The Ketimide Ligand is Not Just an Inert Spectator: Heteroallene Insertion Reactivity of an Actinide–Ketimide Linkage in a Thorium Carbene Amide Ketimide Complex

The ketimide anion R2C—N− is an important class of chemically robust ligand that binds strongly to metal ions and is considered ideal for supporting reactive metal fragments due to its inert spectator nature; this contrasts with R2N− amides that exhibit a wide range of reactivities. Here, we report the synthesis and characterization of a rare example of an actinide ketimide complex [Th(BIPMTMS){N(SiMe3)2}(N—CPh2)] [2, BIPMTMS=C(PPh2NSiMe3)2]. Complex 2 contains Th—Ccarbene, Th—Namide and Th—Nketimide linkages, thereby presenting the opportunity to probe the preferential reactivity of these linkages. Importantly, reactivity studies of 2 with unsaturated substrates shows that insertion reactions occur preferentially at the Th—Nketimide bond rather than at the Th—Ccarbene or Th—Namide bonds. This overturns the established view that metal-ketimide linkages are purely inert spectators.

Amide (R 2 N À ) and ketimide (R 2 C=N À ) (R = alkyl, aryl, or silyl groups) monoanions are two important classes of monodentate nitrogen-donor ligands in coordination and organometallic chemistry. The negative charge of both these types of ligand is N-centered and can form a covalent MÀN bond in metal complex derivatives. However, there is a crucial difference between amides and ketimides: in the former the nitrogen hybridization is sp 2 or sp 3 and it bears two N À C/Si singly bonded groups, whereas in the latter the nitrogen hybridization is sp or sp 2 and it is bonded to only one carbon atom with a N=C double bond. These differences in structure and bonding lead to a significantly different reactivity of these two types of MÀN bond. The MÀN amide bond is reactive, and readily engages in protonolysis and insertion of unsaturated organic substrates; this has been extensively studied for decades and these reactions play a vital role in very important catalytic processes such as hydroamination, hydroalkoxylation, and ring-opening polymerization of lactones. [1] In sharp contrast, the M À N ketimide bond is chemically inert, and resistant to insertion and electrophilic attack. [2] In fact, the chemically inert nature of MÀN ketimide bonds renders ketimides the ligand of choice when spectator ligands are required to stabilize highly reactive species in a broad range of applications including strongly oxidizing high-valent uranium and [3] diuranium inverted-sandwich arene complexes, [4] and olefin polymerization catalysts. [5] To the best of our knowledge, the only reported reactivity of any metal-ketimide, in the absence of acidic hydrogens, involves b-R-group eliminations and free-radical redox CÀC bond homolysis degradation reactions of the ketimide rather than M À N ketimide insertion chemistry. [6] Nonaqueous actinide chemistry has received burgeoning interest in the past decade. [7] Although actinide amides are less well-developed than their transition metal counterparts, they have been known for decades and their reactivity is extensively investigated. [8][9][10] In contrast, actinide ketimides were unknown until 2002. After the first example of a uranium ketimide, [11] a relatively small number of actinide ketimides have been synthesized and characterized. [12] Studies of the AnÀN ketimide (An = U, Th) bond have shed light on the important question of the amount of 5f orbital participation in bonding. [13] However, from a reactivity perspective, and as compared to their transition metal counterparts, the An À N ketimide bond is generally considered to be chemically inert, [11] and considerably stronger than analogous AnÀN amide bonds. Indeed, no insertion reactivity of the AnÀN ketimide spectator ligand linkage with a wide range of substrates has ever been observed, [11,12] despite the fact that the AnÀN ketimide linkage is usually the least sterically hindered linkage in An-complexes. Furthermore, from a general perspective, a direct comparison of bonding character and reactivity of M À N amide /M À N ketimide linkages in one molecule is desirable but still absent. Such studies may provide information on potential catalytic mechanisms and/or deactivation pathways of such complexes, and open new horizons for M À N linkage reactivity.
As part of our investigations of An-ligand multiple bond chemistry, [14] we describe here the synthesis of a thorium carbene amide ketimide that features Th=C carbene , ThÀN amide , and ThÀN ketimide bonds in one molecule. Preliminary reactivity studies unexpectedly revealed that insertion reactions occur at the traditionally inert MÀN ketimide site, rather than at the M= C carbene [15] or M À N amide bonds. This observation overturns the view that ketimides are purely inert spectator ligands.
The characterization data for 1 and 2 are consistent with their formulations. [17] The vivid orange color of 2 (both in the solid-state and in solution) is noteworthy for a 6d 0 5f 0 metal complex. The electronic absorption spectra of 2 exhibits a broad, intense electronic absorption band from the UV to visible wavelength range, and a strong absorption between 450 and 550 nm. Since 1 is pale yellow and the 6d 0 5f 0 electronic configuration of Th IV precludes metal-localized ff, d-f, and d-d transitions, and on the basis of definitive prior work, [12c] we conclude this transition is due to a spin-allowed but orbital-forbidden p ? (N)!p* (N=C) and ligand-to-metal charge transfer (LMCT).
The solid-state structures of 1 and 2 were confirmed by Xray crystallography (1, Figure S1; 2, Figure 1). The salient structural feature of 2 is the two types of Th À N linkage; the Th À N ketimide distance is significantly shorter than the Th À N amide bond (Th-N4 2.265(6) versus Th-N3 2.350 (7) ). The ketimide N=C bond length is 1.279(10) , and Th-N-C bond angle is 171.3(6)8. These parameters suggest that the ThÀ N ketimide interaction may be stronger than the ThÀN amide interaction, [11] and may feature some multiple bond character. The Th = C carbene bond lengths in 1 and 2 are 2.410(8) and 2.474 (8) , respectively, which is similar to other thorium BIPM carbene complexes (2.43-2.50 ). [19] Although the M= C bond in An and rare-earth BIPM carbene complexes is polarized, [14,18] a modest multiple bond character of the Th= C carbene bonds in 1 and 2 is suggested by comparison to the thorium alkyl complex [Th(CH 2 CMe 3 ) 5 ][Li(THF) 4 ], [20] in which the Th À C single bond (2.50-2.56 ) is longer than the Th = C bonds in 1 or 2.
The presence of Th=C carbene , ThÀN amide , and ThÀN ketimide bonds in 2 offers the opportunity to probe their competitive reactivity toward unsaturated organic molecules. For the M= C bond in An and rare-earth metal carbene complexes with BIPM ligands, the cycloaddition and Wittig-type group transfer reaction towards unsaturated organic substrates containing C=E (E = O, N) bonds has been well-documented, even in complexes that can be considered as sterically saturated. [14,15,18] On the other hand, M À N amide bonds (M = d-or f-block metal) are also known to undergo a wide range of reactions with unsaturated substrates. Moreover, irrespective of the predicted reactivity of the Th=C carbene and ThÀ N amide linkages, the ThÀN ketimide bond would be anticipated to be inert. However, we find that when 2 is reacted with one equivalent of aldehyde or isocyanate, insertion reactions occur at the Th À N ketimide linkage (Scheme 2).
Isocyanate is an important heteroallene with versatile reactivity in organic and polymer synthesis and insertions of isocyanates into M À N amide bonds in the d-block are widely reported. [21] We have previously shown that the M = C bonds (M = lanthanide or uranium) in BIPM derivatives readily undergo [2+2] cycloaddition reactions with the C=O bond. [14l, 15] When 2 was treated with one equivalent of tertbutyl isocyanate (tBuN=C=O) in toluene at room temperature, pale-yellow crystals of [Th(BIPM TMS ){N(SiMe 3 ) 2 }{OC-(NtBu)NCPh 2 }] (4) were obtained in 49 % yield (Scheme 2). The moderate crystalline yield is due to the high solubility of 4 in toluene, and 4 was confirmed to be the single product by an NMR-scale reaction with > 95 % conversion. The structure of complex 4 was confirmed by X-ray crystallography as a thorium carbene amide ureate (Figure 3). In this instance, the Th À N ketimide bond was again shown to be active in insertion chemistry. The ureate ligand, which is formed by the selective insertion of C=O into the ThÀN ketimide bond, is coordinated to the thorium center in a k 2 -O, N manner.
To address the issue of whether 3 or 4 can react further we treated them with one more equivalent of 9-anthracene carboxaldehyde (for 3) or tBuNCO (for 4) in C 6 D 6 solvent. In case of 3, heating at 60 8C leads to the slow formation of the alkene Wittig-product ArC(H) = C(PPh 2 NSiMe 3 ) 2 . [14b] In case of 4, heating to 60 8C resulted in an intractable mixture and decomposition. These results indicate that the ThÀN amide and Th=C carbene bonds in 2 are more resistant towards chemical transformations than the Th À N ketimide , which is the opposite of what would be expected.