A Comprehensive Study Concerning the Synthesis, Structure, and Reactivity of Terminal Uranium Oxido, Sulfido, and Selenido Metallocenes

Abstract

Terminal uranium oxido, sulfido, and selenido metallocenes were synthesized, and their reactivity was comprehensively studied. Heating of an equimolar mixture of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂UMe₂ (2) and [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(NH-p-tolyl)₂ (3) in the presence of 4-dimethylaminopyridine (dmap) in refluxing toluene forms [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=N(p-tolyl)(dmap) (4), which is a useful precursor for the preparation of the terminal uranium oxido, sulfido, and selenido metallocenes [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=E(dmap) (E = O (5), S (6), Se (7)) employing a cycloaddition–elimination methodology with Ph₂C=E (E = O, S) or (p-MeOPh)₂CSe, respectively. Metallocenes 5–7 are inert toward alkynes, but they act as nucleophiles in the presence of alkylsilyl halides. The oxido and sulfido metallocenes 5 and 6 undergo [2 + 2] cycloadditions with isothiocyanate PhNCS or CS₂, while the selenido derivative 7 does not. The experimental studies are complemented by density functional theory (DFT) computations.

Introduction

Attributed to their unique structural properties and their potential applications in group transfer and catalysis, organoactinide complexes featuring terminal metal–ligand multiple bonds have been intensively studied over the past two decades.¹ These research activities focused on oxido and chalcogenido organoactinide complexes in particular¹⁻⁵ due to the ubiquity of these functionalities in actinide chemistry, as shown by the prevalence of binary oxides and sulfides in the solid state.⁶ In this context, well-defined molecular structures will not only advance our understanding of the bonding and reactivity of the An=E (O, S, Se, Te) functional groups but also enable us to uncover novel and potentially useful transformations. For example, chlorocarbons convert on solid U₃O₈ to yield CO_x and HCl.⁷ This unusual reaction occurs at the U–O moieties on the U₃O₈ surface. Over the years, many oxido organouranium complexes have been prepared, which certainly enhanced the structural library, but their intrinsic reactivity was rather limited.^2,3 One notable exception, however, constitutes [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂U=O whose reactivity toward alkyl halides was studied in more detail.^3e Moving to the heavier chalcogenides S and Se, structurally authenticated terminal sulfido complexes have so far been limited to the derivatives, such as [Na(18-crown-6)][(η⁵-Me₅C₅)₂U^IV(=S)(SCMe₃)],^5a [Ph₃PMe][U^IV(=S)[N(SiMe₃)₂]₃],^5b [(η⁵-C₅Me₅)₂Co][U^VI(=S)(=O)[N(SiMe₃)₂]₃],^5d [K(18-crown-6)][U^IV(=S)[N(SiMe₃)₂]₃],^5e [(S=)U^IV(OSi(O^tBu)₃)₄K][K(2.2.2-crypt)],^5h [((^Ad,MeArO)₃tacn)U^IV(=S)][K(2.2.2-crypt)],⁵ⁱ [K(2.2.2-cryptand)][U^V(=S){OSi(O^tBu)₃}₄],^5k and (η⁵-C₅Me₅)₂U^VI(=S)( =N-2,6-ⁱPr₂C₆H₃) (Figure 1),^5l and only a few examples of the uranium complexes containing a terminal selenido functionality, such as [Ph₃PMe][U^IV(=Se)[N(SiMe₃)₂]₃],^5b [(η⁵-C₅Me₅)₂Co][U^VI(=Se)( =O)[N(SiMe₃)₂]₃],^5d [K(18-crown-6)][U^IV(=Se)(NR₂)₃],^5f and [K(2.2.2-crypt)][((^Ad,MeArO)₃tacn)U^IV(=Se)],^5m have been reported (Figure 1). Moreover, the reactivity of the terminal sulfido and selenido actinide complexes has so far remained elusive and this also extends to the underlying structure–reactivity relationship.⁵ This renders the synthesis of novel actinide oxido and chalcogenido metallocenes an interesting but still challenging synthetic target for which bulky ligands are indispensable.^3e Besides structural aspects and the reactivity of organoactinide complexes in small molecule activation,⁸ research at the bottom of the periodic table is also driven by the more fundamental question dealing with the impact of the 6d and 5f orbitals on structure, bonding, and reactivity.⁹ Following up on these research lines, we have investigated actinide complexes featuring terminal metal–ligand multiple bonds and explored their reactivity.¹⁰ For these studies, the sterically encumbered cyclopentadienyl ligand, 1,2,4-(Me₃C)₃C₅H₂, has been our ligand of choice stabilizing the terminal actinide oxido metallocenes [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂An^IV=O(dmap) (An = Th, U), which readily react with small molecules such as Me₃SiCl, Ph₂CE (E = O, S) and CS₂.^3e,10b Furthermore, only two examples of terminal actinide sulfido metallocenes and no examples of terminal actinide selenido metallocenes have been structurally authenticated,^5a,5l so we decided to extend these investigations to terminal uranium oxido and chalcogenido metallocenes featuring two 1,2,4-(Me₃Si)₃C₅H₂ ligands. Although 1,2,4-(Me₃Si)₃C₅H₂ is closely related to its counterpart 1,2,4-(Me₃C)₃C₅H₂, this ligand is less sterically demanding and more electron-deficient than 1,2,4-(Me₃C)₃C₅H₂. In this contribution, we detail the synthesis of the terminal uranium oxido, sulfido, and selenido complexes, [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U^IV=E(dmap) (E = O (5), S (6), Se (7)), and their reactivity. We also compare the reactivity of these derivatives to those of thorium(IV) and group 4 metallocenes.

Figure 1.

Selected crystallographically authenticated terminal uranium sulfido and selenido species.

Results and Discussion

Synthesis of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U^IV=E(dmap) (E = O (5), S (6), Se (7))

Salt metathesis between [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂UCl₂ (1) and 2 equiv of MeLi in diethyl ether affords [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂UMe₂ (2) in 82% yield. The subsequent reaction of 2 with 2 equiv of p-toluidine in toluene gives [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(NH-p-tolyl)₂ (3) in excellent yield (93%). Heating of an equimolar mixture of 2 and 3 in the presence of 4-dimethylaminopyridine (dmap) in refluxing toluene proceeds cleanly furnishing the uranium imido complex [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=N(p-tolyl)(dmap) (4) in 87% yield (Scheme 1). The molecular structure of 4 is shown in Figure 2; for selected bond distances and angles, refer to Table 1. Complex 4 features two very different U–N bond distances of 2.426(6) and 2.021(5) Å for U-N(2) and U-N(1), respectively, which are consistent with a dative bond between the dmap ligand and the U(IV) atom and the formation of a uranium–imido complex, respectively. A similar uranium–imido bond distance of 1.988(5) Å was found in the closely related derivative [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂U=N(p-tolyl).^3e In line with the reaction of the actinide imidos [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂An=N(p-tolyl) (An = Th, U) with Ph₂CO,^3e,10b treatment of 4 with 1 equiv of Ph₂CO results in the isolation of the terminal oxido derivative, [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=O(dmap) (5), in good yield (Scheme 1). DFT investigations imply that 4 initially engages in a [2 + 2] cycloaddition with Ph₂C=O to give the heterocyclic intermediate INT5. The formation of INT5 is energetically favorable (ΔG(298 K) = −11.6 kcal/mol) and proceeds via the transition state TS5a with a reaction barrier of ΔG^‡(298 K) = 13.3 kcal/mol (Figure 3). However, the degradation of INT5 to form the oxido compound 5 and Ph₂C=N(p-tolyl) is more thermodynamically preferred (ΔG(298 K) = −20.3 kcal/mol) and proceeds via the transition state TS5b with a reaction barrier of ΔG^‡(298 K) = 25.1 kcal/mol (Figure 3). This profile is consistent with the experimentally observed formation of 5 at ambient temperature. The molecular structure of 5 is provided in Figure 4; for selected bond distances and angles, see Table 1. The U–O and U–N distances are 1.873(2) and 2.526(3) Å, respectively, which are comparable to those established for [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂U=O(dmap) with a U–O distance of 1.860(3) Å and U–N distance of 2.535(4) Å^3e and [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂U=O(py) with a U–O distance of 1.874(4) Å and U–N distance of 2.589(5) Å.¹¹ Nevertheless, the Cp(cent)–U–Cp(cent) angle is 128.2(1)°, which is smaller than those found in [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂U=O(dmap) (141.7(4)°)^3e and [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂U=O(py) (139.2(2)°).¹¹ This suggests a much more open coordination sphere at the uranium atom in 5, which allows stabilizing ligands to coordinate, and therefore this difference reflects the reactivity of these complexes.^3e In contrast to the reaction of the thorium derivative [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂Th=N(p-tolyl) with Ph₂CS in the presence of dmap,^10b the terminal uranium sulfido metallocene, [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=S(dmap) (6), can be isolated from the reaction of 4 with 1 equiv of Ph₂CS in good yield (Scheme 1). Figure 5 shows the molecular structure of 6 and selected bond distances and angles are provided in Table 1. Complex 6 is the third representative of structurally authenticated actinide sulfido metallocenes and is therefore a significant addition to the other two actinide sulfido derivatives, [Na(18-crown-6)][(η⁵-Me₅C₅)₂U^IV(=S)(SCMe₃)] and (η⁵-C₅Me₅)₂U^VI(=S)(=N-2,6-ⁱPr₂C₆H₃).^5a,5l The U–N distance of 2.509(5) Å is comparable to those found for compounds 4 and 5 (Table 1). The U–S distance of 2.437(1) Å can be related to those found in [Na(18-crown-6)][(η⁵-Me₅C₅)₂U^IV(=S)(SCMe₃)] (2.477(2) and 2.462(2) Å),^5a [Ph₃PMe][U^IV(=S)[N(SiMe₃)₂]₃] (2.4805(5) Å),^5b [(C₅Me₅)₂Co][U^VI(=S)(=O)[N(SiMe₃)₂]₃] (2.390(8) Å),^5d [K(18-crown-6)][U^IV(=S)[N(SiMe₃)₂]₃] (2.4463(6) Å),^5e [(S=)U^IV(OSi(O^tBu)₃)₄K][K(2.2.2-crypt)] (2.5220(14) Å),^5h [((^Ad,MeArO)₃tacn)U^IV(=S)][K(2.2.2-crypt)] (2.536(2) Å),⁵ⁱ [K(2.2.2-cryptand)][U^V(=S){OSi(O^tBu)₃}₄] (2.376(5) Å),^5k and (η⁵-C₅Me₅)₂U^VI(=S)(=N-2,6-ⁱPr₂C₆H₃) (2.363(1) Å),^5l further supporting the formation of a uranium metallocene featuring a terminal U=S bond. Moreover, treatment of 4 with 1 equiv of (p-MeOPh)₂CSe yields the terminal selenido derivative, [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=Se(dmap) (7) (Scheme 1). Figure 6 shows the molecular structure of 7; for selected bond distances and angles, see Table 1. To the best of our knowledge, 7 constitutes the first structurally authenticated terminal selenido actinide metallocene, and it also expands the limited family of structurally characterized actinide selenido complexes, comprising [Ph₃PMe][U^IV(=Se)[N(SiMe₃)₂]₃],^5b [(C₅Me₅)₂Co][U^VI(=Se)( =O)[N(SiMe₃)₂]₃],^5d [K(18-crown-6)][U^IV(=Se)(NR₂)₃],^5f and [K(2.2.2-crypt)][((^Ad,MeArO)₃tacn)U^IV(=Se)].^5m The U–N distance of 2.507(4) Å is unremarkable and close to those found for compounds 4–6 (Table 1). Furthermore, the U–Se distance of 2.583(1) Å is in the range established for [Ph₃PMe][U^IV(=Se)[N(SiMe₃)₂]₃] (2.6463(7) Å),^5b [(C₅Me₅)₂Co][U^VI(=Se)( =O)[N(SiMe₃)₂]₃] (2.533(1) Å),^5d [K(18-crown-6)][U^IV(=Se)(NR₂)₃] (2.585(1) and 2.595(1) Å),^5f and [K(2.2.2-crypt)][((^Ad,MeArO)₃tacn)U^IV(=Se)] (2.695(2) Å).^5m

Scheme 1. Synthesis of Complexes 2–7.

Figure 2.

Molecular structure of 4 (thermal ellipsoids drawn at the 35% probability level).

Table 1. Selected Distances (Å) and Angles (°) for Compounds 4–10, 12–15, and 17^a.

compound	C(Cp)–Ub	C(Cp)–Uc	Cp(cent)–Ub	U–X	Cp(cent)–U–Cp(cent)	X–U–X/Y
4	2.809(6)	2.745(6) to 2.864(6)	2.533(6)	N(1) 2.021(5), N(2) 2.426(6)	130.8(2)	93.2(2)
5	2.836(3)	2.741(3) to 2.945(3)	2.564(3)	O(1) 1.873(2), N(1) 2.526(3)	128.2(1)	87.2(1)
6	2.788(7)	2.741(6) to 2.824(6)	2.511(6)	S(1) 2.437(1), N(1) 2.509(5)	134.0(2)	95.9(1)
7	2.784(4)	2.741(5) to 2.827(5)	2.505(5)	Se(1) 2.583(1), N(1) 2.507(4)	135.2(2)	96.5(1)
8	2.758(4)	2.714(4) to 2.803(4)	2.476(4)	O(1) 2.071(3), Cl(1) 2.820(1)	129.2(1)	91.8(1)
9	2.776(6)	2.727(6) to 2.823(5)	2.493(3)	O(1) 2.077(3), I(1) 2.963(1)	128.3(2)	93.3(1)
10	2.757(4)	2.698(3) to 2.807(4)	2.474(3)	O(1) 2.067(2), N(1) 2.525(4)	130.1(1)	92.3(1)
12	2.816(5)	2.745(5) to 2.905(5)	2.541(5)	O(1) 2.114(4), O(2) 2.109(3)	125.2(2)	91.7(1)
13	2.770(13)	2.737(13) to 2.803(12)	2.496(12)	O(1) 2.214(9), S(1) 2.748(3) N(2) 2.530(10)	127.9(3)	61.2(2)d
14	2.737(5)	2.683(4) to 2.783(5)	2.453(4)	S(1) 2.608(2), I(1) 2.944(1)	130.3(2)	100.2(1)
15	2.769(7)	2.722(7) to 2.833(7)	2.489(6)	S(1) 2.755(1), S(2) 2.724(1) N(2) 2.565(5)	131.0(2)	65.1(1)e
17	2.784(7)	2.684(7) to 2.892(7)	2.506(7)	U(1) Se(1) 2.763(1), Se(2) 2.758(1) U(2) Se(1) 2.761(1), Se(2) 2.758(1)	119.5(2)	79.7(1)

^aCp = cyclopentadienyl ring.

^bAverage value.

^cRange.

^dThe angle of O(1)-U(1)-S (1).

^eThe angle of S(1)-U(1)-S(2).

Figure 3.

Energy profile (kcal/mol) for the reaction of 4 + Ph₂CO (computed at T = 298 K). [U] = [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U.

Figure 4.

Molecular structure of 5 (thermal ellipsoids drawn at the 35% probability level).

Figure 5.

Molecular structure of 6 (thermal ellipsoids drawn at the 35% probability level).

Figure 6.

Molecular structure of 7 (thermal ellipsoids drawn at the 35% probability level).

Bonding Studies

To further probe the interaction between the uranium atom and the chalcogenides, oxygen, sulfur, or selenium, density functional theory (DFT) computations at the B3PW91 level of theory were undertaken. While the computed structures of 5–7 are in excellent agreement with the experimental data, computations reveal that the [E]^2– fragment is coordinated to the {[η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂(dmap)U}²⁺ moiety by one U–E σ-bond and two U–E π-bonds, as illustrated in Figure 7. The bonding in [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=E(dmap) was analyzed by a natural localized molecular orbital (NLMO) approach, and the results are summarized in Table 2. The U–E σ-bond comprises a chalcogenide hybrid orbital and a uranium hybrid orbital. Moving from E = O to E = Se, the uranium contribution to the σ-bond increases from 14.1% to 25.7%, while that of the chalcogenide decreases from 85.9% to 74.3%. More importantly, the 6d orbital contribution declines from 69.0% to 59.2%. Notably, the 5f orbital contribution reaches a maximum for E = S (25.8%) but decreases again when moving to E = Se (20.0%). The π bonds (π₁ and π₂) are composed of pure chalcogenide-based p orbitals and uranium hybrid orbitals. The U–E π bonds (π₁ and π₂) also experience an increased uranium contribution (16.5% to 27.2% for π₁ and 18.5% to 32.2% for π₂) when moving from O to Se, while the chalocogenide contribution (83.5% to 72.8% for π₁ and 81.5% to 67.8% for π₂) becomes smaller. This is consistent with the increased π interaction between {[η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂(dmap)U}²⁺ and [E]^2– fragments from O to Se. Differences, however, arise in the contributions of 6d and 5f orbitals to the π₁ and π₂ bonds. Whereas the 6d orbital contributions rises slightly (54.2% to 57.4%), the 5f orbital contribution (42.6% to 37.7%) declines for the π₁ bond. A different trend is, however, observed for the π₂ bond. Here, the 6d orbital contribution reaches a maximum (38.3%) at E = S, while the 5f orbital contribution is at its minimum (59.9%). Overall, the sum of the 6d and 5f contributions varies only slightly between 98.5% and 98.7%, regardless of the chalogenide. However, the electrostatic charge separation between the two fragments {[η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂(dmap)U}²⁺ and [E]^2– becomes progressively smaller along the series from O to Se, that is, 1.76 (for E = O (5)), 0.84 (for E = S (6)), and 0.72 (for E = Se (7)) (Table 2), implying that the electrostatic (ionic) interaction between the chalocogenide atom and the uranium atom is reduced from E = O to E = Se. This is correlated to an increase in the Wiberg U=E bond order from 1.80 (for 5) to 2.24 (for 6) to 2.25 (for 7) (Table 2). This also points to an increased orbital overlap and therefore more covalent bonding between the chalocogenide-based valence orbitals and the uranium atom transitioning from E = O to E = Se. These computations indicate a significant involvement of the uranium 5f orbitals in the bonding between the metallocene {[η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂(dmap)U}²⁺ and [E]^2– fragments, consistent with previous conclusions that the 5f orbitals play a distinct role in the bonding of actinide complexes and that the U=E bonds are generally more polarized than those in d-transition metals.^9c This difference carries on in the reactivity of the uranium complexes 5–7 when compared to their group 4 relatives.^12,13

Figure 7.

Plots of HOMOs for 5–7 (the hydrogen atoms have been omitted for clarity).

Table 2. Natural Localized Molecular Orbital (NLMO) Analysis of U=E Bonds,^a Bond Order, and the Natural Charges for the [Cp₂U(dmap)] and [E] Units.

		5 (O)	6 (S)	7 (Se)
σ U-E	%U	14.1	24.1	25.7
	%s	9.5	11.6	15.7
	%p	3.6	3.6	5.1
	%d	69.0	59.0	59.2
	%f	17.9	25.8	20.0
	%E	85.9	75.9	74.3
	%s	45.9	48.9	47.6
	%p	54.1	51.0	52.4
π1 U=E	%U	16.5	26.5	27.2
	%p	3.2	3.5	4.9
	%d	54.2	56.2	57.4
	%f	42.6	40.3	37.7
	%E	83.5	73.5	72.8
	%p	100	100	100
π2 U=E	%U	18.5	29.5	32.2
	%p	1.5	1.8	2.3
	%d	37.3	38.3	35.6
	%f	61.2	59.9	63.1
	%E	81.5	70.5	67.8
	%p	100	100	100
Wiberg bond order		1.80	2.24	2.25
(U=E)
NBO charge		0.88	0.42	0.36
(Cp2(dmap)U)
NBO charge (E)		–0.88	–0.42	–0.36

^aThe contributions by atom and orbital are averaged over all the ligands of the same character and over alpha and beta orbital contributions.

To further assess the U–E bonding, the quantum theory of atoms in molecules (QTAIM) that focuses on the topology of the electron density rather than the orbital structure was adopted to probe the extent of covalency of actinide-ligand bonds, utilizing diagnostic properties such as the electron density (ρ_b) and energy density (H_b) at the actinide-ligand bond critical points (BCPs) and the delocalization indices (DI) between two atoms.^5d,5j ρ_b and H_b data for the U–E at the BCPs and DI data for the U–E bonds in 5–7 are collected in Table 3: ρ_b values greater than 0.2 eÅ^–3 are typically associated with covalent bonds and smaller density values are indicative of closed-shell (ionic) interactions;^5d,5j the more stabilizing covalent interaction, the more negative energy density H_b,^5d,5j whereas the DIs integrate the electron density in the bonding region between two atoms and its value is closely related to the bond order. However, the computed DI values are always smaller than those expected from the Lewis structures and this difference is often attributed to bond polarity.^5j According to the QTAIM analysis, the U–E bonds in the complexes 5–7 are rather polar and exhibit an appreciable ionic character (Table 3). Moreover, the calculated QTAIM DIs indicate a significant electron density accumulation in the U–E bonding region of the complexes 5–7 (Table 3). In addition, the QTAIM analysis suggests that the overlap decreases down the group 16 from O to Se, whereas the energy matching increases (Table 3). Hence, contrary to the NLMO analyses (Table 2), the computed ρ_b, H_b, and DI values also indicate reduced covalency when moving down in group 16 from O to Se (Table 3), which is also observed for the uranium complexes [{(Me₃Si)₂N}₂U(O)(E)]⁻ (E = O, S, Se).^5j This is chemically counterintuitive, but as stated by Kaltsoyannis and coworkers, the QTAIM parameters evaluated at the BCPs have to be treated with caution,^5c,14 when a series with different bond distances are compared. The U–E distances (computed U–E distance of 1.847, 2.422, and 2.573 Å for 5, 6, and 7, respectively) vary significantly and therefore caution should be exercised in concluding that the U–E covalence for complexes 5–7 decreases as group 16 is descended.

Table 3. Electron Densities (ρ_b) and Energy Densities (H_b) at Selected Bond Critical Points of 5–7 (H in Bold, Both in Atomic Units) and Delocalization Indices (DI) for U=E Bonds.

	5 (O)	6 (S)	7 (Se)
electron densities (ρ)	0.122	0.089	0.018
energy densities (H)	–0.118	–0.019	–0.010
delocalization indices (DI)	2.470	2.312	2.286

Reactivity of Oxido Complex [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂UO(dmap) (5)

The more polarized nature of the moiety U=O may also impact its reactivity. Nevertheless, in contrast to [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂U=O(dmap),^3e the dmap ligand in [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=O(dmap) (5) cannot be removed by Ph₃B addition to yield a base free oxido complex [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂UO, presumably due to the more electron-deficient and less sterically demanding ligand 1,2,4-(Me₃Si)₃C₅H₂, which renders the uranium atom more electrophilic and renders a more open coordination sphere at the uranium atom, therefore binding the dmap more strongly. Nevertheless, in analogy to the oxido compounds [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂An=O(dmap) (An = Th, U),^3e,10b [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=O(dmap) (5) forms with Me₃SiX the metallocenes, [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(OSiMe₃)(X) (X = Cl (8), I (9), NC (10), and N₃ (11)), concomitant with dmap loss (Scheme 2). DFT studies suggest that the reaction of 5 with Me₃SiCl may proceed via two different ways, i.e., an S_N2 (TS8a) or an addition (TS8b) mechanism (Figure 8A). The optimized bond distances of O–Si and Si–Cl in TS8a are 2.479 and 2.245 Å (see the Supporting Information for details), respectively, indicating that the O–Si bond is formed, while the Si–Cl bond is broken simultaneously, and the resulting Cl^– anion migrates to the U atom to establish a U–Cl bond when it leaves the Si atom. The Si atom in Me₃SiCl can readily approach the O atom, and the energy barrier for the S_N2 process is only ΔG^‡(298 K) = 18.3 kcal/mol (Figure 8A), implying that the reaction readily proceeds at ambient temperature. In contrast, the addition reaction occurs via the concerted transition state TS8b, in which the two forming bond distances of U–Cl and Si–O are 4.254 and 2.263 Å, respectively (see the Supporting Information for details). In combination with the Si–Cl distance of 2.167 Å, these metric parameters imply that the O–Si and U–Cl bond formations and Si–Cl bond breakage occur simultaneously, in which the formation of the Si–O bond drives the Si–Cl bond breakage. Nevertheless, the activation barrier of ΔG^‡(298 K) = 28.0 kcal/mol for the addition reaction of 5 with Me₃SiCl (Figure 8A) is energetically less favorable than that of the S_N2 reaction. However, the formation of INT8 is thermodynamically energetically favorable (ΔG(298 K) = −22.4 kcal/mol), but driven by dmap loss, INT8 immediately converts to the even more thermodynamically stable product 8 (ΔG(298 K) = −36.8 kcal/mol) (Figure 8A). Moreover, it is reasonable to assume that the U–Cl and Si–S bond formation energies in the reaction of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂US(dmap) (6) with Me₃SiCl favor the formation of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂UCl₂ and (Me₃Si)₂S (Scheme 3), whereas the reaction of 5 with Me₃SiCl stops with the formation of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(OSiMe₃)Cl (8) attributed to the strong U–O bond (Scheme 2). We also considered that dmap dissociation initiates the formation of 8via either an S_N2 (TS8c) or an addition (TS8d) reaction with Me₃SiCl. However, the activation barriers for these processes of ΔG^‡(298 K) = 29.0 and = 36.7 kcal/mol (Figure 8B), respectively, are significantly larger than that for the S_N2 reaction illustrated in Figure 8A, whose computed reaction profile is in agreement with 8 being formed at ambient temperature. The molecular structures of 8 and 10 are presented in Figures 9 and 10, respectively, and ORTEP of 9 is shown in the Supporting Information. In complex 8, the U–O distance of 2.071(3) Å is close to that in 9 (2.077(3) Å). The U–Cl distance in 8 is 2.820(1) Å, whereas the U–I distance amounts to 2.963(1) Å in 9. In contrast to uranium cyanide complex [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂U(OSiMe₃)(CN)^3e but analogous to uranium isocyanide complex [(Me₃Si)₂N]₃U(NC) and the thorium isocyanide complex [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂Th(OSiMe₃)(NC),^10b,15 the CN^– ligand in 10 coordinates to the U⁴⁺ ion by its nitrogen atom instead of the carbon atom, presumably due to the electron-deficient nature of 1,2,4-(Me₃Si)₃C₅H₂, which increases the Lewis acidity of the metal ion. DFT computations also predict the isocyanide isomer to be energetically more favorable than the cyanide one [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(OSiMe₃)(CN) (ΔG(298 K) = −0.8 kcal/mol) (see the Supporting Information for details), which is in line with the experiment. The U–O distance of 2.067(2) Å is shorter than those in 8 and 9 (Table 1), whereas the U–N distance is 2.525(4) Å. Nevertheless, complexes 8–10 are unstable toward ligand redistribution at high temperature in toluene solution. For example, when heated at 70 °C, 8 yields 1 and [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(OSiMe₃)₂ (12) (Scheme 2). The molecular structure of 12 is provided in Figure 11; for selected bond distances and angles, consult Table 1. The U–O distances are 2.114(4) and 2.109(3) Å, and the angle of O–U–O is 91.7(1)°. Moreover, in analogy to the thorium oxido [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂Th=O(dmap),^10b [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=O(dmap) (5) is reactive toward unsaturated organic substrates. For example, treatment of 5 with PhNCS in toluene rapidly forms the four-membered metallocene [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U[OC(=NPh)S)(dmap) (13) (Scheme 2). DFT investigations imply that the formation of 13 from 5 + PhNCS is exergonic with ΔG(298 K) = −22.2 kcal/mol and proceeds via a transition state TS13 with a low reaction barrier of ΔG^‡(298 K) = +22.7 kcal/mol (Figure 12). Moreover, DFT computations also predict that the degradation of 13 to 6 + PhNCO is energetically unfavorable (ΔG(298 K) = 6.4 kcal/mol) and proceeds via a transition state TS6 with a high reaction barrier of ΔG^‡(298 K) = +29.9 kcal/mol (Figure 12). These results are consistent with the experiment in which complex 13 is formed at ambient temperature. The molecular structure of 13 is shown in Figure 13; for selected bond distances and angles, consult Table 1. The U–N distance of 2.530(10) Å is in line with those found in 4–7 (Table 1). The U–O distance is 2.214(9) Å, whereas the U–S distance of 2.748(3) Å is longer than that found in 6 (2.437(1) Å). However, like for the actinide oxidos [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂An=O(dmap) (An = Th, U)^3e,10b but in contrast to the group 4 derivatives (η⁵-C₅Me₅)₂Ti=O(py) and (η⁵-C₅Me₅)₂Zr=O,^12b,12c,13 no reaction occurs between complex 5 and internal alkynes RC≡CR (R = Ph, Me, p-tolyl) even when heated to 100 °C for 1 week, presumably attributed to the more polarized nature of the actinide oxido bond An⁺-O^–.¹⁶

Scheme 2. Synthesis of Complexes 8–13.

Figure 8.

Energy profile (kcal/mol) for the reaction of 5 + Me₃SiCl (computed at T = 298 K). [U]=[η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U.

Scheme 3. Synthesis of Complexes 13–16.

Figure 9.

Molecular structure of 8 (thermal ellipsoids drawn at the 35% probability level).

Figure 10.

Molecular structure of 10 (thermal ellipsoids drawn at the 35% probability level).

Figure 11.

Molecular structure of 12 (thermal ellipsoids drawn at the 35% probability level).

Figure 12.

Energy profile (kcal/mol) for the reaction of 5 + PhNCS (computed at T = 298 K). [U]=[η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U.

Figure 13.

Molecular structure of 13 (thermal ellipsoids drawn at the 35% probability level).

Reactivity of Sulfido Complex [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=S(dmap) (6)

In contrast to the uranium sulfido complex [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂U=S that readily reacts with one equivalent of Ph₂CS to form [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂U(S₂CPh₂) even in the presence of a Lewis base such as dmap,¹¹ the dmap-stabilized terminal uranium sulfido [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=S(dmap) (6) is formed from the reaction of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=N(p-tolyl)(dmap) (4) and Ph₂CS at room temperature (Scheme 1). We attribute this difference to a combination of steric and electronic effects. The 1,2,4-(Me₃Si)₃C₅H₂ is less bulky and therefore accommodates dmap coordination, but it is also more electron-deficient and hence increases the Lewis acidity of the uranium ion. In analogy to its oxido counterparts [η⁵-1,2,4-(Me₃E)₃C₅H₂]₂U=O(dmap) (E = C,^3e Si (5)), at room temperature, complex 6 reacts immediately upon mixing with Me₃SiI to release the dmap ligand and to give the addition product [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(SSiMe₃)(I) (14) (Scheme 3). The molecular structure of 14 is depicted in Figure 14; for selected bond distances and angles, see Table 1. The U–I distance of 2.944(1) Å is comparable to that found in 9 (2.963(1) Å), whereas the U–S distance of 2.608(2) Å is longer than that found in 6 (2.437(1) Å). Nevertheless, when Me₃SiCl is used as a substrate, dichlorido complex [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂UCl₂ (1) is formed along with dmap and (Me₃Si)₂S loss (Scheme 3). Most likely, this reaction proceeds via a monochlorido intermediate [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(SSiMe₃)(Cl) but it readily converts in the presence of Me₃SiCl via (Me₃Si)₂S loss to the product 1 (Scheme 3). This contrasts to the reactivity of 5 with Me₃SiCl yielding as a stable product [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(OSiMe₃)(Cl) (8) (Scheme 2). Moreover, like its oxido counterpart [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=O(dmap) (5), 6 reacts with unsaturated organic substrates. For example, treatment of 6 with PhNCO or PhNCS in toluene forms the four-membered metallocenes, [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U[EC(=NPh)S)(dmap) (E = O (13), S (15)) (Scheme 3). The formation of 13 from 6 + PhNCO is consistent with the reaction energy profile in Figure 12. The molecular structure of 15 is provided in Figure 15; for selected bond distances and angles, refer to Table 1. The U–N distance is 2.565(5) Å, whereas the U–S distances are 2.755(1) and 2.724(1) Å. Moreover, complex 6 transforms in the presence of CS₂ to a uranium trithiocarbonato intermediate [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(CS₃)(dmap), which readily dimerizes to the trithiocarbonato-bridged complex {[η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U}₂(μ-CS₃)₂ (16)¹⁷ concomitant with dmap release (Scheme 3). However, in analogy to the thorium sulfido [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂Th=S^10b but in contrast to the group 4 derivatives (η⁵-C₅Me₅)₂TiS(py)^12f and (η⁵-C₅Me₅)₂ZrS,^12b,13 no [2 + 2] cycloaddition products [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U[SC(R)=C(R)] are obtained between complex 6 and internal alkynes RC≡CR (R = Ph, Me, p-tolyl) even when the mixture is heated at 100 °C for 1 week, again, presumably due to the more polarized nature of the actinide sulfido bond An⁺–S^–.¹⁶

Figure 14.

Molecular structure of 14 (thermal ellipsoids drawn at the 35% probability level).

Figure 15.

Molecular structure of 15 (thermal ellipsoids drawn at the 35% probability level).

Reactivity of Selenido Complex [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=Se(dmap) (7)

In contrast to the oxido and sulfido derivatives, [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=O(dmap) (5) and [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=S(dmap) (6), the isolated selenido adduct [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=Se(dmap) (7) degrades in toluene solution at 60 °C to the dimer {[η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U}₂(μ-Se)₂ (17) (Scheme 4), presumably due to the larger ionic radius of the Se^2– anion. The molecular structure of 17 is provided in Figure 16; for selected bond distances and angles, see Table 1. The U–Se distances within the U₂Se₂-core are essentially identical and vary between 2.758(1) and 2.763(1) Å. This also extends to the Se–U(1)–Se angles and the Se–U(2)–Se angles; both are 79.7(1)°. Like the oxido [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=O(dmap) (5) and sulfido [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=S(dmap) (6), complex 7 immediately reacts upon mixing with Me₃SiI at room temperature to give the addition product [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(SeSiMe₃)(I) (18) concomitant with dmap loss (Scheme 4). Moreover, analogous to the sulfido derivative [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=S(dmap) (6), when Me₃SiCl is used as a substrate, the dichlorido complex [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂UCl₂ (1) is formed along with dmap and (Me₃Si)₂Se (Scheme 4). Again, this reaction proceeds via a monochlorido intermediate [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(SeSiMe₃)(Cl), which readily converts in the presence of Me₃SiCl releasing (Me₃Si)₂Se to give complex 1 (Scheme 4). However, unlike the oxido [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=O(dmap) (5) and sulfido [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=S(dmap) (6), complex 7 shows no reaction with PhNCS, presumably attributed to the large size of the Se^2– anion, which hinders the approach to the uranium atom. Moreover, like the oxido [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=O(dmap) (5) and sulfido [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=S(dmap) (6), also in the case of 7, no reaction is observed in the presence of internal alkynes RC≡CR (R = Ph, Me, p-tolyl).^.

Scheme 4. Synthesis of Complexes 17 and 18.

Figure 16.

Molecular structure of 17 (thermal ellipsoids drawn at the 35% probability level).

Conclusions

The uranium oxido metallocenes [η⁵-1,2,4-(Me₃E)₃C₅H₂]₂U=O(dmap) (E = C,^3e Si (5)) and the thorium derivative [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂Th=O(dmap)^10b react as nucleophiles toward alkylsilyl halides mimicking the reactivity of (η⁵-C₅Me₅)₂Zr=O(py),^12e but they do not undergo cycloaddition reactions with alkynes in contrast to (η⁵-C₅Me₅)₂Ti=O(py)^12c and (η⁵-C₅Me₅)₂Zr=O.^12b Moreover, both the uranium sulfido [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=S(dmap) (6) and its thorium relative [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂Th=S are inert toward internal alkynes in contrast to (η⁵-C₅Me₅)₂Ti=S(py)^12f and (η⁵-C₅Me₅)₂Zr=S, which participate in cycloaddition reactions.^12b We propose that these differences arise from the more polarized nature of the actinide oxido and sulfido bonds An⁺–E^– (E = O, S).^9c

Moreover, uranium sulfido and selenido metallocenes exhibit distinctly different reactivity patterns, e.g., whereas the sulfido derivative [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=S(dmap) (6) is stable in toluene solution, its selenido counterpart [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=Se(dmap) (7) degrades to the dimer {[η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U}₂(μ-Se)₂ (17). Furthermore, while 6 undergoes a cycloaddition with PhNCS, 7 remains inert toward PhNCS. This shows that the reactivity of actinide metallocenes carrying a terminal An=E (E = heteroatom) functional group is influenced by the size of the heteroatom as well as the polarization of the An=X bond.

Furthermore, this study also illustrates that small variations in the supporting cyclopentadienyl ligand modulate the reactivity of these uranium metallocenes. For example, the dmap ligand in complex [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂U=O(dmap) can be readily removed by Ph₃B,^3e whereas this is not possible for [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=O(dmap) (5). Moreover, in contrast to the uranium cyanide complex [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂U(OSiMe₃)(CN),^3e the uranium isocyanide complex [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(OSiMe₃)(NC) (10) is energetically more favorable than its cyanide isomer [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(OSiMe₃)(CN). Furthermore, while [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=S(dmap) (6) is stable, the complex [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂U=S(dmap) is not.¹¹ Electronic effects of the cyclopentadienyl ligands might also influence the reactivity of these complexes, whereas steric effects seem to prevail in all of these systems, dictating the reaction products being formed. Since the ligand 1,2,4-(Me₃Si)₃C₅H₂ is less sterically demanding than 1,2,4-(Me₃C)₃C₅H₂, this renders the open wedge at the uranium atom more open, inducing smaller Cp(cent)-U-Cp(cent) angles, which allows additional stabilizing ligands to coordinate, which enables the isolation of reaction products and intermediates inaccessible to uranium fragments bearing the more sterically encumbered 1,2,4-(Me₃C)₃C₅H₂ derivative.

In conclusion, the imido uranium metallocene [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=N(p-tolyl)(dmap) (4) is a useful precursor for the synthesis of the terminal oxido, sulfido, and selenido uranium metallocenes, which enabled us to systematically probe the intrinsic reactivity of U=E (E = O, S, Se) moieties. This allowed us to map the space of chemical transformations accessible to organoactinide oxido and chalcogenido complexes, which may also extend to solid-state actinide metal oxides and chalcogenides. Further investigations on the intrinsic reactivity of terminal chalcogenido actinide metallocenes and the uranium imido metallocene 4 are ongoing and will be communicated in due course.

Experimental Section

General Procedures

All reactions and product manipulations were carried out under an atmosphere of dry dinitrogen with rigid exclusion of air and moisture using standard Schlenk or cannula techniques, or in a glove box. All organic solvents were freshly distilled from sodium benzophenone ketyl immediately prior to use. [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂UCl₂ (1)¹⁷ and (p-MeOPh)₂CSe¹⁸ were prepared according to previously reported procedures. All other chemicals were purchased from Aldrich Chemical Co. and Beijing Chemical Co. and used as received unless otherwise noted. Infrared spectra were recorded in KBr pellets on an Avatar 360 Fourier transform spectrometer. ¹H and ¹³C{¹H} NMR spectra were recorded on a Bruker AV 400 spectrometer at 400 and 100 MHz, respectively. ²⁹Si{¹H} NMR spectra were recorded on a JEOL 600 spectrometer at 119.2 MHz. All chemical shifts are reported in δ units with reference to the residual protons of the deuterated solvents, which served as internal standards, for proton and carbon chemical shifts, and to external Me₄Si (0.00 ppm) for silicon chemical shifts. Melting points were obtained on X-6 melting point apparatus and were uncorrected. Elemental analyses were performed on a Vario EL elemental analyzer.

Preparation of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂UMe₂ (2)

A diethyl ether (30.7 mL) solution of MeLi (0.15 M in diethyl ether; 4.6 mmol) was slowly added to a diethyl ether (25 mL) solution of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂UCl₂ (1; 2.00 g, 2.3 mmol) with stirring at room temperature. After the solution was stirred for 1 h at room temperature, the solvent was removed. The residue was extracted with n-hexane (15 mL × 3) and filtered. The volume of the filtrate was reduced to 10 mL, and orange microcrystals of 2 formed when this solution was kept at −20 °C for 2 days. Microcrystals of 2 were isolated by filtration, quickly washed with cooled n-hexane (5 mL), and dried at 50 °C under vacuum overnight. Yield: 1.57 g (82%). M.p.: 150–152 °C (dec.). ¹H NMR (C₆D₆): δ −0.07 (36H, Si(CH₃)₃), −2.32 (18H, Si(CH₃)₃), −52.98 (6H, CH₃) ppm; protons of the ring CH were not observed. ¹³C{¹H} NMR (C₆D₆): δ 273.9 (UCH₃), 246.3 (ring C), 193.0 (ring C), 3.1 (Si(CH₃)₃), −1.9 (Si(CH₃)₃) ppm. ²⁹Si{¹H} NMR (C₆D₆): δ −39.7, −52.8 ppm. IR (KBr, cm^–1): ν 2958 (s), 1253 (s), 1095 (s), 835 (s). Anal. calcd for C₃₀H₆₄Si₆U: C, 43.34; H, 7.76. Found: C, 43.36; H, 7.74.

Preparation of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(NH-p-tolyl)₂ (3)

A toluene (10 mL) solution of p-toluidine (0.26 g, 2.4 mmol) was added to a toluene (10 mL) solution of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂UMe₂ (2; 1.00 g, 1.2 mmol) with stirring at room temperature. After the solution was stirred at 60 °C overnight, the solvent was removed. The residue was extracted with n-hexane (10 mL × 3) and filtered. The volume of the filtrate was reduced to 10 mL, and brown microcrystals of 3 formed when this solution was kept at −20 °C for 1 day. Microcrystals of 3 were isolated by filtration, quickly washed with cooled n-hexane (5 mL), and dried at 50 °C under vacuum overnight. Yield: 1.13 g (93%). M.p.: 170–172 °C (dec.). ¹H NMR (C₆D₆): δ 5.58 (d, J = 5.6 Hz, 4H, phenyl), 5.20 (s, 6H, CH₃), 3.30 (s, 18H, Si(CH₃)₃), −1.12 (s, 36H, Si(CH₃)₃), −12.48 (s, 4H, phenyl) ppm; protons of Cp-ring CH and NH were not observed. ¹³C{¹H} NMR (C₆D₆): δ 269.8 (phenyl C), 226.3 (phenyl C), 185.8 (ring C), 184.4 (ring C), 148.0 (phenyl C), 105.3 (phenyl C), 9.2 (CH₃), 5.3 (Si(CH₃)₃), −0.0 (Si(CH₃)₃) ppm. ²⁹Si{¹H} NMR (C₆D₆): δ −62.4, −64.0 ppm. IR (KBr, cm^–1): ν 2960 (s), 1514 (s), 1383 (s), 1248 (s), 1095 (m), 831 (s). Anal. calcd for C₄₂H₇₄N₂Si₆U: C, 49.77; H, 7.36; N, 2.76. Found: C, 49.75; H, 7.38; N, 2.78.

Preparation of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=N(p-tolyl)(dmap) (4)

A toluene (10 mL) solution of 4-dimethylaminopyridine (dmap; 0.25 g, 2.05 mmol) was added to a toluene (10 mL) solution of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂UMe₂ (2; 0.82 g, 0.99 mmol) and [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(NH-p-tolyl)₂ (3; 1.0 g, 0.99 mmol) with stirring at room temperature. After the solution was stirred at 60 °C overnight, the solution was filtered. The volume of the filtrate was reduced to 5 mL, and brown crystals of 4 formed when this solution was kept at −20 °C for 2 days. Crystals of 4 were isolated by filtration, quickly washed with cooled n-hexane (5 mL), and dried at 50 °C under vacuum overnight. Yield: 1.77 g (87%). M.p.: 202–204 °C (dec.). ¹H NMR (C₆D₆): δ 53.98 (s, 2H, phenyl), 41.91 (s, 2H, phenyl), 30.98 (t, J = 12 Hz, 3H, CH₃), 25.68 (s, 2H, ring CH), 5.61 (s, 18H, Si(CH₃)₃), −3.26 (s, 18H, Si(CH₃)₃), −8.45 (s, 6H, N(CH₃)₂), −9.18 (s, 18H, Si(CH₃)₃), −9.26 (s, 4H, py), −32.81 (s, 2H, ring CH) ppm. ¹³C{¹H} NMR (C₆D₆): δ 196.1 (py C), 151.3 (py C), 150.6 (py C), 149.8 (phenyl C), 142.6 (phenyl C), 141.2 (phenyl C), 117.0 (phenyl C), 116.4 (ring C), 112.4 (ring C), 111.0 (ring C), 102.6 (ring C), 91.1 (ring C), 31.4 (N(CH₃)₂), 31.2 (N(CH₃)₂), 5.3 (CH₃), −5.8 (Si(CH₃)₃), −17.2 (Si(CH₃)₃), −60.4 (Si(CH₃)₃) ppm. ²⁹Si{¹H} NMR (C₆D₆): δ −93.9, −97.4, −115.4 ppm. IR (KBr, cm^–1): ν 2958 (s), 1616 (s), 1384 (s), 1249 (s), 1095 (m), 1006 (s), 829 (s). Anal. calcd for C₄₂H₇₅N₃Si₆U: C, 49.04; H, 7.35; N, 4.09. Found: C, 49.03; H, 7.36; N, 4.06.

Preparation of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=O(dmap)·C₆H₆ (5·C₆H₆)

Method A

A toluene (10 mL) solution of Ph₂CO (91 mg, 0.50 mmol) was added to a toluene (10 mL) solution of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=N(p-tolyl)(dmap) (4; 514 mg, 0.50 mmol) with stirring at room temperature. After this solution was stirred at room temperature overnight, the solvent was removed. The residue was extracted with benzene (10 mL × 3) and filtered. The volume of the filtrate was reduced to 5 mL, and orange crystals of 5·C₆H₆ formed when this solution was kept at 10 °C for 2 days. Crystals of 5·C₆H₆ were isolated by filtration, quickly washed with cooled n-hexane (5 mL), and dried at 50 °C under vacuum overnight. Yield: 432 mg (85%). M.p.: 164–166 °C (dec.). ¹H NMR (C₆D₆): δ 20.19 (s, 18H, Si(CH₃)₃), 7.15 (s, 6H, C₆H₆), −2.26 (s, 18H, Si(CH₃)₃), −7.49 (s, 6H, N(CH₃)₂), −9.38 (s, 18H, Si(CH₃)₃) ppm; other protons were not observed. ¹³C{¹H} NMR (C₆D₆): δ 200.5 (py C), 130.7 (py C), 129.6 (py C), 129.3 (ring C), 128.5 (C₆H₆), 121.3 (ring C), 119.2 (ring C), 96.2 (ring C), 55.0 (ring C), 32.2 (N(CH₃)₂), 22.2 (Si(CH₃)₃), −5.3 (Si(CH₃)₃), −22. 1 (Si(CH₃)₃) ppm. ²⁹Si{¹H} NMR (C₆D₆): δ −35.8, −122.4, −144.9 ppm. IR (KBr, cm^–1): ν 2953 (s), 1616 (s), 1533 (s), 1444 (s), 1384 (s), 1244 (s), 1095 (s), 1006 (s), 937 (s), 833 (s). Anal. calcd for C₄₁H₇₄N₂OSi₆U: C, 48.39; H, 7.33; N, 2.75. Found: C, 48.42; H, 7.36; N, 2.76.

Method B: NMR Scale

A C₆D₆ (0.3 mL) solution of Ph₂CO (3.6 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=N(p-tolyl)(dmap) (4; 20.6 mg, 0.02 mmol) and C₆D₆ (0.2 mL). Resonances of 5 and those of Ph₂C=N(p-tolyl) (¹H NMR (C₆D₆): δ 7.97 (m, 2H, aryl), 7.12 (m, 3H, aryl), 6.98 (m, 2H, aryl), 6.89 (m, 3H, aryl), 6.77 (m, 4H, aryl), 1.97 (s, 3H, CH₃) ppm)^10b were observed by ¹H NMR spectroscopy (100% conversion) when this solution was kept at room temperature overnight.

Preparation of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=S(dmap)·0.5C₆H₆ (6·0.5C₆H₆)

Method A

This compound was prepared as orange crystals from the reaction of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=N(p-tolyl)(dmap) (4; 514 mg, 0.50 mmol) and Ph₂CS (99 mg, 0.50 mmol) in toluene (15 mL) at room temperature and recrystallization from a benzene solution by a similar procedure to that in the synthesis of 5. The product was isolated by filtration, quickly washed with cooled n-hexane (5 mL), and dried at 50 °C under vacuum overnight. Yield: 417 mg (84%). M.p.: 175–177 °C (dec.). ¹H NMR (C₆D₆): δ 9.30 (s, 18H, Si(CH₃)₃), 7.15 (s, 3H, C₆H₆), −0.83 (s, 18H, Si(CH₃)₃), −8.45 (s, 6H, N(CH₃)₂), −9.13 (s, 18H, Si(CH₃)₃) ppm; other protons were not observed. ¹³C{¹H} NMR (C₆D₆): δ 196.9 (py C), 185.2 (py C), 159.8 (py C), 130.7 (ring C), 129.7 (ring C), 129.6 (ring C), 129.4 (ring C), 128.5 (C₆H₆), 121.4 (ring C), 31.9 (N(CH₃)₂), 5.9 (Si(CH₃)₃), −0.7 (Si(CH₃)₃), −8.8 (Si(CH₃)₃) ppm. ²⁹Si{¹H} NMR (C₆D₆): δ −61.6, −81.6, −111.5 ppm. IR (KBr, cm^–1): ν 2957 (s), 1614 (s), 1535 (s), 1444 (s), 1384 (s), 1244 (s), 1095 (s), 1006 (s), 833 (s). Anal. calcd for C₃₈H₇₁N₂SSi₆U: C, 45.89; H, 7.20; N, 2.82. Found: C, 45.92; H, 7.18; N, 2.84.

Method B: NMR Scale

A C₆D₆ (0.3 mL) solution of Ph₂CS (4.0 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=N(p-tolyl)(dmap) (4; 20.6 mg, 0.02 mmol) and C₆D₆ (0.2 mL). Resonances of 6 and those of Ph₂C=N(p-tolyl) were observed by ¹H NMR spectroscopy (100% conversion) when this solution was kept at room temperature overnight.

Preparation of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=Se(dmap)·0.5C₆H₆ (7·0.5C₆H₆)

Method A

This compound was prepared as brown crystals from the reaction of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=N(p-tolyl)(dmap) (4; 514 mg, 0.50 mmol) and (p-MeOPh)₂CSe (153 mg, 0.50 mmol) in toluene (15 mL) at room temperature and recrystallization from a benzene solution by a similar procedure to that in the synthesis of 5. The product was isolated by filtration, quickly washed with cooled n-hexane (5 mL), and dried at room temperature under vacuum overnight. Yield: 453 mg (87%). M.p.: 153–155 °C (dec.). ¹H NMR (C₆D₆): δ 7.15 (s, 3H, C₆H₆), 5.18 (s, 18H, Si(CH₃)₃), 0.92 (s, 18H, Si(CH₃)₃), −7.62 (s, 18H, Si(CH₃)₃), −8.71 (s, 6H, N(CH₃)₂) ppm; other protons were not observed. ¹³C{¹H} NMR (C₆D₆): δ 158.6 (py C), 137.4 (py C), 133.5 (py C), 133.0 (ring C), 132.9 (ring C), 132.0 (ring C), 129.3 (ring C), 128.5 (C₆H₆), 121.7 (ring C), 31.9 (N(CH₃)₂), −0.5 (Si(CH₃)₃), −0.7 (Si(CH₃)₃), −2.2 (Si(CH₃)₃) ppm. ²⁹Si{¹H} NMR (C₆D₆): δ −74.2, −75.6, −103.9 ppm. IR (KBr, cm^–1): ν 2955 (m), 2835 (m), 1604 (s), 1510 (s), 1249 (s), 1170 (s), 1031 (s), 837 (s). Anal. calcd for C₃₈H₇₁N₂SeSi₆U: C, 43.82; H, 6.87; N, 2.69. Found: C, 43.79; H, 6.88; N, 2.73.

Method B: NMR Scale

A C₆D₆ (0.3 mL) solution of (p-MeOPh)₂CSe (6.1 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=N(p-tolyl)(dmap) (4; 20.6 mg, 0.02 mmol) and C₆D₆ (0.2 mL). Resonances of 7 and those of (p-MeOPh)₂C=N(p-tolyl) (¹H NMR (C₆D₆): δ 7.99 (s, 2H, aryl), 7.20 (s, 2H, aryl), 7.02 (s, 2H, aryl), 6.84 (s, 2H, aryl), 6.78 (s, 2H, aryl), 6.63 (s, 2H, aryl), 3.21 (s, 3H, OCH₃), 3.16 (s, 3H, OCH₃), 2.01 (s, 3H, CH₃) ppm)¹⁹ were observed by ¹H NMR spectroscopy (100% conversion) when this solution was kept at room temperature overnight.

Preparation of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(OSiMe₃)(Cl) (8)

Method A

A toluene (10 mL) solution of Me₃SiCl (28 mg, 0.25 mmol) was added to a toluene (10 mL) solution of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=O(dmap) (5; 235 mg, 0.25 mmol) with stirring at room temperature. After the solution was stirred at room temperature for 1 h, the solvent was removed. The residue was extracted with n-hexane (10 mL × 3) and filtered. The volume of the filtrate was reduced to 3 mL, and orange crystals of 8 formed when this solution was kept at −20 °C for 2 days. Crystals of 8 were isolated by decantation of the supernatant, rapidly washed with cooled n-hexane (2 mL), and dried at 50 °C under vacuum overnight. Yield: 199 mg (86%). M.p.: 175–177 °C (dec.). ¹H NMR (C₆D₆): δ 31.85 (s, 9H, Si(CH₃)₃), 8.03 (s, 18H, Si(CH₃)₃), −7.63 (s, 18H, Si(CH₃)₃), −9.78 (s, 18H, Si(CH₃)₃) ppm; the protons of Cp-ring CH were not observed. ¹³C{¹H} NMR (C₆D₆): δ 165.3 (ring C), 157.7 (ring C), 121.3 (ring C), 83.6 (ring C), 82.1 (ring C), 11.1 (Si(CH₃)₃), −6.2 (Si(CH₃)₃), −17.7 (Si(CH₃)₃) ppm. ²⁹Si{¹H} NMR (C₆D₆): δ −31.5, −86.4, −118.4 ppm. IR (KBr, cm^–1): ν 2957 (s), 1248 (s), 1089 (s), 991 (s), 839 (s). Anal. calcd for C₃₁H₆₇ClOSi₇U: C, 40.21; H, 7.29. Found: C, 40.18; H, 7.31.

Method B: NMR Scale

A C₆D₆ (0.3 mL) solution of Me₃SiCl (2.2 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=O(dmap) (5; 18.8 mg, 0.02 mmol) and C₆D₆ (0.2 mL). Resonances of 8 and those of dmap were observed by ¹H NMR spectroscopy (100% conversion) when this solution was kept at room temperature overnight.

Preparation of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(OSiMe₃)(I) (9)

Method A

This compound was prepared as orange crystals from the reaction of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=O(dmap) (5; 235 mg, 0.25 mmol) and Me₃SiI (50 mg, 0.25 mmol) in toluene (15 mL) at room temperature and recrystallization from an n-hexane solution by a similar procedure to that in the synthesis of 8. The product was isolated by decantation of the supernatant, rapidly washed with cooled n-hexane (2 mL), and dried at 50 °C under vacuum overnight. Yield: 214 mg (84%). M.p.: 152–156 °C (dec.). ¹H NMR (C₆D₆): δ 36.48 (s, 9H, Si(CH₃)₃), 10.00 (s, 18H, Si(CH₃)₃), −8.13 (s, 18H, Si(CH₃)₃), −10.42 (s, 18H, Si(CH₃)₃) ppm; the protons of Cp-ring CH were not observed. ¹³C{¹H} NMR (C₆D₆): δ 157.9 (ring C), 155.6 (ring C), 112.6 (ring C), 82.4 (ring C), 81.4 (ring C), 13.8 (Si(CH₃)₃), −6.6 (Si(CH₃)₃), −18.1 (Si(CH₃)₃) ppm. ²⁹Si{¹H} NMR (C₆D₆): δ −35.3, −97.5, −132.9 ppm. IR (KBr, cm^–1): ν 2958 (s), 1248 (s), 1087 (s), 989 (s), 837 (s). Anal. calcd for C₃₁H₆₇IOSi₇U: C, 36.60; H, 6.64. Found: C, 36.58; H, 6.66.

Method B: NMR Scale

A C₆D₆ (0.3 mL) solution of Me₃SiI (4.0 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=O(dmap) (5; 18.8 mg, 0.02 mmol) and C₆D₆ (0.2 mL). Resonances of 9 and those of dmap were observed by ¹H NMR spectroscopy (100% conversion) when this solution was kept at room temperature overnight.

Preparation of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(OSiMe₃)(NC) (10)

Method A

This compound was prepared as yellow crystals from the reaction of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=O(dmap) (5; 235 mg, 0.25 mmol) and Me₃SiCN (25 mg, 0.25 mmol) in toluene (15 mL) at room temperature and recrystallization from an n-hexane solution by a similar procedure to that in the synthesis of 8. The product was isolated by decantation of the supernatant, rapidly washed with cooled n-hexane (2 mL), and dried at 50 °C under vacuum overnight. Yield: 177 mg (78%). M.p.: 163–165 °C (dec.). ¹H NMR (C₆D₆): δ 36.11 (s, 9H, Si(CH₃)₃), 6.76 (s, 18H, Si(CH₃)₃), −8.16 (s, 18H, Si(CH₃)₃), −10.77 (s, 18H, Si(CH₃)₃) ppm; the protons of Cp-ring CH were not observed. ¹³C{¹H} NMR (C₆D₆): δ 180.7 (CN), 175.8 (ring C), 136.6 (ring C), 121.4 (ring C), 80.5 (ring C), 79.3 (ring C), 9.5 (Si(CH₃)₃), −7.4 (Si(CH₃)₃), −20.1 (Si(CH₃)₃) ppm. ²⁹Si{¹H} NMR (C₆D₆): δ −48.5, −100.1, −125.1 ppm. IR (KBr, cm^–1): ν 2954 (s), 2036 (w), 1612 (s), 1249 (s), 1087 (s), 840 (s). Anal. calcd for C₃₂H₆₇NOSi₇U: C, 41.94; H, 7.37; N, 1.53. Found: C, 41.92; H, 7.38; N, 1.56.

Method B: NMR Scale

A C₆D₆ (0.3 mL) solution of Me₃SiCN (2.0 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=O(dmap) (5; 18.8 mg, 0.02 mmol) and C₆D₆ (0.2 mL). Resonances of 10 and those of dmap were observed by ¹H NMR spectroscopy (100% conversion) when this solution was kept at room temperature overnight.

Preparation of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(OSiMe₃)(N₃) (11)

Method A

This compound was prepared as brown microcrystals from the reaction of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=O(dmap) (5; 235 mg, 0.25 mmol) and Me₃SiN₃ (29 mg, 0.25 mmol) in toluene (15 mL) at room temperature and recrystallization from an n-hexane solution by a similar procedure to that in the synthesis of 8. The product was isolated by decantation of the supernatant, rapidly washed with cooled n-hexane (2 mL) and, dried at 50 °C under vacuum overnight. Yield: 172 mg (74%). M.p.: 168–170 °C (dec.). ¹H NMR (C₆D₆): δ 28.67 (s, 9H, Si(CH₃)₃), 6.97 (s, 18H, Si(CH₃)₃), −7.51 (s, 18H, Si(CH₃)₃), −9.25 (s, 18H, Si(CH₃)₃) ppm; the protons of Cp-ring CH were not observed. ¹³C{¹H} NMR (C₆D₆): δ 178.1 (ring C), 163.6 (ring C), 137.0 (ring C), 132.5 (ring C), 130.6 (ring C), 20.8 (Si(CH₃)₃), −0.7 (Si(CH₃)₃), −1.3 (Si(CH₃)₃), −17.0 (Si(CH₃)₃) ppm. ²⁹Si{¹H} NMR (C₆D₆): δ 15.8, −33.7, −83.7, −109.1 ppm. IR (KBr, cm^–1): ν 2955 (s), 2083 (s), 1612 (s), 1249 (s), 1003 (s), 833 (s). Anal. calcd for C₃₁H₆₇N₃OSi₇U: C, 39.93; H, 7.24; N, 4.51. Found: C, 39.92; H, 7.26; N, 4.49.

Method B: NMR Scale

A C₆D₆ (0.3 mL) solution of Me₃SiN₃ (2.3 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=O(dmap) (5; 18.8 mg, 0.02 mmol) and C₆D₆ (0.2 mL). Resonances of 11 and those of dmap were observed by ¹H NMR spectroscopy (100% conversion) when this solution was kept at room temperature overnight.

Preparation of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(OSiMe₃)₂ (12)

Method A

After a toluene (10 mL) solution of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(OSiMe₃)(Cl) (8; 185 mg, 0.2 mmol) was stirred at 70 °C overnight, the solution was filtered. The volume of the filtrate was reduced to 5 mL and cooled to −20 °C, yielding red crystals [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂UCl₂ (1), which were isolated by decantation of the supernatant. Yield: 75 mg (43% based on 8). After the red crystals of 1 were isolated, the solvent of the mother liquid was removed. The residue was extracted with n-hexane (5 mL × 3) and filtered. The volume of the filtrate was reduced to 4 mL, yellow crystals of 12 formed when this solution was kept at −40 °C for 2 days. Crystals of 12 were isolated by decantation of the supernatant, quickly washed with n-hexane (2 mL), and dried at 50 °C under vacuum overnight. Yield: 74 mg (38% based on 8). M.p.: 135–137 °C (dec.). ¹H NMR (C₆D₆): δ 36.12 (s, 18H, Si(CH₃)₃), 8.91 (s, 18H, Si(CH₃)₃), −7.81 (s, 18H, Si(CH₃)₃), −10.15 (s, 18H, Si(CH₃)₃) ppm; the protons of Cp-ring CH were not observed. ¹³C{¹H} NMR (C₆D₆): δ 121.4 (ring C), 120.3 (ring C), 120.2 (ring C), 11.1 (Si(CH₃)₃), −6.5 (Si(CH₃)₃), −7.4 (Si(CH₃)₃) ppm. ²⁹Si{¹H} NMR (C₆D₆): δ −31.5, −32.2, −91.4 ppm. IR (KBr, cm^–1): ν 2957 (s), 1614 (m), 1248 (s), 1093 (m), 831 (s). Anal. calcd for C₃₄H₇₆O₂Si₈U: C, 41.68; H, 7.82. Found: C, 41.71; H, 7.79.

Method B: NMR Scale

After a C₆D₆ (0.5 mL) solution of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(OSiMe₃)(Cl) (8; 185 mg, 0.2 mmol) was kept at 70 °C overnight, resonances of 12 and those of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂UCl₂ (1) (¹ H NMR (C₆D₆): δ 13.70 (br s, 4H, ring CH), 2.79 (s, 36H, Si(CH₃)₃), −13.39 (s, 18H, Si(CH₃)₃) ppm)¹⁷ were observed by ¹H NMR spectroscopy (100% conversion).

Preparation of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U[OC(=NPh)S)(dmap)·0.5C₆H₆ (13·0.5C₆H₆)

Method A

This compound was prepared as orange crystals from the reaction of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=O(dmap) (5; 235 mg, 0.25 mmol) and PhNCS (34 mg, 0.25 mmol) in toluene (15 mL) at room temperature and recrystallization from a benzene solution by a similar procedure as that in the synthesis of 8. The product was isolated by decantation of the supernatant, rapidly washed with cooled n-hexane (2 mL), and dried at 50 °C under vacuum overnight. Yield: 239 mg (86%). M.p.: 159–161 °C (dec.). ¹H NMR (C₆D₆): δ 16.09 (s, 2H, phenyl), 8.83 (s, 2H, phenyl), 7.15 (s, 3H, C₆H₆), 6.39 (br s, 18H, Si(CH₃)₃), 2.47 (s, 2H, ring CH), 1.45 (s, 2H, py H), 0.06 (br s, 18H, Si(CH₃)₃), −2.61 (s, 2H, py H), −3.54 (s, 1H, phenyl), −4.48 (s, 6H, N(CH₃)₂), −7.55 (s, 18H, Si(CH₃)₃) ppm; two Cp-ring CH were not observed. ¹³C{¹H} NMR (C₆D₆): δ 177.2 (CN), 167.3 (py C), 149.5 (py C), 145.3 (py C), 140.3 (phenyl C), 137.0 (phenyl C), 132.5 (phenyl C), 130.6 (phenyl C), 129.6 (ring C), 129.5 (ring C), 129.4 (ring C), 128.5 (C₆H₆), 128.4 (ring C), 127.1 (ring C), 34.7 (N(CH₃)₂), 10.7 (Si(CH₃)₃), 6.7 (Si(CH₃)₃), −2.2 (Si(CH₃)₃) ppm. ²⁹Si{¹H} NMR (C₆D₆): δ −67.9, −97.1, −99.7 ppm. IR (KBr, cm^–1): ν 2954 (s), 2900 (s), 1612 (s), 1573 (s), 1249 (s), 1095 (s), 995 (s), 840 (s). Anal. calcd for C₄₅H₇₆N₃OSSi₆U: C, 48.53; H, 6.88; N, 3.77. Found: C, 48.52; H, 6.86; N, 3.79.

Method B: NMR Scale

A C₆D₆ (0.3 mL) solution of PhNCS (2.7 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=O(dmap) (5; 18.8 mg, 0.02 mmol) and C₆D₆ (0.2 mL). Resonances of 13 were observed by ¹H NMR spectroscopy (100% conversion) when this solution was kept at room temperature overnight.

Preparation of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(SSiMe₃)(I) (14)

Method A

This compound was prepared as orange crystals from the reaction of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=S(dmap) (6; 239 mg, 0.25 mmol) and Me₃SiI (50 mg, 0.25 mmol) in toluene (15 mL) at room temperature and recrystallization from an n-hexane solution by a similar procedure to that in the synthesis of 8. The product was isolated by decantation of the supernatant, rapidly washed with cooled n-hexane (2 mL), and dried at 50 °C under vacuum overnight. Yield: 222 mg (86%). M.p.: 156–158 °C (dec.). ¹H NMR (C₆D₆): δ 13.24 (s, 18H, Si(CH₃)₃), 12.98 (s, 2H, ring CH), −2.00 (s, 18H, Si(CH₃)₃), −3.48 (s, 2H, ring CH), −7.66 (s, 9H, Si(CH₃)₃), −13.46 (s, 18H, Si(CH₃)₃) ppm. ¹³C{¹H} NMR (C₆D₆): δ 130.6 (ring C), 129.7 (ring C), 129.6 (ring C), 129.4 (ring C), 121.4 (ring C), 28.6 (Si(CH₃)₃), 16.8 (Si(CH₃)₃), 13.8 (Si(CH₃)₃), 5.5 (Si(CH₃)₃) ppm. ²⁹Si{¹H} NMR (C₆D₆): δ −11.4, −52.4, −86.6, −114.4 ppm. IR (KBr, cm^–1): ν 2955 (s), 1400 (s), 1244 (s), 1087 (s), 989 (s), 929 (s), 823 (s). Anal. calcd for C₃₁H₆₇ISSi₇U: C, 36.03; H, 6.53. Found: C, 36.05; H, 6.56.

Method B: NMR Scale

A C₆D₆ (0.3 mL) solution of Me₃SiI (4.0 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=S(dmap) (6; 19.1 mg, 0.02 mmol) and C₆D₆ (0.2 mL). Resonances of 14 and those of dmap were observed by ¹H NMR spectroscopy (100% conversion) when this solution was kept at room temperature overnight.

Reaction of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=S(dmap) (6) with Me₃SiCl

NMR Scale

A C₆D₆ (0.3 mL) solution of Me₃SiCl (2.2 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=S(dmap) (6; 19.1 mg, 0.02 mmol) and C₆D₆ (0.2 mL). Resonances of 1 along with those of unreacted 6, (Me₃Si)₂S (¹H NMR (C₆D₆): δ 0.29 (s, 18H, Si(CH₃)₃) ppm), and dmap were observed by ¹H NMR spectroscopy (50% conversion based on 6) when this solution was kept at room temperature overnight. Nevertheless, when 2 equiv of Me₃SiCl (4.4 mg, 0.04 mmol) was added to a C₆D₆ (0.5 mL) solution of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=S(dmap) (6; 19.1 mg, 0.02 mmol), resonances of 1 along with those of (Me₃Si)₂S and dmap were observed by ¹H NMR spectroscopy (100% conversion) when this solution was kept at room temperature overnight.

Reaction of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=S(dmap) (6) with PhNCO

NMR Scale

A C₆D₆ (0.3 mL) solution of PhNCO (2.4 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=S(dmap) (6; 19.1 mg, 0.02 mmol) and C₆D₆ (0.2 mL). Resonances of 13 were observed by ¹H NMR spectroscopy (100% conversion) when this solution was kept at room temperature overnight.

Preparation of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U[SC(=NPh)S)(dmap) (15)

Method A

This compound was prepared as brown crystals from the reaction of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=S(dmap) (6; 239 mg, 0.25 mmol) and PhNCS (34 mg, 0.25 mmol) in toluene (15 mL) at room temperature and recrystallization from a benzene solution by a similar procedure to that in the synthesis of 8. The product was isolated by decantation of the supernatant, rapidly washed with cooled n-hexane (2 mL), and dried at 50 °C under vacuum overnight. Yield: 224 mg (82%). M.p.: 165–167 °C (dec.). ¹H NMR (C₆D₆): δ 9.13 (s, 2H, phenyl), 6.59 (s, 2H, phenyl), 2.41 (s, 18H, Si(CH₃)₃), 1.38 (s, 18H, Si(CH₃)₃), −0.84 (s, 2H, py H), −2.59 (s, 18H, Si(CH₃)₃), −3.75 (s, 6H, N(CH₃)₂), −4.94 (s, 2H, ring CH), −8.72 (s, 1H, phenyl), −9.02 (s, 2H, py H) ppm; two Cp-ring CH protons were not observed. ¹³C{¹H} NMR (C₆D₆): δ 144.7 (CN), 136.6 (py C), 134.8 (py C), 133.4 (py C), 129.5 (phenyl C), 127.2 (phenyl C), 127.1 (phenyl C), 125.7 (phenyl C), 122.5 (ring C), 95.5 (ring C), 35.8 (N(CH₃)₂), 14.0 (Si(CH₃)₃), 12.3 (Si(CH₃)₃), 3.8 (Si(CH₃)₃) ppm. ²⁹Si{¹H} NMR (C₆D₆): δ −46.9, −56.4, −110.6 ppm. IR (KBr, cm^–1): ν 2954 (m), 1612 (s), 1535 (s), 1249 (s), 1087 (s), 995 (s), 833 (s). Anal. calcd for C₄₂H₇₃N₃S₂Si₆U: C, 46.25; H, 6.75; N, 3.85. Found: C, 46.22; H, 6.76; N, 3.87.

Method B: NMR Scale

A C₆D₆ (0.3 mL) solution of PhNCS (2.7 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=S(dmap) (6; 19.1 mg, 0.02 mmol) and C₆D₆ (0.2 mL). Resonances of 15 were observed by ¹H NMR spectroscopy (100% conversion) when this solution was kept at room temperature overnight.

Preparation of {[η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U}₂(μ-CS₃)₂ (16)

Method A

This compound was prepared as brown crystals from the reaction of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=S(dmap) (6; 239 mg, 0.25 mmol) and CS₂ (19 mg, 0.25 mmol) in toluene (15 mL) at room temperature and recrystallization from a benzene solution by a similar procedure to that in the synthesis of 8. The product was isolated by decantation of the supernatant, rapidly washed with cooled n-hexane (2 mL), and dried at 50 °C under vacuum overnight. Yield: 205 mg (90%). ¹H NMR (C₆D₆): δ 20.21 (s, 9H, Si(CH₃)₃), 14.23 (br s, 9H, Si(CH₃)₃), −7.42 (s, 9H, Si(CH₃)₃), −7.68 (s, 9H, Si(CH₃)₃), −10.16 (s, 9H, Si(CH₃)₃), −12.98 (s, 9H, Si(CH₃)₃) ppm; the protons of the Cp-ring CH were not observed. These spectroscopic data agreed with those reported in the literature.¹⁷ Furthermore, this complex was also characterized by X-ray diffraction analysis and its molecular structure is shown in the Supporting Information.

Method B: NMR Scale

A C₆D₆ (0.3 mL) solution of CS₂ (1.5 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=S(dmap) (6; 19.1 mg, 0.02 mmol) and C₆D₆ (0.2 mL). Resonances of 16 and those of dmap were observed by ¹H NMR spectroscopy (100% conversion) when this solution was kept at room temperature overnight.

Preparation of {[η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U}₂(μ-Se)₂ (17)

After a toluene (10 mL) solution of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=Se(dmap) (7; 193 mg, 0.2 mmol) was stirred at 60 °C overnight, the solvent was removed. The residue was extracted with benzene (5 mL × 3) and filtered. The volume of the filtrate was reduced to 4 mL, and brown microcrystals of 17 formed when this solution was kept at 10 °C for 2 days. The product was isolated by decantation of the supernatant, rapidly washed with cooled n-hexane (2 mL), and dried at 50 °C under vacuum overnight. Yield: 144 mg (82%). M.p.: 183–185 °C (dec.). ¹H NMR (C₆D₆): δ 36.44 (s, 18H, Si(CH₃)₃), 13.92 (s, 36H, Si(CH₃)₃) ppm; the protons of Cp-ring CH were not observed. ¹³C{¹H} NMR (C₆D₆): δ 150.1 (ring C), 139.0 (ring C), 137.4 (ring C), 4.6 (Si(CH₃)₃), 2.7 (Si(CH₃)₃) ppm. ²⁹Si{¹H} NMR (C₆D₆): δ −74.4, −75.8, −104.0 ppm. IR (KBr, cm^–1): ν 2957 (s), 1595 (s), 1510 (s), 1249 (s), 1172 (s), 1030 (s), 831 (s). Anal. calcd for C₅₆H₁₁₆Se₂Si₁₂U₂: C, 38.20; H, 6.64. Found: C, 38.23; H, 6.61. Brown crystals of 17·0.5C₆H₁₄ suitable for X-ray structural analysis were isolated from a mixture of benzene and n-hexane (4:1) solution.

Preparation of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(SeSiMe₃)(I) (18)

Method A

This compound was prepared as orange crystals from the reaction of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=Se(dmap) (7; 241 mg, 0.25 mmol) and Me₃SiI (50 mg, 0.25 mmol) in toluene (15 mL) at room temperature and recrystallization from an n-hexane solution by a similar procedure as that in the synthesis of 8. The product was isolated by decantation of the supernatant, rapidly washed with cooled n-hexane (2 mL), and dried at 50 °C under vacuum overnight. Yield: 221 mg (85%). M.p.: 164–166 °C (dec.). ¹H NMR (C₆D₆): δ 14.03 (s, 18H, (CH₃)₃Si), −1.25 (s, 18H, (CH₃)₃Si), −9.50 (s, 9H, (CH₃)₃Si), −13.57 (s, 18H, (CH₃)₃Si) ppm; the protons of Cp-ring CH were not observed. ¹³C{¹H} NMR (C₆D₆): δ 131.4 (ring C), 129.5 (ring C), 121.7 (ring C), 113.7 (ring C), 113.6 (ring C) 21.7 (Si(CH₃)₃), 13.8 (Si(CH₃)₃), 8.4 (Si(CH₃)₃), 6.2 (Si(CH₃)₃) ppm. ²⁹Si{¹H} NMR (C₆D₆): δ −11.5, −35.3, −53.5 ppm. IR (KBr, cm^–1): ν 2954 (s), 1597 (m), 1512 (s), 1381 (s), 1249 (s), 1172 (m), 1087 (m), 1026 (m), 833 (s). Anal. calcd for C₃₁H₆₇ISeSi₇U: C, 34.46; H, 6.25. Found: C, 34.45; H, 6.26. This complex was also characterized by X-ray diffraction analysis and its molecular structure is shown in the Supporting Information. Nevertheless, the quality of the data was rather poor because of crystal twinning, but sufficient to at least establish the overall connectivity.

Method B: NMR Scale

A C₆D₆ (0.3 mL) solution of Me₃SiI (4.0 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=Se(dmap) (7; 19.3 mg, 0.02 mmol) and C₆D₆ (0.2 mL). Resonances of 18 and those of dmap were observed by ¹H NMR spectroscopy (100% conversion) when this solution was kept at room temperature overnight.

Reaction of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=Se(dmap) (7) with Me₃SiCl

NMR Scale

A C₆D₆ (0.3 mL) solution of Me₃SiCl (2.2 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=Se(dmap) (7; 19.3 mg, 0.02 mmol) and C₆D₆ (0.2 mL). Resonances of 1 along with those of unreacted 7, (Me₃Si)₂Se (¹ H NMR (C₆D₆): δ 0.36 (s, 18H, Si(CH₃)₃) ppm), and dmap were observed by ¹H NMR spectroscopy (50% conversion based on 7) when this solution was kept at room temperature overnight. Nevertheless, when 2 equiv of Me₃SiCl (4.4 mg, 0.04 mmol) was added to a C₆D₆ (0.5 mL) solution of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U=Se(dmap) (7; 19.3 mg, 0.02 mmol), resonances of 1 along with those of (Me₃Si)₂Se and dmap were observed by ¹H NMR spectroscopy (100% conversion) when this solution was kept at room temperature overnight.

X-ray Crystallography

Single-crystal X-ray diffraction measurements were carried out on a Rigaku Saturn CCD diffractometer at 100(2) K using Mο Kα radiation (λ = 0.71073 Å) or Cu Kα radiation (λ = 1.54184 Å). An empirical absorption correction was applied using the SADABS program.²⁰ All structures were solved by direct methods and refined by full-matrix least squares on F² using the SHELXL program package.²¹ All the hydrogen atoms were geometrically fixed using the riding model. The crystal data and experimental data for 4–10 and 12–17 are summarized in the Supporting Information. Selected bond lengths and angles are listed in Table 1.

Computational Methods

All calculations were carried out with the Gaussian 09 program (G09),²² employing the B3PW91 functional, plus a polarizable continuum model (PCM) (denoted as B3PW91-PCM), with standard 6-31G(d) basis set for C, H, N, O, S, Se, Si, and Cl elements and a quasi-relativistic 5f-in-valence effective-core potential (ECP60MWB or ECP80MWB) treatment with 60 or 80 electrons in the core region for U and the corresponding optimized segmented ((14s13p10d8f6g)/[10s9p5d4f3g]) basis set for the valence shells of U,²³ to fully optimize the structures of reactants, complexes, transition state, intermediates, and products and also to mimic the experimental toluene-solvent conditions (dielectric constant ε = 2.379). All stationary points were subsequently characterized by vibrational analyses, from which their respective zero-point (vibrational) energy (ZPE) was extracted and used in the relative energy determinations; in addition, frequency calculations were also performed to ensure that the reactant, complex, intermediate, product, and transition state structures resided at minima and 1st-order saddle points on their potential energy hypersurfaces. Please note that to properly describe the reaction profile for the reaction of 4 + Ph₂CO, dispersion effects (D3)²⁴ had to be considered for the sterically encumbered uranium complexes since dispersion contributes significantly to the overall energy profile (see the Supporting Information for details, Figure S4 and Tables S6 and S7) and therefore single-point B3PW91-PCM-D3²⁴ calculations based on B3PW91-PCM geometries were performed for this transformation. Bader’s quantum theory of atoms-in-molecules (QTAIM)²⁵ analyses were performed using the Multiwfn program.²⁶

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.3c03753.

• Crystal parameters for compounds 4–10 and 12–17 (PDF); X-ray crystallographic data, in CIF format

• Cartesian coordinates of all stationary points optimized at the B3PW91-PCM level (XYZ)

The authors declare no competing financial interest.