Insertion of ^tBuNC into thorium–phosphorus and thorium–arsenic bonds: phosphaazaallene and arsaazaallene moieties in f element chemistry

Abstract

The reactivity of thorium–phosphido and thorium–arsenido bonds was probed using tert-butyl isocyanide, ^tBuNC. Reaction of (C₅Me₅)₂Th[E(H)R]₂, E = P, As; R = 2,4,6-ⁱPr₃C₆H₂, 2,4,6-Me₃C₆H₂ with ^tBuNC affords the first phosphaazaallene and arsaazaallene moieties with an f-element.

---

Introduction

Hydroelementation reactions such as hydrophosphination^1,2 are atom efficient processes which are important in developing building blocks containing phosphorus. For example, tertiary phosphines are of interest as ligands^3–5 and for various applications.^6–8 However, the development of these reactions in organoactinide chemistry⁹ has been attenuated by a lack of starting materials.

Despite the intensity with which complexes with actinide–nitrogen bonds have been studied,^10–13 there exists a tremendous knowledge gap with respect to the heavier pnictogen elements. To date, twenty actinide–phosphido or phosphinidene bonds^14–26 and five actinide–arsenido bonds^27–29 have been reported. Of these, only nine thorium–phosphorus and one thorium–arsenic bond are known.

Due to the dearth of actinide–phosphorus and actinide–arsenic bonds, the reactivity of these bonds is unknown. Migratory insertion, the initial step in many catalytic cycles, has been used historically to probe reactivity. In transition metals, primary phosphido complexes are also sparse,^30–37 but Waterman's group has investigated the reactivity of zirconium–phosphorus bonds with ^tBuNC.³⁸ Interestingly, a proton migration from the phosphorus to the carbon of the isocyanide occurs. Here we describe the synthesis of new thorium phosphido and arsenido complexes and their reactivity with ^tBuNC, which differs from their transition metal analogs.

Results and discussion

The synthesis of (C₅Me₅)₂Th[P(H)Tipp]₂, Tipp = 2,4,6-ⁱPr₃C₆H₂, the first primary phosphido complex of thorium, was recently described.²⁶ In the same vein, we have begun investigating the reactivity of primary phosphido and arsenido complexes. In an effort to expand the scope of thorium–pnictogen complexes, we synthesized (C₅Me₅)₂Th[P(H)Mes]₂, 1, Mes = 2,4,6-Me₃C₆H₂, from the stoichiometric salt metathesis reaction between (C₅Me₅)₂ThCl₂ and KP(H)Mes, eqn (1). Complex 1 was isolated as a vibrant orange crystalline solid in 54% yield. The diagnostic spectroscopic features include the doublet P–H resonance and the ν_PH stretch centered at 3.80 ppm with ¹J_P–H = 224 Hz and 2304 cm⁻¹, respectively. The large ¹J_P–H coupling constant reflects the large amount of s-character in the P–H bond.

The ³¹P{¹H} resonance is located at 15.37 ppm and compares well to the ³¹P{¹H} resonance of a structurally similar thorium–phosphido compound, (C₅Me₅)₂Th[P(H)Tipp]₂, located at 1.66 ppm. The molecular structure of 1 is shown in Fig. 1 and mimics the bond distances and angles of (C₅Me₅)₂Th[P(H)Tipp]₂.

Fig. 1 Thermal ellipsoid plot of 1 at the 50% probability level. Hydrogens have been omitted for clarity. Selected bond distances (Å) and angles (°): Th1–P1, 2.872(5); P1–C11, 1.829(3); P1–Th–P1*, 102.68(2); Th1–P1–C11, 128.45(9).

While actinide–phosphido complexes are few in number, the number of structurally characterized actinide–arsenido compounds is five: a bimetallic thorium poly-arsenide cluster, [Cp′₂Th(μ–η^2:1:2:1-As₆)ThCp′₂] (Cp′ = C₅H₃^tBu₂),²⁹ and a series of uranium(IV) complexes: [{U(Tren^TIPS)}₂(μ–η²:η²-As₂H₂)],²⁷ [U(Tren^TIPS)(AsH₂)], [U(Tren^TIPS)(AsH)][K(B15C5)₂], and [{U(Tren^TIPS)(AsK₂)}₄] Tren^TIPS = N(CH₂CH₂NSiPrⁱ₃).²⁸ We hypothesized that the 2,4,6-ⁱPr₃C₆H₂ framework would be sterically large enough to stabilize an actinide metal center such as thorium. Using room temperature σ-bond metathesis between (C₅Me₅)₂ThMe₂ and two equivalents of H₂AsTipp, we successfully isolated the first organothorium primary arsenido complex, (C₅Me₅)₂Th[As(H)Tipp]₂, 2, eqn (2):

Compound 2 was isolated as a ruby red crystalline solid in 70% yield. The diagnostic spectroscopic handles include the As–H resonance at δ 2.61 in the ¹H NMR spectrum and the ν_AsH stretch at 2089 cm⁻¹ in the IR spectrum. The IR stretching frequencies compare well to the ν_AsH stretches at 2061 cm⁻¹ and 2052 cm⁻¹ for zirconium(IV) and uranium(IV) primary arsenido complexes, [(N₃N)ZrAsHR] (R = Mes and Ph; N₃N = N(CH₂CH₂NSiMe₃)₃³⁻),³⁹ [U(Tren^TIPS)(AsH₂)]²⁸ reported by Waterman and Liddle's groups, respectively. The molecular structure of 2 is shown in Fig. 2. The Th1–As1 bond length is 3.0028(6) Å and is slightly longer than the sum of the single bond covalent radii for thorium and arsenic (2.96 Å).⁴⁰ The Th1–As1–C11 bond angle is 116.53(15)°.

Fig. 2 Thermal ellipsoid plot of 2 at the 50% probability level. Hydrogens have been omitted for clarity. Selected bond distances (Å) and angles (°): Th1–As1, 3.0028(6); As1–C11, 1.959(5); As1–Th1–As1*, 88.02(2); Th1–As1–C11, 116.53(15).

We sought to investigate the reactivity of 1 and 2 through insertion reaction with CO surrogates. Waterman's group has reported on the generation of phosphaalkenes and arsaalkenes from the reaction between a primary phosphido/arsenido organometallic complex and an alkyl isocyanide.^38,39 We anticipated a similar reactivity with our thorium complexes. When tert-butyl isocyanide was added to (C₅Me₅)₂Th[P(H)Tipp]₂ or 1 the solution underwent a color change to yellow, eqn (3). Initial spectroscopic experiments showed that one equivalent of the primary phosphine had been formed in the reaction. After recrystallization from a concentrated methylcyclohexane solution, the η²-(N,C)-phosphaazaallene thorium complexes [(C₅Me₅)₂Th(CN^tBu)(η²-N,C)-(^tBuNC=PTipp)], 3, and [(C₅Me₅)₂Th(CN^tBu)(η²-N,C)-(^tBuNC=PMes)], 4, were isolated as yellow solids.

The diagnostic spectroscopic features associated with 3 and 4 include the stretches at 2181 and 2186 cm⁻¹, and 1600 and 1602 cm⁻¹, which can be assigned to the ν_CN and ν_CP stretches, respectively. The ³¹P NMR resonances shifted slightly upfield from the starting material to −21.28 and −10.70 ppm for 3 and 4, respectively. Additionally, the ¹³C NMR resonance of the central carbon of the phosphaazaallene was found at 150.85 and 151.05 ppm with ¹J_P−C = 152.3 Hz and 103.0 Hz, for 3 and 4, respectively. Compound 2 exhibited a similar reactivity to yield a η²-(N,C)-arsaazaallene thorium complex [(C₅Me₅)₂Th(CN^tBu)(η²-N,C)-(^tBuNC=AsTipp)], 5, as an orange solid. The spectroscopic features of 5 can be found in Table 1.

Table 1 Spectroscopic features of compounds 3, 4, and 5

	^31P{^1H} (δ)	^13C{^1H} central allene carbon (δ)	ν CN (cm^−1)	ν CE (E = P, As) (cm^−1)
3	−21.28	150.85, ^1JP–C = 152.3 Hz	2181	1600
4	−10.70	151.05, ^1JP–C = 103.0 Hz	2186	1602
5		154.25	2182	1513

---

The solid-state structures of 4 and 5 were determined using X-ray diffraction studies, Fig. 3. Table 2 lists selected bond distances (Å) and angles (°). Compounds 4 and 5 are isostructural with one another and represent the first examples of actinide phospha- and arsaazaallene complexes. As with transition metals, such complexes are very rare as only two phosphaazaallene compounds have been isolated: (η¹-Nacnac)Ti(CN^tBu)(η²-N,C)-^tBuNC=PMes*)⁴¹ (Nacnac = [2,6-ⁱPr₂C₆H₃]NC(CH₃)CHC(CH₃)N[2,6-ⁱPr₂C₆H₃], Mes* = 2,4,6-^tBu₃C₆H₂) and Cp′₂Nb(Cl)(η²-N,C)-PhNC=PMes*).⁴²

Fig. 3 Thermal ellipsoid plots of 4 and 5 at the 50% probability level. Hydrogens have been omitted for clarity.

Table 2 Selected bond distances (Å) and angles (°) for 4 and 5

	Th1–C11	Th1–N2	Th1–C12	N1–C12	N2–C11	C11–E1	N2–C11–E1	C11–E1–C19
4	2.430(6)	2.346(5)	2.643(6)	1.131(8)	1.348(8)	1.691(6)	152.1(5)	115.8(3)
5	2.419(5)	2.364(4)	2.638(6)	1.128(7)	1.347(7)	1.822(5)	150.3(4)	114.5(2)

---

There is a substantial elongation of the N–C bond (N2–C11) to 1.348(8) and 1.347(7) Å in 4 and 5, respectively, compared to a metal free heterocumulene such as PhN=C=PMes*, with a N–C bond distance of 1.210 Å.⁴³ The N–C bonds in 4 and 5 are also longer than those found in products of isocyanide insertion into actinide–alkyl bonds. For example, 1.276(7) Å was observed in (C₅Me₅)(C₈H₈)U[η²-(N,C)-C(Ph)=N^tBu].⁴⁴ Additionally the N–C–E bond angle is decreased to 152.1(5) and 150.3(4)°, respectively, relative to the N–C–E bond angle of 171.0° of PhN=C=PMes*. The C=P bond distance in 4 of 1.691(6) Å is only slightly longer than the C=P of 1.651(1) Å in PhN=C=PMes* and matches the C=P bond length of 1.688(19) Å in (C₅H₄SiMe₃)₂Nb(Cl)[η²-(N,C)-PhNC=P(2,4,6-^tBu₃C₆H₂)]. The Th1–N2 bond distances of 2.346(5) and 2.364(4) Å in 4 and 5, respectively, compare well to other thorium–amido bond lengths of 2.389(2) Å, [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂Th(Cl)[N(p-tolyl)SiH₂Ph];⁴⁵ 2.322(5) Å, [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂Th[N(p-tolyl(Se–Se))];⁴⁵ and 2.256(8) Å, (C₅Me₅)₂Th[NC(Ph)(CH₂Ph)]₂.⁴⁶

The formation of 4 and 5 is expected to occur through a 1,1 insertion of the alkyl isocyanide in the Th–P bond. Unlike the [(N₃N)ZrEHR] (E = P, As; R = Cy, Ph) complexes which can undergo 1,2 rearrangement to phospha/arsa-azaalkenes, 4 and 5 do not undergo rearrangement, rather a double reduction of the alkyl isocyanide with the concomitant release of H₂ER (E = P, As; R = Tipp, Mes). There are two conceivable reaction pathways for the generation of 4 and 5, Fig. 4. The first involves a transient terminal thorium–phosphinidene intermediate. There is a literature precedent for this route as Mindiola's group have reported the reaction of a terminal titanium phosphinidene, (η¹-Nacnac)Ti(CH₂^tBu)(PMes*), with a tert-butyl isocyanide to yield the titanium phosphaazaallene complex, (η¹-Nacnac)Ti(CN^tBu)(η²-N,C)-^tBuN=C=PMes*).⁴¹ The other route is 1,1 insertion of the isocyanide to form an η²-iminoacyl, followed by an intramolecular deprotonation. To investigate the possible reaction pathway, we attempted the addition of tert-butyl isocyanide to (C₅Me₅)₂Th[P(H)Tipp]₂ at −200 °C and slowly warmed the reaction while monitoring the reaction progress using ³¹P NMR. At −80 °C we observed the formation of 4, H₂PTipp, (C₅Me₅)₂Th[P(H)Tipp]₂, and a singlet at −26.5 ppm. Upon heating to −70 °C the reaction was complete with disappearance of the resonance at −26.5 ppm. This resonance at −26.5 ppm has not been identified but it is possible that it is 4 without a coordinated isocyanide. We saw no evidence of a transient terminal phosphinidene as no resonance >100 ppm was observed (see ESI†).

Fig. 4 Possible reaction pathways for the generation of 3, 4 and 5.

Conclusion

In summary we have broadened the scope of actinide–pnictogenide complexes by the isolation and characterization of new thorium primary phosphido and arsenido compounds. Both compounds exhibited spectroscopic diagnostic features in the infrared and heteronuclear NMR experiments. Insertion reactions of an alkyl isocyanide into the thorium–primary pnictogenide bond resulted in the formation of phospha/arsaazaallene complexes that do not exhibit any type of rearrangement. Further investigation is required to elucidate whether this reactivity is unique to the actinides or Lewis acids coordinated to two primary phosphido or arsenido ligands. Therefore group IV and alternative actinide metals are under investigation.

Experimental

General considerations

The syntheses and manipulations described below were conducted using standard Schlenk and glovebox techniques. All the reactions were conducted in a Vacuum Atmospheres inert atmosphere (N₂) glovebox or a double-manifold Schlenk line. Toluene, 1,2-dimethoxyethane, diethyl ether and hexane were purchased anhydrous, stored over activated 4 Å molecular sieves, and sparged with nitrogen prior to use. Methylcyclohexane was dried over activated 4 Å molecular sieves and sparged with nitrogen for thirty minutes prior to use. tert-Butyl isocyanide was dried over 4 Å molecular sieves, freeze–evacuate–thawed three times, distilled, and stored under nitrogen. All available reactants were purchased from suppliers and used without further purification. ThCl₄(DME)₂,⁴⁷ (C₅Me₅)₂ThCl₂,⁴⁸ (C₅Me₅)₂ThMe₂,⁴⁸ TippPCl₂,⁴⁹ H₂PTipp,⁵⁰ MesPCl₂,⁵¹ MesPH₂,⁵² and (C₅Me₅)₂Th[P(H)Tipp]₂ ²⁶ (Tipp = 2,4,6-ⁱPr₃C₆H₂, Mes = 2,4,6-Me₃C₆H₂) were synthesized as previously described. KPH(Mes) was prepared from H₂PMes and KN(SiMe₃)₂ in THF. Benzene-d₆ and toluene-d₈ (Cambridge Isotope Laboratories) were dried over molecular sieves and degassed with three freeze–evacuate–thaw cycles. All ¹H and ¹³C NMR spectra were obtained on a 500 or 600 MHz DRX Bruker spectrometer. All ³¹P NMR spectra were obtained on a 300 MHz ARX spectrometer at 121 MHz. ¹H NMR shifts given were referenced internally to the residual solvent peak at δ 7.16 ppm (C₆D₅H). ¹³C NMR shifts given were referenced internally to the residual peak at δ 128.0 ppm (C₆D₆). ³¹P NMR spectra were externally referenced to 0.00 ppm with 5% H₃PO₄ in D₂O. Infrared spectra were recorded as KBr pellets on a Perkin-Elmer Spectrum One FT-IR spectrometer. Elemental analyses were performed at the University of California, Berkeley Microanalytical Facility using a Perkin-Elmer Series II 2400 CHNS analyzer.

[TippAsCl₂]. A two-neck 500 mL Schlenk flask was charged with 1-bromo-2,4,6-ⁱPr₃C₆H₂Br (4.00 g, 14.1 mmol), Mg ribbon (polished and cut into small pieces, 377 mg, 15.5 mmol), and THF (75 mL). A reflex condenser was attached under nitrogen and 1–2 mL of 1,2-dibromoethane were added to the reaction. The reaction was heated to reflux and allowed to react for 8 h. The Grignard reaction was allowed to cool to room temperature. A 250 mL Schlenk flask was charged with AsCl₃ (2.56 g, 14.1 mmol) and THF (30 mL) and cooled to −40 °C via a CO₂(s)/CH₃CN bath. The Grignard was added via a cannula to the AsCl₃ followed by ZnCl₂ (3.84 g, 28.2 mmol). The reaction mixture was allowed to stir at −40 °C for 6 h, slowly warmed to room temperature and allowed to stir for an additional 24 h at room temperature. The filtrate was isolated via cannula filtration and the white solid was extracted twice with diethyl ether (2 × 30 mL) and added to the filtrate. The solution was concentrated and placed in a −20 °C freezer. Long colorless crystals grew over a period of 36 h. The crystals were isolated and dried (two crops 3.35 g, 68%). ¹H NMR (C₆D₆, 23 °C): δ 7.04 (s, 2H, ArH), 3.92 (sept, ³J_H–H = 6.5 Hz, 2H, CH_ortho(CH₃)₂), 2.62 (sept, ³J_H–H = 6.5 Hz, 1H, CH_para(CH₃)₂), 1.20 (d, ³J_H–H = 6.5 Hz, 12H, CH(CH₃)_2-ortho), 1.10 (d, ³J_H–H = 6.5 Hz, 6H, CH(CH₃)_2-para).

[TippAsH₂]. A 3-neck 1000 mL round bottom flask was charged with LiAlH₄ (1.80 g, 47.3 mmol) and THF (50 mL). A 100 mL Schlenk flask was charged with TippAsCl₂ (6 g, 17.2 mmol) and THF (50 mL). The LiAlH₄ was cooled to –5 °C via a NaCl(s)/ice bath and TippAsCl₂ was added to the mixture via a cannula and allowed to stir for 10 minutes. The salt bath was removed and the reaction mixture was stirred at room temperature for 12 h. The reaction mixture was placed in an ice bath and the excess LiAlH₄ was quenched dropwise with a degassed HCl/H₂O mixture (20% HCl, extreme care should be taken when first beginning to quench the mixture because the reaction is exothermic and H₂(g) is evolved). The organic phase was separated and the aqueous phase was extracted twice with diethyl ether (2 × 40 mL) and the organic phases were combined. The organic phase was dried over MgSO₄ and the solvent was removed under vacuum to yield the crude product as a colorless viscous solid. The product can be purified via distillation to yield a colorless liquid (4.10 g, 85%). ¹H NMR (C₆D₆, 23 °C): δ 7.10 (s, 2H, ArH), 3.38 (s, 2H, AsH), 3.36 (sept, ³J_H–H = 7.0 Hz, 2H, CH_ortho(CH₃)₂), 2.76 (sept, ³J_H–H = 7.0 Hz, 1H, CH_para(CH₃)₂), 1.22 (d, ³J_H–H = 7.0 Hz, 12H, CH(CH₃)_2-ortho), 1.21 (d, ³J_H–H = 7.0 Hz, 6H, CH(CH₃)_2-para).

(C₅Me₅)₂Th[P(H)Mes]₂, 1. The same procedure was employed as for (C₅Me₅)₂Th[P(H)Tipp]₂ using (C₅Me₅)₂ThCl₂ (287 mg, 0.500 mmol) and KP(H)Mes (200 mg, 1.05 mmol) to yield 1 as a bright orange solid (217 mg, 54%). ¹H NMR (C₆D₆, 23 °C): δ 6.99 (s, 4H, ArH), 3.80 (d, ¹J_P–H = 224 Hz, 2H, PH), 2.67 (s, 12H, CH_3-ortho), 2.33 (s, 6H, CH_3-para), 1.89 (s, 30H, C₅Me₅). ¹³C{¹H} NMR (C₆D₆, 23 °C): δ 140.00 (d, ¹J_P–C = 18.0 Hz), 139.16, 133.90 (the C(sp²)–H resonance was buried under the solvent resonance), 126.77, 25.42, 20.95, 11.47. ³¹P{¹H} NMR (C₆D₆, 23 °C): δ 15.37. IR (cm⁻¹): 2910 (s), 2855 (s), 2341 (m), 1446 (s), 1433 (s), 1378 (m), 1257 (m), 1100 (m), 1067 (m) 1024 (m), 952 (w), 940 (w), 894 (w), 847 (m), 703 (w). Anal. calcd for C₃₈H₅₄P₂Th: C, 56.72%; H, 6.76%. Found: C, 56.61%; H, 6.75%.

(C₅Me₅)₂Th[As(H)Tipp]₂, 2. A 20 mL scintillation vial was charged with (C₅Me₅)₂ThMe₂ (100 mg, 0.188 mmol), toluene (8 mL), and placed in a −23 °C freezer for 30 minutes. The vial was removed from the freezer and H₂AsTipp (106 mg, 0.378 mmol) was added dropwise and allowed to stir at room temperature for 12–14 h to yield a cherry red solution. The solvent was removed under vacuum, extracted with hexane, filtered over Celite, concentrated to 1–2 mL and placed in a −23 °C freezer. Red prism crystals grew after 36 h, and were isolated and dried to yield the product (two crops, 140 mg, 70%). ¹H NMR (C₆D₆, 23 °C): δ 7.26 (s, 4H, ArH), 3.66 (sept, ³J_H–H = 6.6 Hz, 4H, CH_ortho(CH₃)₂), 2.95 (sept, ³J_H–H = 7.2 Hz, 2H, CH_para(CH₃)₂), 2.61 (s, 2H, AsH), 1.98 (s, 30H, C₅Me₅), 1.54 (d, ³J_H–H = 6.6 Hz, 24H, CH(CH₃)_2-ortho), 1.34 (d, ³J_H–H = 7.2 Hz, 12H, CH(CH₃)_2-para). ¹³C{¹H} NMR (C₆D₆, 23 °C): δ 151.07, 146.69, 141.44, 127.33, 120.45, 35.77, 34.51, 24.75, 24.50, 11.99. IR (cm⁻¹): 2959 (s), 2917 (s), 2870 (s), 2089 (m), 1599 (m), 1551 (m), 1455 (s), 1373 (m), 1307 (m), 1245 (w), 1162 (m), 1098 (m), 1061 (m), 1021 (m), 934 (m) 870 (m), 805 (m), 744 (m), 609 (m). Anal. calcd for C₅₀H₇₈As₂Th: C, 56.60%; H, 7.41%. Found: C, 56.21%; H, 7.38%.

(C₅Me₅)₂Th(CN^tBu)(η²-N,C)-(^tBuNCPTipp), 3. A 20 mL scintillation vial was charged with (C₅Me₅)₂Th[P(H)Tipp]₂ (91 mg, 0.0935 mmol), methylcyclohexane (5 mL) and placed in a −23 °C freezer for 30 minutes. The vial was removed from the freezer and tert-butyl isocyanide (16 mg, 0.192 mmol) was added dropwise to the (C₅Me₅)₂Th[P(H)Tipp]₂ and allowed to stir for 2 h to yield a yellow reaction mixture. The mixture was filtered over Celite, concentrated to 1–2 mL and placed in a −23 °C freezer. Yellow needle crystals grew after a 48 h period, and were isolated and dried to yield the product (66 mg, 78%). ¹H NMR (C₆D₆, 23 °C): δ 7.27 (s, 2H, ArH), 4.87 (s, br, 2H, CH_ortho(CH₃)₂), 2.95 (sept, ³J_H–H = 7.2 Hz, 1H, CH_para(CH₃)₂), 2.16 (s, 30H, C₅Me₅), 1.65 (d, ³J_H–H = 7.2 Hz, 12H, CH(CH₃)_2-ortho), 1.36 (d, ³J_H–H = 7.2 Hz, 6H, CH(CH₃)_2-para), 1.22 (s, 9H, CNCMe₃), 1.08 (s, br, 9H, CNCMe₃). ¹³C{¹H} NMR (C₆D₆, 23 °C): δ 164.11 (CNCMe₃), (C_ipso–P could not be located), 152.95, 150.85 (d, ¹J_P–C = 152.3 Hz, Me₃CNCPTipp), 146.56, 122.37, 120.22, 59.80 (d, ³J_P–C = 12.0 Hz, Me₃CNCPTipp), 56.81 (CNCMe₃), 34.90, 33.52 (d, ³J_P–C = 6.0 Hz), 32.41 (Me₃CNCPTipp), 29.14 (CNCMe₃), 24.62, 23.79, 11.77. ³¹P{¹H} NMR (C₆D₆, 23 °C): δ −21.28. IR (cm⁻¹): 2957 (s), 2913 (s), 2864 (s), 2304 (m), 2181(s), 1600 (m), 1450 (s), 1361 (s), 1193 (m), 1092 (m), 1021 (m), 943 (m), 875 (m), 805 (m), 723 (m), 650 (m). Anal. calcd for C₄₅H₇₁N₂PTh: C, 59.85%; H, 7.92%; N, 3.10%. Found: C, 60.05%; H 7.64%; N, 2.71%.

(C₅Me₅)₂Th(CN^tBu)(η²-N,C)-(^tBuNCPMes), 4. The same procedure was employed as for 3 using 1 (100 mg, 0.124 mmol) and tert-butyl isocyanide (21 mg, 0.253 mmol) to yield 4 as a yellow solid (81 mg, 80%). ¹H NMR (C₆D₆, 23 °C): δ 7.05 (s, 2H, ArH), 3.10 (s, 6H, CH_3-ortho), 2.27 (s, 6H, CH_3-para), 2.16 (s, 30H, C₅Me₅), 1.25 (s, 9H, CNCMe₃), 1.02 (s, br, 9H, CNCMe₃). ¹³C{¹H} NMR (C₆D₆, 23 °C): δ 183.56 (d, ¹J_P–C = 56.0 Hz, C_ipso–P), 163.29 (CNCMe₃), 151.05 (d, ¹J_P–C = 103.0 Hz, Me₃CNCPMes), 142.65 (d, ⁴J_P–C = 4.5 Hz), 134.41 (the C(sp²)–H resonance was buried under the solvent resonance), 122.42, 59.47 (d, ³J_P–C = 12.0 Hz, Me₃CNCPMes), 56.66 (CNCMe₃), 31.69 (Me₃CNCPMes) 29.19 (CNCMe₃), 24.58, 21.33, 11.87. ³¹P{¹H} NMR (C₆D₆, 23 °C): δ −10.70. IR (cm⁻¹): 2956 (s), 2913 (s), 2304 (w), 2186 (s), 1602 (m), 1448 (s), 1355 (s), 1191 (s), 1093 (s), 1030 (s), 846 (m), 708 (m), 648 (m). C₃₉H₅₉N₂PTh: C, 57.20%; H, 7.26%; N, 3.42%. Found: C, 57.40%; H, 6.99%; N, 3.30%.

[(C₅Me₅)₂Th(CN^tBu)(η²-N,C)-(^tBuNCAsTipp)], 5. The same procedure was employed as for 3 using 2 (200 mg, 0.188 mmol) and tert-butyl isocyanide (32 mg, 0.385 mmol) to yield 5 as an orange solid (146 mg, 82%). X-ray quality crystals were grown from a toluene/hexane mixture at −23 °C. ¹H NMR (C₆D₆, 23 °C): δ 7.32 (s, 2H, ArH), 4.84 (s, br, 2H, CH_ortho(CH₃)₂), 2.98 (sept, ³J_H–H = 7.2 Hz, 1H, CH_para(CH₃)₂), 2.16 (s, 30H, C₅Me₅), 1.67 (d, ³J_H–H = 7.2 Hz, 6H, CH(CH₃)_2-ortho), 1.38 (d, ³J_H–H = 7.2 Hz, 3H, CH(CH₃)_2-para), 1.24 (s, 9H, CNCMe₃), 0.97 (s, br, 9H, CNCMe₃). ¹³C{¹H} NMR (C₆D₆, 23 °C): δ 164.41 (CNCMe₃), (C_ipso–As could not be located), 154.25 (Me₃CNCAsTipp), 154.05, 146.65, 122.65, 120.24, 60.30 (Me₃CNCAsTipp), 57.01 (CNCMe₃), 35.63, 34.90, 32.03 (Me₃CNCAsTipp), 29.01 (CNCMe₃), 26.91, 24.67, 11.79. IR (cm⁻¹): 2958 (s), 2917 (s), 2865 (s), 2182 (m), 1513 (m), 1452 (s), 1367 (m), 1312 (m), 1214 (m), 1109 (m), 1027 (m), 877 (w), 805 (m), 624 (m). Anal. calcd for C₄₅H₇₁N₂AsTh: C, 57.08%; H, 7.56%; N, 2.96%. Found: C, 58.72%; H, 7.45%; N, 2.71%.

Crystallographic data collection and structure determination

The selected single crystal was mounted on nylon cryoloops using viscous hydrocarbon oil. X-ray data collection was performed at 100(2) K. X-ray data were collected on a Bruker CCD diffractometer with monochromated Mo-Kα radiation (λ = 0.71073 Å). The data collection and processing were performed using a Bruker Apex2 suite of programs.⁵³ The structures were solved by direct methods and refined by full-matrix least-squares methods on F² using the Bruker SHELX-2014/7 program.⁵⁴ All non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atoms were placed at calculated positions and included in the refinement using a riding model. Thermal ellipsoid plots were prepared using X-seed⁵⁵ with 50% of probability displacements for non-hydrogen atoms. Crystal data and details of data collection for complexes 1, 2, 3, and 5 are provided in Table 3.

Table 3 X-ray crystallography data for complexes 1, 2, 3, and 5

	1	2	3	5	TippAsCl2
CCDC deposit number	1455163	1455164	1455165	1455166	1455345
Empirical formula	C38H54P2Th	C50H78As2Th	C45H71N2PTh	C45H71N2AsTh	C15H23AsBr0.55Cl1.45
Formula weight (g mol^−1)	804.79	1061.00	903.04	946.99	349.15
Crystal habit, color	Prism, orange	Prism, red	Needle, yellow	Needle, orange	Prism, colorless
Temperature (K)	100(2)	100(2)	100(2)	100(2)	100(2)
Space group	Pbcn	C2/c	Pnma	Pnma	P1̄
Crystal system	Orthorhombic	Monoclinic	Orthorhombic	Orthorhombic	Triclinic
Volume (Å^3)	3598.3(5)	4809.0(7)	5659.3(6)	5712.6(6)	813.6(2)
a (Å)	11.0230(9)	23.195(2)	28.9553(18)	29.0966(19)	8.3739(12)
b (Å)	15.3941(13)	12.1067(10)	14.5673(9)	14.5564(9)	9.1197(13)
c (Å)	21.2051(18)	17.8732(15)	13.4169(9)	13.4878(9)	11.6208(16)
α (°)	90.00	90.00	90.00	90.00	75.718(2)
β (°)	90.00	106.6350(10)	90.00	90.00	71.5280(10)
γ (°)	90.00	90.00	90.00	90.00	81.712(2)
Z	4	4	4	4	2
Calculated density (Mg m^−3)	1.486	1.465	1.060	1.101	1.425
Absorption coefficient (mm^−1)	4.257	4.497	2.687	3.208	2.400
Final R indices [I > 2σ(I)]	R = 0.0211	R = 0.0195	R = 0.0317	R = 0.0330	R = 0.0627
	R W = 0.0431	R W = 0.0437	R W = 0.0823	R W = 0.0741	R W = 0.1803

---

Acknowledgements

We gratefully acknowledge the U.S. Department of Energy, Office of Science, Early Career Research Program under award DE-SC-0014174 for support of this work.

References

1. V. Koshti, S. Gaikwad and S. H. Chikkali, Coord. Chem. Rev., 2014, 265, 52.
2. L. Rosenberg, ACS Catal., 2013, 3, 2845.
3. K. B. Dillon, F. Mathey and J. F. Nixon, in Phosphorus: The Carbon Copy, Wiley, Chichester, 1998.
4. L. Ackermann, Synthesis, 2006, 1557.
5. P. Garcia, P. R. Payne, E. Chong, R. L. Webster, B. J. Barron, A. C. Behrle, J. A. R. Schmidt and L. L. Schafer, Tetrahedron, 2013, 69, 5737.
6. N. R. Price and J. Chambers, in The Chemistry of Organophosphorus Compounds, ed. F. R. Hartley, Chichester, 1990, vol. 1.
7. D. E. C. Corbridge, in Phosphorus: Chemistry, Biochemistry, and Technology, CRC Press, Boca Raton, FL, 2013, 6th edn, p. 1473.
8. E. Rafter, T. Gutmann, F. Low, G. Buntkowsky, K. Philippot, B. Chaudret and P. W. N. M. van Leeuwen, Catal. Sci. Technol., 2013, 3, 595.
9. I. S. R. Karmel, M. Tamm and M. S. Eisen, Angew. Chem., Int. Ed., 2015, 54, 12422.
10. T. W. Hayton, J. M. Boncella, B. L. Scott, P. D. Palmer, E. R. Batista and P. J. Hay, Science, 2005, 310, 1941.
11. D. M. King, F. Tuna, E. J. L. McInnes, J. McMaster, W. Lewis, A. J. Blake and S. T. Liddle, Science, 2012, 337, 717.
12. N. H. Anderson, S. O. Odoh, Y. Yao, U. J. Williams, B. A. Schaefer, J. J. Kiernicki, A. J. Lewis, M. D. Goshert, P. E. Fanwick, E. J. Schelter, J. R. Walensky, L. Gagliardi and S. C. Bart, Nat. Chem., 2014, 6, 919.
13. N. H. Anderson, H. Yin, J. J. Kiernicki, P. E. Fanwick, E. J. Schelter and S. C. Bart, Angew. Chem., Int. Ed., 2015, 54, 9386.
14. B. M. Gardner, G. Balázs, M. Scheer, F. Tuna, E. J. L. McInnes, J. McMaster, W. Lewis, A. J. Blake and S. T. Liddle, Angew. Chem., Int. Ed., 2014, 53, 4484.
15. D. S. J. Arney, R. C. Schnabel, B. C. Scott and C. J. Burns, J. Am. Chem. Soc., 1996, 118, 6780.
16. M. R. Duttera, V. W. Day and T. J. Marks, J. Am. Chem. Soc., 1984, 106, 2907.
17. D. Patel, F. Tuna, E. J. L. McInnes, W. Lewis, A. J. Blake and S. T. Liddle, Angew. Chem., Int. Ed., 2013, 52, 13334.
18. N. Tsoureas, A. F. R. Kilpatrick, O. T. Summerscales, J. F. Nixon, F. G. N. Cloke and P. B. Hitchcock, Eur. J. Inorg. Chem., 2013, 2013, 4085.
19. A. S. P. Frey, F. G. N. Cloke, P. B. Hitchcock and J. C. Green, New J. Chem., 2011, 35, 2022.
20. O. J. Scherer, B. Werner, G. Heckmann and G. Wolmershäuser, Angew. Chem., Int. Ed., 1991, 30, 553.
21. S. W. Hall, J. C. Huffman, M. M. Miller, L. R. Avens, C. J. Burns, A. P. Sattelberger, D. S. J. Arney and A. F. England, Organometallics, 1993, 12, 752.
22. P. J. Hay, R. R. Ryan, K. V. Salazar, D. A. Wrobleski and A. P. Sattelberger, J. Am. Chem. Soc., 1986, 108, 313.
23. D. A. Wrobleski, R. R. Ryan, H. J. Wasserman, K. V. Salazar, R. T. Paine and D. C. Moody, Organometallics, 1986, 5, 90.
24. J. M. Ritchey, A. J. Zozulin, D. A. Wrobleski, R. R. Ryan, H. J. Wasserman, D. C. Moody and R. T. Paine, J. Am. Chem. Soc., 1985, 107, 501.
25. A. Formanuik, F. Ortu, R. Beekmeyer, A. Kerridge, R. W. Adams and D. P. Mills, Dalton Trans., 2016, 45, 2390.
26. A. C. Behrle, L. Castro, L. Maron and J. R. Walensky, J. Am. Chem. Soc., 2015, 137, 14846.
27. B. M. Gardner, G. Balázs, M. Scheer, A. J. Wooles, F. Tuna, E. J. L. McInnes, J. McMaster, W. Lewis, A. J. Blake and S. T. Liddle, Angew. Chem., Int. Ed., 2015, 54, 15250.
28. B. M. Gardner, G. Balázs, M. Scheer, F. Tuna, J. L. McInnesEric, J. McMaster, W. Lewis, A. J. Blake and S. T. Liddle, Nat. Chem., 2015, 7, 582.
29. O. J. Scherer, J. Schulze and G. Wolmershäuser, J. Organomet. Chem., 1994, 484, c5.
30. B. A. Vaughan, E. M. Arsenault, S. M. Chan and R. Waterman, J. Organomet. Chem., 2012, 696, 4327.
31. A. J. Roering, S. N. MacMillan, J. M. Tanski and R. Waterman, Inorg. Chem., 2007, 46, 6855.
32. M. R. Douglass, C. L. Stern and T. J. Marks, J. Am. Chem. Soc., 2001, 123, 10221.
33. A. V. Firth and D. W. Stephan, Inorg. Chem., 1998, 37, 4732.
34. N. C. Zanetti, R. R. Schrock and W. M. Davis, Angew. Chem., Int. Ed., 1995, 34, 2044.
35. J. Ho, Z. Hou, R. J. Drake and D. W. Stephan, Organometallics, 1993, 12, 3145.
36. J. Ho and D. W. Stephan, Organometallics, 1992, 11, 1014.
37. G. A. Vaughan, G. L. Hillhouse and A. L. Rheingold, Organometallics, 1989, 8, 1760.
38. A. J. Roering, L. T. Elrod, J. K. Pagano, S. L. Guillot, S. M. Chan, J. M. Tanski and R. Waterman, Dalton Trans., 2013, 42, 1159.
39. A. F. Maddox, J. J. Davidson, T. Shalumova, J. M. Tanski and R. Waterman, Inorg. Chem., 2013, 52, 7811.
40. P. Pyykkö and M. Atsumi, Chem. – Eur. J., 2009, 15, 186.
41. F. Basuli, L. A. Watson, J. C. Huffman and D. J. Mindiola, Dalton Trans., 2003, 4228.
42. J. B. Alexander, D. S. Glueck, G. P. A. Yap and A. L. Rheingold, Organometallics, 1995, 14, 3603.
43. M. Yoshifuji, T. Niitsu, K. Toyota, N. Inamoto, K. Hirotsu, Y. Odagaki, T. Higuchi and S. Nagase, Polyhedron, 1988, 7, 2213.
44. W. J. Evans, M. K. Takase, J. W. Ziller and A. L. Rheingold, Organometallics, 2009, 28, 5802.
45. E. Zhou, W. Ren, G. Hou, G. Zi, D.-C. Fang and M. D. Walter, Organometallics, 2015, 34, 3637.
46. K. C. Jantunen, C. J. Burns, I. Castro-Rodriguez, R. E. Da Re, J. T. Golden, D. E. Morris, B. L. Scott, F. L. Taw and J. L. Kiplinger, Organometallics, 2004, 23, 4682.
47. T. Cantat, B. L. Scott and J. L. Kiplinger, Chem. Commun., 2010, 46, 919.
48. P. J. Fagan, J. M. Manriquez, E. A. Maatta, A. M. Seyam and T. J. Marks, J. Am. Chem. Soc., 1981, 103, 6650.
49. V. Chandrasekhar, P. Sasikumar, R. Boomishankar and G. Anantharaman, Inorg. Chem., 2006, 45, 3344.
50. Y. van den Winkel, H. M. M. Bastiaans and F. Bickelhaupt, J. Organomet. Chem., 1991, 405, 183.
51. S. T. Liddle and K. Izod, Organometallics, 2004, 23, 5550.
52. J. D. Masuda, K. C. Jantunen, O. V. Ozerov, K. J. T. Noonan, D. P. Gates, B. L. Scott and J. L. Kiplinger, J. Am. Chem. Soc., 2008, 130, 2408.
53. APEX2 Suite, Bruker AXS Inc., Madison, Wisconsin, USA, 2007.
54. G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr., 2008, 64, 112.
55. L. J. Barbour, J. Supramol. Chem., 2001, 1, 189.

---

Footnote

† Electronic supplementary information (ESI) available. CCDC 1455163–1455166, 1455345. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6dt00776g

---