The reactivity of the cyclopentadienyl uranium(IV) and thorium(IV) amides. An inviting approach to some new organometallic actinide derivatives.

Abstract

Transition metal amides are known to show a very useful reactivity of the metal-nitrogen bonds towards molecules containing acidic hydrogens and dipolar double bonds[1]. This behaviour has been videly used in order to provide a quite easy synthetic approach to many transition metal complexes[2]. The usefulness of bis-cyclopentadienyl-uranium- bis-amides as reactive intermediates, has been firstly tested by Takats et al. [3] towards the carboxylic and thiocarboxylic acids [4] and some inserting molecules like CS 2, COS and CO 2 [5]. With the aim to extend the use of Actinide amides as reagents in the synthesis of new organometallic actinide derivatives, recently we reported the routes to Cp nM)NEt 2 4-n(M = U(IV);Th(IV);n ⩽ 4) [6]. In this paper we report the results concerning the reactivity of uranium mono-, bis-, and tris-cyclopentadienyl amides towards molecules containing acidic hydrogens, and the CO insertion reaction; the Schemes 1 and 2 summarize these results. One of the most interesting aspects resulting from the Scheme 1 is the possibility of obtaining stable Cp 2UX 2 products, without any redistribution of ligands, by using very bulky and/or polydentate X-ligands. This behaviour confirms that the steric hindrance is the limiting factor to the ligands redistribution and that a good stability of the Cp 2UX 2 systems can be obtained either by using polysubstituted cyclopentadienyls or bulky X-ligands. In the Table 1 the 1H NMR spectra of the new Cp 2UX 2 compounds are reported. The CO insertion into the UN bond represents the first example of such an insertion in systems like Cp 3U-X [12, 13], to date considered coordinatively saturated. ▪ This insertion reaction together with the reactivity of Cp 3UNEt 2 and Cp 3UPPh 2 towards MeCN are ▪ t001 1H NMR Spectra of the New Cp 2MX 2 Compounds: M =U(IV), Th(IV) (Benzene-d 6; ppm from Benzene-d 6as Internal Standard). Compounds ppm (T = 27 °C) Cp 2U(Ox) 2 +19.19(d,2H,C 7H,J 6,7 = 7.7Hz);+5.20(t,2H,C 6H, J 5,6 = 8.0;J 6,7 = 7.7 Hz);+1.73(d,2H,C 4H;J 3,4 = 7.5 Hz);+0.16(d,2H,C 5H,J 5,6 = 8.0Hz);−8.9(dd,2H,C 3H,J 2,3 = 4.6;J 3,4 = 7.5 Hz); −10.72(d, 2H, C 2H, J 2,3 = 4.6Hz)−23.01(s,10H,Cp). Cp 2U(Form) 2 +11.59(bs,4H,meta-phenyls);+4.73(bs,4H,meta'-phenyls); -2.08(bs,6H,CH′ 3);-3.86(s,10H,Cp);-10.65(bs,10H ortho′-phenyls + CH 3);-23.86(s,2H,CH);-34.86(bs,4H, ortho-phenyls). Cp 2(Triaz) 2 +10.70(d,4H,meta-phenyl, J = 7.9);+4.04(d,4H,Meta′-phenyls J = 7.8);-2.91(s,6H,CH′ 3);-3.33(s,10H,Cp); -10.39(s,6H,CH 3);-10.90(d,4H,ortho′-phenyll, J = 6.6); -38.92(d,4H,ortho-phenyl, J = 6.4 Hz). Cp 2U(OR) 2 +8.47(m,4H,CH 2CH)+6.41(sept.,2HCH(CH 3) 2); +5.74(s,6H,OCCH 3);+4.62(m,2H,CH 2CHCH 2; +0.85 (d,2H,transCH 2);-0.52(d,2H, cisCH 2,J = 10 Hz); - 2.03(d,12H,CH(CH 3),J = 6.6 Hz); -31.78(s,10H, cp) ▪ Cp 2Th(NEt 2) 2 -1.02(s,10H,Cp);-4.00(q,4H,α-CH 2);-6.17(t,6H,β-CH 3, J = 6.8 Hz) Cp 2Th(Triaz) 2 +0.08(q, 16H,AA′BB′,phen.);-0.84(s,10H,Cp);-4.99(s, 12H, CH 3 The negative sign is indicative of upfield shift, and the positive one downfield shift from benzene -d 6. ▪ further a confirmation that under particular conditions the Cp 3UX compounds bahave as coordinatively unsaturated systems [14]. The CpU(NEt 2) 3, firstly isolated and characterized by us [6], shows an unexpected behaviour towards weakly acidic molecules like HPPh 2. In fact by reacting CpU(NEt 2) 3 at room temperature with one equivalent of HPPh 2, Cp 2U(NEt 2) 2 as the major products is obtained together with a mixture of hardly separable products. Such a behaviour can be interpreted by making two hypotheses: a)CpU(NEt 2) 3 does not react with HPPh 2and completely rearranges to Cp 2U(NEt 2) 2and U(NEt 2) 4 during the reaction time. b)CpU(NEt 2) 3 reacts with HPPh 2 giving rise to an unstable intermediate (A) containing both -NEt 2 and -PPh 2 groups, which rearranges immediately to Cp 2U(NEt 2) 2. By considering the hypothesis (a) the reaction mixture should contain Cp 2U(NEt 2) 2 and U(NEt 2) 4 in the presence of HPPh 2. As we reported, Cp 2U(NEt 2) 2 reacts with HPPh 2, to produce Cp 3UPPh 2 and in addition we observed that U(NEt 2) 4 and HPPh 2 (in molar ratio 1:1) quantitatively afford U(NEt 2) 3(PPh 2) [9]; the lack of the Cp 3UPPh 2 and U(NEt 2) 3(PPh 2) in the reaction mixture suggests that the hypothesis (a) has to be excluded while the (b) one seems to be acceptable. On the other hand, the reaction, of U(NEt 2) 3(PPh 2) and cyclopentadiene in the molar ratio 1:1 produces the same reaction mixture as previously observed for the reactions between CpU(NEt 2) 3 and HPPh 2, making realistic the hypothesis (b) concerning the rearrangement of an unstable intermediate (A): ▪ Analogous studies on reactivity of the corresponding thorium amides are in progress.