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Hafnium alkyl complexes of the anionic PNP pincer ligand and possible alkylidene formation
Journal of Organometallic Chemistry 696 (2011) 4132e4137
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Hafnium alkyl complexes of the anionic PNP pincer ligand and possible alkylidene formation Christina M. Brammell a, Emily J. Pelton b, Chun-Hsing Chen b, Andrey A. Yakovenko a, Wei Weng b, Bruce M. Foxman b, Oleg V. Ozerov a, * a b
Department of Chemistry, Texas A&M University, 3255 TAMU, College Station, TX 77843-3255, USA Department of Chemistry, Brandeis University, MS 015, 415 South Street, Waltham, MA 02454, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 May 2011 Received in revised form 12 July 2011 Accepted 13 July 2011
The diarylamido-based PNP pincer ligand can be used successfully for support of organometallic Hf complexes (PNP ¼ [(4-Me-2-iPr2PeC6H3)2N]). (PNP)HfCl3 (3) was prepared via reaction of (PNP)Li (2) with HfCl4(OEt2). Reactions of (PNP)HfCl3 with alkyl Grignards led to triple alkylation to produce (PNP) HfMe3 (4) with a small methyl or only double alkylation to give (PNP)Hf(CH2SiMe3)2Cl (5) with a larger alkyl. Structures of 3, 4, and 5 in the solid state were established by X-ray diffraction studies. Structures of the alkyl complexes 4 and 5 display remarkably irregular coordination environments about Hf, while in 3 it is approximately octahedral. Compound 4 was found to be thermally stable at 75 C. On the other hand, thermolysis of 5 at similar conditions led to a mixture of products, the major one of which is believed to be a Hf alkylidene on the basis of in situ NMR spectroscopic observations (e.g., d 248.2 ppm in the 13C{1H} NMR spectrum). Ó 2011 Elsevier B.V. All rights reserved.
Keywords: Pincer Hafnium Alkylidene Metal alkyl X-ray crystallography
1. Introduction Alkylidene or Schrock carbene [1] complexes are quite common for the 4d and 5d metals of groups 5 and 6 [2]. On the other hand, examples of alkylidene complexes for their group 4 relatives, Zr and Hf, are much rarer. Alkylidene complex examples are somewhat more numerous for the lighter group 4 element Ti [3e6]. The first examples of the Zr and Hf alkylidene complexes were reported by the Fryzuk group, utilizing a cyclopentadienyl/bis(phosphine) ligand P2Cp (A and B, Fig. 1) [7,8]. In 2004, we reported that a diarylamido/bis(phosphine) PNP pincer ligand [9,10] also can support Zr alkylidene complexes (C, Fig. 1) [11]. The viability of the PNP ligand as an excellent scaffold for exploring the reactivity of metaleligand multiple bonding in the early metal realm has been amply demonstrated through the work of Mindiola et al., especially with Ti [12,13]. The alkyl/alkylidene D (Fig. 1) was shown to generate a transient Ti alkylidyne that possesses rich and remarkable reactivity [14]. The development of the chemistry of the diarylamido/bis(phosphine) PNP ligands owes much to the history of the disilylamido/bis(phosphine) SiPNPR ligands of Fryzuk et al. [15], more recently studied by Caulton et al. [16]. Zr and Hf complexes of
* Corresponding author. Tel.: þ1 979 845 5870. E-mail address: [email protected] (O.V. Ozerov). 0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2011.07.017
Si PNP have been reported, including polyalkyls (E and F, Fig. 1) [17], but not alkylidenes. Alkylidene complexes are most often formed via a-abstraction from polyalkyls [2]. Coordination number considerations ensure that stabilization of group 4 alkylidenes would probably always require neutral donor ligands coordinated to the metal center. The PNP ligand supplies two such phosphine donors. The advantage may be paradoxically in that the early metals have little affinity for phosphorus ligands and thus lack the affinity for destroying a phosphine ligand when reactive species are generated. In contrast, Mindiola showed that b-diketiminate ligands (also known as “Nacnac”) are easily disassembled by Ti alkylidenes, with the Ti center forging an imido ligand out of the b-diketiminate [5]. In this work, we report our exploration of the PNP chemistry of Hf with an eye on the formation of Hf alkylidenes.
2. Results and discussion 2.1. Synthesis and solution characterization of (PNP)HfX3 complexes The introduction of the PNP ligand into the coordination sphere of Hf was successfully accomplished via deprotonation of (PNP)H (1) [10] with n-BuLi in ether and the treatment of the resultant (PNP)Li (2) [11] solution with HfCl4(OEt2)2 (Scheme 1). This reaction produced (PNP)HfCl3 (3), which was isolated in a 69% yield as an
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(d 0.69 ppm, 3JHP ¼ 4 Hz) in the 1H NMR spectrum. In the 13C{1H} spectrum, a corresponding triplet resonance at 60.6 ppm (JPC ¼ 8 Hz) was observed. The equivalence of the three methyl groups on Hf is analogous to the observations for (PNP)ZrMe3 [11] and is indicative of exchange among the methyl group sites that is rapid on the NMR timescale at ambient temperature. No static geometry can result in equivalent methyl groups in 4. The 1H and 13C chemical shifts exhibited by the HfeMe groups in 4 are comparable to those previously reported in similar compounds. For example, Fryzuk’s (SiPNP)HfMe3 compounds (E) resonated in their 1H NMR spectra in the 0.5e0.9 ppm range, depending on the nature of the alkyl group on P (no 13C NMR data reported) [17]. The 13C NMR chemical shift of (tBu2C5H3)HfMe3 was found to be 57.3 ppm by Royo et al. [18]. The ambient-temperature NMR spectra of 5 were consistent with a higher C2v symmetry, with only one kind of methine and two methyl resonances. However, the resonances were somewhat broad, indicative of an exchange process that is only slightly faster than the NMR timescale. The SiMe3 groups gave rise to singlet 1H (d 0.37 ppm) and 13C{1H} (d 4.3 ppm) resonances. The methylene groups of CH2SiMe3 resonated as broad singlets at d 0.64 ppm in the 1 H NMR spectrum and at 77.6 ppm in the 13C{1H} NMR spectrum. These chemical shifts are similar to those for the Hf-bound CH2SiMe3 groups reported in the literature [18e21]. Fig. 1. Key alkylidene complexes of group 4 metals and (SiPNP) complexes of Zr and Hf.
analytically pure orange solid. (PNP)HfCl3 (3) is only modestly soluble in aromatic hydrocarbons. Reaction of 3 with 3 equiv. of MeMgBr led to the formation of yellow trialkyl (PNP)HfMe3 (4), isolated in a 53% yield. On the other hand, the reaction of 3 with 2.5 equiv of Me3SiCH2MgCl led to the formation and isolation of the dialkyl (PNP)Hf(CH2SiMe3)2Cl (5) in a 77% yield. Utilization of a larger Me3SiCH2MgCl-to-3 ratio led only to the increased formation of (PNP)MgX [11] (X ¼ halide or alkyl). Ostensibly, the increased size of Me3SiCH2 vs Me prevents the installation of the third alkyl on Hf in a reaction of 3 with this larger Grignard. Ambient-temperature NMR spectra of 3 and 4 indicated either C2 or Cs time-averaged symmetry: a lone singlet 31P{1H} resonance (d 34.0 for 3 and 18.0 ppm for 4), equivalent aromatic rings in the PNP ligand, as well as a pair of methine and four methyl resonances stemming from iPr groups in both 1H and 13C{1H} NMR spectra. The Hf-bound methyl groups of 4 gave rise to one triplet resonance
Scheme 1. Preparation of the (PNP)HfX3 complexes.
2.2. Thermolysis of (PNP)HfMe3 (4) and (PNP)Hf(CH2SiMe3)2Cl (5) Thermolysis of 4 in C6D6 solution at 75 C for 48 h did not result in the formation of any new products detectable by NMR spectroscopy (Scheme 2). This lack of reactivity is in line with the thermal inertness of (PNP)ZrMe3 [11] but is in contrast to double a-abstraction occurring upon thermolysis of (PNP)TaMe4 to give (PNP)Ta(]CH2)2 [22]. Thermolysis of (PNP)Hf(CH2SiMe3)2Cl (5) at 80 C in C6D6 resulted in 80% conversion overnight and gave rise to a mixture with two dominant P-containing products. In the 31P{1H} NMR spectrum of this mixture, one gave rise to a singlet at 29.4 ppm, while the other, major product, gave rise to an AB system (d 49.0 and 39.0 ppm, 2 JPP ¼ 70 Hz). At the same time, a singlet resonance at d 8.61 ppm was detected by 1H NMR spectroscopy and a multiplet resonance at d 248.2 ppm (identified as a CH by a DEPT experiment) by 13C NMR spectroscopy in this mixture. And HSQC experiment showed correlation between the 1H resonance at d 8.61 ppm and the 13C resonance at d 248.2 ppm. The AB system in the 31P{1H} NMR spectrum, the downfield 1H NMR and 13C NMR resonances are reminiscent of the spectra displayed by the PNP-supported alkylidenes of other group 4
Scheme 2. Thermolysis of (PNP)HfMe3 (4) and of (PNP)Hf(CH2SiMe3)2Cl (5).
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Fig. 2. POV-Ray [29] rendition of an ORTEP [30] drawing (50% probability ellipsoids) of (PNP)HfCl3 (3) with hydrogen atoms and benzene solvent molecule omitted for clarity.
Fig. 4. POV-Ray [29] rendition of an ORTEP [30] drawing (50% probability ellipsoids) of (PNP)Hf(CH2SiMe3)2Cl (5) with hydrogen atoms omitted for clarity.
2.3. Single crystal X-ray diffraction studies metals Ti and Zr. For example, (PNP)Ti(]CHCMe3)(OTf) displayed an AB 31P NMR pattern with 2JPP ¼ 55 Hz and Dd z 10 ppm, an 8.42 ppm singlet 1H NMR resonance and a 301 ppm 13C NMR resonance [23]. (PNP)Zr(]CHPh)(CH2Ph) (C) displayed an AB 31P NMR pattern with 2 JPP ¼ 60 Hz and Dd z 2 ppm, a 7.32 ppm singlet 1H resonance, and a 231 ppm 13C resonance [11]. Likewise, Fryzuk’s A [8] and B [7a] gave rise to singlet 1H NMR resonances at 7.33 and 8.1 ppm, and 13C NMR resonances at 210 and 229.4 ppm, respectively. We thus hypothesized that the new unknown major component of the mixture formed in the thermolysis of 5 is (PNP)Hf(]CHSiMe3)(Cl) (6, Scheme 2), the alkylidene product of a-abstraction. Indeed, the required SiMe4 by-product of a-abstraction was identified by 1H and 13C NMR in the reaction mixture. Unfortunately, the high and apparently similar solubility of the components of the mixture has precluded isolation of the proposed 6 in a pure state.
Fig. 3. POV-Ray [29] rendition of an ORTEP [30] drawing (50% probability ellipsoids) of (PNP)HfMe3 (4) with hydrogen atoms omitted for clarity.
We were able to obtain X-ray quality single crystals of 3, 4, and 5 and establish their solid-state structures by X-ray diffraction methods. The individual molecular structures are depicted in Figs. 2e4. The bond distances and angles associated with the Hf immediate coordination environment are shown in Table 1, while Fig. 5 shows the immediate six-coordinate environment about the Hf center in these three molecules side by side from analogous points of view. Octahedral geometry is sterically (and often, electronically) preferred for six ligands about a metal center. However, only 3 possesses a geometry about Hf that can be described as even
Table 1 Summary of select bond distances and angles in (PNP)HfCl3 (3), (PNP)HfMe3 (4), and (PNP)Hf(CH2SiMe3)2Cl (5). Bond distances (Å)
(PNP)HfCl3
(PNP)HfMe3
(PNP)Hf(CH2SiMe3)2Cl
P1eHf1 P2eHf1 N1eHf1 R1-Hf1 R2-Hf1 R3-Hf1 Bond Angles ( ) P1eHf1eP2 P1eHf1eN P1eHf1eR1 P1eHf1eR2 P1eHf1eR3 P2eHf1eN P2eHf1eR1 P2eHf1eR2 P2eHf1eR3 N1eHf1eR1 N1eHf1eR2 N1eHf1eR3 R1eHf1eR2 R1eHf1eR3 R2eHf1eR3
2.6954 (11) 2.6954 (11) 2.173 (5) 2.4082 (13) 2.412 (2) 2.4082 (13)
2.7360 (14) 2.8034 (14) 2.214 (4) 2.234 (5) 2.301 (5) 2.251 (6)
2.7817 2.8309 2.1936 2.2478 2.2283 2.4575
146.32 (5) 73.16 (2) 89.97 (4) 106.84 (2) 90.90 (4) 73.16 (2) 90.90 (4) 106.84 (2) 89.97 (4) 91.50 (3) 180 91.50 (3) 88.50 (3) 177.00 (6) 88.50 (3)
132.83 (4) 70.78 (11) 126.77 (15) 78.50 (14) 104.45 (16) 70.39 (11) 82.77 (15) 147.22 (14) 78.04 (15) 95.63 (19) 140.63 (18) 125.53 (19) 83.3 (2) 123.4 (2) 85.2 (2)
124.363 (13) 70.00 (3) 97.72 (4) 139.08 (4) 76.929 (14) 69.09 (3) 79.94 (4) 82.15 (4) 156.209 (13) 128.18 (5) 97.24 (6) 133.56 (3) 118.97 (6) 87.14 (5) 86.86 (4)
Cl1 Cl2 Cl10
C27 C28 C29
C27 C31 C11
R1¼ R2¼ R3¼
(4) (4) (13) (17) (16) (4)
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Fig. 5. Views [29,30] down the NeHf axis in (PNP)HfCl3 (3, left), (PNP)HfMe3 (4, center) and (PNP)Hf(CH2SiMe3)2Cl (5, right). Only the atoms directly bound to the Hf centers shown.
a distorted octahedral. The only source of major deviation from the octahedron in 3 has to do with the inability of the PNP ligand to accommodate a 180 PeHfeP angle. This angle is merely 146.32(5) , but the Namido and the three chlorides form a nearly perfectly square-planar “midsection” of this distorted octahedron. The low PeHfeP angle is observed despite the strict meridionality of the PNP ligand. The PeMeP angles in meridional PNP complexes of late transition metals are typically in the 160e170 range [9]. Hafnium is considerably larger and the longer HfeN bond positions it farther away from the phosphines in the PNP/Hf plane, resulting in a narrower PeHfeP angle. The geometry about Hf in 4 and 5 is irregular and cannot be easily described in terms of simple polyhedra. The molecular structure of 4 is nearly superimposable [24] with that of (PNP) ZrMe3 we reported in 2004 [11]. We described [11] the latter as having “both the PNP and the Me3 sets of donors. halfway between a facial and a meridional arrangement.” This same description applies well to 4. The coordination environment about Hf in 5 can be described similarly, although it is not superimposable with (PNP)HfMe3. Fryzuk et al. analyzed the structure of the closely related (SiPNPMe)HfMe3 as a bicapped tetrahedron, with the two neutral P donors capping the faces of the NHfMe3 tetrahedron [17]. This description is less apt for (PNP)MMe3 (M ¼ Zr or Hf) because the requisite angles deviate to a significantly greater degree from the idealized bicapped tetrahedron. It is possible that the bicapped tetrahedron motif is in fact desirable for these systems, but that the greater rigidity of the diarylamido PNP causes greater distortions in the structure. Polyalkyl complexes of d0 metals frequently deviate from the sterically preferred geometries and it has been proposed that maximization of orbital overlap for the strongly covalent MeC bonds may be responsible [25]. The HfeC distances in 4 and 5 are in the 2.23e2.30 Å range that is typical for Hf alkyls. The HfeMe bond distances in Fryzuk’s (SiPNPMe)HfMe3 [17] and (P2N2)HfMe2 [26] are in the 2.24e2.27 Å range, and the ZreMe bond distances in our (PNP)ZrMe3 are in the 2.25e2.29 Å range [11]. Scott and Lippard reported a series of Zr and Hf bis(trimethylsilylmethyl) complexes supported by N4 dianionic tropocoronand ligands with MeC bonds ranging from 2.24 to 2.35 Å [21]. The HfeN and HfeCl bond distances are unremarkable. The HfeP distances are notably shorter in 3 (2.6954(11) Å) than in the alkyl complexes, where they range from 2.73 to 2.83 Å. That is likely a consequence of the stronger Lewis acidity of the Hf center in 3. The longest HfeP distances were found in 5 which probably reflects the increased steric pressure from the larger alkyl groups, as well. This trend and the approximate values are reminiscent of Fryzuk’s (P2N2)HfX2 [26] and and (SiPNPMe)HfX3 [17,27] complexes where for X ¼ Cl, the HfeP distances (ca. 2.7 Å) were ca. 0.05e0.10 Å shorter than for X ¼ Me (ca. 2.8 Å). On the other hand, the HfeP distances in (P2Cp)HfCl3 (ca. 2.85e2.88 Å) were considerably longer [8], despite being bound to a HfCl3 center. The difference illustrates
the greater donicity (and imposed electron count at Hf) of the Cp ligand vs amido. In Hf(IV) complexes, there is no possibility of backdonation and the bonding between P and Hf must be of a purely Lewis acidebase adduct nature. The mismatch between soft phosphine donors and a hard Hf(IV) center probably results in a rather weak bond whose length may change considerably with minimal change in overall energy. It is worth noting that the PNP ligand resonances in the 1H and 13 C NMR spectra of 3 displayed virtual triplet coupling [28] to the two 31P nuclei. This phenomenon is typically associated with transdisposition of equivalent phosphines. More precisely, it stems from a large JPP value that tends to be greater as the PeMeP angle approaches 180 . The coupling to the pair of 31P nuclei in 4 and 5 manifested itself either simply as doublet splitting or as more complex transitional multiplets, indicative of a smaller JPP value in these compounds. Our structural studies show that the PeHfeP angle in 3 (146.32(5) ) is indeed greater than in the alkyl complexes (132.83(4) and 124.363(13) ), although still considerable smaller than 180 . The combination of the greater PeHfeP angle and tighter HfeP interaction probably gives rise to a greater JPP value in 3 and thus more idealized virtual triplet coupling to the pair of 31P nuclei. 3. Conclusion In summary, we have synthesized and characterized Hf(IV) complexes of the diarylamido-based PNP ligand. The structure of (PNP)HfCl3 (3) displays deviations from octahedral geometry about Hf that can be simply explained by the constraint of the PNP ligand. The deviations from octahedral geometry about Hf in (PNP)HfMe3 (4) and (PNP)Hf(CH2SiMe3)2Cl (5) are much more severe and stem from a combination of the effect of the chelate constraint and from the preference for accommodating stronger bonds to anionic ligands. Thermolysis of 5 led to the formation of a new compound that displays spectroscopic characteristics consistent with a Hf alkylidene, however, attempts at isolation and full characterization have not proven successful. 4. Experimental section 4.1. General considerations Unless specified otherwise, all manipulations were performed under an argon atmosphere using standard Schlenk or glovebox techniques. Dioxane, diethyl ether, tetrahydrofuran and benzene were dried over sodium-benzophenone ketyl, distilled or vacuum transferred and stored over molecular sieves in an Ar-filled glovebox; (PNP)H (1) was synthesized according to published procedures [10], HfCl4(OEt2)2 was synthesized in a manner analogous to published procedures [31] and all other chemicals were used as received from commercial vendors. All NMR spectra were recorded
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on a Varian iNova 300 spectrometer (1H NMR, 299.951 MHz; 31P NMR, 121.425 MHz; 13C NMR, 75.413 MHz), Varian Mercury 300 spectrometer (13C NMR 75.426 MHz), Varian iNova 400 spectrometer (1H NMR, 399.755 MHz; 13C NMR, 100.518 MHz; 31P NMR 181.822 MHz), or a Varian iNova NMR 500 (1H NMR, 499.425 MHz/ 499.683 MHz; 13C NMR, 75.424 MHz/125.580 MHz; 31P NMR, 202.171 MHz) spectrometer. Chemical shifts are reported in d/ppm. For 1H and 13C NMR spectra, the residual solvent peak was used as an internal reference. 31P NMR spectra were referenced externally using 85% H3PO4 at d 0 ppm. Elemental analysis was performed by CALI Labs, Parsippany, NJ, USA.
(dd, 6H, JHH ¼ 7 Hz, JPH ¼ 14 Hz, CHMe2), 1.13 (apparent t overlapping with 1.10, 6H, JHH ¼ 7 Hz, JPH ¼ 17 Hz, CHMe2), 1.10 (apparent t overlapping with 1.10, 6H, JHH ¼ 7 Hz, JPH ¼ 17 Hz, CHMe2), 0.94 (apparent t, 6H, JHH ¼ 8 Hz, CHMe2), 0.69 (t, 9H, JPH ¼ 4, HfMe3). 31P{1H} NMR (C6D6): d 18.4 (s). 31P{1H} NMR (toluene): d 17.7. 13C{1H} NMR (C6D6): d 161.0 (d, JPC ¼ 23 Hz), 133.3 (s), 132.7 (s), 122.2 (s), 121.8 (s), 121.7 (d, JPC ¼ 8 Hz), 60.6 (t, JPC ¼ 8 Hz, HfMe3), 25.0 (br s, CHMe2), 21.6 (m, CHMe2), 21.1 (s, ArCH3), 20.0 (br s, CHMe2), 19.6 (br s, CHMe2), 18.7 (br s, CHMe2), 17.6 (br s, CHMe2). Elem. Anal., found(calc) for C29H49HfNP2: C, 53.29(53.27); H, 7.47(7.56).
4.2. Synthesis of (PNP)HfCl3 (3)
4.4. Synthesis of (PNP)Hf(CH2SiMe3)2Cl (5)
(PNP)H (1) (4.39 g, 10.2 mmol) was suspended in diethyl ether at 35 C. In a slow dropwise fashion n-butyllithium (4.1 mL of a 2.5 M solution in hexanes, 10.2 mmol) was added to the suspension. The reaction was left to stir for 30 min and then placed under vacuum to remove all volatiles. The oily residue (presumed to contain 2) was dissolved in toluene. HfCl4$Et2O (4.80 g, 10.2 mmol) was added to the solution. After stirring overnight a deep orange solution was observed with a precipitate (LiCl). The solution was stripped down and CH2Cl2 was added and the solution was passed through a plug of Celite. 31P{1H} NMR spectroscopic analysis revealed >98% content of 3. The filtrate was recrystallized from pentane at 35 C. The collected crystals were washed with cold pentane and dried under vacuum (69% yield). 1H NMR (C6D6): d 6.94 (dvt, 2H, JHH ¼ 8 Hz, JPH ¼ 8 Hz, AreH), 6.89 (ddd, 2H, JHH ¼ 2 Hz, JHH ¼ 8 Hz, AreH), 6.86 (br, 2H, AreH), 2.36 (m, 2H, CHMe2), 2.12 (m, 2H, CHMe2), 2.09 (s, 6H, AreCH3), 1.40 (apparent q, 6H, JHH ¼ 7 Hz, CHMe2), 1.21 (dd overlapping with 1.18, 6H, JHH ¼ 7 Hz, CHMe2), 1.18 (dd overlapping with 1.21, 6H, JHH ¼ 7 Hz, CHMe2), 0.93 (apparent q, 6H, JHH ¼ 7 Hz, CHMe2). 1H{31P} NMR (C6D6): d 6.94 (br d, 2H, JHH ¼ 8 Hz, AreH), 6.90 (dd, 2H, JHH ¼ 2, 7 Hz, AreH), 6.86 (br d, 2H, JHH ¼ 2 Hz, AreH), 2.36 (m, 2H, CHMe2), 2.12 (m, 2H, CHMe2), 2.09 (s, 6H, AreCH3), 1.40 (d, 6H, JHH ¼ 7 Hz, CHMe2), 1.21 (d overlapping with 1.18, 6H, JHH ¼ 7 Hz, CHMe2), 1.18 (d overlapping with 1.21, 6H, JHH ¼ 7 Hz, CHMe2), 0.93 (d, 6H, JHH ¼ 7 Hz, CHMe2). 31P{1H} NMR (C6D6): d 34.0 (s). 13C{1H} (CDCl3): d 157.9 (vt, JPC ¼ 11 Hz), 133.4 (s), 132.1 (s), 131.4 (s), 121.0 (vt, JPC ¼ 17 Hz), 120.0 (s), 26.4 (br s, CHMe2), 21.7 (vt, JPC ¼ 9 Hz, CHMe2), 21.1 (s, AreMe), 21.0 (s, CHMe2), 19.1 (s, CHMe2), 18.9 (s, CHMe2), 17.6 (s, CHMe2). Elem. Anal., Calculated for C26H40Cl3HfNP2: C, 43.75; H, 5.65; Cl, 14.71. Found, batch 1: C, 43.73; H, 5.62 and batch 2: C, 43.49; H, 5.57; Cl, 14.64.
(PNP)HfCl3 (3) (500 mg, 0.708 mmol) was dissolved in 11 mL of diethyl ether. To this was added 62 mL of dioxane and Me3SiCH2MgCl (1.75 mL, 1.0 M in Et2O, 1.75 mmol). The color of the solution changed from yellow to faint green. The mixture was stirred until no starting material was present by 31P{1H} NMR analysis. After that, the solution was passed through a pad of Celite. The filtrate was placed under vacuum to remove all volatiles. Twice, pentane was added to the yellow residue and placed under vacuum to remove volatiles. The product was recrystallized from pentane at 35 C. The yellow crystalline solid was washed twice with cold pentane and dried under vacuum. (446 mg, 77% yield). 1H NMR (C6D6): d 6.98 (br dd, 2H, JHH ¼ 2 Hz, JPH ¼ 5 Hz, AreH), 6.90 (br dd, 2H, JHH ¼ 2 Hz, JHH ¼ 8 Hz, AreH), 6.79 (dd, 2H, JHH ¼ 8 Hz, JPH ¼ 5 Hz, AreH), 2.35 (br m, 4H, CHMe2), 2.10 (s, 6H, AreCH3), 1.16 (dd, 12H, JHH ¼ 7 Hz, JPH ¼ 13 Hz CHMe2), 1.05 (br, 12H, CHMe2), 0.64 (br s, 4H, HfCH2SiMe3), 0.37 (s, 18H, SiMe3). 31P{1H} NMR (Et2O): d 11.2 (s). 31P{1H} NMR (C6D6): d 12.0 (s). 31P{1H} NMR (THF): d 12.4 (s). 13C{1H} NMR (C6D6): d 158.7 (d, JPC ¼ 16 Hz), 133.2 (s), 131.1 (s), 128.3 (additional aromatic signal overlapping with internal standard), 124.6 (d, JPC ¼ 14 Hz), 123.5 (s), 77. 6 (t, JPC ¼ 6 Hz, HfCH2), 24.3 (br s, CHMe2), 21.3 (s, ArCH3), 20.2 (br s, CHMe2), 19.3 (br s, CHMe2), 4.5 (s, SiMe3). Elem. Anal., found(calc) for C34H62ClHfNP2Si2: C, 49.88(49.92); H, 7.53(7.65).
4.3. Synthesis of (PNP)HfMe3 (4)
4.6. Thermolysis of (PNP)Hf(CH2SiMe3)2Cl (5)
(PNP)HfCl3 (3) (0.31 g, 0.44 mmol) was suspended in 75 mL of toluene. CH3MgBr (0.44 mL, 3 M solution in diethyl ether, 1.3 mmol) was added dropwise. The orange solution turned lime yellow. 50 mL of dioxane was added to precipitate out the MgBr2 and MgCl2. The reaction was stirred overnight. The volatiles were removed from the solution under vacuum and then approximately 5 mL of isooctane was added to help precipitate magnesium halides. The volatiles were then removed under vacuum and the residue was dissolved in toluene. The solution was passed through a pad of Celite and the volatiles were again removed under vacuum. Approximately 4 mL of pentane was added to dissolve the yellow residue and the flask was placed in the freezer at 35 C for recrystallization. Yellow crystals of 4 were collected, washed with cold pentane and the solid was placed under vacuum to dry (53% yield). 1H NMR (C6D6): d 6.96 (dd, 2H, JHH ¼ 8 Hz, JPH ¼ 5 Hz, AreH), 6.92 (br d, 2H, JHH ¼ 8 Hz, AreH), 6.88 (br d, 2H, JHH ¼ 8 Hz, AreH), 2.37 (m, 2H, CHMe2), 2.21 (m, 2H, CHMe2), 2.11 (s, 6H, AreMe), 1.22
(PNP)Hf(CH2SiMe3)2Cl (5) (48.0 mg, 0.612 mmol) was dissolved in C6D6 in a J-Young NMR tube yielding a yellow solution. The NMR tube was placed into an 80 C oil bath overnight, during which time it became a dark orange solution. 31P{1H} NMR revealed 80% conversion of 5 and the appearance of a major new product (6) displaying an AB system (two doublets at 49.0 ppm and 39.0 ppm, JPP ¼ 70 Hz) and another, lesser product displaying a singlet resonance at 29.4 ppm. In the 1H NMR spectrum, a new singlet downfield resonance (d 8.61 ppm) was detected. In the 13C{1H} NMR spectrum, a new resonance at 248.2 ppm (br m) was detected. 1 H and 13C NMR spectroscopic analysis also indicated the presence of substantial amount of SiMe4 (0 ppm in both).
4.5. Thermolysis of (PNP)HfMe3 (4) (PNP)HfMe3 (4) was dissolved in C6D6, forming a yellow solution, and was placed into a 75 C oil bath for 48 h, during which time it became a dark reddish brown solution. 31P{1H} NMR revealed no new compounds. Only the resonance of 4 was observed at 18.2 ppm.
4.7. X-ray crystallography For the structures of 3 and 5, all operations were performed on a Bruker-Nonius Kappa Apex2 diffractometer at 120 K, using
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graphite-monochromated MoKa radiation. All diffractometer manipulations, including data collection, integration, scaling, and absorption corrections were carried out using the Bruker Apex2 software [32]. The structures were solved using SIR-92 [33], and refined (full-matrix-least squares) using the Oxford University Crystals for Windows program [34]. For the structure of 4, the single crystal X-ray diffraction experiment was carried out with a Bruker SMART 1000 diffractometer with a CCD area detector (graphite-monochromated Mo Ka radiation) at 110 K. Semiempirical method SADABS [35] was applied for absorption correction. The structure was solved by direct methods and refined by the full-matrix least-squares technique against F2 with anisotropic temperature parameters for all non-hydrogen atoms. Data reduction and further calculations were performed using the Bruker SAINTþ [36] and SHELXTL NT [37] program packages. Acknowledgment We are grateful to the US National Science Foundation for support of this research through grants CHE-0809522 and CHE0944634 to O.V.O., through support of E.J.P. as an REU participant in 2007 (grant CHE-0552767 for an REU site at Brandeis University), through support of C.M.H. as an REU participant in 2008 (grant CHE-0552767 for an REU site at Brandeis University) and 2009 (grant CHE-0755207 for an REU site at Texas A&M University), and through grant CHE-0521047 for the purchase of a new X-ray diffractometer at Brandeis University. We are also grateful for the support from the Welch Foundation (grant A-1717 to O.V.O), the Dreyfus Foundation (Camille Dreyfus Teacher-Scholar Award to O.V.O.), and the Sloan Foundation (Alfred P. Sloan Research Fellowship to O.V.O.). We thank Dr. Christos Douvris and Yanjun Zhu for help with some of the experiments in this paper. Appendix. Supporting material CCDC 833482 ((PNP)HfMe3), 833492 ((PNP)HfCl3), 833493 ((PNP)Hf(CH2SiMe3)2Cl) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data_request/cif. Appendix. Supplementary material
[5] [6]
[7]
[8] [9]
[10] [11] [12] [13]
[14]
[15]
[16]
[17] [18] [19] [20] [21] [22] [23] [24] [25]
Supplementary material associated with this article can be found in online version at doi:10.1016/j.jorganchem.2011.07.017. References [1] J.F. Hartwig, Organotransition Metal Chemistry: From Bonding to Catalysis. University Science Books, Sausalito, CA, 2009, 41e45. [2] (a) R.R. Schrock, Chem. Rev. 102 (2002) 145; (b) R.R. Schrock, in: P.R. Braterman (Ed.), Reactions of Coordinated Ligands, Plenum, New York, 1986; (c) J. Feldman, R.R. Schrock, Prog. Inorg. Chem. 39 (1991) 1; (d) R.R. Schrock, Pure Appl. Chem. 66 (1994) 1447; (e) R. Beckhaus, C. Santamaria, J. Organomet. Chem. 617e618 (2001) 81. [3] (a) H. van der Heijden, B. Hessen, J. Chem. Soc. Chem. Commun. (1995) 145; (b) P.-J. Sinnema, L. van der Veen, A.L. Spek, N. Veldman, J.H. Teuben, Organometallics 16 (1997) 4245; (c) S. Kahlert, H. Goerls, J. Scholz, Angew. Chem., Int. Ed. 37 (1998) 1857; (d) C. Kruger, R. Mynott, C. Siedenbiedel, L. Stehling, G. Wilke, Angew. Chem. Int. Ed. Engl. 30 (1991) 1668; (e) See also Tebbe’s reagent: F.N. Tebbe, G.W. Parshall, G.S. Reddy J. Am. Chem. Soc. 100 (1978) 3611; (f) For studies of the thermal decomposition of Ti(CH2CMe3)4, see: J. Cheon, D.M. Rogers, G.S. Girolami J. Am. Chem. Soc. 119 (1997) 6804. [4] (a) J.A. van Doorn, H. van der Heijden, A.G. Orpen, Organometallics 13 (1994) 4271; (b) J.A. van Doorn, H. van der Heijden, A.G. Orpen, Organometallics 14 (1995) 1278;
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(c) R. Baumann, R. Stumpf, W.M. Davis, L.-C. Liang, R.R. Schrock, J. Am. Chem. Soc. 121 (1999) 7822; (d) Coville et al. described a series of bis(iminophosphorano)methanediide complexes of the group 4 metals that resemble metal alkylidenes R.P.K. Babu, R. McDonald, R.G. Cavell, Organometallics 19 (2000) 3462. F. Basuli, B.C. Bailey, J. Tomaszewski, J.C. Huffman, D.J. Mindiola, J. Am. Chem. Soc. 125 (2003) 6052. Andrews et al. have studied reactions of group 4 metal atoms with CX4 molecules that give M]CX2 products: (a) J.T. Lyon, L. Andrews, Organometallics 26 (2007) 332; (b) H.-G. Cho, T.-H. Kim, L. Andrews, Chem. Asian J. 1 (2006) 404; (c) H.-G. Cho, X. Wang, L. Andrews, Organometallics 24 (2005) 2854; (d) H.-G. Cho, L. Andrews, Organometallics 23 (2004) 4357. (a) M.D. Fryzuk, S.S.H. Mao, M.J. Zaworotko, L.R. MacGillivray, J. Am. Chem. Soc. 115 (1993) 5336; (b) M.D. Fryzuk, P.B. Duval, S.S.H. Mao, M.J. Zaworotko, L.R. MacGillivray, J. Am. Chem. Soc. 121 (1999) 2478; (c) M.D. Fryzuk, P.B. Duval, S.S.H. Mao, S.J. Rettig, M.J. Zaworotko, L.R. MacGillivray, J. Am. Chem. Soc. 121 (1999) 1707. M.D. Fryzuk, P.B. Duval, B.O. Patrick, S.J. Rettig, Organometallics 20 (2001) 1608. (a) L.-C. Liang, Coord. Chem. Rev. 250 (2006) 1152; (b) O.V. Ozerov, in: D. Morales-Morales, C. Jensen (Eds.), The Chemistry of Pincer Compounds, Elsevier, Amsterdam, 2007, pp. 287e309. L. Fan, B.M. Foxman, O.V. Ozerov, Organometallics 23 (2004) 326. W. Weng, L. Yang, B.M. Foxman, O.V. Ozerov, Organometallics 23 (2004) 4700. (a) D.J. Mindiola, Acc. Chem. Res. 39 (2006) 813; (b) D.J. Mindiola, B.C. Bailey, F. Basuli, Eur. J. Inorg. Chem. (2006) 3135. (a) U.J. Kilgore, J. Tomaszewski, H. Fan, J.C. Huffman, D.J. Mindiola, Organometallics 26 (2007) 6132; (b) U.J. Kilgore, C.A. Sengelaub, H. Fan, J. Tomaszewski, M. Pink, J.A. Karty, M.H. Baik, D.J. Mindiola, Organometallics 28 (2009) 843. (a) A.R. Fout, J.L. Scott, D.L. Miller, B.C. Bailey, J.C. Huffman, M. Pink, D.J. Mindiola, Organometallics 28 (2009) 331; (b) B.C. Bailey, H. Fan, J.C. Huffman, M.-H. Baik, D.J. Mindiola, J. Am, Chem. Soc. 129 (2007) 8781. (a) M.D. Fryzuk, Can. J. Chem. 70 (1992) 2839; (b) M.D. Fryzuk, D.J. Berg, T.S. Haddad, Coord. Chem. Rev. 99 (1990) 137; (c) M.D. Fryzuk, C.D. Montgomery, Coord. Chem. Rev. 95 (1989) 1. (a) B.C. Fullmer, H. Fan, M. Pink, J.C. Huffman, N.P. Tsvetkov, K.G. Caulton, J. Am. Chem. Soc. 133 (2011) 2571; (b) O.V. Ozerov, L.A. Watson, M. Pink, K.G. Caulton, J. Am. Chem. Soc. 129 (2007) 6003. M.D. Fryzuk, A. Carter, S.J. Rettig, Organometallics 11 (1992) 469. T. Cuenca, C. Montejano, P. Royo, J. Organomet. Chem. 514 (1996) 93. T. Ishiyama, T. Mizuta, K. Miyoshi, H. Nakazawa, Organometallics 22 (2003) 1096. T.H. Warren, R.R. Schrock, W.M. Davis, Organometallics 17 (1998) 308. M.J. Scott, S.J. Lippard, Inorg. Chim. Acta 263 (1997) 287. L.C.H. Gerber, L.A. Watson, S. Parkin, W. Weng, B.M. Foxman, O.V. Ozerov, Organometallics 26 (2007) 4866. B.C. Bailey, J.C. Huffman, D.J. Mindiola, W. Weng, O.V. Ozerov, Organometallics 24 (2005) 1390. See Fig. S25 in the Supporting material. See for example: (a) M. Kaupp, Chem. Eur. J. 4 (1998) 1678; (b) M. Kaupp, Angew. Chem., Int. Ed. 40 (2001) 3534; (c) L. Perrin, L. Maron, O. Eisenstein, Faraday Discussions 124 (2003) 25; (d) [(tBu3SiCC)6Ta] is trigonal prismatic but [(tBu3SiCC)6Zr]2 is octahedral: T.P. Vaid, A.S. Veige, E.B. Lobkovsky, W.V. Glassey, P.T. Wolczanski, L.M. LiableSands, A.L. Rheingold, T.R. Cundari J. Am. Chem. Soc. 120 (1998) 10067; (e) [Li(tmeda)]2[ZrMe6] was shown to have a trigonal prismatic environment about Zr in the solid state: P.M. Morse, G.S. Girolami J. Am. Chem. Soc. 111 (1989) 4114. M.D. Fryzuk, J.R. Corkin, B.O. Patrick, Can. J. Chem. 81 (2003) 1376. M.D. Fryzuk, A. Carter, A. Westerhaus, Inorg. Chem. 24 (1985) 642. R.H. Crabtree, The Organometallic Chemistry of the Transition Metals, third ed. Wiley-Interscience, New York, 2001, pp. 260e262. POV-Ray e The Persistence of Vision Raytracer, available at http://www. povray.org/. Ortep-3 for Windows L. Farugia, J. Appl. Crystallogr. 30 (1997) 565. W. Weng, Ph.D. Thesis, Brandeis University, Waltham, MA, 2007. Apex2, Version 2 User Manual, M86-E01078. Bruker Analytical X-ray Systems, Madison, WI, June 2006. A. Altomare, G. Cascarano, G. Giacovazzo, A. Guagliardi, M.C. Burla, G. Polidori, M. Camalli, J. Appl. Cryst. 27 (1994) 435. P.W. Betteridge, J.R. Carruthers, R.I. Cooper, K. Prout, D.J. Watkin, J. Appl. Cryst. 36 (2003) 1487. G.M. Sheldrick, SADABS, Version 2007/4. Bruker Analytical X-ray Systems, Inc., Madison, WI, 2007. Bruker, SAINTPlus, Data Reduction and Correction Program v. 6.2. Bruker AXS, Madison, WI, 2001. G.M. Sheldrick, SHELXTL-2008/2004, Structure Determination Software Suite. Bruker AXS, Madison, WI, 2008.
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem
Hafnium alkyl complexes of the anionic PNP pincer ligand and possible alkylidene formation Christina M. Brammell a, Emily J. Pelton b, Chun-Hsing Chen b, Andrey A. Yakovenko a, Wei Weng b, Bruce M. Foxman b, Oleg V. Ozerov a, * a b
Department of Chemistry, Texas A&M University, 3255 TAMU, College Station, TX 77843-3255, USA Department of Chemistry, Brandeis University, MS 015, 415 South Street, Waltham, MA 02454, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 May 2011 Received in revised form 12 July 2011 Accepted 13 July 2011
The diarylamido-based PNP pincer ligand can be used successfully for support of organometallic Hf complexes (PNP ¼ [(4-Me-2-iPr2PeC6H3)2N]). (PNP)HfCl3 (3) was prepared via reaction of (PNP)Li (2) with HfCl4(OEt2). Reactions of (PNP)HfCl3 with alkyl Grignards led to triple alkylation to produce (PNP) HfMe3 (4) with a small methyl or only double alkylation to give (PNP)Hf(CH2SiMe3)2Cl (5) with a larger alkyl. Structures of 3, 4, and 5 in the solid state were established by X-ray diffraction studies. Structures of the alkyl complexes 4 and 5 display remarkably irregular coordination environments about Hf, while in 3 it is approximately octahedral. Compound 4 was found to be thermally stable at 75 C. On the other hand, thermolysis of 5 at similar conditions led to a mixture of products, the major one of which is believed to be a Hf alkylidene on the basis of in situ NMR spectroscopic observations (e.g., d 248.2 ppm in the 13C{1H} NMR spectrum). Ó 2011 Elsevier B.V. All rights reserved.
Keywords: Pincer Hafnium Alkylidene Metal alkyl X-ray crystallography
1. Introduction Alkylidene or Schrock carbene [1] complexes are quite common for the 4d and 5d metals of groups 5 and 6 [2]. On the other hand, examples of alkylidene complexes for their group 4 relatives, Zr and Hf, are much rarer. Alkylidene complex examples are somewhat more numerous for the lighter group 4 element Ti [3e6]. The first examples of the Zr and Hf alkylidene complexes were reported by the Fryzuk group, utilizing a cyclopentadienyl/bis(phosphine) ligand P2Cp (A and B, Fig. 1) [7,8]. In 2004, we reported that a diarylamido/bis(phosphine) PNP pincer ligand [9,10] also can support Zr alkylidene complexes (C, Fig. 1) [11]. The viability of the PNP ligand as an excellent scaffold for exploring the reactivity of metaleligand multiple bonding in the early metal realm has been amply demonstrated through the work of Mindiola et al., especially with Ti [12,13]. The alkyl/alkylidene D (Fig. 1) was shown to generate a transient Ti alkylidyne that possesses rich and remarkable reactivity [14]. The development of the chemistry of the diarylamido/bis(phosphine) PNP ligands owes much to the history of the disilylamido/bis(phosphine) SiPNPR ligands of Fryzuk et al. [15], more recently studied by Caulton et al. [16]. Zr and Hf complexes of
* Corresponding author. Tel.: þ1 979 845 5870. E-mail address: [email protected] (O.V. Ozerov). 0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2011.07.017
Si PNP have been reported, including polyalkyls (E and F, Fig. 1) [17], but not alkylidenes. Alkylidene complexes are most often formed via a-abstraction from polyalkyls [2]. Coordination number considerations ensure that stabilization of group 4 alkylidenes would probably always require neutral donor ligands coordinated to the metal center. The PNP ligand supplies two such phosphine donors. The advantage may be paradoxically in that the early metals have little affinity for phosphorus ligands and thus lack the affinity for destroying a phosphine ligand when reactive species are generated. In contrast, Mindiola showed that b-diketiminate ligands (also known as “Nacnac”) are easily disassembled by Ti alkylidenes, with the Ti center forging an imido ligand out of the b-diketiminate [5]. In this work, we report our exploration of the PNP chemistry of Hf with an eye on the formation of Hf alkylidenes.
2. Results and discussion 2.1. Synthesis and solution characterization of (PNP)HfX3 complexes The introduction of the PNP ligand into the coordination sphere of Hf was successfully accomplished via deprotonation of (PNP)H (1) [10] with n-BuLi in ether and the treatment of the resultant (PNP)Li (2) [11] solution with HfCl4(OEt2)2 (Scheme 1). This reaction produced (PNP)HfCl3 (3), which was isolated in a 69% yield as an
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(d 0.69 ppm, 3JHP ¼ 4 Hz) in the 1H NMR spectrum. In the 13C{1H} spectrum, a corresponding triplet resonance at 60.6 ppm (JPC ¼ 8 Hz) was observed. The equivalence of the three methyl groups on Hf is analogous to the observations for (PNP)ZrMe3 [11] and is indicative of exchange among the methyl group sites that is rapid on the NMR timescale at ambient temperature. No static geometry can result in equivalent methyl groups in 4. The 1H and 13C chemical shifts exhibited by the HfeMe groups in 4 are comparable to those previously reported in similar compounds. For example, Fryzuk’s (SiPNP)HfMe3 compounds (E) resonated in their 1H NMR spectra in the 0.5e0.9 ppm range, depending on the nature of the alkyl group on P (no 13C NMR data reported) [17]. The 13C NMR chemical shift of (tBu2C5H3)HfMe3 was found to be 57.3 ppm by Royo et al. [18]. The ambient-temperature NMR spectra of 5 were consistent with a higher C2v symmetry, with only one kind of methine and two methyl resonances. However, the resonances were somewhat broad, indicative of an exchange process that is only slightly faster than the NMR timescale. The SiMe3 groups gave rise to singlet 1H (d 0.37 ppm) and 13C{1H} (d 4.3 ppm) resonances. The methylene groups of CH2SiMe3 resonated as broad singlets at d 0.64 ppm in the 1 H NMR spectrum and at 77.6 ppm in the 13C{1H} NMR spectrum. These chemical shifts are similar to those for the Hf-bound CH2SiMe3 groups reported in the literature [18e21]. Fig. 1. Key alkylidene complexes of group 4 metals and (SiPNP) complexes of Zr and Hf.
analytically pure orange solid. (PNP)HfCl3 (3) is only modestly soluble in aromatic hydrocarbons. Reaction of 3 with 3 equiv. of MeMgBr led to the formation of yellow trialkyl (PNP)HfMe3 (4), isolated in a 53% yield. On the other hand, the reaction of 3 with 2.5 equiv of Me3SiCH2MgCl led to the formation and isolation of the dialkyl (PNP)Hf(CH2SiMe3)2Cl (5) in a 77% yield. Utilization of a larger Me3SiCH2MgCl-to-3 ratio led only to the increased formation of (PNP)MgX [11] (X ¼ halide or alkyl). Ostensibly, the increased size of Me3SiCH2 vs Me prevents the installation of the third alkyl on Hf in a reaction of 3 with this larger Grignard. Ambient-temperature NMR spectra of 3 and 4 indicated either C2 or Cs time-averaged symmetry: a lone singlet 31P{1H} resonance (d 34.0 for 3 and 18.0 ppm for 4), equivalent aromatic rings in the PNP ligand, as well as a pair of methine and four methyl resonances stemming from iPr groups in both 1H and 13C{1H} NMR spectra. The Hf-bound methyl groups of 4 gave rise to one triplet resonance
Scheme 1. Preparation of the (PNP)HfX3 complexes.
2.2. Thermolysis of (PNP)HfMe3 (4) and (PNP)Hf(CH2SiMe3)2Cl (5) Thermolysis of 4 in C6D6 solution at 75 C for 48 h did not result in the formation of any new products detectable by NMR spectroscopy (Scheme 2). This lack of reactivity is in line with the thermal inertness of (PNP)ZrMe3 [11] but is in contrast to double a-abstraction occurring upon thermolysis of (PNP)TaMe4 to give (PNP)Ta(]CH2)2 [22]. Thermolysis of (PNP)Hf(CH2SiMe3)2Cl (5) at 80 C in C6D6 resulted in 80% conversion overnight and gave rise to a mixture with two dominant P-containing products. In the 31P{1H} NMR spectrum of this mixture, one gave rise to a singlet at 29.4 ppm, while the other, major product, gave rise to an AB system (d 49.0 and 39.0 ppm, 2 JPP ¼ 70 Hz). At the same time, a singlet resonance at d 8.61 ppm was detected by 1H NMR spectroscopy and a multiplet resonance at d 248.2 ppm (identified as a CH by a DEPT experiment) by 13C NMR spectroscopy in this mixture. And HSQC experiment showed correlation between the 1H resonance at d 8.61 ppm and the 13C resonance at d 248.2 ppm. The AB system in the 31P{1H} NMR spectrum, the downfield 1H NMR and 13C NMR resonances are reminiscent of the spectra displayed by the PNP-supported alkylidenes of other group 4
Scheme 2. Thermolysis of (PNP)HfMe3 (4) and of (PNP)Hf(CH2SiMe3)2Cl (5).
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Fig. 2. POV-Ray [29] rendition of an ORTEP [30] drawing (50% probability ellipsoids) of (PNP)HfCl3 (3) with hydrogen atoms and benzene solvent molecule omitted for clarity.
Fig. 4. POV-Ray [29] rendition of an ORTEP [30] drawing (50% probability ellipsoids) of (PNP)Hf(CH2SiMe3)2Cl (5) with hydrogen atoms omitted for clarity.
2.3. Single crystal X-ray diffraction studies metals Ti and Zr. For example, (PNP)Ti(]CHCMe3)(OTf) displayed an AB 31P NMR pattern with 2JPP ¼ 55 Hz and Dd z 10 ppm, an 8.42 ppm singlet 1H NMR resonance and a 301 ppm 13C NMR resonance [23]. (PNP)Zr(]CHPh)(CH2Ph) (C) displayed an AB 31P NMR pattern with 2 JPP ¼ 60 Hz and Dd z 2 ppm, a 7.32 ppm singlet 1H resonance, and a 231 ppm 13C resonance [11]. Likewise, Fryzuk’s A [8] and B [7a] gave rise to singlet 1H NMR resonances at 7.33 and 8.1 ppm, and 13C NMR resonances at 210 and 229.4 ppm, respectively. We thus hypothesized that the new unknown major component of the mixture formed in the thermolysis of 5 is (PNP)Hf(]CHSiMe3)(Cl) (6, Scheme 2), the alkylidene product of a-abstraction. Indeed, the required SiMe4 by-product of a-abstraction was identified by 1H and 13C NMR in the reaction mixture. Unfortunately, the high and apparently similar solubility of the components of the mixture has precluded isolation of the proposed 6 in a pure state.
Fig. 3. POV-Ray [29] rendition of an ORTEP [30] drawing (50% probability ellipsoids) of (PNP)HfMe3 (4) with hydrogen atoms omitted for clarity.
We were able to obtain X-ray quality single crystals of 3, 4, and 5 and establish their solid-state structures by X-ray diffraction methods. The individual molecular structures are depicted in Figs. 2e4. The bond distances and angles associated with the Hf immediate coordination environment are shown in Table 1, while Fig. 5 shows the immediate six-coordinate environment about the Hf center in these three molecules side by side from analogous points of view. Octahedral geometry is sterically (and often, electronically) preferred for six ligands about a metal center. However, only 3 possesses a geometry about Hf that can be described as even
Table 1 Summary of select bond distances and angles in (PNP)HfCl3 (3), (PNP)HfMe3 (4), and (PNP)Hf(CH2SiMe3)2Cl (5). Bond distances (Å)
(PNP)HfCl3
(PNP)HfMe3
(PNP)Hf(CH2SiMe3)2Cl
P1eHf1 P2eHf1 N1eHf1 R1-Hf1 R2-Hf1 R3-Hf1 Bond Angles ( ) P1eHf1eP2 P1eHf1eN P1eHf1eR1 P1eHf1eR2 P1eHf1eR3 P2eHf1eN P2eHf1eR1 P2eHf1eR2 P2eHf1eR3 N1eHf1eR1 N1eHf1eR2 N1eHf1eR3 R1eHf1eR2 R1eHf1eR3 R2eHf1eR3
2.6954 (11) 2.6954 (11) 2.173 (5) 2.4082 (13) 2.412 (2) 2.4082 (13)
2.7360 (14) 2.8034 (14) 2.214 (4) 2.234 (5) 2.301 (5) 2.251 (6)
2.7817 2.8309 2.1936 2.2478 2.2283 2.4575
146.32 (5) 73.16 (2) 89.97 (4) 106.84 (2) 90.90 (4) 73.16 (2) 90.90 (4) 106.84 (2) 89.97 (4) 91.50 (3) 180 91.50 (3) 88.50 (3) 177.00 (6) 88.50 (3)
132.83 (4) 70.78 (11) 126.77 (15) 78.50 (14) 104.45 (16) 70.39 (11) 82.77 (15) 147.22 (14) 78.04 (15) 95.63 (19) 140.63 (18) 125.53 (19) 83.3 (2) 123.4 (2) 85.2 (2)
124.363 (13) 70.00 (3) 97.72 (4) 139.08 (4) 76.929 (14) 69.09 (3) 79.94 (4) 82.15 (4) 156.209 (13) 128.18 (5) 97.24 (6) 133.56 (3) 118.97 (6) 87.14 (5) 86.86 (4)
Cl1 Cl2 Cl10
C27 C28 C29
C27 C31 C11
R1¼ R2¼ R3¼
(4) (4) (13) (17) (16) (4)
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Fig. 5. Views [29,30] down the NeHf axis in (PNP)HfCl3 (3, left), (PNP)HfMe3 (4, center) and (PNP)Hf(CH2SiMe3)2Cl (5, right). Only the atoms directly bound to the Hf centers shown.
a distorted octahedral. The only source of major deviation from the octahedron in 3 has to do with the inability of the PNP ligand to accommodate a 180 PeHfeP angle. This angle is merely 146.32(5) , but the Namido and the three chlorides form a nearly perfectly square-planar “midsection” of this distorted octahedron. The low PeHfeP angle is observed despite the strict meridionality of the PNP ligand. The PeMeP angles in meridional PNP complexes of late transition metals are typically in the 160e170 range [9]. Hafnium is considerably larger and the longer HfeN bond positions it farther away from the phosphines in the PNP/Hf plane, resulting in a narrower PeHfeP angle. The geometry about Hf in 4 and 5 is irregular and cannot be easily described in terms of simple polyhedra. The molecular structure of 4 is nearly superimposable [24] with that of (PNP) ZrMe3 we reported in 2004 [11]. We described [11] the latter as having “both the PNP and the Me3 sets of donors. halfway between a facial and a meridional arrangement.” This same description applies well to 4. The coordination environment about Hf in 5 can be described similarly, although it is not superimposable with (PNP)HfMe3. Fryzuk et al. analyzed the structure of the closely related (SiPNPMe)HfMe3 as a bicapped tetrahedron, with the two neutral P donors capping the faces of the NHfMe3 tetrahedron [17]. This description is less apt for (PNP)MMe3 (M ¼ Zr or Hf) because the requisite angles deviate to a significantly greater degree from the idealized bicapped tetrahedron. It is possible that the bicapped tetrahedron motif is in fact desirable for these systems, but that the greater rigidity of the diarylamido PNP causes greater distortions in the structure. Polyalkyl complexes of d0 metals frequently deviate from the sterically preferred geometries and it has been proposed that maximization of orbital overlap for the strongly covalent MeC bonds may be responsible [25]. The HfeC distances in 4 and 5 are in the 2.23e2.30 Å range that is typical for Hf alkyls. The HfeMe bond distances in Fryzuk’s (SiPNPMe)HfMe3 [17] and (P2N2)HfMe2 [26] are in the 2.24e2.27 Å range, and the ZreMe bond distances in our (PNP)ZrMe3 are in the 2.25e2.29 Å range [11]. Scott and Lippard reported a series of Zr and Hf bis(trimethylsilylmethyl) complexes supported by N4 dianionic tropocoronand ligands with MeC bonds ranging from 2.24 to 2.35 Å [21]. The HfeN and HfeCl bond distances are unremarkable. The HfeP distances are notably shorter in 3 (2.6954(11) Å) than in the alkyl complexes, where they range from 2.73 to 2.83 Å. That is likely a consequence of the stronger Lewis acidity of the Hf center in 3. The longest HfeP distances were found in 5 which probably reflects the increased steric pressure from the larger alkyl groups, as well. This trend and the approximate values are reminiscent of Fryzuk’s (P2N2)HfX2 [26] and and (SiPNPMe)HfX3 [17,27] complexes where for X ¼ Cl, the HfeP distances (ca. 2.7 Å) were ca. 0.05e0.10 Å shorter than for X ¼ Me (ca. 2.8 Å). On the other hand, the HfeP distances in (P2Cp)HfCl3 (ca. 2.85e2.88 Å) were considerably longer [8], despite being bound to a HfCl3 center. The difference illustrates
the greater donicity (and imposed electron count at Hf) of the Cp ligand vs amido. In Hf(IV) complexes, there is no possibility of backdonation and the bonding between P and Hf must be of a purely Lewis acidebase adduct nature. The mismatch between soft phosphine donors and a hard Hf(IV) center probably results in a rather weak bond whose length may change considerably with minimal change in overall energy. It is worth noting that the PNP ligand resonances in the 1H and 13 C NMR spectra of 3 displayed virtual triplet coupling [28] to the two 31P nuclei. This phenomenon is typically associated with transdisposition of equivalent phosphines. More precisely, it stems from a large JPP value that tends to be greater as the PeMeP angle approaches 180 . The coupling to the pair of 31P nuclei in 4 and 5 manifested itself either simply as doublet splitting or as more complex transitional multiplets, indicative of a smaller JPP value in these compounds. Our structural studies show that the PeHfeP angle in 3 (146.32(5) ) is indeed greater than in the alkyl complexes (132.83(4) and 124.363(13) ), although still considerable smaller than 180 . The combination of the greater PeHfeP angle and tighter HfeP interaction probably gives rise to a greater JPP value in 3 and thus more idealized virtual triplet coupling to the pair of 31P nuclei. 3. Conclusion In summary, we have synthesized and characterized Hf(IV) complexes of the diarylamido-based PNP ligand. The structure of (PNP)HfCl3 (3) displays deviations from octahedral geometry about Hf that can be simply explained by the constraint of the PNP ligand. The deviations from octahedral geometry about Hf in (PNP)HfMe3 (4) and (PNP)Hf(CH2SiMe3)2Cl (5) are much more severe and stem from a combination of the effect of the chelate constraint and from the preference for accommodating stronger bonds to anionic ligands. Thermolysis of 5 led to the formation of a new compound that displays spectroscopic characteristics consistent with a Hf alkylidene, however, attempts at isolation and full characterization have not proven successful. 4. Experimental section 4.1. General considerations Unless specified otherwise, all manipulations were performed under an argon atmosphere using standard Schlenk or glovebox techniques. Dioxane, diethyl ether, tetrahydrofuran and benzene were dried over sodium-benzophenone ketyl, distilled or vacuum transferred and stored over molecular sieves in an Ar-filled glovebox; (PNP)H (1) was synthesized according to published procedures [10], HfCl4(OEt2)2 was synthesized in a manner analogous to published procedures [31] and all other chemicals were used as received from commercial vendors. All NMR spectra were recorded
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on a Varian iNova 300 spectrometer (1H NMR, 299.951 MHz; 31P NMR, 121.425 MHz; 13C NMR, 75.413 MHz), Varian Mercury 300 spectrometer (13C NMR 75.426 MHz), Varian iNova 400 spectrometer (1H NMR, 399.755 MHz; 13C NMR, 100.518 MHz; 31P NMR 181.822 MHz), or a Varian iNova NMR 500 (1H NMR, 499.425 MHz/ 499.683 MHz; 13C NMR, 75.424 MHz/125.580 MHz; 31P NMR, 202.171 MHz) spectrometer. Chemical shifts are reported in d/ppm. For 1H and 13C NMR spectra, the residual solvent peak was used as an internal reference. 31P NMR spectra were referenced externally using 85% H3PO4 at d 0 ppm. Elemental analysis was performed by CALI Labs, Parsippany, NJ, USA.
(dd, 6H, JHH ¼ 7 Hz, JPH ¼ 14 Hz, CHMe2), 1.13 (apparent t overlapping with 1.10, 6H, JHH ¼ 7 Hz, JPH ¼ 17 Hz, CHMe2), 1.10 (apparent t overlapping with 1.10, 6H, JHH ¼ 7 Hz, JPH ¼ 17 Hz, CHMe2), 0.94 (apparent t, 6H, JHH ¼ 8 Hz, CHMe2), 0.69 (t, 9H, JPH ¼ 4, HfMe3). 31P{1H} NMR (C6D6): d 18.4 (s). 31P{1H} NMR (toluene): d 17.7. 13C{1H} NMR (C6D6): d 161.0 (d, JPC ¼ 23 Hz), 133.3 (s), 132.7 (s), 122.2 (s), 121.8 (s), 121.7 (d, JPC ¼ 8 Hz), 60.6 (t, JPC ¼ 8 Hz, HfMe3), 25.0 (br s, CHMe2), 21.6 (m, CHMe2), 21.1 (s, ArCH3), 20.0 (br s, CHMe2), 19.6 (br s, CHMe2), 18.7 (br s, CHMe2), 17.6 (br s, CHMe2). Elem. Anal., found(calc) for C29H49HfNP2: C, 53.29(53.27); H, 7.47(7.56).
4.2. Synthesis of (PNP)HfCl3 (3)
4.4. Synthesis of (PNP)Hf(CH2SiMe3)2Cl (5)
(PNP)H (1) (4.39 g, 10.2 mmol) was suspended in diethyl ether at 35 C. In a slow dropwise fashion n-butyllithium (4.1 mL of a 2.5 M solution in hexanes, 10.2 mmol) was added to the suspension. The reaction was left to stir for 30 min and then placed under vacuum to remove all volatiles. The oily residue (presumed to contain 2) was dissolved in toluene. HfCl4$Et2O (4.80 g, 10.2 mmol) was added to the solution. After stirring overnight a deep orange solution was observed with a precipitate (LiCl). The solution was stripped down and CH2Cl2 was added and the solution was passed through a plug of Celite. 31P{1H} NMR spectroscopic analysis revealed >98% content of 3. The filtrate was recrystallized from pentane at 35 C. The collected crystals were washed with cold pentane and dried under vacuum (69% yield). 1H NMR (C6D6): d 6.94 (dvt, 2H, JHH ¼ 8 Hz, JPH ¼ 8 Hz, AreH), 6.89 (ddd, 2H, JHH ¼ 2 Hz, JHH ¼ 8 Hz, AreH), 6.86 (br, 2H, AreH), 2.36 (m, 2H, CHMe2), 2.12 (m, 2H, CHMe2), 2.09 (s, 6H, AreCH3), 1.40 (apparent q, 6H, JHH ¼ 7 Hz, CHMe2), 1.21 (dd overlapping with 1.18, 6H, JHH ¼ 7 Hz, CHMe2), 1.18 (dd overlapping with 1.21, 6H, JHH ¼ 7 Hz, CHMe2), 0.93 (apparent q, 6H, JHH ¼ 7 Hz, CHMe2). 1H{31P} NMR (C6D6): d 6.94 (br d, 2H, JHH ¼ 8 Hz, AreH), 6.90 (dd, 2H, JHH ¼ 2, 7 Hz, AreH), 6.86 (br d, 2H, JHH ¼ 2 Hz, AreH), 2.36 (m, 2H, CHMe2), 2.12 (m, 2H, CHMe2), 2.09 (s, 6H, AreCH3), 1.40 (d, 6H, JHH ¼ 7 Hz, CHMe2), 1.21 (d overlapping with 1.18, 6H, JHH ¼ 7 Hz, CHMe2), 1.18 (d overlapping with 1.21, 6H, JHH ¼ 7 Hz, CHMe2), 0.93 (d, 6H, JHH ¼ 7 Hz, CHMe2). 31P{1H} NMR (C6D6): d 34.0 (s). 13C{1H} (CDCl3): d 157.9 (vt, JPC ¼ 11 Hz), 133.4 (s), 132.1 (s), 131.4 (s), 121.0 (vt, JPC ¼ 17 Hz), 120.0 (s), 26.4 (br s, CHMe2), 21.7 (vt, JPC ¼ 9 Hz, CHMe2), 21.1 (s, AreMe), 21.0 (s, CHMe2), 19.1 (s, CHMe2), 18.9 (s, CHMe2), 17.6 (s, CHMe2). Elem. Anal., Calculated for C26H40Cl3HfNP2: C, 43.75; H, 5.65; Cl, 14.71. Found, batch 1: C, 43.73; H, 5.62 and batch 2: C, 43.49; H, 5.57; Cl, 14.64.
(PNP)HfCl3 (3) (500 mg, 0.708 mmol) was dissolved in 11 mL of diethyl ether. To this was added 62 mL of dioxane and Me3SiCH2MgCl (1.75 mL, 1.0 M in Et2O, 1.75 mmol). The color of the solution changed from yellow to faint green. The mixture was stirred until no starting material was present by 31P{1H} NMR analysis. After that, the solution was passed through a pad of Celite. The filtrate was placed under vacuum to remove all volatiles. Twice, pentane was added to the yellow residue and placed under vacuum to remove volatiles. The product was recrystallized from pentane at 35 C. The yellow crystalline solid was washed twice with cold pentane and dried under vacuum. (446 mg, 77% yield). 1H NMR (C6D6): d 6.98 (br dd, 2H, JHH ¼ 2 Hz, JPH ¼ 5 Hz, AreH), 6.90 (br dd, 2H, JHH ¼ 2 Hz, JHH ¼ 8 Hz, AreH), 6.79 (dd, 2H, JHH ¼ 8 Hz, JPH ¼ 5 Hz, AreH), 2.35 (br m, 4H, CHMe2), 2.10 (s, 6H, AreCH3), 1.16 (dd, 12H, JHH ¼ 7 Hz, JPH ¼ 13 Hz CHMe2), 1.05 (br, 12H, CHMe2), 0.64 (br s, 4H, HfCH2SiMe3), 0.37 (s, 18H, SiMe3). 31P{1H} NMR (Et2O): d 11.2 (s). 31P{1H} NMR (C6D6): d 12.0 (s). 31P{1H} NMR (THF): d 12.4 (s). 13C{1H} NMR (C6D6): d 158.7 (d, JPC ¼ 16 Hz), 133.2 (s), 131.1 (s), 128.3 (additional aromatic signal overlapping with internal standard), 124.6 (d, JPC ¼ 14 Hz), 123.5 (s), 77. 6 (t, JPC ¼ 6 Hz, HfCH2), 24.3 (br s, CHMe2), 21.3 (s, ArCH3), 20.2 (br s, CHMe2), 19.3 (br s, CHMe2), 4.5 (s, SiMe3). Elem. Anal., found(calc) for C34H62ClHfNP2Si2: C, 49.88(49.92); H, 7.53(7.65).
4.3. Synthesis of (PNP)HfMe3 (4)
4.6. Thermolysis of (PNP)Hf(CH2SiMe3)2Cl (5)
(PNP)HfCl3 (3) (0.31 g, 0.44 mmol) was suspended in 75 mL of toluene. CH3MgBr (0.44 mL, 3 M solution in diethyl ether, 1.3 mmol) was added dropwise. The orange solution turned lime yellow. 50 mL of dioxane was added to precipitate out the MgBr2 and MgCl2. The reaction was stirred overnight. The volatiles were removed from the solution under vacuum and then approximately 5 mL of isooctane was added to help precipitate magnesium halides. The volatiles were then removed under vacuum and the residue was dissolved in toluene. The solution was passed through a pad of Celite and the volatiles were again removed under vacuum. Approximately 4 mL of pentane was added to dissolve the yellow residue and the flask was placed in the freezer at 35 C for recrystallization. Yellow crystals of 4 were collected, washed with cold pentane and the solid was placed under vacuum to dry (53% yield). 1H NMR (C6D6): d 6.96 (dd, 2H, JHH ¼ 8 Hz, JPH ¼ 5 Hz, AreH), 6.92 (br d, 2H, JHH ¼ 8 Hz, AreH), 6.88 (br d, 2H, JHH ¼ 8 Hz, AreH), 2.37 (m, 2H, CHMe2), 2.21 (m, 2H, CHMe2), 2.11 (s, 6H, AreMe), 1.22
(PNP)Hf(CH2SiMe3)2Cl (5) (48.0 mg, 0.612 mmol) was dissolved in C6D6 in a J-Young NMR tube yielding a yellow solution. The NMR tube was placed into an 80 C oil bath overnight, during which time it became a dark orange solution. 31P{1H} NMR revealed 80% conversion of 5 and the appearance of a major new product (6) displaying an AB system (two doublets at 49.0 ppm and 39.0 ppm, JPP ¼ 70 Hz) and another, lesser product displaying a singlet resonance at 29.4 ppm. In the 1H NMR spectrum, a new singlet downfield resonance (d 8.61 ppm) was detected. In the 13C{1H} NMR spectrum, a new resonance at 248.2 ppm (br m) was detected. 1 H and 13C NMR spectroscopic analysis also indicated the presence of substantial amount of SiMe4 (0 ppm in both).
4.5. Thermolysis of (PNP)HfMe3 (4) (PNP)HfMe3 (4) was dissolved in C6D6, forming a yellow solution, and was placed into a 75 C oil bath for 48 h, during which time it became a dark reddish brown solution. 31P{1H} NMR revealed no new compounds. Only the resonance of 4 was observed at 18.2 ppm.
4.7. X-ray crystallography For the structures of 3 and 5, all operations were performed on a Bruker-Nonius Kappa Apex2 diffractometer at 120 K, using
C.M. Brammell et al. / Journal of Organometallic Chemistry 696 (2011) 4132e4137
graphite-monochromated MoKa radiation. All diffractometer manipulations, including data collection, integration, scaling, and absorption corrections were carried out using the Bruker Apex2 software [32]. The structures were solved using SIR-92 [33], and refined (full-matrix-least squares) using the Oxford University Crystals for Windows program [34]. For the structure of 4, the single crystal X-ray diffraction experiment was carried out with a Bruker SMART 1000 diffractometer with a CCD area detector (graphite-monochromated Mo Ka radiation) at 110 K. Semiempirical method SADABS [35] was applied for absorption correction. The structure was solved by direct methods and refined by the full-matrix least-squares technique against F2 with anisotropic temperature parameters for all non-hydrogen atoms. Data reduction and further calculations were performed using the Bruker SAINTþ [36] and SHELXTL NT [37] program packages. Acknowledgment We are grateful to the US National Science Foundation for support of this research through grants CHE-0809522 and CHE0944634 to O.V.O., through support of E.J.P. as an REU participant in 2007 (grant CHE-0552767 for an REU site at Brandeis University), through support of C.M.H. as an REU participant in 2008 (grant CHE-0552767 for an REU site at Brandeis University) and 2009 (grant CHE-0755207 for an REU site at Texas A&M University), and through grant CHE-0521047 for the purchase of a new X-ray diffractometer at Brandeis University. We are also grateful for the support from the Welch Foundation (grant A-1717 to O.V.O), the Dreyfus Foundation (Camille Dreyfus Teacher-Scholar Award to O.V.O.), and the Sloan Foundation (Alfred P. Sloan Research Fellowship to O.V.O.). We thank Dr. Christos Douvris and Yanjun Zhu for help with some of the experiments in this paper. Appendix. Supporting material CCDC 833482 ((PNP)HfMe3), 833492 ((PNP)HfCl3), 833493 ((PNP)Hf(CH2SiMe3)2Cl) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data_request/cif. Appendix. Supplementary material
[5] [6]
[7]
[8] [9]
[10] [11] [12] [13]
[14]
[15]
[16]
[17] [18] [19] [20] [21] [22] [23] [24] [25]
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