Experimental and Computational Studies on a Base‐Free Terminal Uranium Phosphinidene Metallocene

Abstract

The first stable base‐free terminal uranium phosphinidene metallocene is presented; and its structure and reactivity have been studied in detail and compared to that of the corresponding thorium derivative. Salt metathesis reaction of the methyl iodide uranium metallocene Cp’’’₂U(I)Me (2, Cp’’’=η ⁵‐1,2,4‐(Me₃C)₃C₅H₂) with Mes*PHK (Mes*=2,4,6‐(Me₃C)₃C₆H₂) in THF yields the base‐free terminal uranium phosphinidene metallocene, Cp’’’₂U=PMes* (3). In addition, density functional theory (DFT) studies suggest substantial 5f orbital contributions to the bonding within the uranium phosphinidene [U]=PAr moiety, which results in a more covalent bonding between the [Cp’’’₂U]²⁺ and [Mes*P]²⁻ fragments than that for the related thorium derivative. This difference in bonding besides steric reasons causes different reactivity patterns for both molecules. Therefore, the uranium derivative 3 may act as a Cp’’’₂U(II) synthon releasing the phosphinidene moiety (Mes*P:) when treated with alkynes or a variety of hetero‐unsaturated molecules such as imines, thiazoles, ketazines, bipy, organic azides, diazene derivatives, ketones, and carbodiimides.

The first base‐free terminal uranium phosphinidene metallocene was prepared and it showed a distinctively different reactivity compared to the related thorium phosphinidenes.

Introduction

For more than 20 years d‐transition‐metal phosphinidene complexes have been extensively studied and various derivatives have been prepared and their intrinsic reactivity and chemical and physical properties are now thoroughly investigated,[ ¹ , ² , ³ , ⁴ ] resulting in interesting applications in the synthesis of phosphorus compounds, organometallic derivatives, and new materials.[ ¹ , ² , ³ ] Furthermore, because of the high reactivity terminal phosphinidene compounds have been particularly sought‐after, since they are also more efficient in phosphorus‐element bond synthesis and more useful in catalytic transformations than bridged phosphinidene derivatives.[ ¹ , ² , ³ ] So while the field of phosphinidene complexes of d‐transition metals has flourished, only a few derivatives containing 5f‐elements have emerged over the last two decades.[ ⁴ , ⁵ ] The scarcity of actinide derivatives can be traced to the sensitivity of these multiple‐bonded actinide complexes to steric effects imposed by the ligand environment at the metal atom, ^[6] which makes a judicial choice of the employed ligand set imperative. Attributed to these challenges a more detailed reactivity study of these species is still missing. ^[5] Moreover, this may also present a rewarding endeavor since organoactinide chemistry has not only witnessed a renaissance in recent years attributed to potential applications of organoactinides in small molecule activation and functionalization, ^[7] but it also addresses the more fundamental question concerning the influence of 5f orbital occupation on bonding and reactivity in general. ^[8] Several studies have confirmed that already subtle changes in the 5f orbital contributions can have a significant influence on the reactivity of organoactinide compounds. ^[8] In this context, we compared the reactivity of thorium and uranium metallacyclopropene complexes and noted some remarkable divergence in their reactivity. ^[9] For example, while the alkyne moiety in the thorium metallacyclopropene Cp’’’₂Th(η ²‐C₂Ph₂) (Cp’’’=η ⁵‐1,2,4‐(Me₃C)₃C₅H₂) reacts as a nucleophile towards hetero‐unsaturated molecules or as a strong base inducing the intermolecular C−H bond activations,[ ^9e , ^9f ] the related uranium metallacyclopropene Cp*₂U[η ²‐C₂(SiMe₃)₂] (Cp*=η ⁵‐C₅Me₅) serves as an efficient synthon for the Cp*₂U(II) fragment when reacted with unsaturated molecules. ^[9j] More recently we have also reported on terminal phosphinidene thorium complexes including the first isolable base‐free terminal actinide phosphinidene metallocene Cp’’’₂Th=PMes* (3’; Mes*=2,4,6‐(Me₃C)₃C₆H₂). ^[10a] The strong coordination of the phosphinidene moiety in 3’ resulted in unusual reactivity toward various small molecules such as CS₂, isothiocyanate, nitriles, isonitriles, and organic azides, yielding carbodithioates, imido complexes, metallaaziridines, and azido compounds. ^[10a] To evaluate the difference between terminal thorium and uranium phosphinidenes, we set out to prepare the first stable base‐free terminal uranium phosphinidene metallocene, Cp’’’₂U=PMes* (3), which indeed shows distinctively different reactivity patterns to those found for its thorium counterpart Cp’’’₂Th=PMes* (3’).

Results and Discussion

Synthesis of Cp’’’₂U=PMes* (3)

Addition of CuI (1 equiv) to the uranium dimethyl complex Cp’’’₂UMe₂ (1) in toluene forms the methyl iodide complex Cp’’’₂U(I)Me (2) in 85 % yield (Scheme 1). The molecular structure of 2 is shown in Figure 1, and selected bond distances and angles are listed in Table 1. The U−C(35) distance is 2.423(5) Å, whereas the U−I distance is 2.990(1) Å, and the angle of C(35)‐U‐I is 92.5(2)°. Subsequent treatment of 2 with 1 equiv of Mes*PHK in THF allows the isolation of the targeted base‐free terminal phosphinidene uranium metallocene, Cp’’’₂U=PMes* (3), in 75 % yield (Scheme 1). ^[11] The molecular structure of 3 is presented in Figure 2, and selected bond distances and angles are given in Table 1. To the best of our knowledge, complex 3 represents the first structurally authenticated base‐free terminal phosphinidene uranium metallocene, and therefore constitutes a notable addition to the class of other structurally characterized actinide metallocenes featuring a phosphinidene functionality, Cp’’’₂Th=PMes*, ^[10a] {Cp’’₂Th(=PMes*)(ClK)}₂ (Cp’’=η ⁵‐1,3‐(Me₃C)₂C₅H₃), ^[10c] {Cp’’₂Th(=P‐2,4,6‐iPr₃C₆H₂)(ClK)}₂, ^[10d] Cp*₂U(=PMes*)(OPMe₃), ^[5b] {[Cp*₂Th(=P‐2,4,6‐iPr₃C₆H₂)(PH‐2,4,6‐iPr₃C₆H₂)]K}₂, ^[5g] and [Cp*₂Th(=P‐2,4,6‐iPr₃C₆H₂)(PH‐2,4,6‐iPr₃C₆H₂)][K(2,2,2‐cryptand)]. ^[5g] The short U−P distance of 2.495(1) Å and the essentially linear U‐P‐C(35) angle (177.4(1)°) are in line with a U=P double bond. ^[12] Furthermore, the U−P distance of 2.495(1) Å is only moderately elongated relative to the predicted value by Pyykkö for a U=P double bond (2.36 Å), ^[13] but it is shorter than those values found for Cp*₂U(=PMes*)(OPMe₃) (2.562(3) Å), ^[5b] [(iPr₃SiNCH₂CH₂)₃NU=PH][K(B15C5)₂] (2.613(2) Å), ^[5c] and [(iPr₃SiNCH₂CH₂)₃NU=PH][Na(12C4)₂] (2.685(2) Å). ^[5f] Overall, these structural parameters observed for 3 fully support the description of a uranium phosphinidene.

Scheme 1.

Synthesis of complexes 2 and 3.

Figure 1.

Molecular structure of 2 (thermal ellipsoids drawn at the 35 % probability level).

Table 1.

Selected distances (Å) and Angles (deg) for compounds 2–4, 6–12 and 14.^[a]

compound	C(Cp)−U[b]	C(Cp)−U[c]	Cp(cent)−U[b]	U−X	Cp (cent)‐U‐Cp(cent)	X‐U‐X/Y
2	2.790(4)	2.733(4) to 2.870(4)	2.514(4)	C(35) 2.423(5), I(1) 2.990(1)	140.8(2)	92.5(2)
3	2.793(4)	2.719(3) to 2.899(4)	2.517(4)	P(1) 2.495(1)	138.0(1)
4	2.804(3)	2.744(3) to 2.884(3)	2.530(3)	C(35) 2.318(3), C(36) 2.331(3)	139.1(1)	33.6(1)
6	2.808(5)	2.678(5) to 2.950(5)	2.538(5)	C(18) 2.439(8), N(1) 2.227(6)	138.0(2)	35.6(2)
7	2.803(4)	2.763(4) to 2.857(4)	2.528(4)	C(37) 2.389(11), N(1) 2.243(4) S(1) 2.813(2)	143.7(2)	66.6(1)[d], 33.3(3)[e]
8	2.835(4)	2.776(4) to 2.898(4)	2.563(4)	N(1) 2.214(3), N(2) 2.227(3)	140.2(1)	111.4(1)
9	2.843(5)	2.790(5) to 2.904(5)	2.573(5)	N(1) 2.429(4), N(1A) 2.429(4)	140.6(1)	66.1(2)
10	2.800(5)	2.760(5) to 2.833(5)	2.526(5)	N(1) 1.985(4), N(2) 1.981(4)	141.9(2)	99.8(2)
11	2.844(3)	2.715(3) to 3.040(3)	2.634(3)	O(1) 2.132(2), O(2) 2.146(2)	121.3(1)	68.0(1)
12	2.818(3)	2.718(3) to 2.960(3)	2.545(3)	N(1) 2.245(2), C(35) 2.369(3)	133.4(1)	33.8(1)
14	2.831(4)	2.724(4) to 2.958(4)	2.560(4)	N(1) 2.250(3), C(35) 2.353(4)	133.2(1)	34.0(1)

[a] Cp=cyclopentadienyl ring. [b] Average value. [c] Range. [d] The angle of S(1)‐U(1)‐N(1). [e] The angle of N(1)‐U(1)‐C(37).

Figure 2.

Molecular structure of 3 (thermal ellipsoids drawn at the 35 % probability level).

Bonding studies

Density functional theory (DFT) computations at the B3PW91 level of theory were performed to evaluate the interaction between the [Cp’’’₂U]²⁺ and the [PMes*]²⁻ fragments, which also allows the bonding in 3 to be compared to its thorium analogue Cp’’’₂Th=PMes* (3’). ^[10a] The computed structures for 3 and 3’ in gas phase reproduce the experimental solid‐state data very well and show that the [Mes*P]²⁻ fragment is coordinated to the [Cp’’’₂An]²⁺ moiety by one An−P σ‐bond and two An−P π‐bonds, as illustrated in Figure 3. The natural localized molecular orbital (NLMO) analysis (Table 2) performed on 3 reveals that U−P σ‐bond, σ(U=P), combines a phosphorus hybrid orbital (73.4 %; 78.5 % 3s and 21.5 % 3p) and a uranium hybrid orbital (25.3 %; 55.8 % 6d and 26.0 % 5f). The two orthogonal U−P π bonds, π ₁ and π ₂, have similar compositions and consist of a pure 3p phosphorus‐based orbital (62.7 %) and a uranium hybrid orbital (32.0 %; 53.1 % 6d and 44.7 % 5f) and a pure 3p phosphorus‐based orbital (58.9 %) and a uranium hybrid orbital (37.1 %; 47.2 % 6d and 51.1 % 5f), respectively. Within this description additional electron density is transferred from the π‐orbitals of [Mes*P]²⁻ fragment to the electron deficient metallocene unit [Cp’’’₂U]²⁺. Nevertheless, in the related thorium complex 3’, the metal contribution to the bonding of the Th=PMes* moiety decreases notably (20.6 % Th for Th=P σ bond, and 25.2 % and 30.9 % Th for Th=P π ₁ and π ₂ bonds, respectively) (Table 2). A direct comparison of the 5f orbital contributions to the bonding in the uranium complex 3 (U=P σ (26.0 %) and U=P π bonds (44.7 % and 51.1 % for π ₁ and π ₂ bonds, respectively) and its thorium analogue 3’ (15.0 % for the Th=P σ bond and 33.9 % and 24.0 % for the Th=P π ₁ and π ₂ bonds, respectively) shows a significantly larger 5f orbital contribution in 3 than that in 3’, which is consistent with the previously investigated systems.[ ^8d , ^8e , ^9g , ^9j , ^9m ] Therefore the thorium derivative shows an increased charge separation, and hence an increased electrostatic interaction between the individual [Cp’’’₂An]²⁺ and [Mes*P]²⁻ fragments, that is, 0.58 for the uranium complex (3) and 0.80 for thorium complex (3’) (Table 2). The decreased Mayer bond order of the An=P of 1.76 (for 3) to 1.53 (for 3’)) (Table 2) points in the same direction. These render the bonding between the metallocene [Cp’’’₂Th]²⁺ and the [Mes*P]²⁻ fragment more ionic, which is also consistent with an increased 5f orbital energy of the thorium atom relative to those of the uranium atom,[ ^8f , ^8g ] the efficiency of the π‐donation from the π‐MO of the phosphinidene fragment to the thorium atom decreases. Hence a different reactivity of the uranium complex 3 can be expected compared to related thorium phosphinidenes.[ ¹⁰ , ¹⁴ ]

Figure 3.

Plots of HOMOs for 3 (the hydrogen atoms have been omitted for clarity).

Table 2.

Natural localized molecular orbital (NLMO) analysis of An=PAr Bonds,^[a] bond order, and the natural charges for the [Cp’’’₂An] and [ArP] units.

		3 (U)	3’ (Th) [10a]
σ An−P	%An	25.3	20.6
	%s	7.8	9.4
	%p	10.4	10.1
	%d	55.8	65.5
	%f	26.0	15.0
	%P	73.4	78.7
	%s	78.5	76.7
	%p	21.5	23.3
π 1 An=P	%An	32.0	25.2
	%p	2.2	1.8
	%d	53.1	64.3
	%f	44.7	33.9
	%P	62.7	70.7
	%p	100	100
π 2 An=P	%An	37.1	30.9
	%p	1.7	2.0
	%d	47.2	74.0
	%f	51.1	24.0
	%P	58.9	66.0
	%p	100	100
Mayer bond order (An=P)		1.76	1.53
NBO charge (An)		0.52	0.62
NBO charge (P)		−0.01	−0.12
NBO charge (Cp2An)		0.29	0.40
NBO charge (ArP)		−0.29	−0.40

[a] The contributions by atom and orbital are averaged over all the ligands of the same character (complexes of U and Th) and over alpha and beta orbital contributions (complex of U).

Reactivity studies

As previously established for the base‐free thorium derivative, Cp’’’₂Th=PMes* (3’), ^[10a] no phosphinidene dissociation occurs when 3 is heated to 100 °C in toluene solution, which is in line with a strong coordination of the phosphinidene moiety to the uranium atom. However, in contrast to the thorium derivatives (see Figure S1),[ ¹⁰ , ¹⁴ ] the coordinated phosphinidene in the uranium species 3 is susceptible to exchange with internal alkynes. For example, addition of PhC≡CPh at 50 °C yields the—so far unknown—uranium(IV) metallacyclopropene Cp’’’₂U(η ²‐C₂Ph₂) (4) besides the phosphaindane derivative 3,3‐Me₂‐5,7‐tBu₂C₈H₅P (5) in quantitative conversion (Scheme 2). According to DFT computations, it can be assumed that 3 initially reacts with PhC≡CPh to yield a metallacyclopropene adduct INT4 (Figure 4), then the phosphinidene 2,4,6‐tBu₃C₆H₂P dissociates from INT4 to give the metallacyclopropene 4. Finally, the phosphinidene 2,4,6‐tBu₃C₆H₂P converts to the phosphaindane 3,3‐Me₂‐5,7‐tBu₂C₈H₅P (5) via C−H bond activation. The energetically favorable (ΔG(298 K)=−152.2 kJ mol⁻¹) formation of 4 + 5 and the overall reaction barrier of ΔG ^≠(298 K)=112.4 kJ mol⁻¹ agree with the experimental observations. Figure 5 shows the molecular structure of 4, whereas selected bond distances and angles can be found in Table 1. The C(35)−C(36) distance is 1.342(4) Å, and the U−C distances are 2.318(3) Å for C(35) and 2.331(3) Å for C(36), and the angle of C(35)‐U‐C(36) is 33.6(1)°. These structural parameters are essentially identical to those observed for the known Cp*₂U[η ²‐C₂(SiMe₃)₂] with the U−C distances of 2.315(9) and 2.350(9) Å, and with a C=C distance of 1.338(11) Å, and a C‐U‐C angle of 33.3(3)°. ^[9j] Nevertheless, contrary to the reactivity of the thorium phosphinidene complex {Cp’’₂Th(=PMes*)(ClK)}₂ toward PhC≡CPh, ^[10c] no [2+2] cycloaddition product is formed between complex 3 and PhC≡CPh, presumably caused by steric hindrance. ^[10a] However, while no reaction was observed for the thorium phosphinidene complex Cp’’’₂Th=PMes* (3’), ^[10a] the sterically more encumbered uranium species 3 undergoes a ligand replacement reaction, which cannot be exclusively attributed to steric effects, and the different electronic structures at the metal atoms also need to be taken into considerations.

Scheme 2.

Synthesis of complexes 4 and 6.

Figure 4.

Energy profile (kJ mol⁻¹) for the reaction of 3+PhC≡CPh (computed at T=298 K). [U]=Cp’’’₂U. Ar=2,4,6‐tBu₃C₆H₂.

Figure 5.

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

Moreover, hetero‐unsaturated organic molecules can also replace the phosphinidene moiety in 3. For example, contrary to the reactivity of the thorium phosphinidene complex {Cp’’₂Th(=PMes*)(ClK)}₂ toward PhCH=NPh (see Supporting Information, Figure S1),[ ^10c , ¹⁴ ] complex 3 reacts with this substrate to yield the metallaaziridine Cp’’’₂U(η ²‐CHPhNPh) (6) and the phosphaindane 3,3‐Me₂‐5,7‐tBu₂C₈H₅P (5) (Scheme 2). Figure 6 shows the molecular structure of 6, while selected bond distances and angles are provided in Table 1. The U−N and U−C(18) distances are 2.227(6) and 2.439(8) Å, respectively, whereas the N(1)‐U‐C(18) angle amounts to 35.6(2)°. Also in contrast to the reactivity of the thorium phosphinidene complexes 3’ and {Cp’’₂Th(=PMes*)(ClK)}₂ with thiazole (see Supporting Information, Figure S1),[ ^10b , ^10c ] thiazole replaces the phosphinidene fragment in the uranium derivative 3 to yield the six‐membered heterocyclic complex Cp’’’₂U(SCH=CHN=CH) (7) and the phosphaindane 5 (Scheme 3). To account for this reactivity we propose a mechanism similar to the reaction with PhCH=NPh, in the first step thiazole substitutes the phosphinidene fragment to yield a metallaaziridine complex, which is, however, unstable and converts via C‐S cleavage to a zwitterionic intermediate, which then spontaneously forms 7 (Scheme 3). Figure 7 shows the molecular structure of 7 and selected bond distances and angles are listed in Table 1. The U−N and U−C(37) distances amount to 2.243(6) and 2.389(11) Å, respectively, whereas the U−S distance is much longer with 2.813(2) Å. Nevertheless, under similar reaction conditions, treatment of 3 with (Ph₂C=N)₂ gives a diiminato complex Cp’’’₂U(N=CPh₂)₂ (8) and phosphaindane 3,3‐Me₂‐5,7‐tBu₂C₈H₅P (5) in quantitative conversion (Scheme 4). A plausible mechanism may include phosphinidene exchange with (Ph₂C=N)₂ to also furnish a metallaaziridine, which opens via N‐N cleavage to 8 (Scheme 4). The solid‐state molecular structure of 8 is provided in Figure 8, while selected bond distances and angles are given in Table 1. The U−N distances are 2.214(3) Å for N(1) and 2.227(3) Å for N(2), and the U‐N‐C angles are 177.0(3)° for N(1) and 176.6(3)° for N(2), and the N(1)‐U‐N(2) angle is 111.4(1)°. These structural parameters may be compared to those found in Cp*₂U(N=CPh₂)₂ with the U−N distances of 2.172(7) and 2.169(6) Å, and the U‐N‐C angles of 172.8(6) and 174.7(6)°, and the N‐U‐N angle of 107.2(2)°. ^[9j] In analogy to the bis(phosphide) thorium complex [H₂B(3‐Mes‐C₃H₂N₂)₂]₂Th(PHMes)₂ (Mes=2,4,6‐Me₃Ph) towards bipy, ^[15] reductive elimination occurs in the reaction of compound 3 and bipy, that is, the known compound Cp’’’₂U(bipy) (9) ^[6a] is accessible by the addition of 2,2’‐bipyridine (bipy) to compound 3 (Scheme 4). Figure 9 presents the molecular structure of 9 and selected bond distances and angles are listed in Table 1. The U−N(1) and U−N(1A) distances are 2.420(6) Å, and the N(1)‐U‐N(1A) angle is 66.1(2)°.

Figure 6.

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

Scheme 3.

Synthesis of complex 7.

Figure 7.

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

Scheme 4.

Synthesis of complexes 8 and 9.

Figure 8.

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

Figure 9.

Molecular structure of 9 (thermal ellipsoids drawn at the 35 % probability level).

In contrast, treatment of 3 with 1,2‐diphenyldiazene (PhN=NPh) gives the uranium(VI) bisimido species Cp’’’₂U(=NPh)₂ (10) besides the phosphaindane 5 in quantitative conversion (Scheme 5). It is reasonable to postulate that the reaction sequence commences with a substitution of phosphinidene fragment by PhN=NPh to form a metalladiazirine, then an electron transfer ensues to cleave the N−N bond to give the bisimido complex 10 (Scheme 5). Complex 10 may also be prepared by the reaction of 3 with phenyl azide (PhN₃) in quantitative conversion (Scheme 5), and the reaction outcome remains unaffected regardless of the amount of azide employed. It is reasonable to propose that PhN₃ displaces the phosphinidene fragment in 3 and releases N₂ to give a uranium(IV) imido complex, which subsequently reacts with a second molecule of PhN₃ to yield the bisimido uranium(VI) compound 10 and N₂ (Scheme 5). The molecular structure of 10 can be found in Figure 10, while the selected bond distances and angles are available in Table 1. The short U−N distances (1.985(4) Å for N(1) and 1.981(4) Å for N(2)) and the angles of U‐N(1)‐C(35) (171.4(4)°) and U‐N(2)‐C(41) (172.8(4)°) are in line with a U=N double bond description. ^[12] These structural parameters matches those previously found in related compounds such as Cp*₂U(=N‐p‐tolyl)₂ with the U−N distances of 1.971(4) and 1.975(3) Å and the U‐N‐C angles of 178.8(3) and 179.1(3)°, ^[9j] Cp*₂U(=NPh)₂ with the U−N distance of 1.952(7) Å and the U‐N‐C angle of 177.8(6)°, ^[16] and Cp’’’₂U=N(p‐tolyl) with the U‐N distance of 1.988(5) Å and the U‐N‐C angle of 172.3(5)°. ^[6a]

Scheme 5.

Synthesis of complex 10.

Figure 10.

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

Addition of Ph₂CO to 3 also releases the coordinated phosphinidene to yield the uranium pinacolate Cp’’’₂U[(OCPh₂)₂] (11) and the phosphaindane 5 (Scheme 6). However, no change in product formation is observed when the equivalents of Ph₂CO added to the reaction are varied. Presumably, on replacement of phosphinidene fragment with Ph₂CO an unstable metallaoxirane intermediate forms,[ ^9e , ^10c ] which subsequently couples with a second molecule of Ph₂CO to furnish the pinacolate 11 (Scheme 6). Figure 11 shows the molecular structure of 11 and selected bond distances and angles are given in Table 1. The U−O distances are 2.132(2) Å for O(1) and 2.146(2) Å for O(2), and the O(1)‐U‐O(2) angle is 68.0(1)°.

Scheme 6.

Synthesis of complex 11.

Figure 11.

Molecular structure of 11 (thermal ellipsoids drawn at the 35 % probability level).

However, when 3 is exposed to carbodiimides (RN=)₂C a mixture of products is formed, which consists of the metallaaziridines Cp’’’₂U[C(=PMes*)N(R)] (R=iPr (12), C₆H₁₁ (14)), the imido complexes Cp’’’₂U=NR (R=iPr (13), C₆H₁₁ (15)) and phosphaindane 5 (Scheme 7). The ¹H NMR spectroscopy data show that the complexes 12 and 13 as well as 14 and 15 are formed in a 1:1 ratio. Again, we assume that the initial step involves the replacement of the phosphinidene fragment by (RN=)₂C to give a metallaaziridine, which gives rise to the imido complexes 13 and 15 by isonitrile RNC loss (Scheme 7). However, the released isonitrile RNC may also react with a second molecule of 3 in a [2+1] cycloaddition to furnish the three‐membered metallaheterocycles, followed by a [1,3]‐U migration to yield the metallaaziridines 12 and 14 (Scheme 7). To verify this conjecture, we established that complexes 12 and 14 may also be accessed by the direct reaction of 3 with isonitriles RNC (for details see Experimental Section). It should also be noted that the similar actinide metallaaziridines can also be accessed by the reaction of bis(phosphido) actinide complexes with isocyanides. ^[17] The molecular structure of 14 is shown in Figure 12, whereas the structure of 12 is provided in the Supporting Information. The U−N distances are 2.245(2) Å for 12 and 2.250(3) Å for 14, whereas the U−C distances are 2.369(3) Å for 12 and 2.353(3) Å for 14. These structural parameters are comparable to those found in Cp*₂U[C=P(2,4,6‐Me₃Ph)NtBu](CNtBu) with the U−C distance of 2.369(4) Å, and the U−N distance of 2.293(4) Å, ^[17c] and Cp*₂U[C=P(Ph)NtBu](CNtBu) with the U−C distance of 2.383(3) Å, and the U−N distance of 2.273(2) Å. ^[17b]

Scheme 7.

Synthesis of complexes 12–15.

Figure 12.

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

Conclusions

In summary, the first stable base‐free terminal phosphinidene uranium metallocene, Cp’’’₂U=PMes* (3), was comprehensively studied. Density functional theory (DFT) shows that 5f orbitals contribute substantially to the σ and π‐bonds of the uranium phosphinidene U=PAr moiety and that the bonds between the [Cp’’’₂U]²⁺ and [Mes*P]²⁻ fragments are more covalent than those of the related thorium phosphinidene complex. The coordinated phosphinidene in the terminal phosphinidene thorium metallocenes is inert to ligand exchange,[ ¹⁰ , ¹⁴ ] but it reacts with unsaturated molecules via a [2+2], [2+1] or [2+3] cycloaddition process or acts as a strong base inducing the intermolecular E−H (E=C, Si, N) bond activations.[ ¹⁰ , ¹⁴ ] In contrast, the uranium phosphinidene complex 3 behaves differently, it serves as a synthetically useful Cp’’’₂U(II) synthon in the reactions with unsaturated molecules such as alkynes, imines, thiazoles, ketazines, bipy, organic azides, diazene derivatives, ketones, and carbodiimides, in which the coordinated phosphinidene is readily replaced during the reactions. It is interesting to note that 3 adds to the series of uranium metallocenes which may act as Cp₂U(II) synthons such as Cp’’’₂U(bipy), ^[6a] Cp*₂U[(μ‐Ph)₂BPh₂], ^[18] and Cp*₂U[P(SiMe₃)(2,4,6‐Me₃Ph)](THF). ^[19] Although no phosphorus‐containing species were obtained, it allows us to isolate species which are so far not accessible by other synthetic routes. Further investigations on the intrinsic reactivity of terminal phosphinidene actinide metallocenes and uranium metallacyclopropene complex 4 are in progress and will be detailed 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. Cp’’’₂UMe₂ (1), ^[6a] 2,4,6‐tBu₃C₆H₂PH₂ (Mes*PH₂), ^[20] and 2,4,6‐tBu₃C₆H₂PHK (Mes*PHK) ^[21] were prepared according to literature methods. All other chemicals were purchased from Sigma–Aldrich 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, ¹³C{¹H} and ³¹P{¹H} NMR spectra were recorded on a Bruker AV 400 spectrometer at 400, 100 and 162 MHz, respectively. 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 85 % H₃PO₄ (0.00 ppm) for phosphorus chemical shifts. Melting points were measured on an X‐6 melting point apparatus and were uncorrected. Elemental analyses were performed on a Vario EL elemental analyzer.

Preparation of Cp’’’₂U(I)Me (2): Solid CuI (0.38 g, 2.0 mmol) was slowly added to a stirred toluene (20 mL) solution of Cp’’’₂UMe₂ (1; 1.47 g, 2.0 mmol) at room temperature. During the reaction copper metal (Cu) and ethane CH₃CH₃ were formed. After this solution was stirred at room temperature 3 days, the solvent was removed. The residue was extracted with n‐hexane (10 mL×3) and filtered. The volume of the combined filtrate was reduced to 10 mL, orange crystals of 2 were isolated after this solution was kept at −20 °C for one day. Yield: 1.44 mg (85 %). M.p.: 139–141 °C (dec.). ¹H NMR (400 MHz, C₆D₆): δ=9.12 (s, 18 H, C(CH ₃)₃), 6.45 (s, 18 H, C(CH ₃)₃), −13.02 (s, 18 H, C(CH ₃)₃), −102.51 (s, 3 H, UCH ₃) ppm; ring C−H atoms were not observed. ¹³C{¹H} NMR (100 MHz, C₆D₆): δ=64.0 (C(CH₃)₃), 46.4 (C(CH₃)₃), 44.0 (C(CH₃)₃), 42.0 (C(CH₃)₃), 41.3 (C(CH₃)₃), 40.6 (C(CH₃)₃), −20.9 (UCH₃) ppm; ring C atoms were not observed. IR (KBr): $\tilde{\nu }$ =2958 (s), 1479 (s), 1458 (s), 1363 (s), 1238 (s), 1107 (s), 1020 (s), 997 (s), 837 (s), 808 (s) cm⁻¹. Anal. Calcd for C₃₅H₆₁IU: C, 49.64; H, 7.26. Found: C, 49.73; H, 7.30.

Preparation of Cp’’’₂U=PMes* (3): A THF (10 mL) solution of Mes*PHK (316 mg, 1.0 mmol) was added to a THF (10 mL) solution of Cp’’’₂U(I)Me (2; 847 mg, 1.0 mmol) with stirring at room temperature. After the solution was stirred at room temperature 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, brown crystals of 3 were isolated when this solution was kept at −20 °C for two days. Yield: 736 mg (75 %). M.p.: 130–132 °C (dec.). ¹H NMR (400 MHz, C₆D₆): δ=44.95 (s, 2 H, ring CH), 21.71 (s, 2 H, ring CH), 19.26 (s, 2 H, phenyl), 3.50 (s, 9 H, C(CH ₃)₃), 1.73 (s, 18 H, C(CH ₃)₃), 0.48 (s, 18 H, C(CH ₃)₃), −0.99 (s, 18 H, C(CH ₃)₃), −35.78 (s, 18 H, C(CH ₃)₃) ppm. ¹³C{¹H} NMR (100 MHz, C₆D₆): δ=311.4 (phenyl C), 281.2 (phenyl C), 251.5 (phenyl C), 172.6 (phenyl C), 89.1 (C(CH₃)₃), 61.6 (C(CH₃)₃), 50.2 (C(CH₃)₃), 44.6 (C(CH₃)₃), 31.6 (C(CH₃)₃), 31.2 (d, J _P‐C=10.0 Hz, C(CH₃)₃), 8.6 (C(CH₃)₃), 7.5 (C(CH₃)₃), 3.3 (C(CH₃)₃), 1.5 (C(CH₃)₃), −57.5 (ring C), −58.5 (ring C), −59.5 (ring C), −60.8 (ring C) ppm. IR (KBr): $\tilde{\nu }$ =2955 (s), 1477 (s), 1384 (s), 1357 (s), 1259 (s), 1070 (s), 1016 (s), 812 (s) cm⁻¹. Anal. Calcd for C₅₂H₈₇PU: C, 63.65; H, 8.94. Found: C, 63.68; H, 8.96. Please note that we also attempted to record a ³¹P{¹H} NMR spectrum, however, no resonances were observed even when the sample was measured for two days.

Preparation of Cp’’’₂U(η²‐C₂Ph₂) (4). Method A: A toluene (5 mL) solution of PhC≡CPh (45 mg, 0.25 mmol) was added to a toluene (10 mL) solution of Cp’’’₂U=PMes* (3; 245 mg, 0.25 mmol) with stirring at room temperature. After the solution was stirred at 50 °C two days, the solvent was removed. The residue was extracted with n‐hexane (10 mL × 3) and filtered. The volume of the filtrate was reduced to 2 mL, brown crystals of 4 were isolated when this solution was kept at room temperature for two days. Yield: 245 mg (82 %). M.p.: 178–180 °C (dec.). ¹H NMR (400 MHz, C₆D₆): δ=26.59 (s, 4 H, phenyl), 16.62 (s, 4 H, phenyl), 10.79 (d, J=5.6 Hz, 2 H, phenyl), 9.30 (br s, 18 H, C(CH ₃)₃), −15.00 (br s, 18 H, C(CH ₃)₃), −32.03 (s, 18 H, C(CH ₃)₃) ppm; ring C−H atoms were not observed. ¹³C{¹H} NMR (100 MHz, C₆D₆): δ=202.7 (UC), 201.8 (phenyl C), 201.0 (phenyl C), 151.4 (phenyl C), 138.4 (phenyl C), 137.9 (C(CH₃)₃), 137.3 (C(CH₃)₃), 136.7 (C(CH₃)₃), 85.8 (C(CH₃)₃), −50.1 (ring C), −51.1 (ring C) ppm; one ring C overlapped. IR (KBr): $\tilde{\nu }$ =2960 (s), 1460 (m), 1384 (m), 1259 (s), 1093 (s), 1020 (s), 800 (s) cm⁻¹. Anal. Calcd for C₄₈H₆₈U: C, 65.28; H, 7.76. Found: C, 65.35; H, 7.73. Brown crystals of 4⋅4C₆H₆ suitable for X‐ray structural analysis were grown from a benzene solution.

Method B. NMR Scale: A C₆D₆ (0.3 mL) solution of PhC≡CPh (3.6 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with Cp’’’₂U=PMes* (3; 20 mg, 0.02 mmol) and C₆D₆ (0.2 mL). Resonances of 4 along with those of 3,3‐Me₂‐5,7‐tBu₂C₈H₅P (5) (¹H NMR (400 MHz, C₆D₆): δ=7.46 (dd, J=3.8, 1.5 Hz, 2 H, phenyl), 4.39 (ddd, J=181.6, 11.9, 7.9 Hz, 1 H, PH), 1.59 (d, J=3.6 Hz, 1 H, CH ₂), 1.56 (s, 9 H, (CH ₃)₃C), 1.34 (s, 3 H, CH ₃), 1.31 (s, 9 H, (CH ₃)₃C), 1.29 (d, J=3.6 Hz, 1 H, CH ₂), 1.11 (s, 3 H, CH ₃) ppm. ³¹P{¹H} NMR (162 MHz, C₆D₆): δ=−79.5 ppm) ^[10a] were observed by NMR spectroscopy (100 % conversion) when this solution was kept at 50 °C for 2 days.

Preparation of Cp’’’₂U(η²‐CHPhNPh) (6). Method A: This compound was obtained as brown crystals from the reaction of Cp’’’₂U=PMes* (3; 245 mg, 0.25 mmol) and PhCH=NPh (46 mg, 0.25 mmol) in toluene (15 mL) at 50 °C and recrystallization from an n‐hexane solution by a similar procedure as that in the synthesis of 4. Yield: 184 mg (83 %). M.p.: 215–217 °C (dec.). ¹H NMR (400 MHz, C₆D₆): δ=129.18 (s, 1 H, CHPh), 34.03 (s, 1 H, phenyl), 26.49 (s, 2 H, phenyl), 23.78 (s, 1 H, phenyl), 13.63 (s, 9 H, C(CH ₃)₃), 13.36 (s, 2 H, phenyl), 12.24 (s, 9 H, C(CH ₃)₃), 7.42 (s, 1 H, phenyl), −0.60 (s, 1 H, phenyl), −2.57 (s, 1 H, phenyl), −10.04 (s, 9 H, C(CH ₃)₃), −17.56 (s, 9 H, C(CH ₃)₃), −35.03 (s, 9 H, C(CH ₃)₃), −42.50 (s, 9 H, C(CH ₃)₃), −68.53 (s, 1 H, phenyl) ppm; ring C−H atoms were not observed. ¹³C{¹H} NMR (100 MHz, C₆D₆): δ=228.0 (UC), 213.3 (phenyl C), 201.5 (phenyl C), 173.3 (phenyl C), 173.1 (phenyl C), 159.7 (phenyl C), 139.6 (phenyl C), 134.4 (phenyl C), 125.8 (phenyl C), 54.6 (C(CH₃)₃), 35.1 (C(CH₃)₃), 21.1 (C(CH₃)₃), 20.2 (C(CH₃)₃), −43.5 (ring C), −66.1 (ring C), −78.5 (ring C) ppm; other carbons overlapped. IR (KBr): $\tilde{\nu }$ =2958 (s), 1602 (s), 1506 (s), 1359 (s), 1261 (s), 1097 (s), 1028 (s), 748 (s) cm⁻¹. Anal. Calcd for C₄₇H₆₉NU: C, 63.71; H, 7.85; N, 1.58. Found: C, 63.75; H, 7.83; N, 1.52. Method B (NMR scale): A C₆D₆ (0.3 mL) solution of PhCH=NPh (3.6 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with Cp’’’₂U=PMes* (3; 20 mg, 0.02 mmol) and C₆D₆ (0.2 mL). Resonances of 6 along with those of 3,3‐Me₂‐5,7‐tBu₂C₈H₅P (5) were observed by ¹H NMR spectroscopy (100 % conversion) when this solution was kept at 50 °C for 2 days.

Preparation of Cp’’’₂U(SCH=CHN=CH) (7): Method A: This compound was obtained as brown crystals from the reaction of Cp’’’₂U=PMes* (3; 245 mg, 0.25 mmol) and thiazole (22 mg, 0.25 mmol) in toluene (15 mL) at 50 °C and recrystallization from an n‐hexane solution by a similar procedure as that in the synthesis of 4. Yield: 198 mg (86 %). M.p.: 98–100 °C (dec.). ¹H NMR (C₆D₆): δ 70.47 (s, 1 H, CH), 40.47 (s, 1 H, CH), 17.59 (br s, 2 H, ring CH), 15.97 (br s, 2 H, ring CH), 7.47 (s, 1 H, CH), 0.38 (s, 18 H, C(CH ₃)₃), −1.87 (s, 18 H, C(CH ₃)₃), −10.49 (s, 18 H, C(CH ₃)₃) ppm. ¹³C{¹H} NMR (C₆D₆): δ 271.5 (UC), 158.9 (CH=CHN), 152.2 (CH=CHN), 46.9 (C(CH₃)₃), 41.9 (C(CH₃)₃), 36.8 (C(CH₃)₃), 35.6 (C(CH₃)₃), 34.8 (C(CH₃)₃), −4.4 (ring C), −20.6 (ring C), −20.7 (ring C), −71.2 (ring C) ppm; other C atoms overlapped. IR (KBr, cm⁻¹): $\tilde{\nu }$ =2957 (s), 1595 (m), 1479 (s), 1460 (s), 1390 (s), 1359 (s), 1261 (s), 1240 (s), 1095 (s), 1020 (s), 808 (s). Anal. Calcd for C₃₇H₆₁NSU: C, 56.25; H, 7.78; N, 1.77. Found: C, 56.22; H, 7.83; N, 1.72. Method B (NMR scale): A C₆D₆ (0.3 mL) solution of thiazole (1.7 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with Cp’’’₂U=PMes* (3; 20 mg, 0.02 mmol) and C₆D₆ (0.2 mL). Resonances of 7 along with those of 3,3‐Me₂‐5,7‐tBu₂C₈H₅P (5) were observed by ¹H NMR spectroscopy (100 % conversion) when this solution was kept at 50 °C for 2 days.

Preparation of Cp’’’₂U(N=CPh₂)₂ (8): Method A: This compound was obtained as brown crystals from the reaction of Cp’’’₂U=PMes* (3; 245 mg, 0.25 mmol) and (Ph₂C=N)₂ (90 mg, 0.25 mmol) in toluene (15 mL) at 50 °C and recrystallization from a benzene solution by a similar procedure as that in the synthesis of 4. Yield: 226 mg (85 %). M.p.: 155–157 °C (dec.). ¹H NMR (400 MHz, C₆D₆): δ=33.06 (br s, 2 H, ring CH), 14.59 (br s, 6 H, C(CH ₃)₃), 12.45 (br s, 18 H, C(CH ₃)₃), 7.70 (s, 1 H, phenyl), 7.41 (s, 2 H, phenyl), 7.37 (s, 1 H, phenyl), 7.04 (s, 2 H, phenyl), 2.29 (s, 18 H, C(CH ₃)₃), 1.45 (s, 9 H, phenyl), 1.28 (s, 5 H, phenyl), −23.34 (br s, 12 H, C(CH ₃)₃), −75.71 (br s, 2 H, ring CH) ppm. ¹³C{¹H} NMR (100 MHz, C₆D₆): δ=160.1 (phenyl C), 155.3 (phenyl C), 151.7 (phenyl C), 148.8 (phenyl C), 138.8 (phenyl C), 136.4 (phenyl C), 129.7 (phenyl C), 122.6 (phenyl C), 104.7 (N=C), 56.2 (C(CH₃)₃), 38.5 (C(CH₃)₃), 34.9 (C(CH₃)₃), 34.8 (C(CH₃)₃), 34.2 (C(CH₃)₃), 34.1 (C(CH₃)₃), −28.4 (ring C) ppm; other carbons were not observed. IR (KBr): $\tilde{\nu }$ =2958 (s), 1600 (s), 1583 (s), 1562 (s), 1359 (s), 1238 (s), 1028 (m), 825 (s) cm⁻¹. Anal. Calcd for C₆₀H₇₈N₂U: C, 67.65; H, 7.38; N, 2.63. Found: C, 67.68; H, 7.33; N, 2.62. Method B (NMR scale): A C₆D₆ (0.3 mL) solution of (Ph₂C=N)₂ (7.2 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with Cp’’’₂U=PMes* (3; 20 mg, 0.02 mmol) and C₆D₆ (0.2 mL). Resonances of 8 along with those of 3,3‐Me₂‐5,7‐tBu₂C₈H₅P (5) were observed by ¹H NMR spectroscopy (100 % conversion) when this solution was kept at 50 °C for 36 h.

Preparation of Cp’’’₂U(bipy) (9): Method A: This compound was obtained as green crystals from the reaction of Cp’’’₂U=PMes* (3; 245 mg, 0.25 mmol) and bipy (39 mg, 0.25 mmol) in toluene (15 mL) at 50 °C and recrystallization from a benzene solution by a similar procedure as that in the synthesis of 4. Yield: 177 mg (82 %). ¹H NMR (400 MHz, C₆D₆): δ=1.26 (s, 4 H, ring CH), 1.17 (s, 36 H, C(CH ₃)₃), −7.47 (d, J=4.9 Hz, 2 H, bipy), −9.01 (s, 18 H, C(CH ₃)₃), −58.93 (s, 2 H, bipy), −99.40 (s, 2 H, bipy), −125.80 (s, 2 H, bipy) ppm. These spectroscopic data agreed with those reported in the literature. ^[6a] Method B (NMR scale): A C₆D₆ (0.3 mL) solution of bipy (3.1 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with Cp’’’₂U=PMes* (3; 20 mg, 0.02 mmol) and C₆D₆ (0.2 mL). Resonances of 9 along with those of 3,3‐Me₂‐5,7‐tBu₂C₈H₅P (5) were observed by ¹H NMR spectroscopy (100 % conversion) when this solution was kept at 50 °C 36 h.

Preparation of Cp’’’₂U(=NPh)₂ (10): Method A: This compound was obtained as brown crystals from the reaction of Cp’’’₂U=PMes* (3; 245 mg, 0.25 mmol) and PhN=NPh (46 mg, 0.25 mmol) in toluene (15 mL) at 50 °C and recrystallization from a benzene solution by a similar procedure as that in the synthesis of 4. Yield: 177 mg (80 %). M.p.: 195–197 °C (dec.). ¹H NMR (400 MHz, C₆D₆): δ=9.48 (t, J=7.5 Hz, 4 H, phenyl), 4.99 (s, 4 H, ring CH), 3.07 (d, J=6.6 Hz, 4 H, phenyl), 1.65 (s, 36 H, C(CH ₃)₃), 1.62 (s, 18 H, C(CH ₃)₃), 0.17 (t, J=7.2 Hz, 2 H, phenyl) ppm. ¹³C{¹H} NMR (100 MHz, C₆D₆): δ=186.6 (phenyl C), 167.4 (phenyl C), 156.4 (phenyl C), 146.7 (phenyl C), 125.5 (ring C), 107.9 (ring C), 107.5 (ring C), 38.1 (C(CH₃)₃), 38.0 (C(CH₃)₃), 31.4 (C(CH₃)₃), 29.5 (C(CH₃)₃) ppm. IR (KBr): $\tilde{\nu }$ =2951 (s), 1573 (m), 1464 (s), 1357 (s), 1261 (s), 1236 (s), 1093 (s), 1020 (s), 800 (s) cm⁻¹. Anal. Calcd for C₄₆H₆₈N₂U: C, 62.28; H, 7.73; N, 3.16. Found: C, 62.31; H, 7.72; N, 3.12. Method B (NMR scale): A C₆D₆ (0.3 mL) solution of PhN=NPh (3.6 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with Cp’’’₂U=PMes* (3; 20 mg, 0.02 mmol) and C₆D₆ (0.2 mL). Resonances of 10 along with those of 3,3‐Me₂‐5,7‐tBu₂C₈H₅P (5) were observed by ¹H NMR spectroscopy (100 % conversion) when this solution was kept at 50 °C for 2 days. Method C (NMR scale): A C₆D₆ (0.3 mL) solution of PhN₃ (4.8 mg, 0.04 mmol) was slowly added to a J. Young NMR tube charged with Cp’’’₂U=PMes* (3; 20 mg, 0.02 mmol) and C₆D₆ (0.2 mL). Resonances of 10 along with those of 3,3‐Me₂‐5,7‐tBu₂C₈H₅P (5) were observed by ¹H NMR spectroscopy (100 % conversion in 10 min).

Reaction of Cp’’’₂U=PMes* (3) with PhN₃: NMR Scale: A C₆D₆ (0.2 mL) solution of PhN₃ (2.4 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with Cp’’’₂U=PMes* (3; 20 mg, 0.02 mmol) and C₆D₆ (0.3 mL). Resonances of 10 along with those of unreacted 3 and 3,3‐Me₂‐5,7‐tBu₂C₈H₅P (5) were observed by ¹H NMR spectroscopy (50 % conversion based on 3, in 10 min).

Preparation of Cp’’’₂U[(OCPh₂)₂]⋅0.5C₆H₆ (11⋅0.5C₆H₆): Method A: This compound was obtained as orange crystals from the reaction of Cp’’’₂U=PMes* (3; 245 mg, 0.25 mmol) and Ph₂CO (91 mg, 0.50 mmol) in toluene (15 mL) at 50 °C and recrystallization from a benzene solution by a similar procedure as that in the synthesis of 4. Yield: 227 mg (82 %). M.p.: 154–156 °C (dec.). ¹H NMR (400 MHz, C₆D₆): δ=61.95 (s, 2 H, ring CH), 27.41 (s, 1 H, phenyl), 23.01 (s, 1 H, phenyl), 16.08 (s, 1 H, phenyl), 13.84 (s, 18 H, C(CH ₃)₃), 13.42 (s, 1 H, phenyl), 9.72 (s, 2 H, phenyl), 8.43 (s, 2 H, phenyl), 7.66 (s, 4 H, phenyl), 7.15 (s, 3 H, C₆ H ₆), 7.01 (s, 5 H, phenyl), 4.61 (s, 1 H, phenyl), 2.90 (s, 1 H, phenyl), −1.30 (s, 1 H, phenyl), −5.29 (s, 18 H, C(CH ₃)₃), −23.24 (s, 2 H, ring CH), −43.97 (s, 18 H, C(CH ₃)₃) ppm. ¹³C{¹H} NMR (100 MHz, C₆D₆): δ=138.3 (phenyl C), 137.9 (phenyl C), 136.7 (phenyl C), 132.0 (phenyl C), 130.2 (phenyl C), 130.1 (phenyl C), 128.5 (C ₆H₆), 89.4 (OC), 65.3 (C(CH₃)₃), 63.4 (C(CH₃)₃), 33.9 (C(CH₃)₃), 32.2 (C(CH₃)₃), 29.7 (C(CH₃)₃), 28.8 (C(CH₃)₃), −65.3 (ring C) ppm; other carbons were not observed. IR (KBr): $\tilde{\nu }$ =2958 (s), 1599 (s), 1446 (s), 1317 (s), 1276 (s), 1028 (s), 920 (s), 763 (s) cm⁻¹. Anal. Calcd for C₆₃H₈₁O₂U: C, 68.27; H, 7.37. Found: C, 68.31; H, 7.32. Method B (NMR scale): A C₆D₆ (0.3 mL) solution of Ph₂CO (7.3 mg, 0.04 mmol) was slowly added to a J. Young NMR tube charged with Cp’’’₂U=PMes* (3; 20 mg, 0.02 mmol) and C₆D₆ (0.2 mL). Resonances of 11 along with those of 3,3‐Me₂‐5,7‐tBu₂C₈H₅P (5) were observed by ¹H NMR spectroscopy (100 % conversion) when this solution was kept at 50 °C for 36 h.

Reaction of Cp’’’₂U=PMes* (3) with Ph₂CO: NMR Scale: A C₆D₆ (0.2 mL) solution of Ph₂CO (3.6 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with Cp’’’₂U=PMes* (3; 20 mg, 0.02 mmol) and C₆D₆ (0.3 mL). Resonances of 11 along with those of unreacted 3 and 3,3‐Me₂‐5,7‐tBu₂C₈H₅P (5) were observed by ¹H NMR spectroscopy (50 % conversion based on 3) after the sample was kept at 50 °C for 36 h.

Preparation of Cp’’’₂U[C(=PMes*)N(iPr)] (12): Method A: This compound was obtained as brown crystals from the reaction of Cp’’’₂U=PMes* (3; 245 mg, 0.25 mmol) and (iPrN=)₂C (17 mg, 0.13 mmol) in toluene (15 mL) at 50 °C and recrystallization from an n‐hexane solution by a similar procedure as that in the synthesis of 4. Yield: 100 mg (38 % based on U). M.p.: 115–117 °C (dec.). ¹H NMR (400 MHz, C₆D₆): δ=116.29 (s, 1 H, NCH), 41.10 (s, 3 H, CH ₃), 29.49 (s, 3 H, CH ₃), 22.45 (s, 1 H, phenyl), 17.61 (s, 1 H, phenyl), 16.05 (s, 18 H, C(CH ₃)₃), 10.99 (s, 9 H, C(CH ₃)₃), 8.76 (s, 9 H, C(CH ₃)₃), 3.56 (s, 9 H, C(CH ₃)₃), 3.54 (s, 9 H, C(CH ₃)₃), −19.83 (br s, 18 H, C(CH ₃)₃), −38.81 (s, 9 H, C(CH ₃)₃) ppm; protons of the rings were not observed. ¹³C{¹H} NMR (100 MHz, C₆D₆): δ=129.3 (phenyl C), 126.7 (phenyl C), 125.6 (phenyl C), 121.8 (phenyl C), 59.7 (NC), 40.1 (C(CH₃)₃), 37.8 (C(CH₃)₃), 32.7 (CH₃), 32.6 (CH₃), 31.5 (C(CH₃)₃), 30.9 (C(CH₃)₃) ppm; other carbons were not observed. ³¹P{¹H} NMR (162 MHz, C₆D₆): δ=857.4 ppm. IR (KBr): $\tilde{\nu }$ =2960 (s), 1591 (m), 1512 (s), 1460 (s), 1388 (s), 1359 (s), 1238 (s), 1020 (m), 812 (s) cm⁻¹. Anal. Calcd for C₅₆H₉₄NPU: C, 64.04; H, 9.02; N, 1.33. Found: C, 64.08; H, 8.99; N, 1.32. Method B (NMR scale): A C₆D₆ (0.3 mL) solution of (iPrN=)₂C (2.6 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with Cp’’’₂U=PMes* (3; 20 mg, 0.02 mmol) and C₆D₆ (0.2 mL). Resonances of 12 along with those of Cp’’’₂U=NiPr (13) (¹H NMR (400 MHz, C₆D₆): δ=37.20 (br s, 1H NCH), 1.71 (s, 3 H, CH ₃), 1.67 (s, 3 H, CH ₃), −18.33 (br s, 18 H, C(CH ₃)₃), −25.45 (br s, 18 H, C(CH ₃)₃), −35.10 (br s, 18 H, C(CH ₃)₃) ppm; protons of the rings were not observed) and 3,3‐Me₂‐5,7‐tBu₂C₈H₅P (5) were observed by ¹H NMR spectroscopy (100 % conversion) when this solution was kept at 50 °C overnight. Complex 13 was not isolated as a pure compound on a synthetic scale, since it was an oily residue and very soluble in solvents such as benzene and n‐hexane and 12 could not be removed completely. Method C (NMR scale): A C₆D₆ (0.3 mL) solution of iPrNC (1.4 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with Cp’’’₂U=PMes* (3; 20 mg, 0.02 mmol) and C₆D₆ (0.2 mL). Resonances of 12 were observed by ¹H NMR spectroscopy (100 % conversion) after the sample was kept at room temperature overnight.

Preparation of Cp’’’₂U[C(=PMes*)N(C₆H₁₁)] (14): Method A: This compound was obtained as brown crystals from the reaction of Cp’’’₂U=PMes* (3; 245 mg, 0.25 mmol) and DCC (52 mg, 0.25 mmol) in toluene (15 mL) at 50 °C and recrystallization from an n‐hexane solution by a similar procedure as that in the synthesis of 4. Yield: 93 mg (34 % based on U). M.p.: 170–172 °C (dec.). ¹H NMR (400 MHz, C₆D₆): δ=115.37 (s, 1 H, NCH), 54.77 (s, 1 H, Cy), 37.96 (s, 1 H, Cy), 30.76 (s, 1 H, Cy), 26.32 (s, 1 H, Cy), 22. 65 (s, 1 H, Cy), 21.23 (s, 2 H, phenyl), 20.27 (s, 1 H, Cy), 17.22 (s, 1 H, Cy), 16.53 (s, 18 H, C(CH ₃)₃), 15.79 (s, 1 H, Cy), 12.73 (s, 1 H, Cy), 12.37 (s, 1 H, Cy), 10.70 (s, 9 H, C(CH ₃)₃), 8.10 (s, 9 H, C(CH ₃)₃), 4.02 (s, 9 H, C(CH ₃)₃), −19.91 (br s, 18 H, C(CH ₃)₃), −38.77 (s, 18 H, C(CH ₃)₃) ppm; protons of the rings were not observed. ¹³C{¹H} NMR (100 MHz, C₆D₆): δ=165.7 (C=P), 161.9 (phenyl C), 150.4 (phenyl C), 145.9 (phenyl C), 136.9 (phenyl C), 63.0 (NCH), 60.7 (Cy C), 54.7 (Cy C), 46.9 (Cy C), 43.4 (C(CH₃)₃), 43.2 (C(CH₃)₃), 39.6 (C(CH₃)₃), 37.0 (C(CH₃)₃), 31.4 (C(CH₃)₃), 19.7 (C(CH₃)₃), −49.4 (ring C), ppm; other carbons were not observed. ³¹P{¹H} NMR (162 MHz, C₆D₆): δ=874.6 ppm. IR (KBr): $\tilde{\nu }$ =2957 (s), 2928 (s), 1506 (s), 1460 (s), 1386 (s), 1357 (s), 1292 (s), 1226 (s), 1097 (s), 1022 (s), 875 (s), 808 (s) cm⁻¹. Anal. Calcd for C₅₉H₉₈NPU: C, 64.99; H, 9.06; N, 1.28. Found: C, 64.98; H, 9.09; N, 1.30. Method B (NMR scale): A C₆D₆ (0.3 mL) solution of DCC (2.1 mg, 0.01 mmol) was slowly added to a J. Young NMR tube charged with Cp’’’₂U=PMes* (3; 20 mg, 0.02 mmol) and C₆D₆ (0.2 mL). Resonances of 14 along with those of Cp’’’₂U=NC₆H₁₁ (15) (¹H NMR (400 MHz, C₆D₆): δ=31.26 (br s, 2 H, CH ₂), 25.10 (br s, 2 H, CH ₂), 21.48 (s, 1 H, CHN), 20.12 (s, 2 H, CH ₂), 16.10 (s, 2 H, CH ₂), 14.24 (s, 2 H, CH ₂), 12.65 (br s, 18 H, C(CH ₃)₃), −14.39 (br s, 18 H, C(CH ₃)₃), −35.89 (s, 18 H, C(CH ₃)₃) ppm; protons of the rings were not observed) and 3,3‐Me₂‐5,7‐tBu₂C₈H₅P (5) were observed by ¹H NMR spectroscopy (100 % conversion) when this solution was kept at 50 °C overnight. Complex 15 was not isolated as a pure compound on a synthetic scale, since it was an oily residue and very soluble in solvents such as benzene and n‐hexane and 14 could not be removed completely. Method C (NMR Scale): A C₆D₆ (0.3 mL) solution of C₆H₁₁NC (2.2 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with Cp’’’₂U=PMes* (3; 20 mg, 0.02 mmol) and C₆D₆ (0.2 mL). Resonances of 14 were observed by ¹H NMR spectroscopy (100 % conversion) after the sample 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 Cu Kα radiation (λ=1.54184 Å). An empirical absorption correction was applied using the SADABS program. ^[22] All structures were solved by direct methods and refined by full‐matrix least squares on F ² using the SHELXL program package. ^[23] The hydrogen atoms were geometrically fixed using a riding model. The crystal data and experimental data for 2–4, 6–12, 14 and 17 are summarized in Tables S1–S3. Selected bond lengths and angles are listed in Table 1.

Deposition numbers 2002932 (2), 2002931 (3), 2002929 (4), 2002930 (6), 2002928 (7), 2002936 (8), 2002933 (9), 2002934 (10), 2002939 (11), 2002937 (12), 2002942 (14), and 2004059 (17) contain the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service.

Computational methods: All calculations were carried out with the Gaussian 09 program (G09), ^[24] employing the B3PW91 functional, plus a polarizable continuum model (PCM) (denoted as B3PW91‐PCM), with standard 6‐31G(d) basis set for C, H and P elements and a quasi‐relativistic 5f‐in‐valence effective‐core potential (ECP60MWB) treatment with 60 electrons in the core region for U and the corresponding optimized segmented ((14s13p10d8f6g)/[10s9p5d4f3g]) basis set for the valence shells of U, ^[25] 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) were 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, respectively, on their potential energy hypersurfaces. In order to consider the dispersion effect for the reactions 3+PhC≡CPh, single‐point B3PW91‐PCM‐D3 ^[26] calculations, based on B3PW91‐PCM geometries, have been performed.

Conflict of interest

The authors declare no conflict of interest.

Supporting information

As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.

Supplementary

D. Wang, W. Ding, G. Hou, G. Zi, M. D. Walter, Chem. Eur. J. 2020, 26, 16888.