, and rinsed with hexanes (2  10 mL). The resulting green solution (orange to transmitted) was concentrated and cooled to -20 °C to yield black crystals of 4d (4.11 g, 79.4%) isolated in three crops over several days. Anal. Calcd for C30H38N2CrCl: C, 70.09; H, 7.45; N, 5.45. Found: C, 69.82; H, 7.17; N, 5.07. Mp: 195-197 °C. UV/vis (hexanes; λmax, nm (ε, M-1 cm-1)): 420 (8670), 585 (875). Kinetics Measurements. All kinetics measurements were performed according to the same experimental protocol. A typical experiment is described as a representative example with 1b. In a glovebox, 19.1 mg (0.0452 mmol) of 1b was dissolved in pentane and diluted to the mark in a 100 mL volumetric flask. An aliquot of 5 mL of this 4.52  10-4 M solution was transferred using a pipet to a 25 mL volumetric flask. Pentane was added, and then 400 μL of a 2.0 M solution of MeI in MTBE (0.80 mmol) was added by microliter syringe. Pentane was added up to the mark, and the solution was thoroughly mixed. A portion of the 9.04  10-5 M solution was loaded into the UV-visible cell for airsensitive samples (described above) and transferred from the glovebox to the spectrophotometer, where the absorption at 530 nm was recorded every 0.5 min for 60 min. The resulting kinetics trace was fit to a first-order decay curve to give the rate constant kobs = 8.05  10-5 s-1. The pseudo-first-order experiment was repeated three more times with varying excess concentrations of MeI. The second-order rate constant, k = (2.4 ( 0.3)  10-2 M-1 s-1, for the reaction was extracted from the slope of the straight line plot of kobs vs MeI concentration (R2 = 0.9674). The reactions with 1c and 1d were performed in the same manner, with R2 =0.9801 and R2 =0.9966, respectively. In

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the previously described kinetics measurements for 1a, the reaction was monitored by the disappearance of the strong peak of the Cr(II) starting material at 430 nm, rather than the growth of the Cr(III) methyl product at 530 nm. Since 2 equiv of Cr(II) are consumed for each equivalent of Cr(III) methyl produced (Scheme 1 and eq 5), the rate constant previously reported for 1a was twice as large as those determined in the current study. The previous value for 1a was divided by 2 in the comparisons with 1b-d, above. X-ray Crystallography. Data Collection: All crystals were mounted on a glass fiber and measurements were made on a Rigaku/ADSC diffractometer for 2b, 2c, and 3b (Table 1) and on a Bruker X8 APEX diffractometer for 3c, 3d, 4c, and 4d (Table 2) with graphite-monochromated Mo KR radiation. The data were collected at a temperature of -100.0 ( 0.1 °C. Data Reduction: Data for 2b, 2c, and 3b were collected using the d*TREK61 software package and processed using TwinSolve.62 Data for 3c, 3d, 4c, and 4d were collected and integrated using the Bruker SAINT63 software package. Data were corrected for absorption effects using the multiscan technique (TwinSolve for 2b, 2c, and 3b and SADABS64 for 3c, 3d, 4c, and 4d). The data were corrected for Lorentz and polarization effects. Structure Solution and Refinement: The structures were solved by direct methods.65 All non-hydrogen atoms were refined anisotropically (unless otherwise mentioned below). All hydrogen atoms were included in calculated positions but not refined (unless otherwise mentioned below). All refinements were performed using SHELXL-9766 for 2b, 2c, and 3b and SHELXTL67 for 3c, 3d, 4c, and 4d. For complexes 2b and 3b, all Cp hydrogen atoms were located in difference maps and refined isotropically. Complex 2c crystallizes with a half-molecule on a mirror plane perpendicular to the b-axis. All four methyl groups on the xylyl substituents in complex 3b (C7, C8, C20, and C21) had disordered methyl hydrogens, which were modeled in two orientations with equal populations. The methyl hydrogens on C12 and C22 in complex 3c were modeled as disordered in two orientations with 50% occupancy. In complex 3d, one ethyl substituent (C25) was disordered and was modeled in two orientations. The possibility that complex 3d contains a very small amount (∼1%) of 2d as a cocrystallized impurity50 is discussed above. Complex 4c crystallizes with unresolvable residual electron density, likely from disordered solvent, in the lattice. The structure was refined without modeling any solvent molecules; then the PLATON/SQUEEZE68 program was employed to search the cell for solvent accessible voids and then to correct the raw diffraction data to eliminate any residual electron density found in those voids. The results from this procedure removed 127 residual electron density from the unit cell, or approximately 16 electrons per asymmetric unit. This is less than one molecule of hexane or CH2Cl2 per asymmetric unit. Since it is not possible to properly identify the solvent, the values for the formula weight, etc., reflect only those atoms found in the atom list; no assumptions were made as to the identity of the lattice solvent. Complex 4c crystallizes as a (61) d*TREK, Area Detector Software. Version 4.13; Molecular Structure Corporation, 1996-1998. (62) CrystalClear 1.3.6: Mar 19 2004; Rigaku: 1998-2004. (63) SAINT, Version 7.46A; Bruker AXS Inc.: Madison, WI, 1997-2007. (64) SADABS. Bruker Nonius area detector scaling and absorption correction, V2.10; Bruker AXS Inc.: Madison, WI, 2003. (65) SIR97. Altomare, A.; Burla, M. C.; Camalli, M.; Gascarano, G. L.; Giacovazzo, C.; Gualiardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 1999, 32, 115-119. (66) Sheldrick, G. M. SHELXL-97, Programs for Crystal Structure Analysis (Release 97-2); University of G€ottingen: Germany, 1997. (67) SHELXTL, Version 5.1; Bruker AXS Inc.: Madison, WI, 1997. (68) SQUEEZE. Sluis, P. v. d.; Spek, A. L. Acta Crystallogr. A 1990, 46, 194-201.

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racemic twin, with both enantiomers present in the crystal. In complex 4d, one ethyl substituent (C14 and C15) was disordered and was modeled in two orientations with roughly equivalent populations. Refinements converged with R1 = 12.4%. The program ROTAX69 was used to look for possible nonmerohedral twinning. The twin law corresponding to 180 degree twinning about the 0 1 1 reciprocal lattice direction was used to generate an HKLF5 format data set. Refinements using this (69) Parsons, S.; Gould, B. ROTAX; University of Edinburgh with additions by Cooper, R. (Oxford) and Farrugia, L. (Glasgow), Version Nov 26, 2001.

MacLeod et al. data along with a refined batch scale factor allowed the model to converge with R1 = 5.8%.

Acknowledgment. We are grateful to the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canadian Foundation for Innovation, and the University of British Columbia for financial support. Supporting Information Available: Complete crystallographic data for complexes 2b, 2c, 3b, 3c, 3d, 4c, and 4d. This material is available free of charge via the Internet at http://pubs.acs.org.