Structure and Bonding of Titanocene Amidoborane Complexes: A

Oct 25, 2010 - Structure and Bonding of Titanocene Amidoborane Complexes: A Common Bonding Motif with Their β-Agostic Organometallic Counterparts...
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Organometallics 2010, 29, 5769–5772 DOI: 10.1021/om100848c

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Structure and Bonding of Titanocene Amidoborane Complexes: A Common Bonding Motif with Their β-Agostic Organometallic Counterparts David J. Wolstenholme,* Kyle T. Traboulsee, Andreas Decken, and G. Sean McGrady* Department of Chemistry, University of New Brunswick, P.O. Box 4400, Fredericton, New Brunswick E3B 5A3, Canada Received September 1, 2010 The β-H elimination of metal alkyl and related complexes is a central phenomenon in organometallic chemistry and plays an essential role in a number of fundamental reactions of academic and industrial importance.1-3 It is generally considered that the β-H elimination of a C-H bond proceeds through an agostic intermediate. In recent years, studies have shown that β-agostic complexes of early transition metal complexes are stabilized through delocalization of the M-C bonding electrons over the alkyl moiety, as reflected in the increased olefinic character of the CR-Cβ bond.2,4 The related metal amidoalkyl complexes, which also exhibit a β-agostic C-H interaction, show no such delocalization on account of the significant π-character of the M-N bond.3b These systems possess activated NR-Cβ bonds and potentially represent model systems for R-elimination of the N-C moiety. However, a number of metal amido complexes containing a B-H moiety are reported to form 3c,2e M 3 3 3 H-B interactions and tend to undergo β-H elimination in preference to R-elimination of the N-B bond.5 These and related metal amidoborane complexes are currently of interest as key intermediates in the dehydrogenation of ammonia-borane, a leading candidate for chemical hydrogen storage.5 However, the nature of the chemical bonding and the driving forces that underlie these 3c,2e interactions are poorly understood.

Here we report the experimental and/or theoretical structures of the archetypal amidoborane complexes [Cp 2 TiNH 2 BH 3 ] (I) and [Cp 2 Ti(H)NH 2 BH 3 ] (II), which exhibit a 3c,2e Ti 3 3 3 H-B interaction (Scheme 1). 6 In addition, we have analyzed the electron distributions (AIM)7 for these benchmark complexes, along with their related metal alkyl [Cp 2 TiCH 2 CH 3 ] (III), 8 [EtTiCl 3 (dmpe)] (IV), 2a and amidoalkyl [CpTi(NiPr 2 )Cl 2 ] (V)3b counterparts. The goal of this study was to explore the structural and electronic features of the Ti 3 3 3 H-B moieties of the amidoborane systems while comparing these results with those of their organometallic Ti 3 3 3 H-C counterparts. A detailed analysis of the electronic behavior of these systems provides insight into the differences and similarities between metal amidoborane and alkyl complexes and allows for a deeper understanding of their β-H elimination processes. The single-crystal X-ray structure of I (Figure 1) represents a rare example of a 3c,2e metal amidoborane complex, in which the titanocene coordinates to the NH2BH3- ligand through Ti-N and Ti 3 3 3 H-B bonds.9 The resulting planar TiNBH ring system possesses a similar geometry to the recently published zirconocene amidoborane hydride analogue of II (VI).10 The M 3 3 3 H-B and N-B distances in I and VI are statistically identical (dTi 3 3 3 H = 1.95(3) and dN-B = 1.534(5) A˚ in I, vs dZr 3 3 3 H = 2.02(4), dN-B = 1.539(7) A˚ in VI). Furthermore, the geometries of the d1 (I) and d0 (II) titanocene amidoborane complexes exhibit nearly identical Ti 3 3 3 H-B moieties, as evident from the DFT calculations. In spite of the isoelectronic and isostructural relationship between ammonia-borane and ethane, their physical and chemical properties differ dramatically, due in large part to

*Corresponding authors. E-mail: [email protected]. (1) (a) Brookhart, M.; Green, M. L. H. J. Organomet. Chem. 1983, 250, 395–408. (b) Brookhart, M.; Green, M. L. H.; Wong, L.-L. Prog. Inorg. Chem. 1988, 36, 1–124. (c) Shultz, L. H.; Brookhart, M. Organometallics 2001, 20, 3975–3982. (d) Brookhart, M.; Green, M. L. H.; Parkin, G. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 6908–6914. (e) Pantazis, D. A.; McGrady, J. E.; Besora, M.; Maseras, F.; Etienne, M. Organometallics 2008, 27, 1128–1134. (f ) Etienne, M.; McGrady, J. E.; Maseras, F. Coord. Chem. Rev. 2009, 253, 635–646. (2) (a) Haaland, A.; Scherer, W.; Ruud, K.; McGrady, G. S.; Downs, J.; Swang, O. J. Am. Chem. Soc. 1998, 120, 3762–3772. (b) Scherer, W.; Sirsch, P.; Shorokhov, D.; Tafipolsky, M.; McGrady, G. S.; Gullo, E. Chem.;Eur. J. 2003, 9, 6057–6070. (c) Scherer, W.; McGrady, G. S. Angew. Chem., Int. Ed. 2004, 43, 1782–1806. (3) (a) Pupi, R. M.; Coalter, J. N.; Petersen, J. L. J. Organomet. Chem. 1995, 497, 17–25. (b) Scherer, W.; Wolstenholme, D. J.; Herz, V.; Eickerling, G.; Br€ uck, A.; Benndorf, P.; Roesky, P. W. Angew. Chem., Int. Ed. 2010, 49, 2242–2246. (4) (a) Scherer, W.; Sirsch, P.; Grosche, M.; Spiegler, M.; Mason, S. A.; Gardiner, M. G. Chem. Commun. 2001, 2072–2073. (b) Scherer, W.; Sirsch, P.; Shorokhov, D.; McGrady, G. S.; Mason, S. A.; Gardiner, M. Chem.;Eur. J. 2002, 8, 2324–2334. (5) (a) Keaton, R. J.; Blacquiere, J. M.; Baker, R. T. J. Am. Chem. Soc. 2007, 129, 1844–1845. (b) Hamilton, C. W.; Baker, R. T.; Staubitz, A.; Manners, I. Chem. Soc. Rev. 2009, 38, 279–293. (c) Sloan, M. E.; Staubitz, A.; Clark, T. J.; Russell, C. A.; Lloyd-Jones, G. C.; Manners, I. J. Am. Chem. Soc. 2010, 132, 3831–3841.

(6) The optimized structures of CH3CH3, NH3BH3, [Cp2TiH2], and complexes I-VI were calculated using DFT methods at the BP86/ 6-311G(d,p) level of approximation. (7) Bader, R.F.W. Atoms in Molecules: A Quantum Theory; Oxford University Press: Oxford, U.K., 1990. (8) Klei, R.; Telgen, J. H.; Teuben, J. H. J. Organomet. Chem. 1981, 209, 297–307. (9) (a) Wilson, D. C.; Shore, S. G. “Novel Coordination Modes of Amidotrihydroborate: Synthesis and Structure of Metallocene and Lanthanide Compounds Containing Amidotrihydroborate”; Saint Louis, MO; Boron in the Americas XI; 2008. (b) Wilson, D. C.; Hoy, J. M.; Potratz, C. M.; Shore, S. G. Synthesis and Structure of Metallocene and Lanthanide Compounds Containing Amidotrihydroborate: A Study of Amidotrihydroborate Coordination Modes; Ohio State University; CERMACS, 2008. (c) Wilson, D.C. Synthesis, Structure, and Characterization of Rare Earth (II ) Transition Metal Cyanides; Lanthanide(II) and Metallocene Amidotrihydroborates; Dissertation, Ohio State University, Chemistry, 2009. (10) Forster, T. D.; Tuononen, H. M.; Parvez, M.; Roesler, R. J. Am. Chem. Soc. 2009, 131, 6689–6691.

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Wolstenholme et al. Table 1. Theoretical Atomic Charges (q) for [NH3BH3], [CH3CH3], [Cp2TiNH2BH3] (I), [Cp2Ti(H)NH2BH3] (II), [Cp2TiCH2CH3] (III), [EtTiCl3(dmpe)] (IV), and [CpTi(NiPr2)Cl2] (V)a

Figure 1. Single crystal X-ray structure of I with thermal ellipsoids at 50% probability. Selected bond lengths (A˚) and angles (deg): Ti1-H11 1.95(3), Ti1-N1 2.153(3), Ti1-B1 2.520(4), N1-B1=1.534(5), B1-H11=1.21(3), Ti1-N1-B1=84.5(2), N1-B1-H11 107.1(14).

Figure 2. Atomic charges (AIM) for ammonia-borane and ethane, and the E-H 3 3 3 Ti (E = B or C) moiety in their corresponding metal complexes (gray=C, white=H, blue=N, and orange=B). Scheme 1. Isolated and Postulated Titanocene Amidoborane Complexes [Cp2TiNH2BH3] (I) and [Cp2Ti(H)NH2BH3] (II)

the protic N-H and hydridic B-H bonds in the former species, as compared to the nonpolar C-H bonds in the latter molecule.11 However, coordination of the amidoborane and ethyl ligands to a metal center results in significant changes to their charge distribution (Figure 2 and Table 1), according to our DFT calculations. The coordinated B-H bond of the Ti 3 3 3 H-B moieties in I and II becomes less polar than in the parent ammonia-borane, whereas the β-agostic C-H moiety in the related metal alkyl complexes III and IV is more polarized than in ethane. These results indicate that the Hβ atoms in the amidoborane systems acquire a certain degree of metal hydride character, as revealed by their atomic charges (q(H 3 3 3 Ti) = -0.531 au in I, -0.506 au in IIa, -0.465 au in IIb vs -0.389 au in [Cp2TiH2]). In addition, the metal-coordinated (N and C) atoms in I-IV acquire approximately 0.3 unit of charge in transition from a neutral molecule to the metal complex, (11) (a) Shriver, D.F.; Atkins, P.W. Inorganic Chemistry, 3rd ed.; Oxford University Press: Oxford, U.K., 1999. (b) Staubitz, A.; Robertson, A. P. M.; Sloan, M. E.; Manners, I. Chem. Rev. 2010, 110, 4023–4078.

compound

q(H-X)

q(H-Y)

q(X)

q(Y)

q(Ti)

[NH3BH3] [CH3CH3] I IIa IIb III IV V

-0.604 0.003 -0.531 -0.506 -0.465 -0.085 -0.013 0.013

0.381 0.003 0.346 0.353 0.346 -0.008 0.019

1.717 -0.009 1.730 1.743 1.708 -0.071 -0.084 0.270

-1.030 -0.009 -1.301 -1.284 -1.295 -0.322 -0.320 -1.002

1.498 1.578 1.571 1.439 1.670 1.730

a

Values are in au (X = B or C and Y = N or C).

due to the electropositive nature of the Ti centers. The only noticeable difference between the amidoborane and alkyl complexes is the nature of the E (B or C) atoms of the 3c,2e Ti 3 3 3 H-E moieties, in which the B atoms in I and II carry a significant positive charge, while the C atoms in III and IV are negatively charged. The effect of this global charge redistribution is a convergence in electronic structures of the amidoborane and ethyl ligands toward a common bonding motif, in spite of the major electronic differences shown by their parent compounds ammonia-borane and ethane. This redistribution of charge upon coordination to a transition metal center results in close structural similarities between the amidoborane and ethyl complexes (I-IV). In general, β-agostic interactions in metal alkyl and amidoalkyl species are characterized by short M 3 3 3 H distances (d ) and tight — MXC (X = C or N) angles.1-4 In the case of our benchmark amidoborane complexes I and II, the Ti 3 3 3 H-B and — TiNB angles are comparable with typical values observed for β-agostic M 3 3 3 H-C interactions (Table 2). The activation of a C-H bond in metal alkyl complexes is generally accompanied by a shortening of the CR-Cβ linkage, whereas metal amidoalkyl complexes experience a slight elongation of the NR-Cβ bonds. The N-B bonds in I and II (1.530-1.555 A˚) experience a shortening relative to ammonia-borane (1.597(3) A˚), which appears to be a characteristic feature of β-agostic amidoborane systems. Moreover, the Ti-N bonds in I and II are slightly longer than those found in related Ti-N-B complexes that do not possess a 3c,2e moiety.12 Therefore, the Ti 3 3 3 H-B interactions in these systems behave in a similar manner as metal alkyls, in which the Ti-coordinated B-H bonds lengthen and develop Ti-H characteristics, while the N-B bonds shorten and acquire double-bond properties. Finally, the geometrical parameters for I and II (Table 2) indicate that the β-H elimination of their B-H bonds is more advanced than that of the C-H bonds in their β-agostic metal alkyl (III, IV) or amidoalkyl (V) counterparts. The reaction coordinate that characterizes β-H elimination involves significant redistribution of the bonding electron density as the M 3 3 3 H-E (E = C or B) interaction develops,2,3b as revealed from an AIM analysis of the electron distributions of I and II. In these instances, the presence of a bond path between the Ti and the Hβ atom signals an increase in Ti 3 3 3 H-B bonding relative to a noninteracting B-H group (Figure 3a). This bonding motif closely mimics that (12) The Ti-N bonds in [(NEt2)3TiNH2B(C6F5)3] and [CpTi(NMe2)2NH2B(C6F5)3] are 2.169 and 2.163 A˚, respectively. Mountford, A. J.; Clegg, W.; Coles, S. J.; Harrington, R. W.; Horton, P. N.; Humphrey, S. M.; Hursthouse, M. B.; Wright, J. A.; Lancaster, S. J. Chem.;Eur. J. 2007, 13, 4535–4547.

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Table 2. Topological Properties of the Electron Density for [Cp2TiNH2BH3], I, [Cp2Ti(H)NH2BH3], II, [Cp2TiCH2CH3], III, [EtTiCl3(dmpe)] IV, and [CpTi(iPr2N)Cl2], V. Ti 3 3 3 H(B/C) complex

d (A˚)

I IIa IIb III IV V

1.880 1.889 1.856 2.022 2.103 2.324

Ti-N/C

Fb(r) (e A˚-3)

r2Fb(r) (e A˚-5)

0.404 0.401 0.423 0.251 0.231

3.490 3.151 3.628 3.526 3.077

N-B/C-C/N-C

B-H/C-Hagostic

d (A˚)

Fb(r) (e A˚-3)

r2Fb(r) (e A˚-5)

d (A˚)

Fb(r) (e A˚-3)

r2Fb(r) (e A˚-5)

d (A˚)

Fb(r) (e A˚-3)

r2Fb(r) (e A˚-5)

2.180 2.207 2.226 2.188 2.158 1.897

0.464 0.443 0.420 0.568 0.631 0.909

5.331 4.804 4.665 1.986 1.132 8.860

1.555 1.530 1.539 1.516 1.519 1.501

0.969 1.023 0.998 1.594 1.589 1.619

7.947 8.731 8.595 -11.533 -11.589 -12.200

1.315 1.315 1.325 1.160 1.135 1.110

0.807 0.799 0.805 1.532 1.639 1.810

-0.141 -0.015 -0.949 -14.479 -16.777 -20.740

Figure 3. Theoretical [BP86/6-311G(d,p)] 2D Laplacian maps of (a) [Cp2TiNH2BH3], I, (b) [EtTiCl3(dmpe)], IV, and (c) [CpTi(iPr2N)Cl2], V, in the Ti-X-Y (X = N, C and Y = B, C) plane. Positive (solid red lines) and negative (dashed blue lines) contour lines are drawn at 0, (2.0  10n, (4.0  10n, (8.0  10n e A˚-5 with n = (3, (2, (1; extra levels at 0.001, 2.4, 2.8, 15, 150, 180, and 700 e A˚-5. Bond critical points (BCPs) are represented by a circle, while ring critical points (RCPs) are denoted by a square.

of the related β-agostic metal alkyl complexes, as shown in the topology of the corresponding moieties in III and IV (Figure 3b). However, the four-center bonding in I and II [ 3 3 3 Ti-N-B-H 3 3 3 ] appears to be more stable than the related metal alkyl III and IV [ 3 3 3 Ti-C-C-H 3 3 3 ] ring systems. Thus, the central location of the RCPs in I and II shows that these systems compensate for the electrostatic repulsion between the Ti and B atoms, whereas the close proximity of the RCP and Ti 3 3 3 H BCP in III and IV signals an unstable interaction close to bond fission. In the case of our benchmark metal amidoalkyl system, V, no Ti-Hβ bond path is observed (Figure 3c), except when the — TiNC angle is artificially constrained below 85.3b Thus, AIM analysis of I-V indicates that the bonding in titanocene amidoborane complexes resembles more closely β-agostic metal alkyls than their amidoalkyl counterparts. The strength of a covalent bonding interaction is closely correlated with the electron density at the BCP, Fb(r).13 Although the Fb(r) values for the Ti 3 3 3 H-B interactions in I and II are an order of magnitude smaller than that of a fully fledged Ti-H bond (Fb(r) = 0.696 in [Cp2TiH2]), these Ti 3 3 3 H interactions nevertheless accumulate a significant amount of electron density and are comparable to the; albeit polar;Ti-N bond (Table 2). In addition, the Ticoordinated B-H bonds are significantly weaker than their noninteracting counterparts. The Fb(r) values for the N-B bonds point toward a strengthening of these interactions, as they develop a degree of double-bond character typical of corresponding β-agostic systems. Overall, the bonding motif (13) Matta, C. F. In Hydrogen Bonding-New Insight (Challenges and Advances in Computational Chemistry and Physics Series); Grabowski, S., Ed.; Springer: New York, 2006.

in I and II attests to an advanced state of β-H elimination of the B-H bond, with a concomitant delocalization of the Ti-N bonding electron density over the whole amidoborane ligand. A comparison of Ti 3 3 3 H-B interactions in I and II with the corresponding Ti 3 3 3 H-C interactions in III and IV reveals a more advanced stage of the β-H elimination for the B-H bonds of our benchmark titanocene amidoborane complexes I and II. The high degree of Ti-H character developed in the Ti 3 3 3 H-B interactions in I and II is also reflected in the fine structure of the negative Laplacian, L(r)=-r2F(r), of the charge density.3b,4 Here, positive L(r) values represent regions of space defined by local charge concentrations (CC), whereas negative L(r) values correspond to sites of local charge depletion (CD). The valence shells of the Ti atoms in I-V are characterized by regions of CC and CD (Figure 3). The Hβ atoms of the appended ligands in these systems oppose local zones of CD, implying that the Hβ atoms seek out a region of CD to help promote the activation of the B-H and C-H bonds. The magnitude of the CD is most pronounced for titanocene amidoborane complexes (I and II), the systems that exhibit the most advanced β-H elimination. This conclusion is supported by the polarization patterns surrounding the Hβ atoms, which reveal a greater degree of perturbation for the Hβ atom in I and II than the related metal alkyl complexes III and IV. The remarkable similarities between the geometries and electron distributions of the amidoborane complexes I and II and their ethyl counterparts III and IV provide a persuasive argument for a common bonding type and support the classification of these titanocene amidoborane complexes as β-agostic in nature. In summary, despite the major differences in the chemical and physical properties of ammonia-borane and ethane, the

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geometric and electronic structures of the titanocene amidoborane complexes I and II display all the features characteristic of a β-agostic system, albeit closer to β-H elimination product than in their ethyl III and IV counterparts. The driving force behind the formation of these 3c,2e interactions appears to be delocalization of the Ti-N bonding electrons over the whole amidoborane ligand, in a manner that closely resembles the delocalization of the Ti-C electron density in corresponding metal alkyl complexes. The global redistribution of electron density in the Ti 3 3 3 H-B-N and Ti 3 3 3 H-C-C moieties works in opposite directions to reduce the differences between the two types of complexes relative to their parent molecules ammonia-borane and ethane. This feature is highlighted by the atomic charges

Wolstenholme et al.

calculated for the amidoborane and ethyl ligands, which converge toward a common bonding scenario. Finally, the β-H elimination in I and II appears to be controlled, at least in part, by the regions of CD that directly oppose the agostic B-H moieties.

Acknowledgment. This work was supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada. We thank ACEnet for providing computational facilities. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.