Reactions of Oxygen with Metallaheterocyclic Alkyl Amide Complexes

Nov 4, 2009 - Department of Chemistry, The University of Tennessee, Knoxville, Tennessee 37996- ... Nanjing University, Nanjing 210093, People's Repub...
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Organometallics 2009, 28, 6642–6645 DOI: 10.1021/om900773w

Reactions of Oxygen with Metallaheterocyclic Alkyl Amide Complexes. Selective Insertion of Oxygen into Metal-Carbon Bonds Xianghua Yu,† Xue-Tai Chen,‡ and Zi-Ling Xue*,† †

Department of Chemistry, The University of Tennessee, Knoxville, Tennessee 37996-1600, and ‡State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, People’s Republic of China Received September 5, 2009

Summary: Reactions of O2 with the four-membered cyclic complexes {(Me2N)[(Me3Si)2N]M[N(SiMe3)SiMe2CH2]}2 (M= Zr (1a), Hf (1b)), containing both M-C and M-N bonds, give the five-membered cyclic complexes {(Me2N)[(Me3Si)2N]M[N(SiMe3)SiMe2CH2O]}2 (M=Zr (2a), Hf (2b)) with rarely observed selective O insertion into the M-C bonds. Both dimeric products exist as interesting cis and trans isomers. Reactions of O2 with transition-metal complexes have been actively investigated to understand the biological roles of these reactions and their applications in catalysis.1-3 Most of these studies focus on dn metal complexes,1-3 and there are relatively few studies of reactions of d0 transition-metal complexes with O2.4 Many of these d0 early-transition-metal complexes are oxygen sensitive, and the nature of the sensitivity is not well understood. Schwartz,4a,b Gibson,4c Brindley,4d Wolczanski,4e and co-workers reported oxygen insertion into Zr-R bonds in the reaction of Zr alkyl complexes with O2. Tilley reported O insertion into a Zr-Si *To whom correspondence should be addressed. E-mail: xue@utk. edu. (1) (a) Feig, A. L.; Lippard, S. J. Chem. Rev. 1994, 94, 759. (b) Klotz, I. M.; Kurtz, D. M. Chem. Rev. 1994, 94, 567. (c) Theopold, K. H.; Reinaud, O. M.; Blanchard, S.; Leelasubeharoen, S.; Hess, A.; Thyagarajan, S. ACS Symp. Ser. 2002, No. 823, 75. (d) Kopp, D. A.; Lippard, S. J. Curr. Opin. Chem. Biol. 2002, 6, 568. (e) Que, L. Jr.; Tolman, W. B. Angew. Chem., Int. Ed. 2002, 41, 1114. (f) Brown, S. N.; Mayer, J. M. Inorg. Chem. 1992, 31, 4091. (g) Balch, A. L.; Cornman, C. R.; Olmstead, M. M. J. Am. Chem. Soc. 1990, 112, 2963. (h) Popp, B. V.; Stahl, S. S. Top. Organomet. Chem. 2007, 22, 149. (i) Grice, K. A.; Goldberg, K. I. Organometallics 2009, 28, 953. (2) (a) Ezhova, M. B.; James, B. R. In Advances in Catalytic Activation of Dioxygen by Metal Complexes; Simandi, L. I., Ed.; Kluwer: Boston, 2003; Catalysis by Metal Complexes Vol. 26, pp 1-77. (b) Zhang, C. X.; Liang, H.-C.; Humphreys, K. J.; Karlin, K. D. In ref 2a, pp 79-121. (c) Funabiki, T. In ref 2a, pp 157-226. (d) Boring, E.; Geletii, Y. V.; Hill, C. L. In ref 2a, pp 227-264. (e) Simandi, L. I. In ref 2a, pp 265-328. (3) (a) The Activation of Dioxygen and Homogeneous Catalytic Oxidation; Barton, D. H. R., Martell, A. E., Sawyer, D. T., Eds.; Plenum: New York, 1993. (b) Theopold, K. H.; Reinaud, O. M.; Blanchard, S.; Leelasubeharoen, S.; Hess, A.; Thyagarajan, S. ACS Symp. Ser. 2002, No. 823, 75. (4) (a) Labinger, J. A.; Hart, D. W.; Seibert, W. E.; Schwartz, J. J. Am. Chem. Soc. 1975, 97, 3851. (b) Blackburn, T. F.; Labinger, J. A.; Schwartz, J. Tetrahedron Lett. 1975, 35, 3041. (c) Gibson, T. Organometallics 1987, 6, 918. (d) Brindley, P. B.; Scotton, M. J. J. Chem. Soc., Perkin Trans. 2 1981, 419. (e) Lubben, T. V.; Wolczanski, P. T. J. Am. Chem. Soc. 1987, 109, 424. (f) Tilley, T. D. Organometallics 1985, 4, 1452. (g) Kim, S.-J.; Jung, I. N.; Yoo, B. R.; Cho, S.; Ko, J.; Kim, S. H.; Kang, S. O. Organometallics 2001, 20, 1501. (h) Kim, S.-J.; Choi, D.-W.; Lee, Y.-J.; Chae, B.-H.; Ko, J.; Kang, S. O. Organometallics 2004, 23, 559. (i) Stanciu, C.; Jones, M. E.; Fanwick, P. E.; Abu-Omar, M. M. J. Am. Chem. Soc. 2007, 129, 12400. (j) Morris, A. M.; Pierpont, C. G.; Finke, R. G. Inorg. Chem. 2009, 48, 3496. pubs.acs.org/Organometallics

Published on Web 11/04/2009

bond in the reaction between Cp2Zr(SiMe3)Cl and O2.4f Kim and co-workers found that reactions of O2 with chelating diamides (N∼N)TiMeX and (N∼N)TiCl2 (X = Me, Cl; N∼ N = silacycloalkane or -alkene bridge) lead to selective O insertion to give [(N∼N)Ti(μ-OMe)X]2 and oxidation of the diamide ligands, respectively.4g,h Abu-Omar and coworkers recently reported a unique bis-peroxo Zr(IV) complex from the reaction of a bis-diamido complex with O2.4i There is currently intense interest in developing metal oxide thin films as gate insulating materials in the new generations of microelectronic devices.5-8 These metal oxide thin films, such as HfO2, with much larger dielectric constants have been chosen to replace SiO2 so that the insulating film thickness could be reduced to 2 nm or below. These better insulating materials reduce the dimensions of the microelectronic devices and leakage currents. Reactions of O2 with d0 transition-metal complexes, such as metal amides and imides, have been used to make microelectronic metal oxide thin films by chemical vapor deposition (CVD) and atomic layer deposition (ALD) processes.6a,b,e,f,7 We have recently studied reactions of O2 with d0 transition-metal complexes to understand the nature of the reactions and pathways to metal oxides.9-13 Reactions of O2 with (5) (a) Wallace, R. M.; Wilk, G. D. Crit. Rev. Solid State Mater. Sci. 2003, 28, 231. (b) Smith, R. C.; Ma, T.; Hoilien, N.; Tsung, L. Y.; Bevan, M. J.; Colombo, L.; Roberts, J.; Campbell, S. A.; Gladfelter, W. L. Adv. Mater. Opt. Electron. 2000, 10, 105. (c) Senzaki, Y.; Hochberg, A. K.; Norman, J. A. T. Adv. Mater. Opt. Electron. 2000, 10, 93. (d) Lucovsky, G.; Phillips, J. C. Mater. Res. Soc. Symp. Proc. 1999, 567, 201(Ultrathin SiO2 and Highκ Materials for ULSI Gate Dielectrics). (6) (a) Bastianini, A.; Battiston, G. A.; Gerbasi, R.; Porchia, M.; Daolio, S. J. Phys. IV 1995, 5, C5–525. (b) Chiu, H.-T.; Wang, C.-N.; Chuang, S.-H. Chem. Vap. Deposition 2000, 6, 223. (c) Ohshita, Y.; Ogura, A.; Hoshino, A.; Hiiro, S.; Machida, H. J. Cryst. Growth 2001, 233, 292. (d) Hendrix, B. C.; Borovik, A. S.; Xu, C.; Roeder, J. F.; Baum, T. H.; Bevan, M. J.; Visokay, M. R.; Chambers, J. J.; Rotondaro, A. L. P.; Bu, H.; Colombo, L. Appl. Phys. Lett. 2002, 80, 2362. (e) Lehn, J.-S. M.; Javed, S.; Hoffman, D. M. Chem. Vap. Deposition 2006, 12, 280. (f) Jimenez, E. D.; Javed, S.; Hoffman, D. M. Inorg. Chim. Acta 2009, 362, 385. (g) Schlom, D. G.; Guha, S.; Datta, S. MRS Bull. 2008, 33, 1017. (7) Son, K.-A.; Mao, A. Y.; Sun, Y.-M.; Kim, B. Y.; Liu, F.; Kamath, A.; White, J. M.; Kwong, D. L.; Roberts, D. A.; Vrtis, R. N. Appl. Phys. Lett. 1998, 72, 1187. (8) (a) Miyazaki, S. J. Vac. Sci. Technol. 2001, B19, 2212. (b) Yu, J. J.; Boyd, I. W. Phys. Status Solidi A 2006, 203, R9. (c) Robertson, J. NonCryst. Sol. 2002, 303, 94. (9) Wang, R.-T.; Zhang, X.-H.; Chen, S.-J.; Yu, X.-H.; Wang, C.-S.; Beach, D. B.; Wu, Y.-D.; Xue, Z.-L. J. Am. Chem. Soc. 2005, 127, 5204. (10) Chen, S.-J.; Zhang, X.-H.; Yu, X.; Qiu, H.; Yap, G. P. A.; Guzei, I. A.; Lin, Z.; Wu, Y. D.; Xue, Z.-L. J. Am. Chem. Soc. 2007, 129, 14408. (11) Wu, Z.-Z.; Cai, H.; Yu, X.-H.; Blanton, J. R.; Diminnie, J. B.; Pan, H.-J.; Xue, Z.-L. Organometallics 2002, 21, 3973. (12) Qiu, H.; Chen, S.-J.; Wang, C.-S.; Wu, Y.-D.; Guzei, I. A.; Chen, X.-T.; Xue, Z.-L. Inorg. Chem. 2009, 48, 3073. r 2009 American Chemical Society

Communication Scheme 1. Preparation of cis-2a,b and trans-2a,b

homoleptic M(NMe2)4 (M = Zr, Hf) with O2 are fast, yielding the oxo aminoxo complexes M3(NMe2)6(μ-NMe2)3(μ3O)(μ3-ONMe2) and Me2N-NMe2.9 Reaction of Ta(NMe2)5 with O2 is also fast, giving the aminoxy complexes (Me2N)nTa(η2-ONMe2)5-n (n=4, 3) as well as (Me2N)4Ta2[η2-N(Me)CH2NMe2]2(μ-O)2 and (Me2N)6Ta3[η2-N(Me)CH2NMe2]2(η2-ONMe2)(μ-O)3 containing novel (aminomethyl)amide N(Me)CH2NMe2 ligands.10 For amide silyl complexes such as (Me2N)4Ta-SiR3 (R3 = (SiMe3)3,11 ButPh212), their reactions with O2 give (Me2N)3Ta(η2-ONMe2)(OSiR3) as well as (Me2N)4Ta(OSiButPh2) and (Me2N)2(Ph2ButSiO)2(μ,η2-Me2NCH2NMe)2Ta2(μ-O)2 when R3 = ButPh2. Insertion into both Ta-Si and Ta-N bonds is observed, and the reactions reveal no selectivity of the oxygen insertion.12 Many d0 organometallic transition-metal complexes are oxygen sensitive, and their reactivities are little understood. We have been interested in the reactions of O2 with complexes containing mixed ligands and selective oxygen insertion. The four-membered cyclic complexes {(Me2N)[(Me3Si)2N]M[N(SiMe3)SiMe2CH2]}2 (M = Zr (1a), Hf (1b)), containing both M-C and M-N bonds, react with O2 to give the five-membered cyclic complexes {(Me2N)[(Me3Si)2N]M[N(SiMe3)SiMe2CH2O]}2 (M = Zr (cis-2a and trans-2a), Hf (cis-2b and trans-2a); Scheme 1) with rarely observed selective O insertion into the M-C bonds.4g The -NMe2 and -N(SiMe3)2 ligands are in interesting cis and trans configurations. Our studies of the reactions and characterization of the products are presented. Complexes 1a,b have been reported,14 and they have been prepared by the reactions summarized in Scheme 2 through γ-H abstraction by silyl ligands in the unstable silyl amide complexes. Interestingly, the reactions of 1a,b with O2 immediately give the five-membered-ring metallaheterocyclic dimeric complexes 2a,b (Scheme 1). Oxygen insertion into the M-CH2 bonds in 1a,b is observed in the reactions. The O insertion products cis-2a,b and trans-2a,b exist as dimers. Two isomers of the products with configurations of the two (13) (a) Chen, T.-N.; Wu, Z.-Z.; Li, L.-T.; Sorasaenee, K. R.; Diminnie, J. B.; Pan, H.-J.; Guzei, I. A.; Rheingold, A. L.; Xue, Z.-L. J. Am. Chem. Soc. 1998, 120, 13519. (b) Chen, T.-N.; Zhang, X.-H.; Wang, C.-S.; Chen, S.-J.; Wu, Z.-Z.; Li, L.-T.; Sorasaenee, K. R.; Diminnie, J. B.; Pan, H.-J.; Guzei, I. A.; Rheingold, A. L.; Wu, Y.-D.; Xue, Z.-L. Organometallics 2005, 24, 1214. (14) Yu, X.-H.; Bi, S.-W.; Guzei, I. A.; Lin, Z.-Y.; Xue, Z.-L. Inorg. Chem. 2004, 43, 7111.

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Scheme 2

-NMe2 ligands cis and trans to each other were isolated in 28% and 36% yields, respectively, and characterized. The core X-ray structures of the two Zr isomers cis-2a and trans2a are shown in Figure 1. In cis-2a, the two -NMe2 ligands are on the same side of the four-membered Zr2O2 ring. The Zr2O2 ring adopts a butterfly structure with a dihedral angle of 157.74° between the two Zr2O planes. In trans-2a, the two amide ligands are on different sides of the central Zr2O2 ring. The Zr2O2 ring is planar. The cis and trans isomers were separated by fractional crystallization. As trans-2a and trans-2b are much less soluble in pentane than cis-2a and cis-2b, they were separated first. In the reaction of 1a with O2 in a J. Young NMR tube, a total 64% yield of 2a (cis-2a:trans-2a = 28%:36%) in 1H NMR spectra was observed. The isolated yields of cis-2a and trans-2a are 20% and 23%, respectively. For their Hf analogues, the isolated yields of cis-2b and trans-2b are 17% and 22%, respectively. After O insertion, cis-2a,b and trans-2a,b are inert to O2. No interconversion of the cis and trans isomers at 23 °C was observed. Recrystallization of crystals of trans-2b in THF only gave trans-2b, and no cis-2b was observed in the crystals from the recrystallization. As in 1a,b, electron deficiency and open coordination sphere lead to the dimerization of the monomer “(Me2N)[(Me3Si)2N]M[N(SiMe3)SiMe2CH2O]” in the solid state. The dimerization is achieved through the bridging -Omoiety (Scheme 3), giving 18e for each M atom if lone-pair electrons on N and O atoms are donated through the p-d π bonds. The solubility of both trans-2a and trans-2b is low in benzene-d6. They, however, dissolve well in THF. The NMR spectra of these new complexes were thus recorded in THF-d8. 1H-coupled 13C NMR spectra of these complexes also confirmed the presence of the -CH2- moiety in cis-2a,b and trans-2a,b. The insertion of the O atoms into the M-CH2 bonds leads to a significant downfield shift of the -CH2- resonances from 0.25 ppm in 1a to 4.25 ppm in cis2a and 4.33 ppm in trans-2a in 1H NMR. The -CH2resonances of 72.56 ppm in cis-2a and 72.06 ppm in trans2a in 13C NMR are also significantly shifted from 37.2 ppm in 1a. Similar downfield shifts were observed in cis-2b and trans-2b. The Zr and Hf complexes cis-2a,b and trans-2a,b have similar structures. A molecular drawing and selected bond distances and angles of the Hf complex cis-2b are given in Figure 2. A drawing of the Zr complex cis-2a is given in the Supporting Information. In the structure of cis-2b, the three rings defined by N(1)-Hf(1)-O(1)-C(1)-Si(1), Hf(1)-O(1)-Hf(2)-O(2), and N(4)-Hf(2)-O(2)-C(15)-Si(15) are each nearly coplanar. The Hf(1) 3 3 3 C(9) and Hf(1) 3 3 3 Si(3) distances of 3.256 and 3.229 A˚ are significantly shorter than those of Hf(1) 3 3 3 C(11)

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Figure 1. Molecular drawings of cores of cis-2a (left) and trans-2a (right).

Figure 2. Molecular drawing of cis-2b showing 30% probability thermal ellipsoids. Selected bond distances (A˚) and angles (deg): Hf(1)-N(1) = 2.100(5), Hf(1)-N(2) = 2.100(5), Hf(1)N(3) = 2.034(6), Hf(1)-O(1) = 2.145(4), Hf(1)-O(2) =2.157(4), Hf(2)-O(1) = 2.138(4), Hf(2)-O(2) = 2.150(4), Si(1)-N(1) = 1.729(5), C(1)-Si(1) = 1.888(7), C(1)-O(1) = 1.461(7); N(1)Hf(1)-N(2)=105.3(2), N(1)-Hf(1)-N(3)=98.5(2), N(2)-Hf(1)-N(3)=114.6(2), N(1)-Hf(1)-O(1)=84.13(18), N(2)-Hf(1)-O(1) = 133.16(18), N(3)-Hf(1)-O(1) = 108.9(2), N(1)Hf(1)-O(2) = 150.21(18), N(2)-Hf(1)-O(2) = 92.80(17), N(3)-Hf(1)-O(2) = 95.0(2), O(1)-Hf(1)-O(2) = 66.58(15), Hf(1)-O(1)-Hf(2) = 111.69(18), Si(1)-N(1)-Hf(1) = 116.9(3), Si(2)-N(1)-Hf(1) = 125.4(3), N(1)-Si(1)-C(1) = 103.5(3), O(1)-C(1)-Si(1) = 109.4(4), C(1)-O(1)-Hf(1) = 122.2(4), Si(3)-N(2)-Hf(1) = 114.4(3), Si(4)-N(2)-Hf(1) = 128.8(3). Scheme 3. Dimerization in cis-/trans-2a,ba

a The arrows represent dative bonds from lone pair electrons of N and O atoms.

(3.783 A˚) and Hf(1) 3 3 3 Si(4) (3.468 A˚). The Si(3)-N(2)-Hf(1) and N(2)-Si(3)-C(9) bond angles of 114.4(3) and 109.3(19)° are both smaller than those of Si(4)-N(2)-Hf(1) (128.8(3)°) and N(2)-Si(4)-C(11) (112.9(3)°), respectively.

Figure 3. Molecular drawing of trans-2b showing 30% probability thermal ellipsoids. Selected bond distances (A˚) and angles (deg): Hf(1)-N(1) = 2.117(5), Hf(1)-N(2) = 2.106(5), Hf(1)-N(3) = 2.019(5), Hf(1)-O(1) = 2.149(4), Hf(1)-O(1A) = 2.177(4), Si(1)-N(1) = 1.741(5), C(1)-Si(1) = 1.869(7), C(1)O(1A)=1.458(8), Si(2)-N(1)=1.738(5), Si(3)-N(2) =1.744(5), Si(4)-N(2)=1.735(5), C(2)-Si(1)= 1.860(7); N(1)-Hf(1)-N(2) = 105.68(18), N(1)-Hf(1)-N(3) = 101.2(2), N(2)-Hf(1)N(3)=106.2(2), N(1)-Hf(1)-O(1)=146.18(17), N(2)-Hf(1)O(1)=93.40(17), N(3)-Hf(1)-O(1)=99.82(19), N(1)-Hf(1)O(1A) = 83.64(16), N(2)-Hf(1)-O(1A) = 146.67(17), N(3)Hf(1)-O(1A) = 103.07(18), O(1)-Hf(1)-O(1A) = 65.92(18), Hf(1)-O(1)-Hf(1A)=114.07(18), Si(1)-N(1)-Hf(1)=114.8(2), Si(2)-N(1)-Hf(1) = 130.1(3), N(1)-Si(1)-C(1) = 102.4(3), O(1A)-C(1)-Si(1) = 106.1(4), C(1A)-O(1)-Hf(1) = 128.5(3), Si(3)-N(2)-Hf(1) = 111.5(3), Si(4)-N(2)-Hf(1) = 129.2(3).

These observations suggest an agostic Siβ-Cγ interaction in cis-2b. The Zr and Hf complexes trans-2a and trans-2b are isomorphous as well. A molecular drawing and selected bond distances and angles of the Hf complex trans-2b are given in Figure 3. A drawing of the Zr complex trans2a is given in the Supporting Information. As in cis-2b, the three rings defined by N(1)-Hf(1)-O(1A)-C(1)-Si(1), Hf(1)-O(1A)-Hf(1A)-O(1), and N(1A)-Hf(1A)-O(1)-C(1A)-Si(1A) respectively are also each nearly coplanar in trans-2b. The Hf(1) 3 3 3 C(8) and Hf(1) 3 3 3 Si(4) distances of 3.986 and 3.474 A˚ are significantly longer than those of Hf(1) 3 3 3 C(11) (3.113 A˚) and Hf(1) 3 3 3 Si(3) (3.190 A˚). The Si(4)-N(2)-Hf(1) and

Communication

N(2)-Si(4)-C(8) bond angles of 129.2(3) and 114.3(3)° are both larger than those of Si(3)-N(2)-Hf(1) (111.5(3)°) and N(2)-Si(3)-C(11) (108.2(3)°), respectively. These observations similarly suggest an agostic Siβ-C γ interaction in trans-2b. Differences in bond energies between M-C and M-N bonds as well as between C-O and N-O bonds may be used to explain the selective O insertion into M-C bonds in the reactions of alkyl amide complexes with O 2. Lappert and co-workers have reported that the Zr-C bond energy of 74 kcal mol -1 is significantly lower compared to that (90 kcal mol-1) of the Zr-N bond.15 The C-O and N-O bond energies are 85.5 and 48 kcal mol -1 , respectively. 16 In other words, the formation of C-O and N-O bonds releases ca. 85.5 and 48 kcal mol -1 , respectively. Using the Zr-O and OdO bond energies of 13215 and 118 kcal mol-1, 16 enthalpies of the O insertion into Zr-C vs Zr-N bonds (eqs 11 and 2) may be estimated: ΔH = -169 and -62 kcal for the insertion into Zr-C and Zr-N bonds in 1a, respectively. The former is much more exothermic than the latter. Thus, O insertion into Zr-C bonds is thermodynamically more favored than the insertion into Zr-N bonds. In essence, both smaller Zr-C (than Zr-N) bond energy in the reactant 1a and larger C-O (than N-O) bond energy in the products cis-2a and trans-2a contribute to the difference (ΔΔH = -107 kcal) in the enthalpies between the Zr-C and Zr-N insertions. In other words, the overall process of the O insertion into Zr-C bonds is thermodynamically more favored than the insertion into Zr-N bonds. It should be pointed out that the use of bond energies here to estimate the reaction enthalpies assumes that the bond energy for a Zr-C(alkyl) single bond is the same as that for the Zr-C bond in a fourmembered azazirconacyclic ring and the Zr-N(amido) bond is the same as the Zr-N bond in a four-membered azazirconacylic ring. In addition, the ring strain associated with the four azametallacyclic rings of 1a,b and the lack of a large substituent on the carbon may further help promote (15) Lappert, M. F.; Patil, D. S.; Pedley, J. B. J. Chem. Soc., Chem. Commun. 1975, 830. (16) Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorg. Chem., 4th ed.; HarperCollins College: 1993; Appendix E, pp A-21-A-34.

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preferential O-insertion into the M-C bond over the M-N bond in these reactions.

Our earlier DFT studies of the reactions of O2 with M(NMe2)n (M = Zr, n = 4;9 M = Ta, n = 510) suggest that O2 binds to d0 M atoms in the complexes using its lone pair electrons. Subsequent O2 insertion occurs to give an active peroxide (M-O-O-NMe2) complex. The R-O atom may insert into another M-NMe2 bond to give a M(ONMe2)2 moiety. It is not clear if the current reaction adopts a similar pathway in the insertion of O2 into the two M-C bonds in 1a,b. In summary, the work here reveals a rare selective O insertion into M-alkyl rather than M-amide bonds in the reactions of O2 with Zr and Hf four-membered cyclic alkyl amides, yielding interesting cis and trans five-membered cyclic complexes.

Acknowledgment. Acknowledgment is made to the National Science Foundation (Grant No. CHE-0516928 to Z.-L.X.), National Basic Research Program of China (No. 2006CB806104 to X.-T.C.), Distinguished Overseas Young Investigators program of the Natural Science Foundation of China (No. 20028101 to Z.-L.X.), and Changjiang (Cheung Kong) Lecture Professorship (to Z.-L.X.) for financial support of the research. We thank Prof. Zhenyang Lin for helpful discussions. Note Added after ASAP Publication. In the version of this paper published on the Web on Nov 4, 2009, the National Science Foundation grant number was incorrect. The grant number that appears on the web as of Nov 13, 2009, is correct. Supporting Information Available: Text, figures, tables, and CIF files giving experimental details, molecular drawings of cis2a and trans-2a, crystal data and structure refinement details for cis-2a,b and trans-2a,b, crystallographic data, and diastereotopic -CHaHb- peaks in the 1H NMR spectra of cis-2a,b and trans-2a,b. This material is available free of charge via the Internet at http://pubs.acs.org.