Synthesis and Characterization of Group 4 Trianionic ONO3–Pincer

Mar 4, 2015 - (2-10) Recently, group 4 complexes featuring a variety of trianionic pincer .... Labeling scheme for 1H and 13C NMR resonances of compou...
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Synthesis and Characterization of Group 4 Trianionic ONO3− PincerType Ligand Complexes and a Rare Case of Through-Space 19F−19F Coupling Soufiane S. Nadif, Jakub Pedziwiatr, Ion Ghiviriga, Khalil A. Abboud, and Adam S. Veige* Center for Catalysis, University of Florida, P.O. Box 117200, Gainesville, Florida 32611, United States S Supporting Information *

ABSTRACT: This report describes the synthesis and characterization of a new series of group 4 complexes supported by a trianionic ONO3− pincer-type ligand. Treating TiCl4 with the proligand [CF3−ONO]H3 (1) and NEt3 in benzene afforded {[CF3−ONO]TiCl3}{HNEt3}2 (2). By means of a lithium transmetalation route, the neutral monochloride complex [CF3−ONO]TiCl(THF) (3) was synthesized in 91% yield. The analogous Hf(IV) derivative could not be obtained using this method. Instead, transmetalation with thallium(I) resulted in the formation of the seven-coordinate complex [CF3−ONHO]HfCl2(THF)2 (4(THF)2), which was characterized by combustion analysis and X-ray crystallography. Applying vacuum to 4-(THF)2 liberated the THF ligands to provide the five-coordinate THF-free complex [CF3−ONHO]HfCl2 (4). Alkylation of complex 4 with alkyllithium or Grignard reagents resulted in a mixture of unidentifiable products. However, access to the neutral complex 3 enabled the subsequent preparation of organotitanium complexes [CF3−ONO]TiR(THF) (5-R; R = Me, Bn, Mes). Singlecrystal X-ray analysis of 5-Me indicated that the organotitanium complexes are mononuclear. Single-crystal X-ray diffraction and NMR studies in solution confirmed that complex 5-Mes exhibits rare through-space 19F−19F coupling (5 Hz).

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framework, with [2,6-iPrNCN]HfX2− (X = Cl, Me) and [3,5-MeNCN]2Hf being the only reported examples.13 In this work, the ONO3− trianionic pincer ligand 1 (shown complexed with a metal M in Figure 1)14 was exploited to

INTRODUCTION Trianionic pincer ligands1 fuse three anionic donor groups and impart a predisposition for meridional coordination. Combining the inherent coordination geometry constraint with metal ions having low d-electron counts offers an opportunity to synthesize electronically and coordinatively unsaturated metal complexes poised to act as catalysts.2−10 Recently, group 4 complexes featuring a variety of trianionic pincer ligands have come to the fore. The terphenyl OCO3− ligand11 stabilizes Zr(IV) within several interesting coordination environments, including the dimeric zirconium benzyl complex {[tBuOCO]ZrCH2Ph}2, the bis(trimethylphosphine) zirconium benzyl complex [tBuOCO]Zr(CH2Ph)(PMe3)2, the bis(tetrahydrofuran) zirconium benzyl complex [tBuOCO]Zr(CH2Ph)(THF)2, the pyridinyl bis(pyridine) zirconium complex [tBuOCO]Zr(η2-C5H4N)(py)2, and the α-picoline zirconium dibenzyl complex [tBuOCHO]Zr(CH2Ph)2(α-picoline). Zirconium complexes featuring trianionic pincer-type ligands capable of storing electron equivalents provide an additional element of functionality. For example, Heyduk and co-workers8 reported the NNN3− complex and demonstrated its role in catalytic nitrene transfer. Titanium trianionic pincer complexes are known as well, including the analogous OCO3− dimeric titanium benzyl complex {[tBu(Me)OCO]TiCH2Ph}2 reported by Golisz and Bercaw12 and alkyltitanium complexes bearing an unsymmetrical ONO3− trianionic pincer-type ligand reported by Nifant’ev, Nagy, and co-workers.10 Hafnium species are poorly represented within this © 2015 American Chemical Society

Figure 1. Illustration of the [CF3−ONO]M fragment.

synthesize titanium(IV) and hafnium(IV) complexes. The important features of ligand 1 are (1) ease of synthesis, (2) protic OH groups, making metalation straightforward, and (3) electron-withdrawing CF3 groups that decrease the donating ability of the alkoxides. One of the pitfalls of employing the terphenyl OCO3− ligand is the proclivity to form inert dimers, such as the zirconium and titanium dimers {[tBuOCO]ZrCH2Ph}211 and {[tBu(Me)OCO]TiCH2Ph}2.12 These complexes indicate very clearly that catalyst deactivation can occur via Received: January 20, 2015 Published: March 4, 2015 1107

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Table 1. 1H, 13C{1H}, 19F, and 15N Chemical Shift Data (ppm)

alkoxide bridges leading to dimerization. Methods to avoid the formation of dimeric species include the use of very weakly donating alkoxides or steric prevention of any possibility of dimerization. One goal for this work was to synthesize mononuclear group 4 alkyl complexes that do not dimerize. To accomplish this goal, presented within are the syntheses and characterization of mononuclear [CF3−ONO]TiR(THF) complexes (R = alkyl or aryl). In addition, during this study a case of an unusual “through-space” 19F−19F coupling was discovered.

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RESULTS AND DISCUSSION Straightforward metalation of [CF3−ONO]H3 (1) involves adding the ligand to a toluene solution of TiCl4 at −35 °C and then stirring the reaction mixture for 12 h. After 12 h, adding triethylamine completes the reaction and serves to eliminate residual HCl to yield the pincer ate complex {[CF3−ONO]TiCl3}{HNEt3}2 (2). Analytically pure dark-red microcrystalline material precipitates upon the addition of a concentrated benzene solution of 2 to cold pentane (51% yield; Scheme 1). Scheme 1. Synthesis of 2

Combustion analysis and multinuclear and multidimensional NMR spectroscopy are consistent with the assignment of complex 2 as a complex salt. Table 1 lists the NMR spectroscopic data for 2, and Figure 2 provides an atom-labeling scheme. The 1 H NMR spectrum of 2 (C6D6) exhibits one resonance for the pincer arylmethyl protons at 2.03 ppm and is indicative of the assigned C2 symmetry. Complexes bound by the ONO3− ligand are not able to adopt C2v symmetry because the N-aryl rings cannot achieve a coplanar orientation. The complex [CF3− ONO]WCtBu(OEt2), a previously reported trianionic pincer tungsten alkylidyne, exhibits nearly overlapping arylmethyl groups despite being C1-symmetric,14 and thus, additional evidence was sought to confirm the C2 symmetry of 2. The 19F NMR spectrum of 2 exhibits only two quartets for the CF3 groups at −71.51 and −75.54 ppm, which is a hallmark of C2 symmetry for metal ions coordinated with 1.14−16 Confirming the presence of 2 equiv of triethylammonium countercation (HNEt3+), the N−H protons appear at 9.74 ppm and the ethyl protons resonate at 2.48 ppm (NCH2CH3) and 0.86 ppm (NCH2CH3). These NMR signatures are consistent with those of HNEt3+ countercations present in other titanium complex salts.17,18 In addition, the recently reported 1H NMR spectrum (C6D6) of {[CF3−ONO]TaCl3}{HNEt3}x containing ligand 1 exhibits similar resonances (2.43 ppm (NCH2CH3); 0.81 ppm (NCH2CH3)).15 However, for {[CF3−ONO]TaCl3}{HNEt3}{HNEt3Cl}x, an excess of HNEt3Cl cocrystallizes during purification and is batch-dependent (i.e., x >1 and is variable). Fortunately, complex 2 does not suffer from this complication, as only the expected 2 equiv of HNEt3+ are present in the complex. Attempts to alkylate 2 with either RLi or RMgCl reagents led to multiple products that were inseparable. The complication likely arises from the presence of 2 equiv of protons in the

1

2

3

4b

H4 H5 H7 H10 H14 H15 H17 H20 H21 H22 H23 H24 H25

6.69 6.63 7.47 1.83 6.69 6.63 7.47 1.83 − − − − −

6.57 6.70 7.76 2.04 6.57 6.70 7.76 2.04 2.53 0.92 9.69 − −

6.79 6.83 7.82 1,93 6.07 6.60 7.68 1.91 3.96 1.16 1.16 3.96 −

6.82 6.71 7.57 1.95 6.82 6.71 7.57 1.95 3.07 − − − −

6.86 6.83 7.84 2.01 6.41 6.65 7.69 1.97 3.83 1.06 1.06 3.83 1.13

H27 H28 H29 H30 H31 C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15 C16 C17 C18 C19 C20 C21 C22 C23 C24 C25 C26 C27 C28 C29 C30 C31 Npincer F8 F9 F18 F19

− − − − − 80.6 120.9 143.0 126.1 132.1 134.4 128.3 123.2 123.3 20.2 80.6 120.9 143.0 126.1 132.1 134.4 128.3 123.2 123.3 20.2 − − − − − − − − − − − 66.2 −74.16 −75.69 −74.16 −75.69

− − − − − 83.3 125.5 150.5 121.2 130.8 131.6 127.0 124.3 123.9 20.3 83.3 125.5 150.5 121.2 130.8 131.6 127.0 124.3 123.9 20.3 46.2 8.2 − − − − − − − − − − −70.9 −74.9 −70.9 −74.9

− − − − − 86.1 128.3 139.8 127.7 132.9 137.1 128.3 123.2 123.8 20.7 87.6 122.9 128.4 133.6 131.0 113.8 147.3 123.4 123.7 20.0 44.8 39.0 − − − − − − − − − 119.4 −75.9 −71.6 −73.3 −75.9

− − − − − 83.2 124.5 149.3 126.7 131.9 130.7 126.8 123.9 123.2 20.2 83.2 124.5 149.3 126.7 131.9 130.7 126.8 123.9 123.2 20.2 43.9 − − − − − − − − − − 180.2 −71.5 −76 −71.5 −76

− − − − − 85.1 127.9 145.3 126.1 132.2 133.4 127.9 123.3 124.3 20.5 86.0 123.3 148.8 117.1 130.9 130.8 127.7 123.9 124.1 20.1 72.3 24.8 24.8 72.3 60.3 − − − − − − 279.7 −76.57 −71.75 −72.51 −76.17

5-Me

5-Bn 6.90 6.83 7.84 2.06 6.55 6.67 7.71 1.98 3.59 0.99 0.99 3.59 2.94, 3.49 7.27 6.98 6.68 − − 85.1 126.5 147.9 124.8 131.9 132.3 127.5 123.6 124.4 20.4 85.4 123.8 150.0 119.4 131.0 130.9 127.3 123.8 124.3 20.1 a a a a 86.1 140.0 130.8 128.8 124.5 − − 278.1 −76.46 −71.01 −75.04 −71.47

5-Mes 7.03 6.60 7.73 1.79 6.46 6.68 7.71 1.97 3.60 1.13 1.20 4.26 − 6.42 − − 2.69 1.87 85.3 129.5 140.9 128.4 131.3 134.7 128.3 123.2 124.1 20.5 87.1 119.2 147.8 133.2 131.2 131.0 128.9 123.6 124.1 19.9 72.6 25.0 25.0 72.6 203.1 139.7 126.6 139.3 − 24.8 20.8 289.3 −75.01 −70.32 −78.37 −70.43

a

Not measured because the lines of the coupling protons were too broad. bMeasured in toluene-d8.

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evaporation of the solvent. Therefore, characterization efforts focused on complex 3. Recrystallization via slow evaporation from a concentrated Et2O solution of 3 provides single crystals amenable to an X-ray diffraction experiment. Figure 3 depicts the solid-state molecular

Figure 3. (left) Solid-state structure of [CF3−ONO]TiCl(THF) (3) with hydrogen atoms removed for clarity. (right) Truncated ellipsoid plot (50%) of the Ti(IV) core. Selected bond distances [Å]: Ti1−O1 1.8544(14), Ti1−O2 1.8131(13), Ti1−O3 2.1278(14), Ti1−N1 1.8941(16), Ti1−Cl1 2.2593(6). Selected bond angles [deg]: Cl1− Ti1−O1 115.10(5), O1−Ti1−O2 126.19(7), O2−Ti1−Cl1 117.84(5).

Figure 2. Labeling scheme for 1H and compounds 1, 2, 3, 4, and 5-R.

13

structure of 3, and Table 2 lists the crystallographic refinement data. Complex 3 crystallizes in the P1̅ space group. The complex is C1-symmetric, and the geometry of the titanium ion is a distorted trigonal bipyramid (Addison parameter τ = 0.68)19 with the nitrogen (N1) and the THF (O3) occupying the apical positions. The THF ligand was disordered in the crystal lattice and was partitioned into two parts and refined. The observation of a trigonal-bipyramidal geometry for titanium(IV) is unremarkable, as many other titanium complexes adopt this geometry.20−24 The interesting feature of this titanium complex is the orientation of the trianionic pincer within the coordination sphere. Occupying the equatorial sites are the chloride (Cl1) and the two alkoxide O atoms from the pincer (O3 and O4). With the exception of the OCO3− chromium oxide complex [tBuOCO]Cr(O)(THF)2 and the iron(III) complex [CF3−ONO]Fe(bipy),25 the dominant orientation of the trianionic pincers is to occupy meridional positions. However, in 3 the equatorialplane angles are Cl1−Ti1−O1 = 115.10(5)°, O1−Ti1−O2 = 126.19(7)°, and O2−Ti2−Cl1 = 117.84(5)°. The C1-symmetric solid-state structure is consistent with solution-phase NMR data (Table 1). In particular, the 19F NMR spectrum of 3 exhibits four quartets at −71.59, −73.27, and −75.89 ppm, with the latter resonance containing two coincidentally overlapping signals. In the 1H NMR spectrum of 3, the pincer arylmethyl protons also overlap but are distinguishable, appearing at 1.91 and 1.93 ppm. Resonances for the coordinated THF protons appear at 3.94 and 1.16 ppm. Applying the synthetic protocol for the synthesis of complex 3 to the preparation of a Hf(IV) derivative did not work. Instead, NMR characterization of the reaction mixture indicated the

C NMR resonances of

countercation, with which the alkylating agent reacts first. Circumventing this complication requires a metalation strategy that produces a neutral ONO3− pincer complex. Deprotonation of 1 in THF with n-butyllithium at −35 °C results in an orange-colored solution and yields the trilithio salt [CF3−ONO]Li3 in situ. Dropwise addition of the THF trilithio salt solution to a cold mixture of TiCl4(THF)2 in THF (Scheme 2) produces a color change from bright yellow to dark purple. The purple color indicates the formation of the trianionic pincer titanium chloride complex [CF3−ONO]TiCl(THF) (3). Purifying 3 involves removing all of the volatiles and triturating the residual solid with pentane (3 × 0.5 mL). Taking up the purple solid in Et2O and filtering separates any inorganic salts and provides a filtrate containing 3. Removal of all volatiles in vacuo provides 3 in 91% yield. In addition to signals attributed to complex 3, broad 19F NMR signals (C6D6) at −71.92, −73.60, −74.27, and −75.26 ppm are indicative of the bis(THF) complex 3-(THF)2. This was confirmed by addition of a drop of THF and the concomitant increase in those 19F signals. Moreover, placing complex 3-(THF)2 under vacuum results in partial removal of the extra THF. Isolation and purification of 3-(THF)2 is not possible since loss of THF occurs instantaneously upon Scheme 2. Synthesis of 3

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Organometallics Table 2. X-ray Crystallographic Structure Parameters and Refinement Data empirical formula formula weight crystal system space group dimensions (mm) a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z (Å) abs coeff (mm−1) F(000) Dcalcd (g/cm3) γ(Mo Kα) (Å) T (K) θ range (deg) completeness to θmax reflections collected indep. reflections [Rint] data/restraints/param. final R1, wR2 [I >2σ(I)] R1, wR2 (all data) largest diff peak/hole (e/Å3) goodness of fit on F2

3

4-(THF)2

5-Me

5-Mes

C20H20ClF1 NO3Ti 681.76 triclinic P1̅ 0.33 × 0.25 × 0.05 9.8325(12) 9.8559(12) 14.8535(18) 108.978(2) 99.517(2) 90.588(2) 1339.3(3) 2 0.532 684 1.691 0.71073 100(2) 1.47 to 27.50 100% 23729 6155 [0.0228] 6155/0/378 0.0372, 0.0977 [4983] 0.0485, 0.1026 0.744/−0.518 1.071

C32H39Cl2F12Hf NO5Zr 995.03 triclinic P1̅ 0.17 × 0.16 × 0.14 11.4548(4) 12.7684(5) 12.8371(5) 99.5340(10). 92.2050(10) 91.9520(10) 1848.68(12) 2 3.067 984 1.788 0.71073 100(2) 1.61 to 27.50 100% 49360 8495 [0.0327] 8495/0/486 0.0149, 0.0383 [8093] 0.0161, 0.0387 1.132/−0.504 1.046

C25H23F12NO3Ti 661.34 triclinic P1̅ 0.174 × 0.083 × 0.070 8.7650(7) 11.8664(9) 13.6840(11) 84.0275(13) 89.6253(13) 71.6678(13). 1343.18(18) 2 0.431 668 1.635 0.71073 100(2) 1.497 to 27.50 100% 20310 6165 [0.0305] 6165/16/399 0.0460, 0.1150 [4686] 0.0654, 0.1232 1.073/−0.695 1.054

C33H3F12NO3Ti 765.49 monoclinic P1̅ 0.185 × 0.128 × 0.015 9.253(4) 41.867(16 17.033(7) 90 94.549(7) 90 6578(4) 8 0.364 3120 1.546 0.71073 100(2) 2.544 to 25.000 97.1% 54007 11557 [0.2552] 11557/0/911 0.0965, 0.1566 [5298] 0.2014, 0.1897 0.441/−0.530 0.958

Scheme 3. Synthesis of 4-(THF)2 and 4

angles around N1 equals 348.66(12)°, which is indicative of sp3 hybridization or pyramidalization of the N atom. In contrast, complex 3 contains the ONO ligand in its trianionic form with a planar N atom and a sum of angles equal to 359.59(13)°. Figure 4 (right) depicts a truncated view of the Hf(IV) core to highlight its pentagonal-bipyramidal geometry. The five atoms comprising the pentagonal plane are separated by an average angle of 72.70(4)°. Complex 4-(THF)2 is the only mononuclear sevencoordinate Hf(IV) pincer complex to be structurally characterized. The only related complex is a seven-coordinate pincer complex that exists as a dimer.26 One rationale for the observation of the high coordination number is the presence of poorly donating fluorinated alkoxide groups. The poor donation renders the complex electrophilic, thus requiring the additional donation from a second THF. Also, in comparison to the fivecoordinate titanium derivative 3, the larger Hf(IV) ion permits the expanded coordination. Refluxing 4 in toluene, in an attempt to eliminate HCl, does not provide the trianionic pincer complex. Also, addition of base to trap HCl leads to decomposition, and conditions for the preparation of a trianionic form of the ligand bound to Hf(IV) remain elusive.

presence of unidentifiable products. Thus, a milder transmetalating agent was sought. Exposure of 1 in THF to 2.1 equiv of TlOEt results in an instantaneous color change from orange to green-brown (a thallium salt of the ligand is presumably the intermediate, but its identity was not pursued). Addition of 1.1 equiv of a HfCl4 suspension in THF results in instantaneous formation of a white precipitate (TlCl), and the solution color changes from green-brown to orange-red. Removal of TlCl by filtration, removal of all volatiles in vacuo, and trituration with pentane (3 × 1 mL) provides the dichloride complex [CF3−ONHO]HfCl2(THF)2 (4-(THF)2) as an orange microcrystalline powder (Scheme 3). Single crystals grow in a concentrated Et2O solution of 4-(THF)2. Figure 4 depicts the results of a single-crystal X-ray diffraction experiment, and Table 2 lists crystallographic refinement data. In the solid state, complex 4-(THF)2 is C1-symmetric and seven-coordinate, with the Hf(IV) ion in a pentagonal-bipyramidal geometry. The pentagonal plane contains the ONO ligand in its dianionic form and two THF molecules. Trans chloride ligands complete the coordination sphere. Evidence that the proton resides on N1 rather than O1 or O2 comes from metrical data for N1 and confirms that the ligand is in its dianionic form. The sum of 1110

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Å, Hf−O1 1.94/2.0164(11) Å, Hf1−O2 1.93/1.9821(11) Å, Hf1−Cl1 2.39/2.4416(4) Å, and Hf1−Cl2 2.40/2.4578(4) Å. Bond length differences are expected considering that 4-(THF)2 is seven-coordinate whereas 4′ is only five-coordinate. Overall, the calculations provide bond lengths that are shorter than those observed in the X-ray structure of 4-(THF)2 and may reflect, in part, the less crowded coordination sphere of 4′. The NMR spectroscopic data for 4 indicate that a fluxional process occurs in solution. At 25 °C, the 1H NMR spectrum reveals resonances attributable to a C2-symmetric complex. For example, only one resonance appears at 1.86 ppm for the two arylmethyl protons, and three resonances appear at 6.50, 6.68, and 7.67 pm for the pincer aromatic protons. Consistent with C2 symmetry, the 19F NMR spectrum reveals only two broad resonances at −72.42 and −75.80 ppm attributable to the CF3 groups. Cooling the solution to −60 °C slows the dynamic process and reveals eight 19F signals attributable to two species, as depicted in Figure 6. For 4 to be C2-symmetric and exhibit only two signals in the 19 F NMR spectrum at 30 °C, two fluxional processes must occur in solution. The proton must shuttle between the two faces of the biaryl backbone according to fluxional process A in Scheme 4. The second process must render the complex C1-symmetric and slow the interconversion of two conformers. One mechanism is that at low temperature the proton shuttle slows and the chlorides lock in to apical and equatorial positions relative to the N−H proton according to fluxional process B in Scheme 1. Attempts to alkylate complex 4 with organolithium and Grignard reagents provided reaction mixtures containing multiple inseparable products. One reason for the complication could be the presence of the proton on the pincer N atom. Any alkylating agent would first deprotonate the N−H, but unfortunately, adding a second equivalent of alkylating agent to compensate did not improve the synthetic control. Attempts to synthesize a Hf(IV) hydride using NaBH4, LiAlH4, or DIBALH resulted in equally complicated reaction mixtures. In contrast to the attempts to alkylate complex 4, alkylations of [CF3−ONO]TiCl(THF) (3) were remarkably successful. Treating complex 3 with the appropriate Grignard reagent at ambient temperature results in the clean production of titanium(IV) complexes of the general formula [CF3−ONO]TiR(THF) (5-R, R = Me, Bn, Mes) according Scheme 5. Table 1 lists the NMR data for complexes 5-R. Our group11 and Bercaw’s group27 independently synthesized and characterized the OCO3− trianionic pincer group 4 benzyl complexes {[OCO]MBn(THF)}2 (M = Ti,27 Zr11), which are dimers in the solid and solution state. The dimerization occurs via an O atom bridge from aryl oxides in the OCO ligand. With the knowledge that dimers readily form, it was imperative to determine the structure of complexes 5-R. The methyl complex was scrutinized, as it has the smallest alkyl ligand and is most likely to dimerize. Bright-red single crystals deposit from a concentrated Et2O solution of 5-Me at −35 °C. Figure 7 depicts the molecular structure, and Table 2 lists crystallographic refinement data. Clearly, 5-Me is a C1-symmetric mononuclear complex comprising a titanium(IV) ion in a square-pyramidal geometry (Addison parameter τ = 0.10).19 This contrasts with the structure of 3, which distorts toward a trigonal-bipyramidal geometry (τ = 0.68).19 The preference for a square-pyramidal geometry in 5Me is due to the stronger trans influence of the methyl ligand versus the chloride ligand in 3. Presenting an opportunity to compare bond lengths, Table 3 lists the titanium-core bond lengths for 3 and 5-Me. The

Figure 4. (left) Solid-state structure of [CF3−ONHO]HfCl2(THF)2 (4-(THF)2), with hydrogen atoms (except for H1) removed for clarity. (right) Thermal ellipsoid plot of the Hf(IV) core. Selected bond distances [Å]: Hf1−O1 2.0164(11), Hf1−O2 1.9821(11), Hf1−O3 2.3004(11), Hf1−O4 2.304(12), Hf1−Cl1 2.4416(4), Hf1−Cl2 2.4578(4), Hf1−N1 2.4045(13). Selected bond angles [deg]: O1− Hf1−O2 140.22(5), N1−Hf1−O2 73.96(5), O2−Hf1−O3 76.31(4), O3−Hf1−O4 69.12(4), O4−Hf1−O1 74.32(4), O1−Hf1−N1 69.78(5), Cl1−Hf1−Cl2 171.615(13).

Although combustion analysis was obtained for the single crystals of 4-(THF)2, its preparation is not reproducible, and therefore, NMR characterization is not possible. The problem is that with such a crowded coordination sphere, loss of THF is rather facile. These observations led to the isolation of a second hafnium complex by simply subjecting the reaction mixture to vacuum to remove the THF ligands, providing complex 4 (Scheme 3). However, complex 4 was not analyzed by singlecrystal X-ray diffraction. Instead, DFT calculations were performed, providing 4′ (Figure 5) as an estimate of the

Figure 5. DFT-optimized structure complex [CF3−ONHO]HfCl2 4′. Selected bond distances [Å]: Hf1−O1 1.94, Hf1−O2 1.93, Hf1−N1 2.43, Hf1−Cl1 2.39, Hf1−Cl2 2.40. Selected bond angles [deg]: O1− Hf1−O2 130.6, N1−Hf1−Cl2 166.6.

ground-state geometry of 4 (see the Supporting Information for computational details). The results indicate that the chloride ligands reorient into cis positions and that overall the complex is a distorted trigonal bipyramid (Addison parameter τ = 0.60).19 The calculation provides good estimates for the bond lengths compared to 4-(THF)2, for example, N1−Hf 2.39/2.4045(13) 1111

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Figure 6. Variable-temperature 19F NMR spectra of complex 4 (top 30 °C, middle −40 °C, bottom −60 °C).

Scheme 4. Illustration of the Proposed Fluxional Processes for Complex 4

Figure 7. Solid-state structure of [CF3−ONO]TiMe(THF) (5-Me), with hydrogen atoms (except on C25), a disordered (4%) titanium atom, two disordered CF3 groups, and a disordered THF ligand removed for clarity. Selected bond distances [Å]: Ti1−O1 1.8501(16), Ti1−O2 1.8468(16), Ti1−O3 2.0955(17), Ti1−N1 1.9371(18), Ti1− C25 2.074(2). Selected bond angles [deg]: C25−Ti1−O1 100.07(9), O1−Ti1−O2 156.21(7), O2−Ti1−C25 103.68(9), N1−Ti1−O3 150.17(8).

Scheme 5. Synthesis of 5-R

Table 3. Comparison of Core Bond Lengths in 3 and 5-Me (Å) 3 Ti1−O1 Ti1−O2 Ti1−O3 Ti1−N1 Ti1−Cl1

important difference between the core bond lengths in 3 and 5Me is the Ti−THF bond length, which is 0.0323(16) Å shorter for 5-Me. The relatively stronger bond between Ti and THF can help explain some of the reactivity of 5-Me. Treating 5-Me with 1 atm H2 in C6D6 does not result in hydrogenation of the Ti−Me bond; in fact, 5-Me does not react with H2 even at 100 °C. Presumably the THF ligand prevents H2 from accessing the metal ion to complete a σ-bond metathesis. The shorter Ti− OTHF bond is compensation for the loss of π donation upon replacement of the chloride with a methyl.

5-Me 1.8544(14) 1.8131(13) 2.1278(14) 1.8941(16) 2.2593(6)

Ti1−O1 Ti1−O2 Ti1−O3 Ti1−N1 Ti1−C25

1.8501(16) 1.8468(16) 2.0955(17) 1.9371(18) 2.074(2)

Δ 0.0043(15) −0.0337(15) 0.0323(16) −0.0430(17) −

NMR spectroscopic data agree with the solid-state C1symmetric structure of 5-Me. The 1H NMR spectrum of 5-Me in C6D6 exhibits two arylmethyl resonances at 1.97 and 2.01 ppm, a clear indication of C1 symmetry, and the Ti−Me protons resonate as a singlet at 1.13 ppm. As expected, the 19F NMR spectrum exhibits a characteristic set of four quartets for the CF3 groups at −71.75, −72.51, −76.17, and −76.57 ppm. The NMR data for the benzyl complex 5-Bn are also consistent with a C11112

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Figure 8. 19F NMR spectra of 5-Mes in C6D6 (25 °C) without (bottom) and with selective decoupling at −75.01 ppm (middle) and −78.37 ppm (top).

Figure 9. 19F NMR spectra of 5-Me in C6D6 (25 °C) without (bottom) and with selective decoupling at −71.75 ppm (top).

symmetric structure. For example, the pincer-ligand methyl

methylene protons from the benzyl ligand at 3.49 and 2.94 ppm (J = 8.7 Hz). Complex 5-Mes exhibits NMR data distinctly different from those for 5-Me and 5-Bn. For example, in 5-Me and 5-Bn, the THF protons appear as broad singlets in the 1H NMR spectrum

protons appear as two singlets at 2.06 and 1.98 ppm, and the 19F NMR spectrum exhibits four quartets at −76.46, −71.01, −75.04, and −71.47 ppm. In addition, 5-Bn exhibits diastereotopic 1113

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Organometallics (C6D6). However, the 1H NMR spectrum of 5-Mes reveals sharp and diastereotopic resonances for both sets of THF protons at 4.26 and 3.60 ppm (OCH2CH2), and 1.13 and 1.20 ppm (OCH2CH2). The sharp signals and diastereotopic resonances are a consequence of inhibited substitution with trace THF. Substitution likely occurs via an associative process that is not accessible at ambient temperatures for the bulky 5-Mes. A second intriguing consequence of the mesityl ligand is that complex 5-Mes exhibits “through-space” 19F−19F coupling. Evidence and proof of through-space 19F−19F spin−spin coupling is well-documented in organofluorine compounds.28 The mechanism was first proposed by Petrakis and Sederholm in 196129 and later refined by Ng and Sederholm.30 A few cases of through-space 19F−19F coupling within organometallic complexes have been reported previously.31−33 The most relevant example is a dinuclear Pd(II) complex bearing 3,5-(CF3)2C6H3− (“fluoromesityl”) ligands that exhibits through-space coupling between the CF3 groups with a coupling constant of 8 Hz.34 Figure 8 depicts the results of selective decoupling of the resonances at −75.01 and −78.37 ppm. Decoupling the signal at −78.37 ppm affects the signals at −70.43 and −75.01 ppm, and decoupling the signal at −75.01 effects the signals at −70.32 ppm −78.37 ppm. This experiment indicates that coupling occurs between sets of CF3 groups across the ligand with a coupling constant of 5.7 Hz. The coupling must be through-space since the sets of C−F bonds are separated by eight bonds. To demonstrate that this does not occur in the Ti−Me complex, Figure 9 depicts the spin saturation of a CF3 group in 5-Me at −71.75, but only the adjacent CF3 group is decoupled at −76.57 ppm. Compelling evidence for the through-space 19F−19F coupling in 5-Mes comes from its molecular structure, which is depicted in Figure 10 (left). Single crystals amenable to an X-ray diffraction

the ligand are held approximately 2.8 Å from each other, allowing the 19F nuclei to couple. This conformation contrasts dramatically with the one in 5-Me, where the closest opposing CF3 groups are over 5 Å away from each other. Figure 11 depicts the overlapping structures of 5-Me and 5-Mes and emphasizes the dramatic contortion within the pincer ligand for 5-Mes.

Figure 11. Overlapped structures (through C25−Ti1−N1) of 5-Mes and 5-Me.

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CONCLUSIONS When confined to within 2.8 Å, two fluorine nuclei will magnetically couple even though no direct bonding between the atoms exists. This through-space coupling is a rare phenomenon, and one of the conclusions of this work is that the elephantine-sized mesityl group forces the pendant arms of the ONO pincer group into close proximity. Size matters in this case because the analogous but smaller methyl group in 5-Me does not result in close F−F contacts in the solid state nor in solution as by determined by 19F NMR spectroscopy. The fluorine atoms within the ONO3− trianionic pincer ligand seem to be particularly promiscuous since this actually marks the second unusual 19F coupling interaction observed. Previously, we reported the hydrogen-bonding interaction between the backbone pincer NH proton and one of CF3 groups (N−H····F− CF2) within the ONO ligand when bound to tantalum(V).15 Importantly, as in the case of the NH····F interaction within the tantalum complexes, elucidating the through-space interaction by 19F NMR spectroscopy establishes a convenient spectroscopic handle for detecting this interaction in the future. Any deviation from the typical quartet observed for the CF3 groups should elicit the understanding that either an NH···F or a through-space 19 F−19F interaction is present.

Figure 10. (left) Solid-state structure of [CF3−ONO]TiMes(THF) (5Mes), with hydrogen atoms removed for clarity. (right) Space-filling model of 5-Mes, including hydrogen atoms. Selected bond distances [Å]: Ti1−O1 1.873(4), Ti1−O2 1.831(4), Ti1−O3 2.121 (4), Ti1−N1 1.904(5), Ti1−C25 2.093(7), F1−F2 2.756(6). Selected bond angles [deg]: C25−Ti1−O1 109.4(2), O1−Ti1−O2 137.7(2), O2−Ti1−C25 109.4(2), N1−Ti1−O3 162.2(2).

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experiment grow by the slow evaporation of a saturated pentane solution of 5-Mes. Complex 5-Mes is clearly C1-symmetric, with the titanium(IV) ion at the center of a distorted square-pyramidal geometry (Addison parameter τ = 0.34). As expected from the NMR spectroscopy data, the space-filling model in Figure 10 (right) shows that the methyl groups of the mesityl group and the flanking CF3 groups effectively block any access to the metal center, thus preventing associative substitution of the THF and rendering the THF protons diastereotopic. Finally, the mesityl group forces the [CF3−ONO]3− ligand to fold into a conformation where two CF3 groups from opposite sides of

SYNTHETIC PROCEDURES

General Considerations. Unless specified otherwise, all manipulations were performed under an inert atmosphere using standard Schlenk or glovebox techniques. Glassware was oven-dried before use. Pentane, toluene, diethyl ether (Et2O), and tetrahydrofuran (THF) were dried using a GlassContours drying column. Benzene-d6 and toluene-d8 (Cambridge Isotopes) were dried over sodium benzophenone ketyl and distilled or vacuum-transferred and stored over 4 Å molecular sieves. Commercially available TiCl4 and Grignard reagents were used without further purification. [CF3−ONO]H3 (1) was prepared according to the literature procedure.14 Elemental analyses 1114

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°C, δ (ppm)): 7.82 (s, 1H, H7), 7.68 (s, 1H, H17), 6.83 (d, 1H, J = 7.8 Hz, H5), 6.79 (d, 1H, J = 9.0 Hz, H4), 6.60 (d, 1H, J = 5.9 Hz, H15), 6.07 (d, 1H, J = 8.9 Hz, H14), 3.96 (s, 4H, H21/H24), 1.93 (s, 3H, H10), 1.91 (s, 3H, H20), 1.16 (s, 4H, H22/H23). 1H−13C gHMBC NMR (300 MHz, benzene-d6, 25 °C, δ (ppm)): 147.3 (s, C17), 139.8 (s, C3), 137.1 (s, C6), 133.6 (s, C16), 132.9 (s, C5), 131.0 (s, C15), 128.7 (s, C2), 128.4 (s, C13), 128.3 (s, C7), 127.7 (s, C4), 123.2 (s, C8 or C18), 123.7 (C8 or C18), 123.4 (s, C19), 123.8 (s, C9), 122.9 (s, C12), 113.8 (s, C14), 87.6 (s, C11), 86.1 (s, C1), 20.7 (s, C10), 20.0 (s, C20). 19F{1H} NMR (282 MHz, benzened6, 25 °C, δ (ppm)): −71.56 (m, 3F, J = 9.8 Hz, F9), −73.27 (q, 3F, J = 9.7 Hz, F19), −75.89 (q, 6F, J = 9.3 Hz, F18 and F8). 1H−15N gHMBC (300 MHz, benzene-d6, 25 °C, δ (ppm)): 119.4 (Npincer). Anal. Calcd for C24H20ClF12NO3Ti: C, 42.28; H, 2.96; N, 2.05. Found: C, 42.27; H, 2.67; N, 2.01. Synthesis of {[CF3−ONHO]HfCl2 (4). In a nitrogen-filled glovebox, a glass vial was charged with [CF3−ONO]H3 (1) (100 mg, 0.189 mmol) and 3 mL of THF and then cooled to −35 °C. In another vial, a suspension of HfCl4 (67.0 mg, 0.209 mmol) in THF was cooled to −35 °C. Tl(OEt) (0.028 mL, 4.0 × 102 mmol) was added dropwise to the stirring solution of 1. The reaction mixture color changed instantaneously from orange to green-brown. After 15 min of stirring, the previously prepared suspension of HfCl4 was added dropwise, resulting in a color change from green-brown to orange-red and the formation of a white microcrystalline powder. After 10 min, the orange-red solution was filtered through Celite. The volatiles in the resulting filtrate were removed in vacuo, and the resulting sticky residue was triturated with pentane (3 × 2 mL). Single crystals were obtained by slow evaporation of a concentrated Et2O solution of 4 (yield: 0.091 g, 62%). 1H NMR (300 MHz, toluene-d8, 30 °C, δ (ppm)): 1.86 (s, 6H, H10), 6.50 (d, 2H, J = 7.7 Hz, H4), 6.67 (d, 2H, J = 7.7 Hz, H5), 7.31 (s, 1H, N−H), 7.67 (s, 2H, H7). 1H−13C gHMBC NMR (300 MHz, toluene-d8, 30 °C, δ (ppm)): 20.1 (s, C10), 83.1 (s, C1), 128.3 (s, C2), 129.1 (s, C7), 131.7 (s, C5), 137.5 (s, C6), 143.1 (s, C3). 19F{1H} NMR (282 MHz, toluene-d8, 30 °C, δ (ppm)): −72.42 (bs, 6F, CF3), −75.80 (bs, 6F, CF3). Anal. Calcd for C20H13Cl2F12NO2Hf: C, 30.93; H, 1.69; N, 1.80. Found: C, 30.81; H, 1.71; N, 1.80. Note: 4-(THF)2 could not be obtained reproducibly. The X-ray structure and combustion analysis come from a single batch. Anal. Calcd for C28H29Cl2F12NO4Hf: C, 36.52; H, 3.17; N, 1.52. Found: C, 36.36; H, 3.03; N, 1.64. Synthesis of [CF3−ONO]TiMe(THF) (5-Me). In a vial, [CF3− ONO]TiCl(THF) (3) (132 mg, 0.194 mmol, 1 equiv) was dissolved in 2 mL of benzene. To this solution, a 3.0 M ether solution of MeMgCl (72 μL, 0.22 mmol, 1.1 equiv) was added dropwise while stirring at room temperature. After 3 h, the solvent was removed, and the product was triturated with pentane three times. The red powder was taken up in pentane and filtered. The solvent was removed in vacuo, yielding 5-Me as a microcrystalline red powder. Cooling a concentrated pentane solution of 5-Me to −35 °C precipitated single crystals. A second batch of crystals was obtained after further concentrating and cooling the solution to −35 °C (yield: 50 mg, 39%). 1H NMR (300 MHz, benzened6, 25 °C, δ (ppm)): 7.84 (s, 1H, H7), 7.69 (s, 1H, H17), 6.86 (d, 1H, 3J = 8.4 Hz, H4), 6.83 (dd, 1H, 3J = 8.3 Hz, 4J = 1.7 Hz, H5), 6.65 (dd, 1H, 3J = 8.5 Hz, 4J = 1.7 Hz, H15), 6.41 (d, 1H, 3J = 8.5 Hz, H14), 3.83 (broad, 4H, H21), 1.97 (s, 3H, H20), 2.01 (s, 3H, H10), 1.13 (s, 3H, H25), 1.06 (broad, 4H, H22/23). 1H−13C gHMBC NMR (300 MHz, benzene-d6, 25 °C, δ (ppm)): 148.8 (s, C13), 145.3 (s, C3), 133.4 (s, C6), 132.2 (s, C5), 130.9 (s, C15), 130.8 (s, C16), 127.9 (s, C2), 127.9 (s, C7), 127.7 (s, C17), 126.1 (s, C4), 124.3 (s, C9), 124.1 (s, C19), 123.9 (s, C18), 123.3 (s, C8), 123.3 (s, C12), 117.1 (s, C14), 86.0 (s, C11), 85.1 (s, C1), 72.3 (s, C21/24), 60.3 (s, C25), 24.8 (s, C22/23), 20.5 (s, C10), 20.1 (s, C20). 19F{1H} NMR (282 Hz, benzene-d6, 25 °C, δ (ppm)): −71.75 (m, 3F, J = 9.5 Hz, F9), −72.51 (q, J = 9.3 Hz, F18), −76.17 (q, 3F, J = 9.2 Hz, F19), −76.57 (q, 3F, J = 9.7 Hz, F8). 1H−15N gHMBC (300 MHz, benzene-d6, 25 °C, δ (ppm): 279.7 (Npincer). Anal. Calcd for C25H23F12NO3Ti: C, 45.41; H, 3.51; N, 2.12. Found: C, 45.50; H, 3.36; N, 2.07. Synthesis of [CF3−ONO]TiBn(THF) (5-Bn). In a vial, [CF3− ONO]TiCl(THF) (3) (53 mg, 0.078 mmol, 1 equiv) was dissolved in 2 mL of benzene. To this solution, a 1.0 M Et2O solution of BnMgBr (88 μL, 0.088 mmol, 1.1 equiv) was added dropwise while stirring at room

were performed at Complete Analysis Laboratory, Inc. (Parsippany, NJ). NMR Spectroscopy Characterization and Method. NMR spectra were obtained on a Varian Mercury spectrometer operating at 300 MHz for 1H or a Varian Inova spectrometer operating at 500 MHz for 1H. The chemical shifts (δ) are reported in parts per million and are referenced to the lock signal on the tetramethylsilane scale for 1H and 13 C NMR spectra, the CFCl3 scale for 19F NMR spectra, and the neat NH3 scale for 15N NMR spectra. Compounds 2, 3, 4, and 5-R were characterized by 1H, 13C, 19F, and 15N NMR spectroscopy, and the chemical shifts are presented in Table 1. The assignments were made primarily on the basis of the cross-peaks observed in the 1H−13C gHMBC spectra. The chemical shifts of the fluorinated carbons were measured in the 19F−13C gHSQC spectra, and their assignment to positions 8 and 9 versus 18 and 19 was made on the basis of the longrange coupling of the fluorine atoms to the quaternary carbon two bonds away, as observed in the 19F−13C gHMBC spectra. Cross-peaks with H4 and H14 revealed the 15N chemical shifts in the 1H−15N gHMBC spectra. No stereochemical assignments were made for H7 and H17 or C8 and C9 since they are interchangeable. Some assignments were made on the basis of relative shielding. For example, C1 and C2 were assigned as the more shielded atoms in the C1/C11 and C2/C12 pairs, respectively. For the pairs of fluorine atoms F8/F18 and F9/F19, F8 and F9 were assigned as the more deshielded atoms. In a typical assignment procedure, H7 was found to display crosspeaks with C10 (∼20 ppm), C1 (80−85 ppm), C3 (145−160 ppm), and C5 (130−140 ppm). H10, H5, and C7 were then identified by one-bond correlations or by the couplings H10−C5, H10−C7, and H5−C7. H4 was identified by its coupling with H5 or C6. A similar method was applied to C1-symmetric complexes that have unique atoms in positions 11−20. One coupling of F8 or F9 with C1 was sufficient to identify these fluorine atoms, since the pairs F8/F9 and F18/F19 were revealed by selective decoupling in the 19F spectra. For complexes featuring NMRactive nuclei such as coordinated THF and Et3NH+, the proton signals were assigned on the basis of their intensities and multiplicities. The carbons in these positions were assigned on the basis of their one-bond and long-range couplings to protons. Synthesis of {[CF3−ONO]TiCl3}{HNEt3}2 (2). In a nitrogen-filled glovebox, a glass vial was charged with TiCl4 (23 mg, 0.12 mmol) and toluene (1 mL) and cooled to −35 °C. In another vial, [CF3−ONO]H3 (1) (64 mg, 0.12 mmol) was dissolved in toluene (1 mL), and this solution was cooled to −35 °C and then added dropwise to the first solution while stirring. After 12 h, Et3N (0.013 mL, 1.8 mmol) was added dropwise. Thick white smoke escaped from the vial, and the suspension turned to a dark-red solution. After 30 min of stirring, all of the volatiles were removed, and the residue was dissolved in a minimal amount of benzene. Then the mixture was dropped into a cold pentane solution to precipitate 2 as a dark-purple solid (yield: 42 mg, 51%). 1H NMR (300 MHz, benzene-d6, 25 °C, δ (ppm)): 9.69 (s, 2H, H23), 7.76 (s, 2H, H7/17), 6.70 (d, 2H, J = 8.5 Hz, H5/15), 6.57 (d, 2H, J = 8.5 Hz, H4/14), 2.53 (q, 12H, J = 6.45 Hz, H21), 2.04 (s, 6H, H20), 0.92 (t, 18H, J = 6.7 Hz, H22). 1H−13C gHMBC NMR (300 MHz, benzene-d6, 25 °C, δ (ppm)): 20.3 (s, 2C, H20/10), 83.3 (s, 2C, H1/11), 121.2 (s, 2C, H4/14), 123.9 (s, 2C, H9/19), 124.3 (s, 2C, H8/18), 125.5 (s, 2C, H2/12), 127.0 (s, 2C, H7/17), 130.8 (s, 2C, H5/15) 131.6 (s, 2C, H6/16). 19F{1H} NMR (282 MHz, benzene-d6, 25 °C, δ (ppm)): −70.92 (q, 6F, J = 9.4 Hz, F8/18), −74.9 (q, 6F, J = 9.7 Hz, F9/19). Anal. Calcd for C32H44Cl3F12N3O2Ti: C, 43.43; H, 5.01; N, 4.75. Found: C, 43.37; H, 5.04; N, 4.66. Synthesis of [CF3−ONO]TiCl(THF) (3). In a nitrogen-filled glovebox, a glass vial was charged with [CF3−ONO]H3 (1) (115 mg, 0.370 mmol) and THF (1 mL) and then cooled to −35 °C. To the mixture was added a cold solution of nBuLi (0.26 mL, 1.6 M) while stirring. The mixture was cooled again to −35 °C and was then added dropwise to a cold THF solution of TiCl4(THF)2 (84 mg, 0.37 mmol). After 30 min of stirring, all of the volatiles were removed in vacuo, and the residue was taken up in a minimal amount of Et2O and filtered. The filtrate was evaporated in vacuo, and the resulting residue was triturated with pentane (2 × 2 mL) and filtered to obtain 3 as a dark-red powder (yield: 0.23 g, 91%). Single crystals were obtained by slow evaporation of a concentrated Et2O solution of 3. 1H NMR (300 MHz, benzene-d6, 25 1115

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Organometallics temperature. After 3 h, the solvent was removed, and the product was triturated with pentane three times. The red powder was taken up in pentane and filtered. The solvent was removed in vacuo, yielding 5-Bn as a microcrystalline red powder (yield: 39 mg, 69%). 1H NMR (300 MHz, benzene-d6, 25 °C, δ (ppm)): 7.84 (s, 1H, H7), 7.71 (s, 1H, H17), 7.27 (d, 1H, 3J = 7.7 Hz, H27), 6.98 (t, 1H, 3J = 7.5 Hz, H28), 6.90 (d, 1H, 3 J = 8.3 Hz, H4), 6.83 (d, 1H, 3J = 8.3 Hz, H5), 6.68 (d, 1H, 3J = 6.2 Hz, H29), 6.67 (d, 1H, 3J = 6.8 Hz, H15), 6.55 (d, 1H, 3J = 8.5 Hz, H14), 3.59 (bs, 4H, H21), 3.49 (d, 1H, 3J = 8.7 Hz, H25), 2.06 (s, 3H, H10), 1.98 (s, 3H, H20), 0.99 (bs, 4H, H22/23). 1H−13C gHMBC NMR (300 MHz, benzene-d6, 25 °C, δ (ppm)): 150.0 (s, C13), 147.9 (s, C3), 140.0 (s, C26), 132.3 (s, C6), 131.9 (s, C5), 131.0 (s, C15), 130.9 (s, C16), 130.8 (s, C27), 128.8 (s, C28), 127.5 (s, C7), 127.3 (s, C17), 126.5 (s, C2), 124.8 (s, C4), 124.5 (s, C29), 124.4 (s, C9), 124.3 (s, C19), 123.8 (s, C12), 123.8 (s, C18), 123.6 (s, C8), 119.4 (s, C14), 86.1 (s, C25), 85.4 (s, C11), 85.1 (s, C1), 20.4 (s, C20). 19F{1H} NMR (282 MHz, benzene-d6, 25 °C, δ (ppm)): −71.01 (m, 3F, J = 9.5 Hz, F9), −71.47 (q, 3F, J = 9.7 Hz, F19), −75.04 (q, 3F, J = 9.8 Hz, F18), −76.46 (q, 3F, J = 9.6 Hz, F8). 1H−15N gHMBC (300 MHz, benzene-d6, 25 °C, δ (ppm): 278.1 (Npincer). Anal. Calcd for C31H27F12NO3Ti: C, 50.49; H, 3.69; N, 1.90. Found: C, 50.35; H, 3.69; N, 1.95. Synthesis of [CF3−ONO]TiMes(THF) (5-Mes). In a vial, [CF3− ONO]TiCl(THF) (3) (120 mg, 0.176 mmol, 1.00 equiv) was dissolved in 2 mL of benzene. To this solution, a 1.0 M Et2O solution of MesMgBr (199 μL, 0.20 mmol, 1.1 equiv) was added dropwise while stirring at room temperature. After 3 h, the solvent was removed, and the product was triturated with pentane three times. The red powder was taken up in pentanes and filtered, and then all of the volatiles were removed in vacuo, yielding 5-Mes as a microcrystalline red powder. Slow evaporation of a concentrated pentane solution of 5-Mes precipitated crystals (yield: 35 mg, 26%). 1H NMR (300 MHz, benzene-d6, 25 °C, δ (ppm)): 7.73 (s, 1H, H7), 7.71 (s, 1H, H17), 7.03 (d, 1H, 3J = 8.3 Hz, H4), 6.68 (dd, 1H, 3J = 8.4 Hz, 4J = 1.6 Hz, H15), 6.60 (dd, 1H, 3J = 8.3 Hz, 4J = 1.8 Hz, H5), 6.46 (d, 1H, 3J = 8.7 Hz, H14), 6.42 (s, 2H, H27), 4.26 (dd, 2H, 3J = 8.3 Hz, 2J = 6.5 Hz, H24), 3.60 (dd, 2H, 3J = 8.8 Hz, 2J = 6.0 Hz, H21), 2.69 (s, 6H, H31), 1.97 (s, 3H, H20), 1.87 (s, 3H, H32), 1.79 (s, 3H, H10), 1.20 (m, 2H, H23), 1.13 (m, 2H, H22). 1H−13C gHMBC NMR (300 MHz, benzene-d6, 25 °C, δ (ppm)): 203.1 (s, C25), 147.8 (s, C13), 140.9 (s, C3), 139.7 (s, C26), 139.3 (s, C28), 134.7 (s, C6), 133.2 (s, C14), 131.3 (s, C5), 131.2 (s, C15), 131.0 (s, C16), 129.5 (s, C2), 128.9 (s, C17), 128.4 (s, C4), 128.3 (s, C7), 126.6 (s, C27), 124.1 (s, C9), 124.1 (s, C19), 123.6 (s, C18), 123.2 (s, C8), 119.2 (s, C12), 87.1 (s, C11), 85.3 (s, C1), 72.6 (s, C21), 25.0 (s, C22/23), 24.8 (s, C31), 20.8 (s, C33), 20.5 (s, C10), 19.9 (s, C20). 19F{1H} NMR (282 Hz, benzene-d6, 25 °C, δ (ppm)): −70.32 (q, 3F, J = 11.1 Hz, F9), −70.43 (q, 3F, J = 8.4 Hz, F19), −75.01 (qq, 3F, J = 11.4 Hz, J = 5.7 Hz, F8), −78.37 (qq, 3F, J = 8.5 Hz, J = 5.6 Hz, F18). Anal. Calcd for C33H31F12NO3Ti: C, 51.78; H, 4.08; N, 1.83. Found: C, 51.69; H, 4.26; N, 1.80.

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by the University of Florida High-Performance Computing Center. Synquest Labs, Inc. provided hexfluoroacetone.

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ASSOCIATED CONTENT

S Supporting Information *

Full experimental procedures, NMR spectra, and X-ray crystallographic details (CIF). This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]fl.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.S.V. thanks UF and the National Science Foundation (CHE1265993) for financial support of this project. K.A.A. thanks UF and the NSF (CHE-0821346) for funding the purchase of X-ray equipment. Computational resources and support were provided 1116

DOI: 10.1021/acs.organomet.5b00036 Organometallics 2015, 34, 1107−1117

Article

Organometallics (33) Clark, H. C.; Manzer, L. E. Inorg. Chem. 1974, 13, 1996−2004. (34) Bartolomé, C.; Espinet, P.; Martin-Á lvarez, J. M.; Villafañe, F. Eur. J. Inorg. Chem. 2004, 2326−2337.

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DOI: 10.1021/acs.organomet.5b00036 Organometallics 2015, 34, 1107−1117