PRUEBAS DE ACEPTACIÓN

75

Figure 2.5. Solid-state structures of complexes 2.9 (upper left), 2.10 (upper right) displaying, and 2.12 (bottom) with thermal ellipsoids at the 50% probability level. H- atoms, disorder of one DME group of 2.9, and disorder of the adamantyl group in 2.12

have been excluded for clarity.

Using these titanium complexes that are extremely nucleophilic at nitrogen might present a way to access these organic compounds if cleanly oxidizing them from the metal precursor is possible. From a solution of complex 2.10 stored over periods of time in the glove box freezer, a color change to a lighter brown was noted over time, and the formation of some plate shaped crystals on the wall of the vial. Examination of this material by single crystal X-ray diffraction revealed that the complex had been oxidized to a Ti(III) (PN)2Ti(NCNAd) (2.12) (Figure 2.5, Scheme 2.8). While few metrical parameters have changed, some slight changes include a minor decrease is observed of the Ti1−N3 bond (1.948(2) Å). A slight increase in the N3−C45 bond

76

(1.227(4) Å) coupled with a slight decrease in the C45−N4 bond (1.229(34) Å) results in more double bond character in both of these bonding interactions. The N3−C45−N4 angle is essentially unchanged at 174.8(4)o _{while more importantly the C45−N4−C46} angle of 129.5(4)o _{is slightly less bent, likely as a result of the elimination of anionic} charge residing on this terminal nitrogen. The other marked difference is that this structure is closer to a trigonal bipyramid in character (t5 = 0.82) Although unfortunately this complex was in poor yield and likely just a side product of eventual oxidation in the glove box atmosphere overtime, this demonstrated that oxidation to Ti(III) could be achieved. The reproduction of this complex by intentional chemical oxidation has unfortunately not yet been accessible, but if possible in future studies, this complex would provide a nice electronic comparison of the Ti(II) vs. Ti(III) complex constrained to the same ligand environment.

Conclusion.

This chapter has revealed the completely unexpected reactivity of a titanium nitride to be a source of nitridyl radicals. Since little is know of these extremely reactive radicals, this study provides the first detailed mechanistic study of the behavior of early transition metal nitridyl radicals in solution, as well as a glass in matrix EPR studies. It has also been shown that this titanium nitride can be electrophilic in character in reactivity with various isocyanides. Given the novelty of all reactivity discussed in this chapter, and our continued interest to develop the scope of reactivity in early transition metal nitride chemistry, we questioned if we could design zirconium nitride complexes.

Reprinted with permission from {G. B. Wijeratne, E. M. Zolnhofer, S. Fortier, L. N. Grant, P. J. Carroll, C.-H. Chen, K. Meyer, J. Krzystek, A. Ozarowski, T. A. Jackson, D. J. Mindiola, and Joshua Telser, Inorg. Chem., 2015, 54, 10380-10397}. Copyright {2015} American Chemical Society.

77

Experimental Details

General Procedures

Unless otherwise stated, all operations were performed in a M. Braun Lab Master double-dry box under an atmosphere of purified dinitrogen or using high vacuum standard Schlenk techniques under an argon or dinitrogen atmosphere. Hexanes, pentane, tetrahydrofuran (THF) and toluene were purchased from Fisher Scientific and Et2O was purchased from Sigma Aldrich. Solvents were sparged with argon for 20 minutes and dried using a two-column solvent purification system where columns designated for hexanes and toluene were packed with Q5 and alumina respectively, and columns designated for Et2O and THF were packed with alumina. Deuterated benzene was purchased from Cambridge Isotope Laboratories (CIL) and was sparged with nitrogen for 20 minutes, then was dried over a potassium mirror, vacuum transferred to a collection flask, and degassed by freeze–pump–thaw cycles. All solvents were transferred into a dry box and were stored over 4 Å sieves. Tritylchloride and iodine were purchased and used without further purification. mer- [TiCl3(THF)3]106 _{and N2CPh2107 were prepared according to literature procedures. 1.3} and 1.9 were prepared according to published literature procedures.49,50 _{NaN3 was} dried by evacuating in a schlenk flask overnight, followed by stirring overnight in THF after bringing into the glovebox. This solution was decanted, and then suspended and stirred in toluene overnight. Finally, after decanting this solution, it was suspended in pentane, stirred overnight, and then this was taken to dryness for use. KC8 was prepared with by mixing carb

on with potassium and heating over a week at 150 °C in a well-sealed, high pressure flask, with good stirring with a metal stirbar. Each night, the reaction vessel was brought back into the glovebox to manually mix the two reagents to ensure good mixing. All sieves were heated to 200 °C under vacuum overnight prior to use. Celite used for filtrations was heated to 200 °C under vacuum overnight prior to use. 1_H NMR spectra were recorded on a Bruker AVIII 400, AVII 500 MHz spectrometer, DMX

78

300 MHz spectrometer, or a Bruker DMX 360 MHz spectrometer. 13_{C spectra were} recorded on a Bruker AVII 500 MHz spectrometer, 31_P{1_{H} spectra were recorded on} a Bruker AVIII 400 spectrometer, and 7_{Li NMR spectra were recorded on a Bruker} AVIII 400 spectrometer, and 15_{N NMR spectra were recorded on a Bruker AVIII 400} spectrometer. 1_{H NMR spectra are reported with reference to residual proteo solvent} resonances of C6D6 at 7.16 ppm. 31_P{1_{H} NMR spectra were referenced to external} H3PO4 (0.0 ppm). Elemental analyses were measured by Midwest Microlab for all PN ligand chemistry and in the inorganic chemistry department at the Friedrich- Alexander University Erlangen-Nürnberg, Germany, and also by Robertson Microlit Laboratories, Ledgewood, NJ, USA for the Ti(II) reactivity studies from 2.4.

Synthesis of(NPN’)(PN)TiI (2.1)

To a 10 mL orange solution of 1.3 (201 mg, 0.106 mmol, 1 equiv.) in toluene in a 20 mL vial was added 5 mL solution I2, (253.8 mg, 0.212 mmol, 2 equiv.). The reaction was allowed to stir at room temperature overnight at room temperature. The reaction can then be driven to complextion by one of the following methods: the solution can be placed into a 25 mL Schlenk flask and photolyzed for fifteen minutes, or alternatively (though not recommended, the reaction will also go if left stirring in ambient light for three days). After completion, all volatiles were removed in vacuo. The deep red residue was extracted into toluene and filtered over Celite. The toluene was taken to a minimum (less than 2 mL) in a 20 mL vial and layered with 5 mL hexane. This vial was stored at −35 o_{C overnight, resulting in the deposition of red} crystals suitable for XRD (184 mg, 0.101 mmol, 48% yield). Due to the nature of the proposed mechanism, involving both parent imide and hydride complexes en route to

2.2, there is inevitably always a small amount (5-10% contamination) of these species in isolated samples. Between this contamination, and the assymetry of the resulting complex, specific resonances are nearly impossible to assign due to overlapping peaks. Reliable J values cannot be determined for most resonances for this reason. 1_H

79

2H, C-H, Ar), 6.17 (dd, J = 8.6, 3.5 Hz, 1H, C-H, Ar), 5.66 (dd, J = 8.6, 3.7 Hz, 1H, C-H, Ar), 3.27 (sept, 3_{JH-H = 6.9 Hz, 2H, P-CHMe2), 2.95 (sept, J value not possible to measure,} 1H, P-CHMe2),3.63 (s, Δν1/2 = 4.0 Hz, 6H, CH3, mesityl Ar), 2.67 (s, Δν1/2 = 3.8 Hz, 6H,

CH3, mesityl Ar), 2.3-0.79 (multiple overlapping peaks from P-CHMe2 and Ar-Me, 36

H). 31_P{1_{H} NMR (162 MHz, 298 K, benzene-d6): δ 34.9 (1P, NPN’), 14.7 (1P, PN).}

Multiple attempts to obtain satisfactory elemental analysis were unsuccessful.

Synthesis of(NPN’)(PN)TiCl(2.2)

To a 10 mL orange solution of 1.3 (10 mg, 0.005 mmol, 1 equiv.) in C6D6 in a 20 mL vial was added 5 mL solution ClCPh3, (5.88 mg, 0.02 mmol, 4 equiv.). Triphenylphosphine was added to monitor yield by 31_{P NMR (2.77 mg, 0.01 mmol, 2} equiv.). The reaction was allowed to stir at room temperature overnight at room temperature. The reaction can then be driven to complextion by one of the following methods: the solution can be photolyzed for fifteen minutes with a Xenon lamp, or alternatively (though not recommended, the reaction will also go if left stirring in ambient light for three days). This NMR reaction was used to determine best yield with the internal standard. This reaction can be scaled up to ~200-300 mg of 1.3, with slightly lowere yields than in the NMR reaction due to increased solubility of 2.2. If conducted on a larger scale, the same procedure is followed, with the exception of the use of toluene as a solvent. After completion, all volatiles were removed in vacuo. The red residue was extracted into toluene and filtered over Celite. The toluene was taken to a minimum (less than 2 mL) in a 20 mL vial and layered with 5 mL hexane. This vial was stored at −35 o_{C overnight, resulting in the deposition of red crystals suitable} for XRD (4 mg, 0.005 mmol, 49% yield). Due to the nature of the proposed mechanism, involving both parent imide and hydride complexes en route to 2.2, there is inevitably always a small amount (5-10% contamination) of these species in isolated samples. Between this contamination, and the assymetry of the resulting complex, specific resonances are nearly impossible to assign due to overlapping peaks. Reliable J values cannot be determined for most resonances for this reason. 1_{H NMR (400 MHz, 298}

80

= 10.2, 2.0 Hz, 1H, C-H, Ar),5.92 (dd, J = 10.3.6, 3.7 Hz, 1H, C-H, Ar), 2.8-0.82 (multiple overlapping peaks from P-CHMe2 and Ar-Me). 31P{1H} NMR (162 MHz, 298 K,

benzene-d6): δ 35.0 (1P, NPN’), 12.7 (1P, PN). Multiple attempts to obtain

satisfactory elemental analysis were unsuccessful.

Synthesis of (NPN’)(PN)TiH (2.3)

A 5 mL orange solution of 1.2 in C6D6 (10 mg, 0.012 mmol, 1 eq.) was photolyzed by a Xenon lamp for 15 minutes in a J-Young NMR tube and conversion was monitored by 31_{P NMR. The solution turns slightly darker. After completion, all volatiles were} removed in vacuo and the reaction mixture was filtered over Celite. While stability and increased solubility of this complex prevents bulk characterization, cooling a solution of 2.3 in 1 mL n-hexane at −35 o_{C overnight resulted in the deposition of} reddish brown crystals of 2.3 suitable for single crystal XRD. While yield from crystallization was not reliable, monitoring the solution with added triphenyl phosphine (2.6 mg, 0. 012 mmol, 1 eq.) determined the yield to be just less than 49%, as expected given the determined degradation of 50% of the ligand scaffold (4.5 mg, 0.006 mmol, ~50%), However, since bulk isolation of this complex free from starting materials has not been possible, characterization by 1_{H NMR has not been possible.}

31_P{1_{H} NMR (162 MHz, 298 K, benzene-d6): δ 22.3 (1P, NPN’), 7.4 (1P, PN).}

Experiments to Convert 2.3 to 2.2

A solution of 2.3 (10 mg, 0.013, 1 equiv.) in benzene-d6 in a J-Young NMR tube was treated with tritylchloride (3.8 mg, 0.12 mmol, 1 equiv.). Immediately 1_{H and} 31_{P NMR} spectra showed clean and quantitative formation of tritylmethane and 2.2.

Modified Protocol to Prepare trans-[(py)4TiCl2] (2.4)

In a 250 mL round-bottom flask, 1.000 g (2.70 mmol) of mer-[TiCl3(THF)3], prepared as described by Jones et al.102 _{was dissolved in 100 mL of THF, giving a pale blue} solution. With vigorous stirring, 1.1 mL (13.5 mmol) of pyridine was added, turning

81

the solution dark green instantaneously. A sample of 0.5472 g (4.049 mmol) of KC8 was then added to the flask. The reaction mixture was stirred for 15 min and then filtered through a medium porosity frit layered with a bed of Celite. The Celite was washed with 10 mL of THF, and the combined dark blue filtrate was concentrated in vacuo to ∼20 mL. The supernatant was decanted off, and the dark blue product 2.4

was dried in vacuo (yield: 880 mg, 75%, one crop). Attempts to isolate more compound from the filtrate did not produce pure material. Elemental analysis (%), calc for C12H32Cl2N4Ti (MW = 351.18 g/mol): C, 55.20; N, 12.87; H, 4.63. Found: C, 47.27; N, 10.51; H, 4.04. The lower carbon content (as well as H and N) is most likely due to pyridine loss (and possible replacement by oxygen and/or water during handling) and the overall thermally unstable nature of this species.

Synthesis of trans-[Ti(η2_{-PhCCPh)(Cl)2(py)3] (2.5)}

In a 20 mL vial containing a 1/2-in.-long Teflon-coated magnetic stir bar, toluene (4 mL) was added to trans-[(py)4TiCl2] (107 mg, 246 μmol), producing a dark blue slurry. In a separate 20 mL vial, toluene (1 mL) was added to PhCCPh (44 mg, 246 μmol), producing a colorless solution. Both vials were left in a freezer at −35 °C for 30 min. The vials were removed from the freezer, and the PhCCPh solution was added dropwise over 5 min to the stirred 2.4 slurry, producing a dark, amber-colored slurry, which was stirred for 1 h at ambient temperature before filtering through a plug of Celite in a pipet, resulting in separation of a small amount of dark green solid from the amber-colored filtrate. The solvent volume was reduced to ∼2 mL in vacuo and n- pentane (10 mL) was added, resulting in the precipitation of a light-green solid. The supernatant was decanted, and the solid was dried in vacuo and isolated as an olive- green microcrystalline powder (99 mg, 75% yield). 1_{H NMR (300 MHz, 298 K,}

benzene-d6): δ 9.28 (ν1/2 = 34 Hz, 6H, o-Hpy), 7.48 (d, 3JHH = 8 Hz, 4H, o-HPh),

7.13−7.02 (5H, Htol) 7.01 (pseudo-t, 3_{JHH = 8 Hz, 4H, m-HPh), 6.89 (t,} 3_{JHH = 7 Hz, 2H, p-} HPh), 6.69 (ν1/2 = 44 Hz, 3H, p-Hpy), 6.42 (ν1/2 = 32 Hz, 6H, m-Hpy), 2.11 (s, 3H, Htol). 13_C{1_{H} NMR (75 MHz, 298 K, benzene-d6): δ 152.0 (ν1/2 = 58 Hz), 141.5 (ν1/2 = 1}

82

Hz), 136.8 (ν1/2 = 41 Hz), 131.9 (ν1/2 = 1 Hz), 128.1 (ν1/2 = 1 Hz), 127.6 (ν1/2 = 1 Hz), 126.5 (ν1/2 = 2 Hz), 123.4 (ν1/2 = 19 Hz). Crystals suited for X-ray diffraction (XRD) analysis were obtained by layering a saturated solution in toluene with n-pentane. Multiple attempts to obtain satisfactory elemental analysis were unsuccessful.

Synthesis of Complex (py)2(Cl)2Ti(μ2:η2-N2CPh2)2Ti(Cl)2 (2.6)

In a 20 mL vial containing a 1/2-in.-long Teflon-coated magnetic stir bar, toluene (4 mL) was added to 2.4 (135 mg, 310 μmol), producing a dark blue slurry. In a separate 20 mL vial, toluene (2 mL) was added to N2CPh2 (60 mg, 310 μmol), producing a fuchsia-colored solution. The N2CPh2 solution was added dropwise to the slurry of

2.4 over 2 min, resulting in a color change to orange-red. The mixture was stirred for