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Computational Study on the Relative Reactivities of Cobalt and Nickel Amidinates via β-H Migration

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Computational Study on the Relative Reactivities of Cobalt and Nickel Amidinates via β-H Migration
  Computational Study on the Relative Reactivities of Cobalt andNickel Amidinates via   -H Migration Jinping Wu,* ,† Jiaye Li, † Chenggang Zhou, † Xinjian Lei, ‡ Thomas Gaffney, ‡ John A. T. Norman, ‡ Zhengwen Li, § Roy Gordon, § and Hansong Cheng* , |  Institute of Theoretical Chemistry and Computational Materials Science, China Uni V  ersity of Geosciences,Wuhan, People’s Republic of China 430074, Air Products and Chemicals, Inc., 1969 Palomar Oaks Way,Carlsbad, California 92011, Department of Chemistry and Chemical Biology, Har  V  ard Uni V  ersity,Cambridge, Massachusetts 02138, and Air Products and Chemicals, Inc., 7201 Hamilton Boule V  ard, Allentown, Pennsyl V  ania 18195 Recei V  ed October 4, 2006 Summary: Density functional theory calculations were per- formed to examine the relati V  e stabilities of se V  eral Co- and  Ni-based amidinates for atomic layer deposition to   -H or    -CH  3  migration from the chelating ligand to the metal center atom. The calculated precursor structures are in excellent agreement with the a V  ailable XRD data. The minimum energy path of the migration process was identified. Our results indicatethat the migration process is thermochemically endothermic;howe V  er, for Ni( i Pr-MeAMD) 2  the reaction is close to thermo-neutral. The relati V  ely moderate acti V  ation barrier allows   -H migration for Ni( i Pr-MeAMD) 2  to occur, leading to instabilityat the ALD process temperature. Replacing the H atoms withmethyl groups can greatly stabilize the precursors to the   -H migration. The results are consistent with the experimentalobser  V  ations. As the feature size of semiconductor devices continues toshrink, 1 atomic layer deposition (ALD) has become the preferredtechnique to deposit extremely thin, conforming metal films onsurfaces of partially fabricated semiconductor substrates, suchas W, Ta, and transition-metal nitrides. 2 - 4 The design anddevelopment of sufficiently stable metal-organic precursors isof great interest for ensuring successful ALD applications.Recently, Gordon et al. 5 synthesized a number of novelamidinates as ALD precursors for deposition of cobalt thin filmson ALD WN as a glue layer for copper interconnects, or nickeland cobalt thin films on silicon surfaces for fabrication of NiSiand CoSi 2  as the contact metals in micro- and nanoelectronics. 6,7 The amidinate ligands in the precursors are chelating  N  ,  N  ′ -diisopropylacetamidinato and  N  ,  N  ′ -di- tert- butylacetamidinatoligands. It was found that bis(  N  ,  N  ′ -diisopropylacetamidinato)-nickel(II) (Ni( i Pr-MeAMD) 2 ) is thermally less stable than bis-(  N  ,  N  ′ -di- tert  -butylacetamidinato)nickel(II) (Ni( t  Bu-MeAMD) 2 ),whereas both bis(  N  ,  N  ′ -diisopropylacetamidinato)cobalt(II)(Co( i Pr-MeAMD) 2 ) and bis(  N  ,  N  ′ -di- tert  -butylacetamidinato)-cobalt(II) (Co( t  Bu-MeAMD) 2 ) are stable. 8,9 It was thus specu-lated that the migration of one of the   -H atoms from thebis(  N  ,  N  ′ -diisopropylacetamidinato) ligands to the metal atomcould be related to the thermal degradation of the precursor.   -H migration has been observed in many organometallicreactions. 10 - 13 However, why the degradation occurs morereadily in Ni( i Pr-MeAMD) 2  than in Co( i Pr-MeAMD) 2  remainsto be addressed. The purpose of this communication is toexamine quantitatively the decomposition mechanism arisingfrom the   -H migration by computing the thermochemicalenergies and reaction pathways using density functional theory(DFT). The results might provide useful insight into the relativereactivity and thermal stability of the ALD precursors and shouldaid the design of novel precursors in the future.All calculations were performed under the generalizedgradients approximation (GGA) with the Perdew - Wang ex-change-correlation functional (PW91) as implemented in theDMol 3 package. 14,15 The spin-polarization scheme was employedto deal with the electronic open-shell systems. A doublenumerical atomic basis set augmented with polarization func-tions (DNP) was used to describe the valence electrons, andthe core electrons were represented by effective core potentials(ECP). All-electron (AE) calculations using the DNP basis setwere also performed for selected precursors, and the differencein the calculated structures and energetics between ECP andAE calculations was found to be relatively small. Full geometryoptimization was performed for all of the reactants and products.The transition state search employed the protocol of completelinear synchronous transit/quadratic synchronous transit (LST/ QST), and the transition state optimization utilized the Newton - Raphson search algorithm. Normal-mode analysis was con-ducted to ensure the correctness of the obtained transition states.The reaction pathway is schematically shown as reactant,transition state, and product in Figure 1, where R representsthe migrating species (R  )  H if the ligand is bis(  N  ,  N  ′ - * To whom correspondence should be addressed. E-mail: chengh@airproducts.com H.C.); jpwu@cug.edu.cn (J.W.) † China University of Geosciences. ‡ Air Products and Chemicals, Inc., Carlsbad, CA. § Harvard University. | Air Products and Chemicals, Inc., Allentown, PA.(1) Moore, G.  Electronics  1965 ,  38  (8).(2) Becker, J. S.; Suh, S.; Wang, S.; Gordon, R. G.  Chem. Mater.  2003 , 15 , 2969.(3) Park, K.-H.; Marshall, W. J.  J. Am. Chem. Soc.  2005 ,  127  , 9330.(4) Kim, H.  J. Vac. Sci. Technol., B  2003 ,  21 (6), 2231 - 2261.(5) Booyong, S. L.; Rahtu, A.; Gordon, R. G.  Nat. Mater.  2003 ,  2 , 749.(6) Zhang, S.; Ostling, M.  Crit. Re V  . Solid State Mater. Sci.  2003 ,  28  (1), 1.(7) Wu, Y.; Xiang, J.; Yang, C.; Lu, W.; Lieber, C. M.  Nature  2004 , 430  (61), 6.(8) Li, Z.; Gordon, R. G.; Farmer, D. B.; Lin, Y.; Vlassak, J.  Electrochem.Solid-State Lett.  2005 ,  8  (7), G182.(9) Gordon, R. G.; Lim, B. S. Atomic Layer Deposition using MetalAmidinates. PCT Int. Patent WO 2004046417, 2004.(10) Mole, L.; Spencer, J. L.; Carr, N.; Orpen, A. G.  Organometallics 1991 ,  10 , 49 - 52.(11) Hartwig, J. F.; Bergman, R. G.; Andersen, R. A.  Organometallics 1991 ,  10 , 3326.(12) Brookhart, M.; Hauptman, E.; Lincoln, D. M.  J. Am. Chem. Soc. 1992 ,  114 , 10394.(13) Doherty, S.; Hogarth, G.; Waugh, M.; Clegg, W.; Elsegood, M. R.J.  Organometallics  2000 ,  19 , 4557.(14) Perdew, J. P.; Wang, Y.  Phys. Re V  . B  1992 ,  45 (13), 244.(15) Delley, B.  J. Chem. Phys.  2000 ,  113 (18), 7756 - 7764. 2803 Organometallics  2007,  26,  2803 - 2805 10.1021/om060910a CCC: $37.00 © 2007 American Chemical SocietyPublication on Web 04/21/2007  diisopropylacetamidinato) and R  )  CH 3  if the ligand is bis-(  N  ,  N  ′ -di- tert  -butylacetamidinato)). M represents Ni or Co. Themain geometric parameters of the fully optimized precursorstructures and a comparison with the available experimental dataare shown in Table 1. Structural information on transition statesand final products is given in the Supporting Information. Theminimum-energy structure of the reactants adopts a distorted-tetrahedral framework, with the dihedral angles formed by fournitrogen atoms around the center metal atom being 83.6 °  and83.8 °  for Co( i Pr-MeAMD) 2  and Ni( i Pr-MeAMD) 2 , respectively,reflecting the steric effect on the backbone. The calculatedreactant structures are in excellent agreement with the XRDresults. 16 We note that there is partial electron delocalizationamong the N - C - N atoms of the two four-membered rings,which results in relatively weak coordination between N atomsand the metal. The R groups on the isopropyl groups of thereactants are out of the plane formed by the four-memberedrings. For one of the R groups, R1, to migrate from the C1 tothe metal center, it is necessary for the isopropyl group toundergo an internal rotation to align it with the four-memberedring in the same plane to minimize the migration barriers.Indeed, this can be readily realized under ambient conditions,since our calculations indicate that the internal rotation resultsin an energy barrier of only a few kcal/mol, consistent with theXRD experiments. The calculated internal rotation barriers areshown in Table 2.Upon R1 group migration, the N1 - M bond of the productsbecomes significantly elongated and the C1 - N1 bond simul-taneously evolves into a double bond; a strong single R1 - Mbond is also formed. The strain imposed by the four-memberedring is relaxed upon the N1 - M bond breaking. As a conse-quence, the metal centers of the products adopt a squareconfiguration.The optimized transition state geometries for both Ni andCo precursors are quite similar, although the structural detailsdiffer. Essentially, the R1 group resides between C1 and M,forming two rather loose single bonds. For R1 ) CH 3 , the C1atom is repelled farther away from the metal atom due to stericeffects.The change of geometric structures along the migrationpathway gives rise to a significant change of electronic structuresof the reactants, products, and transition states. The reactantsall adopt a high-spin state for both R ) H and R ) CH 3  due tothe weak coordination of the ligand field. For the Ni precursors,there are two unpaired electrons occupying HOMO andHOMO1, while for the Co precursors, there are three unpairedelectrons in the low-lying orbitals. Upon R1 group migration,a strong R1 - M bond is formed and the change of metalelectronic configuration from tetrahedral to square gives riseto a low-spin state for the products. Indeed, the calculatedground electronic state of the products yields only one unpairedelectron on Co and zero on Ni. At the transition states, the poorcoordination gives both compounds a high-spin state, the sameas for the reactants. The change of the high-spin states of transition states to the low-spin states of products indicatescrossing of potential energy surfaces. Indeed, our minimumenergy pathway calculations for both high-spin and low-spinstates of Co( i Pr-MeAMD) 2  and Ni( i Pr-MeAMD) 2  show thatcurve crossing occurs near the vicinity of the transition states(Figure 2), after which the potential energies decrease rapidlyalong the low-spin-state pathways. The calculated structures atthe crossing points are shown in the Supporting Information. Itis interesting that the potential energies of the high-spin statesare nearly flat after the transition states. The electronic config-uration of the optimized final products for both Ni and Co of the high-spin states is a distorted tetrahedron, while for the low-spin-state products the electronic configuration changes to asquare. The ligand - metal coordination is also considerablyweakened for the high-spin-state products, with the elonga-tions of the N - M and H - M bond distances being about 0.16and 0.09 Å for Ni( i Pr-MeAMD) 2  and 0.11 and 0.04 Å forCo( i Pr-MeAMD) 2 , respectively.To understand the relative stability, we first consider R  ) H. The calculated thermochemical energies and the activa-tion barriers for the Ni and Co precursors are shown in Table3. The results of our calculations indicate that thermochem-ically the H migration is an endothermic process for bothCo( i Pr-MeAMD) 2  and Ni( i Pr-MeAMD) 2 . However, it is much (16) Lim, B. S.; Rahtu, A.; Park, J. S.; Gordon, R. G.  Inorg. Chem.  2003 , 42 , 7951. Figure 1.  Structures of the reactant, transition state, and product: M  )  Ni, Co and R  )  H, CH 3 . Table 1. Main Optimized Precursor Structural Parametersand Comparison with the Available Experimental Data i Pr-MeAMD  t  Bu-MeAMDCo exptl/ calcdNicalcdCocalcdNi exptl/ calcdM - N1 (Å) 2.004/2.017 2.011 2.021 1.991/2.013M - N2 (Å) 2.020/2.030 2.021 2.037 1.997/2.021M - N3 (Å) 2.004/2.032 2.021 2.037 1.991/2.026M - N4 (Å) 2.020/2.016 2.013 2.024 1.997/2.013C1 - N1 (Å) 1.447/1.461 1.458 1.470 1.466/1.466C2 - N1 (Å) 1.322/1.342 1.338 1.343 1.335/1.339C2 - N2 (Å) 1.318/1.342 1.339 1.341 1.318/1.341N1 - M - N2 (deg) 65.57/65.65 65.94 65.60 66.20/65.87N3 - M - N4 (deg) 65.57/65.69 65.91 65.55 66.20/65.76N1 - N2 - N3 - N4 (deg) 80.58/83.58 83.77 83.95 82.18/83.90 Table 2. Calculated Rotational Barriers and RotationalEnergies of Co( i Pr-MeAMD) 2  and Ni( i Pr-MeAMD) 2 Precursors (kcal/mol)  a precursor rotational barrier rotational energyCo( i Pr-MeAMD) 2  1.5 3.6Ni( i Pr-MeAMD) 2  1.0 6.5 a The rotational energy is defined as the energy difference of isomersupon internal rotation. 2804  Organometallics, Vol. 26, No. 11, 2007 Communications  more endothermic for the former than for the latter. In fact, theH migration process for Ni( i Pr-MeAMD) 2  is very close tothermoneutral, making the metal atom more available for Hattack. Kinetically, the activation barrier of Ni( i Pr-MeAMD) 2 is more than 9 kcal/mol lower than that of Co( i Pr-MeAMD) 2 ,implying that the H migration is more rapid on Ni( i Pr-MeAMD) 2 than on Co( i Pr-MeAMD) 2 . The nearly neutral thermochem-ical energy coupled with a moderate activation barrier makesNi( i Pr-MeAMD) 2  likely to thermally decompose; on the otherhand, the unfavorable thermochemical energy coupled with arelatively high activation barrier for Co( i Pr-MeAMD) 2  pre-vents H migration from occurring and thus stabilizes thecompound. This result is in excellent agreement with experi-mental observations.Qualitatively, the stability difference between Ni( i Pr-MeAMD) 2 and Co( i Pr-MeAMD) 2  can be understood by analyzing thedetailed electronic structure of the transition states and the finalproducts. Our calculations indicate that it is much easier to ionizethe d electron in Ni( i Pr-MeAMD) 2  than in Co( i Pr-MeAMD) 2 .The   -H migration leads to the formation of a metal - H bondat the transition state and in the final product, in which the Hatom withdraws charges from the metal. As a consequence, morecharge transfer from the metal to the H atom occurs inNi( i Pr-MeAMD) 2  than in Co( i Pr-MeAMD) 2 , resulting in theH - Ni bond being stronger than the H - Co bond. Indeed, thefully optimized structures of the transition states and productsexhibit a H - Ni distance much shorter than the H - Co distance.At the transition states, the distances of H - Ni and H - Co are1.577 and 1.607 Å, respectively, while for the final products,these distances are 1.485 and 1.556 Å.To enhance the stability of Ni( i Pr-MeAMD) 2 , we consideredutilizing the  tert  -butyl groups to replace the isopropyl groupsand performed calculations on CH 3  migration for bothNi( t  Bu-MeAMD) 2  and Co( t  Bu-MeAMD) 2 . The calculated ther-mochemical energies and activation barriers are shown in Table3. As expected, the CH 3  migration processes for both com-pounds are very endothermic and their barriers are also exceed-ingly high, indicating great resistance to the migration. Gordonand co-workers prepared the Ni( t  Bu-MeAMD) 2  compound andfound it to be indeed more stable than Ni( i Pr-MeAMD) 2  on thebasis of thermogravimetric analyses (TGA) of the two precur-sors. 17 Here we also note that Co( t  Bu-MeAMD) 2  is much morestable than Ni( i Bu-MeAMD) 2 .It should be pointed out that, although our calculations weredone only for gas-phase molecules while experiments wereperformed in solutions, we expect that the qualitative conclu-sions that can be drawn from the present study remain valid.Nevertheless, it is anticipated that the reaction barriers in allcases could be significantly reduced if the solvation effect isaccounted for, which would cause the   -H migration to occurmore easily. The computational results presented here providethe upper bound of the activation energies. We are currentlypursuing calculations to reevaluate the energetics in relevantsolvents that were used in experiments.In summary, we have performed DFT calculations to examinethe relative stabilities of several Co and Ni ALD precursors to   -H or   -CH 3  migration from the chelating ligand to the centermetal atom. The calculated precursor structures are in excellentagreement with the available experimental results. The minimum-energy path of the migration process involves two steps: (1)internal rotation along the C1 - N1 bond to align the R1 groupin the same plane as defined by the M, N1, and C1 atoms and(2) migration of the R1 group from C1 to the metal atom toform the final product. Our results indicate that the migrationprocess is thermochemically endothermic; however, forNi( i Pr-MeAMD) 2  the reaction energy is close to thermoneutral.The relatively moderate activation barrier allows   -H migrationfor Ni( i Pr-MeAMD) 2  to occur readily, leading to instability atthe ALD process temperatures. Replacing the H atoms withmethyl groups can greatly stabilize the precursors to the   -Hmigration. The results are consistent with the experimentalobservations. Understanding the structure - stability relationshipis of great importance for design of metal-organic precursorsfor specific CVD and ALD applications. Acknowledgment.  This project was partially supported bythe Research Foundation for Outstanding Young Teachers,China University of Geosciences, Wuhan, People’s Republicof China (Grant CUGQNL0519), and Air Products and Chemi-cals, Inc. Supporting Information Available:  Tables giving the maingeometric parameters of the fully optimized reactant, transition-state and final product structures. This material is available free of charge via the Internet at http://pubs.acs.org.OM060910A (17) Li, Z.; Lee, D.; Coulter, M.; Gordon, R. G. Unpublished results. Figure 2.  Calculated minimum-energy pathways of bothhigh-spin and low-spin states of Co( i Pr-MeAMD) 2  andNi( i Pr-MeAMD) 2 . Table 3. Calculated Reaction Energies ( ∆  E r ) and ReactionBarriers (  E a ) for   -Migration and the Unique ImaginaryFrequencies at the Transition States  a precursor ∆  E  r (kcal/mol)  E  a (kcal/mol)imag freq(cm - 1 )Co( i Pr-MeAMD) 2  16.7 33.6 129.8iNi( i Pr-MeAMD) 2  4.7 24.3 111.5iCo( t  Bu-MeAMD) 2  29.2 52.8 309.7iNi( t  Bu-MeAMD) 2  18.5 40.2 295.3i a For M( i Pr-MeAMD) 2 , the reaction is for   -H migration, and forM( i Bu-MeAMD) 2 , it is for   -CH 3  migration (M  )  Ni, Co). Communications Organometallics, Vol. 26, No. 11, 2007   2805 View publication statsView publication stats
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