Insertion of t BuNC into thorium – phosphorus and thorium – arsenic bonds : phosphaazaallene and arsaazaallene moieties in f element chemistry †

Introduction Hydroelementation reactions such as hydrophosphination are atom efficient processes which are important in developing building blocks containing phosphorus. For example, tertiary phosphines are of interest as ligands and for various applications. However, the development of these reactions in organoactinide chemistry has been attenuated by a lack of starting materials. Despite the intensity with which complexes with actinide– nitrogen bonds have been studied, there exists a tremendous knowledge gap with respect to the heavier pnictogen elements. To date, twenty actinide–phosphido or phosphinidene bonds and five actinide–arsenido bonds 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, but Waterman’s group has investigated the reactivity of zirconium–phosphorus bonds with BuNC. 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 BuNC, which differs from their transition metal analogs. Results and discussion


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][4][5] and for various applications. [6][7][8] However, the development of these reactions in organoactinide chemistry 9 has been attenuated by a lack of starting materials.
Due to the dearth of actinide-phosphorus and actinidearsenic 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][31][32][33][34][35][36][37] but Waterman's group has investigated the reactivity of zirconium-phosphorus bonds with t BuNC. 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 t BuNC, which differs from their transition metal analogs.

Results and discussion
The synthesis of (C 5 Me 5 ) 2 Th[P(H)Tipp] 2 , Tipp = 2,4,6-i Pr 3 C 6 H 2 , 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 (C 5 Me 5 ) 2 Th[P(H)Mes] 2 , 1, Mes = 2,4,6-Me 3 C 6 H 2 , from the stoichiometric salt metathesis reaction between (C 5 Me 5 ) 2 ThCl 2 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 1 J P-H = 224 Hz and 2304 cm −1 , respectively. The large 1 J P-H coupling constant reflects the large amount of s-character in the P-H bond.
The 31 P{ 1 H} resonance is located at 15.37 ppm and compares well to the 31 P{ 1 H} resonance of a structurally similar thorium-phosphido compound, (C 5 Me 5 ) 2 Th[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 (C 5 Me 5 ) 2 Th[P(H)Tipp] 2 .

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)°. 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 (C 5 Me 5 ) 2 Th[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 η 2 -(N,C)-phosphaazaallene thorium complexes 3, and [(C 5 Me 5 ) 2 Th(CN t Bu)(η 2 -N,C)-( t BuNCvPMes)], 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 31 P NMR resonances shifted slightly upfield from the starting material to −21.28 and −10.70 ppm for 3 and 4, respectively. Additionally, the 13 C NMR resonance of the central carbon of the phosphaazaallene was found at 150.85 and 151.05 ppm with 1 J P−C = 152.3 Hz and 103.0 Hz, for 3 and 4, respectively. Compound 2 exhibited a similar reactivity to yield a η 2 -(N,C)-arsaazaallene thorium complex [(C 5 Me 5 ) 2 Th(CN t Bu)(η 2 -N,C)-( t BuNCvAsTipp)], 5, as an orange solid. The spectroscopic features of 5 can be found in Table 1.

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.

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 (N 2 ) 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-evacuatethawed three times, distilled, and stored under nitrogen. All available reactants were purchased from suppliers and used without further purification. ThCl 4 (DME) 2 13 C NMR shifts given were referenced internally to the residual peak at δ 128.0 ppm (C 6 D 6 ). 31 P NMR spectra were externally referenced to 0.00 ppm with 5% H 3 PO 4 in D 2 O. 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.
[TippAsCl 2 ]. A two-neck 500 mL Schlenk flask was charged with 1-bromo-2,4,6-i Pr 3 C 6 H 2 Br (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 AsCl 3 (2.56 g, 14.1 mmol) and THF (30 mL) and cooled to −40°C via a CO 2 (s)/CH 3 CN bath. The Grignard was added via a cannula to the AsCl 3 followed by ZnCl 2 (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%). 1  [TippAsH 2 ]. A 3-neck 1000 mL round bottom flask was charged with LiAlH 4 (1.80 g, 47.3 mmol) and THF (50 mL). A 100 mL Schlenk flask was charged with TippAsCl 2 (6 g, 17.2 mmol) and THF (50 mL). The LiAlH 4 was cooled to -5°C via a NaCl(s)/ice bath and TippAsCl 2 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 LiAlH 4 was quenched dropwise with a degassed HCl/H 2 O mixture (20% HCl, extreme care should be taken when first beginning to quench the mixture because the reaction is exothermic and H 2 (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 MgSO 4 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%). 1  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%). 1   [(C 5 Me 5 ) 2 Th(CN t Bu)(η 2 -N,C)-( t BuNCAsTipp)], 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. 1

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 leastsquares methods on F 2 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-seed 55 with 50% of probability displacements for nonhydrogen atoms. Crystal data and details of data collection for complexes 1, 2, 3, and 5 are provided in Table 3.