(dtbpe)Rh Silylamides: CO2 Bond Cleavage by a Rhodium(I)

Article pubs.acs.org/Organometallics

Synthesis and Reactivity of Three-Coordinate (dtbpe)Rh Silylamides: CO2 Bond Cleavage by a Rhodium(I) Disilylamide Matthew T. Whited,*,† Alex J. Kosanovich,† and Daron E. Janzen‡ †

Department of Chemistry, Carleton College, Northfield, Minnesota 55057, United States Department of Chemistry and Biochemistry, St. Catherine University, St. Paul, Minnesota 55105, United States

‡

S Supporting Information *

ABSTRACT: Rhodium(I) silylamide complexes supported by the 1,2-bis(di-tert-butylphosphino)ethane (dtbpe) ligand have been prepared and their structures and reactivity studied. Although the complexes degrade over time to release the corresponding silylamines, they react cleanly with silver(I) salts to transfer the amido group at ambient temperature. The bis(trimethylsilyl)amide complex (dtbpe)Rh−N(TMS)2 reacts with CO2 to form a carbamate complex that decomposes via loss of hexamethyldisiloxane to form a bis(μ-isocyanate) dimer, suggesting that silylamides may be useful nitrene-group and nitrogen-atom sources through selective N−Si bond cleavage.

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Ag(I) salts and a nitrogen-for-oxygen metathesis with carbon dioxide to form a bis(μ-isocyanate) dimer via multiple N−Si bond cleavages.

ate-metal amides are important demonstrated intermediates in numerous C−N bond-forming processes, including cross-coupling of aryl halides and amines, hydroamination, oxidative amination, and aziridination.1 Amides have also been successfully utilized in multidentate supporting ligands with late metals to facilitate a number of transformations, including some in which the amide plays a cooperative role in substrate activation.2 Inspired by the work of Fryzuk and others showing that silylamides are attractive ligands for a variety of metals,2a,3 we have become interested in late-metal complexes containing silylamide ligands. Silicon−nitrogen bonds are unstable under a variety of conditions,4 and we have targeted a scheme where the silyl unit may serve as a readily cleavable protecting group (akin to a large, less reactive proton) en route to the formation of metal−nitrogen multiple bonds in catalytic processes (eq 1).

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SYNTHESIS, DECOMPOSITION, AND ATTEMPTED OXIDATION OF Rh(I) SILYLAMIDES We initially targeted the chloro-bridged dimer [(dtbpe)Rh(μCl)]2 (1) as a precursor to a series of Rh(I) silylamides. Though the complex has been reported previously,8 synthetic details and crystallographic data are not available, so we have provided a synthesis in the Experimental Section and the singlecrystal X-ray structure of 1 in the Supporting Information. We were pleased to find that 1 reacted with either lithium bis(trimethylsilyl)amide (LHMDS) or lithium tert-butyl(trimethylsilyl)amide to afford the corresponding (dtbpe)Rh amide complexes 2a,b as dark green crystalline solids in low to moderate yield (eq 2).

Mindiola has reported a similar oxidatively induced α-silyl abstraction to form a titanium(IV) imide,5 and other reports suggest that Si−N bond scission is a viable route to earlymetal imides.6 However, such a route has not been demonstrated for late transition metals. Mindful of Hillhouse’s successful use of bulky bis(phosphine) ligands to support nickel amido and imido complexes in a variety of oxidation states,7 we have begun our studies by preparing a family of rhodium complexes supported by the 1,2-bis(di-tert-butyl)phosphinoethane (dtbpe) ligand. In this contribution, we report the synthesis and reactivity of three-coordinate dtbpe-supported rhodium silylamides. Specifically, we describe amide-transfer reactions with © 2014 American Chemical Society

Although amide ligands frequently bridge multiple metal centers,1a,3a,b we suspected on the basis of the steric encumbrance of the dtbpe ligand as well as previous results for related systems9 that silylamides 2a,b would exist as threeReceived: November 21, 2013 Published: March 7, 2014 1416

dx.doi.org/10.1021/om401130q | Organometallics 2014, 33, 1416−1422

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The mixed tert-butyl(trimethylsilyl)amide complex 2b exhibited behavior similar to that of 2a. 1H NMR spectroscopy revealed two P−C(CH3)3 resonances, as opposed to one for 2a, indicating that rotation of the Rh−N bond is slow on the NMR time scale, consistent with the steric encumbrance discussed above. Like 2a, complex 2b crystallizes in the Pnna space group, but it does not possess a molecular 2-fold axis of rotational symmetry. Therefore, the solid-state structure of 2b (Figure 2) suffers from disorder of the tert-butyl and trimethylsilyl nitrogen substituents. The amide group is also pushed slightly out of the P−Rh−P plane. In comparison with 2a, complex 2b exhibits a 0.01 Å longer Rh−N bond. This discrepancy may be due to the greater steric effect of a tert-butyl group in comparison with trimethylsilyl, since the N−C bond is ca. 0.2 Å shorter than the N−Si bond (1.5 Å versus 1.7 Å). The (dtbpe)Rh amide complexes 2a,b were thermally unstable and were observed to degrade even at ambient temperature under an inert atmosphere with quantitative expulsion of the corresponding amine, as identified by 1H NMR and quantified by integration relative to benzene solvent. On the basis of previous results from Lappert’s laboratory regarding the decomposition of (Ph3P)2Rh−N(SiMe3)2,3d,9b it is probable that the decomposition occurs through intramolecular proton transfer to the amide via a cyclometalation process.10 However, any decomposition mechanism is speculative at this point, since the only rhodium product identified by NMR upon thermolysis of 2a in benzene-d6 was chlorobridged complex 1, for which the origin of the chlorine is uncertain. On the basis of our proposed oxidation/deprotection route to metal−nitrogen multiple bonds, we were interested in the reactivity of 2a with one-electron oxidants. Although monometallic Rh(II) amides are rare, Ozerov has recently reported that an amido/bis(phosphine) pincer supported rhodium(II) complex is accessible through Ag(I) oxidation of the corresponding Rh(I) precursor.11 On the basis of Ozerov’s findings, we investigated the reactions of bis(trimethylsilyl)amide complex 2a with various silver salts. Reaction of 2a with silver triflate (1 equiv) in THF produced an immediate color change from dark green to orange, along with the appearance of a new product by 31P NMR (δ 128.2, 1JPRh = 224 Hz). Crystallographic analysis of the product revealed that oxidation had not occurred, but instead the amide had cleanly transmetalated from rhodium to silver to form (dtbpe)Rh(OTf) (3) and a putative silver amide byproduct, AgN(SiMe3)2,12 which was not definitively identified (Scheme 1). In the solid state, 3 contains a κ2-triflate ligand connected through two oxygen atoms to a square-planar Rh(I) center (Figure 3). The κ2 coordination (rather than the much more common κ1)13 can be easily rationalized by a preference for Rh(I) centers to adopt square-planar, 16-electron configurations and is similar to the (iPr3P)2Rh(OTf) structure reported by Milstein.14 Complex 2b reacted in a similar fashion to form 3. If complex 3 was left in THF solution for an extended time, the solution gradually gelled as poly(tetrahydrofuran) formed. This process is likely initiated by displacement of triflate to make the tetrahydrofuranate solvento species, which is activated toward ring opening. Consistent with the proposal that (dtbpe)Rh+ is the catalyst for THF polymerization, related metatheses of 2a with silver salts of noncoordinating anions such as hexafluorophosphate and tetrafluoroborate led to nearly instantaneous polymerization of THF. The lability of the triflate ligand was also shown by its facile displacement in more

coordinate monomers. This formulation was supported by Xray crystallography, which showed trigonal-planar geometries for both complexes with Rh−N bond lengths of 2.045(3) Å (2a) and 2.055(2) Å (2b) (Figures 1 and 2). Complex 1 has

Figure 1. Solid-state structure of (dtbpe)Rh−N(SiMe3)2 (2a), with thermal ellipsoids at the 50% probability level and hydrogen atoms omitted for clarity, alongside a space-filling representation of 2a viewed from above the trigonal plane of the complex. Selected bond lengths (Å) and angles (deg): Rh1−N1, 2.045(3); Rh1−P1, 2.1937(7); N1−Si1, 1.724(2); P1−Rh1−P1*, 84.72(4); Si1−N1− Si1*, 120.3(2); Rh1−N1−Si1, 119.86(8).

Figure 2. Solid-state structure of (dtbpe)Rh−N(SiMe3)(CMe3) (2b), with thermal ellipsoids at the 50% probability level and hydrogen atoms omitted for clarity. Only one of two orientations of the disordered −N(SiMe3)(CMe3) group is shown for clarity. Selected bond lengths (Å) and angles (deg): Rh1−N1, 2.055(3); Rh1−P1, 2.1986(5); N1−Si1, 1.733(4); N1−C4, 1.50(1); P1−Rh1−P1*, 84.93(2); Si1−N1−C4, 121.8(4); Rh1−N1−Si1, 113.0(2); Rh1− N1−C4, 125.1(5).

previously been shown to react with excess phenyllithium to generate four-coordinate ate complexes,8a but in this case the use of an excess (2.2 equiv per Rh) of LHMDS afforded only the three-coordinate, neutral complex 2a, as expected for the bulky, π-donating bis(trimethylsilyl)amido ligand. The crystal structure of 2a, depicted in Figure 1, indicates severe encumbrance about the metal center, where the amide ligand is oriented perpendicular to the trigonal plane about rhodium. This orientation not only mitigates steric considerations but also allows N→Rh π donation into the LUMO, which is derived from a dσ*(Rh−P) orbital in the trigonal plane. Hartwig previously observed a similar orientation for the related (Et3P)2Rh−N(SiMePh2)2.9a Though the perpendicular orientation of the amide could allow an agostic interaction between a trimethylsilyl group and rhodium, there is no evidence, by either NMR spectroscopy or X-ray crystallography, for such an interaction. Complex 2a crystallized in the Pnna space group with the Rh−N bond coincident with a 2-fold rotation axis. 1417


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Scheme 1

Figure 3. Solid-state structure of (dtbpe)Rh(κ2-OTf) (3) with thermal ellipsoids at the 50% probability level and hydrogen atoms omitted for clarity. Selected bond lengths (Å) and angles (deg): Rh1−O1, 2.262(2); Rh1−O2, 2.288(2); Rh1−P1, 2.1852(5); Rh1−P2, 2.1857(5); O1−Rh1−O2, 62.23(6); P1−Rh1−P2, 86.74(2); P1− Rh1−O2, 105.29(4); P2−Rh1−O1, 104.74(4).

Figure 4. Solid-state structure of [(dtbpe)Rh(η6-C6H6)]+ (5) with thermal ellipsoids at the 50% probability level. One of two nearly equivalent molecules from the asymmetric unit is represented, and hydrogen atoms, triflate anions, and cocrystallized benzene solvent have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Rh1−P1, 2.2701(2); Rh1−P2, 2.2702(2); Rh1− C6H6(centroid), 1.8717(2); P1−Rh1−P2, 86.32(1).

strongly coordinating solvents. For instance, dissolution of 3 in acetonitrile led to displacement of the triflate and formation of the mono(acetonitrile) adduct [(dtbpe)Rh−NCCH3][OTf] (4) or related species.15 Similarly, reaction of 3 with benzene led to formation of the η6-benzene adduct [(dtbpe)Rh(η6C6H6)][OTf] (5), for which the structure was elucidated by Xray crystallography (Figure 4). Alternatively, benzene adduct 5 could be cleanly accessed via chloride abstraction from dimer 1 with trimethylsilyl triflate (TMS−OTf) in benzene. Triflate complex 3 could be prepared independently and in high yield by the reaction of dtbpe with [(coe)2Rh(μ-OTf)]2 at −35 °C, a route previously reported by Werner et al. for the synthesis of (iPr3P)2Rh(OTf).16 The independently prepared triflate complex 3 could also serve as a precursor for amide complexes 2a,b and the siloxide complex 9 described below, since it is monomeric and contains a weakly bound anion. Additionally, since 3 is reasonably soluble in pentane and diethyl ether, we are currently investigating the reactivity of 3 as

a soluble (dtbpe)Rh + source in various catalytic and stoichiometric reactions. Unfortunately, dtbpe-supported rhodium(II) silylamides have not yet proven accessible. Whereas Ag(I) salts reacted with 2a,b to give salt metathesis products, other one-electron oxidants such as ferrocenium and nitrosonium led to myriad diamagnetic products. Additionally, reaction of amide complex 2a with O2 led to loss of the oxidized dtbpe ligand as (dtbpe)O2 (identified by 31P NMR)17 with no observable intermediates. We are continuing to pursue alternative oxidation conditions as well as the use of redox-active ligands to facilitate a reversible oxidation of Rh(I) silylamides.

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INSERTION REACTIVITY OF (dtbpe)Rh−N(SiMe3)2 AND N/O METATHESIS WITH CARBON DIOXIDE Given the potential importance of amide insertion reactions in catalytic processes such as hydroamination, we pursued migratory insertion reactions involving silylamide 2a. Unlike 1418


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the phosphine-supported bis(μ-arylamido) dirhodium complexes examined by Hartwig,9a complex 2a was not observed to undergo 1,2-insertion of nonpolar alkenes. However, silylamide 2a underwent facile reaction with CO2 (1 atm) in C6D6, resulting in a color change from dark green to yellow with formation of a new C2v-symmetric product (6) characterized by 1 H and 31P (δ 125.9 (d, 1JRhP = 205 Hz)) NMR spectra. The product decomposed at a rate that precluded 13C analysis of the reaction with 12CO2, but reaction with 13CO2 allowed identification of the labeled carbon at δ 175.6, consistent with the assignment of 6 as a Rh(I) carbamate complex.18 Though it is conceivable that the carbamate in 6 binds in κ1 fashion, analogy with the κ2-triflate complex 4 strongly supports a κ2carbamate (Scheme 2). Figure 5. Solid-state structure of [(dtbpe)Rh(μ-NCO)]2 (8) with thermal ellipsoids at the 50% probability level and hydrogen atoms omitted for clarity. Selected bond lengths (Å): Rh1−N1, 2.1593(2); Rh1−N1*, 2.2187(2); N1−C19, 1.159(3); C19−O1, 1.201(3).

Scheme 2

1:1 ratio with another trimethylsilyl-containing intermediate 7 (1H NMR δ 0.26). When the reaction was performed under 13 CO2, it was found that complex 7 incorporated 13CO2 (δ 165.1), with a 13C NMR chemical shift that is only slightly upfield from that of carbamate 6. (b) Independent synthesis of the trimethylsiloxide complex [(dtbpe)Rh(μ-OSiMe3)]2 (9; 31P NMR δ 111.8 (d, 1JRhP = 193 Hz)) from triflate 3 in THF (Scheme 2, top) revealed that it is not the long-lived intermediate 7 in the 6 → 8 conversion. However, exposure of 9 to CO2 (1 atm) led to formation of the observed intermediate, which we assign as the κ2-silylcarbonate species (dtbpe)Rh(O2COSiMe3) (7). Unfortunately, complex 7 was not the only species formed, possibly due to instability under these conditions, and the reaction mixture ultimately converted predominantly to an unidentified product (31P NMR δ 123.3 (d, 1JRhP = 210 Hz)). (c) No reaction occurred when trimethylsilyl isocyanate was added to a solution of siloxide dimer 9 in THF. However, when this mixture was exposed to CO2 (1 atm), the bis(μ-isocyanate) 8 formed. (d) Reaction of the tert-butyl(trimethylsilyl)amide complex 2b with CO2 (1 atm) produced tert-butyl isocyanate (1H NMR δ 1.36) and silylcarbonate complex 7, though the reaction was substantially slower than for the disilylamide 2a (t1/2 > 12 h). This mixture did not convert to the isocyanate complex 8 and instead slowly decomposed primarily to a species tentatively assigned as chloride dimer 1 on the basis of its 1H and 31P NMR spectra. However, as with the decomposition of amides 2a,b, the origin of the chlorine remains uncertain. On the basis of these observations, we favor a mechanism akin to what has been proposed for a related system by Parkin (Scheme 2).20b In this case, carbamate complex 6 quickly eliminates TMS−NCO via 1,3-silyl migration, similar to what is observed in Brook rearrangements of β-keto silanes.22 The liberation of TMS−NCO likely forms the unobserved siloxide (dtbpe)Rh−OSiMe3, which undergoes facile insertion of CO2 to form silylcarbonate complex 7 more quickly than it can dimerize to bis(μ-siloxide) 9. The silylcarbonate complex 7 reacts slowly with free TMS−NCO by an undetermined mechanism to form HMDSO, CO2, and (dtbpe)Rh−NCO, which quickly dimerizes. The fact that model reactions performed during our mechanistic studies proceed less cleanly than the direct reaction of disilylamide 2a with CO2 indicates a

Attempts to crystallize or otherwise purify complex 6 were unsuccessful, as it quickly (t1/2 = 1 h at 22 °C) converted to the closely related complex 7 (31P NMR: δ 126.6 (d, 1JRhP = 207 Hz)), which decomposed over a period of ca. 3 days to the stable product 8 with expulsion of hexamethyldisiloxane (HMDSO), as confirmed by a spiking experiment and comparison with previously reported data (1H NMR, δ 0.12; 13 C NMR, δ 2.08).19 After decomposition was complete, the product was dried in vacuo, reconstituted in dichloromethane, and layered with pentane, affording red crystals of 8, which contains no trimethylsilyl resonances in its 1H NMR spectrum and exhibits a distinct infrared absorbance at 2154 cm−1. Singlecrystal X-ray diffraction confirmed the identity of decomposition product 8 as a (dtbpe)Rh dimer containing two bridging isocyanate ligands (Figure 5). The transformation of CO2 to OCN− has been reported for several transition-metal silylamides.20 Deoxygenation of CO2 by main-group and alkali-metal disilylamides is also known,21 though the major products are normally silylisocyanates or disilylcarbodiimides resulting from a single Si−N cleavage. In this case, we are able to assign a preliminary mechanism for the transformation (Scheme 2) on the basis of the following observations. (a) As the carbamate complex 6 decomposed in C6D6, trimethylsilyl isocyanate (1H NMR δ −0.14) was produced in a 1419


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(d, 2JPH = 11.6 Hz, 4H, P(CH2)2P). 13C{1H}25 NMR (101 MHz, C6D6): δ 22.9−23.9 (m, CH2P), 31.0 (s, C(CH3)3), 36.0−36.5 (m, C(CH3)3). 31P{1H} NMR (162 MHz, C6D6): δ 110.0 (d, 1JRhP = 204 Hz). Anal. Calcd for C36H80Cl2P4Rh2: C,47.33; H, 8.83. Found: C, 47.01; H, 8.70. (dtbpe)Rh−N(SiMe 3 ) 2 (2a). A solution of lithium bis(trimethylsilyl)amide (14.7 mg, 0.0879 mmol) in 2 mL of benzene was added to a stirred suspension of 1 (34.8 mg, 0.0381 mmol) in benzene (8 mL) and heated at 50 °C for 12 h, resulting in a color change from orange to dark green. The reaction mixture was filtered through Celite, and the filtrate was dried in vacuo to give a dark green powder. The powder was dissolved in minimal pentane and placed at −35 °C for 36 h, resulting in the formation of dark prismatic crystals of 2a, which were washed with cold pentane and dried in vacuo. Compound 2a slowly decomposed at ambient temperature with release of HN(SiMe3)2, so although clean NMR spectra were obtained, suitable microanalysis of the product was not obtained. Yield: 24.1 mg, 54.4%. 1H NMR (400 MHz, C6D6): δ 0.64 (s, 18H, Si(CH3)3), 0.97 (d, 3JPH = 12.6 Hz, 4H, P(CH2)2P), 1.23−1.27 (m, 36H, C(CH3)3). 13 C{1H} NMR (101 MHz, C6D6): δ 8.0 (s, Si(CH3)3), 22.0−22.5 (m, CH2P), 31.2 (s, C(CH3)3), 36.0−36.3 (m, C(CH3)3). 31P{1H} NMR (162 MHz, C6D6): δ 138.0 (d, 1JRhP = 174 Hz). (dtbpe)Rh−N(SiMe3)(tBu) (2b). A solution of LiN(TMS)(tBu) (9.8 mg, 0.065 mmol) in 2 mL of benzene was added to a stirred suspension of 1 (28.7 mg, 0.031 mmol) in 8 mL of benzene and heated at 35 °C for 12 h, resulting in a color change from orange to dark green. The reaction mixture was filtered twice through Celite to remove a reddish residue, and the solvent was evaporated in vacuo to give a dark green powder. Complex 2b crystallized as dark blocks by slow evaporation of pentane from a concentrated solution at −35 °C over 72 h, and the crystals were washed with cold pentane and dried in vacuo to yield pure 2b. Decomposition of 2b occurred at a reasonable rate even at ambient temperature; therefore, NMR spectra inevitably showed traces of tert-butyl(trimethylsilyl)amine and a Rh decomposition product and suitable microanalyses could not be obtained. Yield: 9.4 mg, 27%. 1H NMR (400 MHz, C6D6): δ 0.71 (s, 9H, Si(CH3)3), 1.02−1.07 (m, 4H, P(CH2)2P), 1.25−1.32 (m, 36H, −PC(CH3)3), 1.83 (s, 9H, −NC(CH3)3). 13C{1H} NMR (101 MHz, C6D6): δ 8.6 (Si(CH3)3), 22.3−22.8 (m, CH2P), 31.5 (−PC(CH3)3), 35.7−36.5 (m, −PC(CH 3 ) 3 ), 38.4 (−NC(CH 3 ) 3 ), 59.8 (−NC(CH3)3). 31P{1H} NMR (162 MHz, C6D6): δ 135.2 (d, 1JRhP = 172 Hz). (dtbpe)Rh−OTf (3). Procedure A. A solution of silver triflate (9.3 mg, 0.036 mmol) in 2 mL of THF was added to a stirred solution of 2a (16.2 mg, 0.0286 mmol) in 10 mL of THF, causing an immediate color change from dark green to light red with precipitation of dark solids. The reaction mixture was stirred for 5 min and filtered through Celite to afford a light red solution. Evaporation of solvent in vacuo yielded a red powder, which could be crystallized from diethyl ether and pentane at −35 °C to afford pure 3 as dark red prisms. Yield: 9.7 mg, 59%. 1H NMR (400 MHz, C6D6): δ 1.31−1.40 (m, 36H, C(CH3)3), 1.46−1.54 (m, 4H, P(CH2)2P). 13C{1H} NMR (101 MHz, C6D6): δ 22.2−22.9 (m, CH2P), 30.3 (s, C(CH3)3), 36.3−36.8 (m, C(CH3)3). 31P{1H} NMR (162 MHz, C6D6): δ 128.2 (d, 1JRhP = 224 Hz). Anal. Calcd for C19H40F3O3P2RhS: C, 40.01; H, 7.07. Found: C, 40.05; H, 7.18. Procedure B. A solution of dtbpe (41.0 mg, 0.128 mmol) in diethyl ether (2 mL) was added dropwise to a solution of [(C8H14)2Rh(μOTf)]2 (61.0 mg, 0.065 mmol) in diethyl ether (4 mL) at −35 °C, causing an immediate color change from yellow to dark red. The reaction was allowed to proceed with stirring for 5 min, and volatiles were removed in vacuo to afford a red film, which was washed with pentane (2 mL) to give complex 3 in pure form as a dark red powder. Yield: 72 mg, 95%. [(dtbpe)Rh−NCCH3][OTf] (4). A slurry of 3 (26.9 mg, 0.0472 mmol) was stirred in 8 mL of acetonitrile for 3 h, causing the solution to adopt a clear, yellow hue as the red solids dissolved. The solution was filtered through Celite and the solvent removed in vacuo to afford 4 as a pale yellow oil. Compound 3 was insoluble in solvents other than acetonitrile, where exchange of the bound and free acetonitrile

possible important role of monomeric species (as opposed to the dimeric 9) or an additional role of CO2 or TMS−NCO in the reaction shown in Scheme 2. Although Parkin’s system was observed primarily to liberate the disilylcarbonate product (Me3SiO)2CO during a similar reaction at zinc, we do not observe this species by 13C NMR (δ 151.9)20b when the reaction is run with 13CO2. During the conversion, a small peak is observed by 1H NMR (δ 0.21 in C6D6) that is consistent with (Me3SiO)2CO, but it never accounts for more than ca. 5% of the total trimethylsilylcontaining species. Thus, if (Me3SiO)2CO does form, it either disproportionates to HMDSO and CO2 under the reaction conditions or is formed in low quantities through an alternate mechanism.

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CONCLUSIONS In conclusion, we have reported a series of (dtbpe)Rh complexes including two silylamides. The amides are unstable to decomposition via amine loss but exhibit facile amide transfer to Ag(I). Reaction of the disilylamide complex 2a with CO2 forms a spectroscopically characterized carbamate complex that degrades via consecutive silyl migrations to afford a bis(μ-isocyanate) rhodium dimer. Mechanistic studies have shown that oxygen extrusion from CO2 occurs via a 1,3-silyl migration to release trimethylsilyl isocyanate, followed by a second, CO2-promoted silyl group transfer to the bound siloxide. These studies support the potential utility of silylamides as precursors to C−N bond formation, and future studies will be aimed at extending the scope of nitrogendelivery reactions from late-metal silylamides with a particular focus on cationic silylamido and neutral imido complexes.

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EXPERIMENTAL SECTION

General Considerations. All manipulations were carried out under a dinitrogen atmosphere in an MBraun Unilab 2000 glovebox. Routine solvents were purchased from Fisher Scientific and were deoxygenated and dried using a Glass Contour Solvent Purification System, except for anhydrous benzene and pentane, which were used as received from Aldrich. Chloro(1,5-cyclooctadiene)rhodium(I) dimer, silver(I) trifluoromethanesulfonate, lithium bis(trimethylsilyl)amide, carbon dioxide, and carbon-13C dioxide were used as received from Aldrich. Lithium tert-butyl(trimethylsilyl)amide was obtained as a white solid by lithiation of the corresponding amine in pentane, as previously described,23 and [(C8H14)2Rh(μ-OTf)]2 was prepared by the method of Werner et al.16 1,2-Bis(di-tert-butyl)phosphinoethane (dtbpe) was prepared as previously described.24 Hexamethyldisiloxane (Aldrich) and NMR solvents (Cambridge Isotope Laboratories) were degassed and passed through a pad of activated alumina prior to use. Alumina was activated by heating at 250 °C for 4 h under vacuum prior to use. NMR spectra were recorded at ambient temperature on a Varian Unity Plus 400 MHz spectrometer. 1H and 13C NMR chemical shifts were referenced to residual solvent, and 31P NMR chemical shifts are reported relative to an external standard of 85% H3PO4. IR spectra were recorded on a Thermo Scientific Nicolet iS10 FTIR spectrometer in CH2Cl2 using a solution IR cell with KBr windows. Microanalysis was carried out by the microanalysis laboratory at the University of Illinois at Urbana−Champaign. [(dtbpe)Rh(μ-Cl)]2 (1). A solution of dtbpe (69.2 mg, 0.217 mmol) in 3 mL of THF was added to a stirred suspension of [(cod)Rh(μCl)]2 (52.3 mg, 0.106 mmol) in 7 mL of THF, causing an immediate color change from yellow to orange. The reaction mixture was stirred for 24 h, followed by evaporation of solvent in vacuo. The resulting orange powder was dissolved in minimal CH2Cl2 and placed at −35 °C overnight to afford orange needlelike crystals. The crystals were washed with pentane and dried in vacuo. Yield: 64 mg, 66%. 1H NMR (400 MHz, C6D6): δ 1.45 (d, 3JPH = 10.8 Hz, 36 H, −C(CH3)3)), 1.10 1420


Organometallics

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product (31P NMR δ 123.3 (d, 1JRhP = 210 Hz)) in a mixture containing ca. 20% of 7. [(dtbpe)Rh(μ-NCO)]2 (8). A solution of 6 prepared from exposure of 2a (7.4 mg, 0.013 mmol) to CO2 (1 atm), as described above, was allowed to decompose at 30 °C. 31P NMR spectroscopy revealed the relatively fast (t1/2 ≈ 1h) conversion of 6 to an unidentified intermediate (δ 126.6 (d, 1JRhP = 207 Hz)), followed by the slow conversion of the intermediate species to isocyanate complex 8. Volatiles were removed in vacuo to afford an orange powder, which was redissolved in minimal CH2Cl2 and layered with pentane to afford a crop of small red crystals of complex 8. Yield: 2.8 mg, 48%. 1H NMR (400 MHz, CD2Cl2): δ 1.33−1.39 (m, 36H, C(CH3)3), 1.40−1.45 (m, 4H, P(CH2)2P). 13C{1H} NMR (101 MHz, C6D6): δ 23.6 (m, CH2P), 31.2 (s, C(CH3)3), 36.2−36.6 (m, C(CH3)3), 129.0 (s, −NCO). 31 1 P{ H} NMR (162 MHz, CD2Cl2): δ 112.9 (d, 1JRhP = 203 Hz). IR (CH2Cl2, KBr, cm−1): ν(NCO) 2154. X-ray Crystallography. Single-crystal X-ray diffraction data for compounds 1, 2a,b, 3, 5, and 8 were collected on a Rigaku XtaLAB mini diffractometer using Mo Kα radiation (λ = 0.71073 Å). The diffractometer was equipped with an Oxford Cryosystems desktop cooler (Oxford Cryosystems Ltd., Oxford, U.K.) for low-temperature data collection. The crystals were mounted on a MiTeGen micromount (MiTeGen, LLC, Ithaca, NY) using STP oil. The frames were integrated using CrystalClear-SM Expert 3.1 b2726 to give the hkl files corrected for Lp and decay. Data were corrected for absorption effects using a multiscan method (REQAB).26 The structures were solved by direct methods and refined on F2 using the Olex2 software package.27 All non-hydrogen atoms were refined with anisotropic thermal parameters. Special refinement details for complex 2b are provided below, and crystallographic parameters of all complexes are summarized in Table S1 (Supporting Information). ORTEP drawings were prepared using ORTEP-3 for Windows V2013.128 and POV-Ray for Windows v3.6.29 Crystallographic data for the complexes have been deposited at the Cambridge Crystallographic Data Centre (Nos. 972582−972587) and can be obtained free of charge via www.ccdc. cam.ac.uk. Special Crystallographic Refinement Details for (dtbpe)Rh− N(SiMe3)(tBu) (2b). The asymmetric unit of the structure for complex 2b contained half of the molecule and exhibited positional disorder of the tert-butyl and trimethylsilyl amide substituents, which was modeled using occupancies of 0.5. The N1−C4 and N1−Si1 distances for this structure were restrained using a DFIX command.

occurred quickly; therefore, the bound acetonitrile was not observed by NMR. However, dissolution of 4 in CD3CN and integration of the signal for free CH3CN confirmed a 1:1 stoichiometry of CH3CN to (dtbpe)Rh, supporting our assignment of 4. Microanalysis of 4 was not attempted due to its extremely viscous nature. Yield: 12.0 mg, 41.6%. 1 H NMR (400 MHz, CD3CN): δ 1.28−1.35 (m, 36H, C(CH3)3), 1.61−1.70 (m, 4H, P(CH2)2P), 1.96 (s, 3H, f ree CH3CN). 13C{1H} NMR (101 MHz, CD3CN): δ 22.4−23.5 (m, CH2P), 29.8 (s, C(CH3)3), 35.6−36.3 (m, C(CH3)3). 31P{1H} NMR (162 MHz, CD3CN): δ 110.0 (d, 1JRhP = 181 Hz). [(dtbpe)Rh(η6-C6H6)][OTf] (5). A solution of trimethylsilyl trifluoromethanesulfonate (11 mg, 0.049 mmol) in 5 mL of benzene was added to a stirred suspension of 1 (16.0 mg, 0.0175 mmol) in 10 mL of benzene, and the mixture was heated to 60 °C for 6 h, resulting in the precipitation of a red solid. Volatiles were removed in vacuo to afford an orange solid. Pure 5 was isolated as red-orange crystals by layering a concentrated solution of 5 in CH2Cl2 with benzene. Yield: 16.8 mg, 74.0%. 1H NMR (400 MHz, CD2Cl2): 1.26 (d, 3JPH = 13.3 Hz, 36H, C(CH3)3), 1.65−1.79 (m, 4H, P(CH2)2P), 6.70 (s, 6H, η6C6H6). 13C{1H} NMR (101 MHz, C6D6): δ 23.2−24.1 (m, CH2P), 30.2 (s, C(CH3)3), 37.8−38.6 (m, C(CH3)3), 100.7 (η6-C6H6). 31 1 P{ H} NMR (162 MHz, CD2Cl2): δ 119.3 (d, 1JRhP = 207 Hz). Anal. Calcd for C25H46F3O3P2RhS: C, 46.30; H, 7.15. Found: C, 45.58; H, 7.10. (dtbpe)Rh(O2CN(SiMe3)2) (6). In a Wilmad LPV NMR tube, a solution of 2a (7.4 mg, 0.013 mmol) in C6D6 was frozen, and the headspace was evacuated and back-filled with carbon dioxide (1 atm). As the solution thawed, a gradual color change from dark green to pale yellow occurred over ca. 5 min as the carbamate complex 6 formed. 31 P NMR showed the mixture to consist of >90% complex 6 as well as an unidentified but related product (δ 126.6 (d, 1JRhP = 207 Hz)) that we tentatively assign as the silylcarbonate complex (dtbpe)Rh(CO3SiMe3). An identical reaction was run using 13CO2 to allow collection of 13C NMR data, which was otherwise complicated by the instability of complex 6. 1H NMR (400 MHz, C6D6): δ 0.43 (s, 18H, Si(CH3)3), 0.99−1.06 (m, 4H, P(CH2)2P), 1.30−1.37 (m, 36H, C(CH3)3). 13C{1H} NMR (101 MHz, C6D6): δ 3.2 (s, Si(CH3)3), 22.3−23.2 (m, CH2P), 30.7 (s, C(CH3)3), 35.5−36.4 (m, C(CH3)3), 175.6 (Rh−CO2N(SiMe3)2). 31P{1H} NMR (162 MHz, C6D6): δ 125.9 (d, 1JRhP = 205 Hz). (dtbpe)Rh(O2COSiMe3) (7). Observation during Decomposition of 6. In a Wilmad LPV NMR tube, a solution of 2a (5.7 mg, 0.009 mmol) in C6D6 was frozen and the headspace evacuated and backfilled with carbon-13C dioxide (1 atm). As the solution thawed, a gradual color change from dark green to pale yellow occurred over ca. 5 min as the carbamate complex 6 formed. After ca. 5 h, 31P NMR confirmed that the reaction mixture consisted of ca. 90% of silylcarbonate complex 7 (δ 126.6 (d, 1JRhP = 207 Hz)), with a distinct 13C NMR resonance (δ 165.1, Rh−O2COSiMe3) shifted slightly upfield from the related peak for carbamate 6 and a trimethylsilyl 1H resonance (δ 0.26) shifted slightly upfield from that of the carbamate. However, this complex further decomposed to isocyanate 8 over a period of several days under these reaction conditions. Independent Synthesis from (dtbpe)Rh(OTf) (3). A solution of potassium trimethylsilanolate (31.2 mM in THF, 320 μL, 0.010 mmol) was added dropwise to a solution of 3 (5.8 mg, 0.010 mmol) in THF (2 mL), causing an immediate color change from red to green, followed by a second color change to yellow. After 5 min, a portion of the reaction mixture was transferred to a Wilmad LPV NMR tube, and 31 P NMR spectroscopy confirmed the formation of a new complex assigned as [(dtbpe)Rh(μ-OSiMe3)]2 (9) on the basis of the similarity in chemical shift and coupling constant to those of related dimeric complexes 1 and 8 (δ 111.8 (d, 1JRhP = 193 Hz)). The solution was frozen and the headspace evacuated and back-filled with carbon dioxide (1 atm). After 20 min, the partial conversion (ca. 20%) of 6 to 7 was confirmed by 31P NMR (δ 126.6 (d, 1JRhP = 207 Hz) with no significant side products. However, after 5 h, the starting material had completely disappeared and was replaced by an unidentified major

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ASSOCIATED CONTENT

* Supporting Information S

A table giving crystallographic parameters, figures giving NMR spectra for all reported complexes, and CIF files giving crystallographic data for complexes 1−3, 5, and 8. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for M.T.W.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge funding through a Cottrell College Science Award from the Research Corporation for Science Advancement. X-ray crystallography was supported by NSFMRI Award #1125975, “MRI Consortium: Acquisition of a Single Crystal X-ray Diffractometer for a Regional PUI Molecular Structure Facility”. Additional support was provided by startup funds from Carleton College. 1421


Organometallics

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Article

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