Datos de compromiso organizacional

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interesting triphenylmethyl (trityl) imido ligand. Trityl functions as an effective protecting group in a variety of organic reactions, and the cleavage of Ph3C–X bonds is a useful pathway for the

oxidative installation of new uranium-ligand bonds,67, 141, 142 _{including a recent example of the}

formation of a uranium-oxo multiple bond.143 _{In fact, the low-temperature reaction of Ph}

3CN3 with

UIII_[N(SiMe

3)2]3 is known to form the product of one-electron oxidation, UIV(N3)[N(SiMe3)2]3

through loss of trityl radical and formation of Gomberg’s dimer.140 _{Similarly, the reaction of}

UIII_(OAr)

3TACN with Me3SiN3 was reported to produce a mixture of both the uranium(V) mono-

imido through two-electron oxidation as well as the uranium(IV) azide through loss of trimethylsilyl radical and subsequent formation of hexamethyldisilazane.141 _{However, we found that formation}

of UIV_(N

3)[N(SiMe3)2]3 through the low temperature addition of Ph3CN3 followed by addition of N-

methylmorpholine-N-oxide, led to a color change to dark green and vigorous gas formation, to produce UV_(=NCPh

3)[N(SiMe3)2]3 (3.4) in 93% yield (Scheme 3.12.1, Figure 3.12.1). More

conveniently, the room temperature addition of Ph3CN3 to UIII[N(SiMe3)2]3 in Et2O directly

generated 3.4 directly. When this reaction was performed in pyridine at room temperature only the uranium(IV) azide product was generated, and addition of N-methylmorpholine-N-oxide led to decomposition.

Figure 3.12.1 Thermal ellipsoid plot of 3.4 at 30% probability. Hydrogen atoms are omitted for clarity. Bond lengths (Å) and angles (°): U(1)–N(1) 2.264(3), U(1)–N(2) 1.959(5), N(2)–C(1) 1.478(7), N(1)–U(1)–N(2) 113.90(7), U(1)–N(2)–C(1) 180.0.

geometry, in which the U–N–C bond angle was fixed at 180 °. This geometry is likely preferable to satisfy steric demands, but it is noted that the recently reported complex [Li(12-crown- 4)2][UIII(NHCPh3)[N(SiMe3)2]3] exhibited a U–N–C bond angle of 151.2(3) °,143 indicating that

some flexibility is possible, albeit in the coordination sphere of the larger uranium(III) ion. The steric hindrance imposed by the trityl group appeared too great to install a trans-axial ligand, limiting the possibility of synthesizing imido derivatives of the 3.1-X complexes. In spite of this, the cyclic voltammogram of 3.4 was collected to assess the accessibility of the 6+ and 4+ oxidation states, and to detect the possibility for electron transfer processes involving loss of the trityl group (Figure 3.12.2). Reversible UVI/V _{and U}V/IV _{couples were observed at +0.38 V and –1.31 V}

respectively, in the range accessible by chemical means.

Figure 3.12.2 Cyclic voltammogram of 3.4 in CH2Cl2 at a scan rate of 250 mV/s, with 0.1 M

[n_Bu

4N][PF6] supporting electrolyte. A small, unidentified impurity centered at –0.54 V appeared in

multiple experiments despite the use of a sample of high purity confirmed by 1H NMR and elemental analysis.

The sterically hindered environment in 3.4 caused the oxidation reactions with Cu(II) salts to proceed with unexpected results(Scheme 3.12.1). Addition of CuBr2 in THF produced a brown

product that appeared to be diamagnetic by 1_{H NMR, with a curious 2:1 ratio of –N(SiMe} 3)2 to

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hexanes at –21 °C revealed the structure to be UVIBr2(=NSiMe3)[N(SiMe3)2]3 (3.5). Isolation of 3.5

was achieved in 83% yield, ruling out the possibility that 3.5 was a product of ligand redistribution. Additionally, no production of [Cu[N(SiMe3]2]4 was observed,144 ruling out transmetalation.

Formation of 3.5 implies the formal loss of the aminyl radical, ·N(SiMe3)2.72 We have previously

observed loss of aminyl radical in the formation of UVI_O

2(THF)2[N(SiMe3)2]2 from the reaction of

UVI_OCl[N(SiMe

3)2]3 with NaNO2, which was attributed to the strong thermodynamic driving force

of uranyl formation as well as the absence of valence electrons on the 5f0 _{metal center.}103

Similarly, U–N(SiMe3)2 and Ln–N(SiMe3)2 bond homolysis has led to single-electron reduction of

uranyl in the work of Arnold et al,145, 146 _{also to form strong metal-oxo bonds. We were therefore}

surprised at the ability to install weaker bromide ligands, especially considering the relatively poorer stability of the aminyl fragment relative to bromide radical. However, while this reaction proceeds cleanly in THF, no reaction was observed in toluene, a solvent less prone to hydrogen atom donation, implicating the need for hydrogen atom abstraction from solvent to form free HN(SiMe3)2. The synthesis of 3.5 is best described as a sterically induced reduction, a process

common among (C5Me5)3Ln and (C5Me5)3U complexes.147

In contrast to CuBr2, addition of CuCl2 led to an immediate color change to dark red. The 1_{H NMR spectrum of the product showed complete conversion to U}V_Cl

2[N(SiMe3)2]3, a compound

previously reported by us.58 _{This reaction most likely proceeds through transmetalation to form a}

“Cu(NCPh3)” product; however, no other product could be identified from the intractable solid.

With no evidence for redox activity of the trityl functionality observed in the oxidation reactions, we also tested the reduction of 3.4. Addition of KC8 to a THF solution of 3.4 led to a

slow color change to orange. This product was obtained in high purity immediately following filtration through Celite to separate the formed graphite, from which it was readily determined by

1_{H NMR spectroscopy that the reduction product, K(THF)}

6[UIV(=NCPh3)[N(SiMe3)2]3], was

generated (Figure 3.12.3). Therefore, cleavage of the trityl group from the imido ligand will necessitate a different reaction pathway than the oxidation and reduction reactions reported here.

Figure 3.12.3 1_{H NMR of K(THF)}

6[UIV(=NCPh3)[N(SiMe3)2]3] in pyridine-d5.

3.13 Thermodynamics of Uranium Imido Reactions. The unusual reactivity exhibited