Communication pubs.acs.org/Organometallics
Synthesis and Reactivity of a Mixed-Sandwich Uranium(IV) Primary Amido Complex Jessica A. Higgins Frey, F. Geoffrey N. Cloke,* and S. Mark Roe Department of Chemistry, School of Life Sciences, University of Sussex, Brighton BN1 9QJ, U.K. S Supporting Information *
ABSTRACT: Facile N−H bond activation of ammonia by the “tuckedin” mixed-sandwich complex U(η-COTTIPS2)(η5:κ1-C5Me4CH2) (1; COTTIPS2 = 1,4-{SiiPr3}2C8H6) yields a rare example of a primary amido complex, U(COTTIPS2)Cp*(NH2) (2). 2 reacts with CO2 to form the carbamate complex U(COTTIPS2)Cp*(κ2-O2CNH2) (3) and may be deprotonated with KH in the presence of 18-crown-6 to form the primary imido complex [U(COTTIPS2)Cp*(NH)][K(18C6)] (4). 2 and 3 have been crystallophically characterized by X-ray diffraction. hile the study of U−C σ bonds and their ability to insert small molecules has received a significant amount of attention in the literature, U−N σ bonds have been comparatively overlooked1 and are more typically found as part of “inert” multidentate ligand frameworks supporting uranium metal centers in a range of oxidation states.2,3 Some investigations into the insertion chemistry of U−N bonds with respect to CO and CO2 have been reported, establishing the formation of U(IV) carbamoyls and U(III) and U(IV) carbamates.4−7 However, uranium−nitrogen multiple bonds have remained of considerable interest, ever since the first reports of UN double bonds in the mid-1980s,8,9 and the latter have subsequently been synthesized within a range of ligand frameworks;10,11 however, only one example of a primary terminal imido linkage, UNH, exists.12 Molecular, terminal UN triple bonds are extremely rare and until recently had only been observed under matrix isolation or mass spectrometric conditions13−17 or as a proposed intermediate.18 However, examples of molecular, terminal U(V) and U(VI) nitrides within a Tren TIPS2 (where Tren TIPS = {N(CH2CH2NSiiPr3)3}3−) framework have been reported very recently.19 We recently described the synthesis and characterization of several U(IV) monoalkyl complexes supported by a “mixedsandwich” metallocene framework, U(COTTIPS2)Cp*(R) (where R = Me, CH2Ph, CH2TMS, CH{TMS}2).20 A common decomposition product of the alkylsresulting from C−H activation of a Cp* ring Me groupwas the “tuck-in” complex U(COTTIPS2)(η5:κ1-C5Me4CH2) (1) (Scheme 1). We also reported the reaction of 1 with dihydrogen, which readily and reversibly forms the U(IV) hydride U(COTTIPS2)Cp*(H).18 Hence, we decided to explore the reactivity of 1 toward other E−H bonds, and in this communication we report the reaction of 1 with NH3 to form a rare example of a U(IV) primary amido complex, U(COTTIPS2)Cp*(NH2) (2), subsequent insertion chemistry of 2 with CO2 to yield a primary carbamate complex, and deprotonation to form an anionic primary imido complex.
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© XXXX American Chemical Society
Scheme 1. Synthesis of 1
The reaction of 1 with stoichiometric NH3 in an NMR tube resulted in a color change from brown to orange-red, the consumption of NH3, and formation of a product characterized as the mixed-sandwich primary amido complex U(COTTIPS2)Cp*(NH2) (2; Scheme 2). Alternatively, 2 can be obtained by the reaction of the methyl complex U(COTTIPS2)Cp*Me and NH3 or by the reaction of the chloride U(COTTIPS2)Cp*Cl and KNH2 (Scheme 2); further details can be found in the Supporting Information. The IR spectrum of 2 contained vibrational bands at 3582 and 3309 cm−1 corresponding to νN−H symmetric and asymmetric stretches. X-ray diffraction data collected from a sample of 2 recrystallized from tBuOMe (a useful, higher boiling alternative to Et2O, which we have employed with success elsewhere21) confirmed the structure as shown in Figure 1, containing the expected η1-bound amido ligand, with one cocrystallizing molecule of tBuOMe. 2 will also crystallize from Et2O and contains a molecule of Et2O in the asymmetric unit. While U(Cp)3NH2 has been studied theoretically,22 only two previously isolated U(IV) primary amido complexes exist in the current literature, U(1,2,4{tBu}3C5H2)2(NH2)223 and U(TrenTIPS)(NH2),11 and these exhibit U−Namido bond distances (2.228(4) and 2.194(5) Å, respectively) comparable to that of 2.217(4) Å found in 2. The U−centroid distances and angles in 2 are closely comparable to Special Issue: Mike Lappert Memorial Issue Received: November 25, 2014
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DOI: 10.1021/om501190x Organometallics XXXX, XXX, XXX−XXX
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Organometallics Scheme 2. Synthesis and Reactivity of 2a
a
Isolated yields are given in parentheses.
related mixed-sandwich alkyl complexes.18 No reaction occurred when a solution of 2 in C6D6 was exposed to either stoichiometric or excess (>1 bar) CO; however, exposure of a hydrocarbon solution of 2 to 1 equiv or an excess of CO2 resulted in the formation of the primary carbamate complex U(COTTIPS2)Cp*(κ2-O2CNH2) (3), via insertion of CO2 into the U−N bond. Performing the reaction with 13CO2 to form 13 C-3 resulted in the appearance of a resonance at δC 25.8 in the 13 C{ 1 H} NMR spectrum, corresponding to the −O213CNH2 ligand environment. The 1H NMR spectroscopic resonance attributable to the carbamate protons in 3 is observed at δH 15.6, less paramagnetically shifted than in 2 due to the increased distance between the NH2 protons and the U(IV) center. Assignment of this resonance was confirmed by synthesizing U(COTTIPS2)Cp*(κ2-O2CND2) and collecting 1H and 2H NMR spectroscopic data. The IR spectrum of 3 contained a strong vibrational band at 1611 cm−1, along with further broad absorptions at 1520−1410 cm−1, corresponding to symmetric and asymmetric νC−O vibrations (shifted to 1588 and 1509−1375 cm−1 for 13C-3) and consistent with other reported carbamate νC−O bands,4,5,7 along with a band at 3424 cm−1 attributable to νN−H. The resulting κ2 configuration of the carbamate ligand was confirmed by the collection of X-ray diffraction data (Figure 2).
Figure 1. ORTEP representation of the molecular structure of 2, with thermal ellipsoids at the 50% probability level. Hydrogen atoms (except those on N1) and cocrystallizing molecule of tBuOMe are omitted for clarity. Selected bond distances (Å) and angles (deg): U1−N1 2.217(4), Ct(COT)−U1 1.9511(17), Ct(Cp)−U1 2.487(2); Ct(COT)−U1−Ct(Cp) 141.88(7).
those found in other U(IV) complexes containing the COTTIPS2/Cp* ligand framework.2,24 1 H NMR spectroscopic data were consistent with the formulation of 2: resonances attributable to the −NH2 protons at δH 202 were assigned by virtue of relative integral values and by synthesizing the deuterioamide U(COTTIPS2)Cp*(ND2) and collection of complementary 1H and 2H NMR spectroscopic data. Two routes can be used to synthesize the deuterioamide, as shown in Scheme 3, forming 2-d3 or 2-d2. An additional resonance is present at δD −6.05 in the 2H NMR spectrum of 2-d3 corresponding to the partially deuterated Cp* ligand C5Me4CH2D. The reactivity of 2 with small molecules was examined for comparison to the insertion chemistry observed with the Scheme 3. Synthesis of 2-d3 and 2-d2 from ND3
Figure 2. ORTEP representation of the molecular structure of 3. with thermal ellipsoids at the 50% probability level. Hydrogen atoms (except those on N1) and iPr groups are omitted for clarity. Selected bond distances (Å) and angles (deg): Ct1−U1 1.9551(3), Ct2−U1 2.4788(4); Ct1−U1−Ct2 137.076(12).
To the best of our knowledge, 3 is the first example of a primary carbamate ligand bound to a U(IV) metal center in an organometallic system and is one of only a few examples of U(IV) organometallic carbamates.6,7 The two crystallographically characterized U(IV) κ2-carbamates comparable to 3 are ({AdOAr}3tacn)U(κ2-O2CNHMes)25 (A; tacn = triazacyclononane) and U4O2(κ2-O2CNEt2)1226 (B); metrics relating to the B
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Organometallics κ2-carbamate ligands in these complexes and in 3 are provided in Table 1.
In conclusion, a rare primary amido complex (2) has been synthesized from N−H bond activation of NH3 by the “tuckedin” alkyl 1 from the reaction of U(COTTIPS2)Cp*Me and NH3 or from U(COTTIPS2)Cp*Cl and KNH2. The reaction of 1 and NH3 demonstrates the utility of “tucked-in” alkyl complexes as starting materials able to undergo σ-bond metathesis reactions and potentially provides atom efficient routes to new U(IV) compounds. Such behavior has also been demonstrated with other “tucked-in” U(IV) complexes.27−29 Facile CO2 insertion into the U−N bond of 2 resulted in the first crystallographically characterized example of a U(IV) primary carbamate complex; however, 2 is unreactive toward CO. Deprotonation of 2 yielded a rare example of a terminal U(IV) primary imido complex, 4.
Table 1. Selected Bond Distances (Å) and Angles (deg) of “U(κ2-O2CNH2)” Moieties in U(IV) κ2-Carbamate Complexes 3 U−O O−C C−N O−U−O O−C−O
2.399(2), 2.407(2) 1.296(4), 1.269(4) 1.335(5) 54.72(9) 119.0(3)
A 2.434(4), 2.527(4) 1.259(7), 1.278(7) 1.383(7) 52.85(12) 121.1(5)
B 2.442(11), 2.419(10) 1.205(24), 1.275(23), 1.41(2) 51.221(18) 121.0(7)
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ASSOCIATED CONTENT
S Supporting Information *
Text, tables, a figure, and CIF files giving experimental details, the unrefined molecular structure of 4, and crystallographic details and data for compounds 2 and 3. This material is available free of charge via the Internet at http://pubs.acs.org.
In comparison to A and B described above, the “U− O2CNR2” unit of 3 contains a near-symmetric carbamate moiety with almost equal U−O bond distances, in contrast to the asymmetric U−O distances found in the other structures. All four U−O distances in A and B are longer than those in in 3, likely due to the steric demands of the bulky Ad-substituted tacn ligand in A and the other bridging carbamate units in B, whereas the “metallocene wedge” provided by the carbocylic rings in 3 allows for closer approach of the ligand; this is also reflected in the more obtuse O−U−O angle in 3. There are some variations in the O−C bond distances in 3 (0.027 Å), which are also longer than those in A and Bthis may well be a compensatory effect due to the shorter U−O bonds. The carbamate unit in 3 is essentially planar, with a U1−O1−C37− O2 torsion angle of 1.2(2)°. The bond distances and angles in the “U1−O1−C37−O2” fragment are similar to those of the related mixed-sandwich κ2-carboxylates U(COTTIPS2)Cp*(κ2O2CR) (where R = Me, CH2Ph). In comparison to 2, the metal−centroid bond distances remain almost unchanged, while the Ct1−U1−Ct2 angle is more acute (137.076(12) vs 141.88(7)°), as the carbamate ligand demands greater space in the metallocene coordination sphere than the amide ligand. Deprotonation of 2 was attempted as a route to imido or nitrido complexes: the synthesis of an anionic primary imido complex from a parent amido, U(TrenTIPS)(NH2), was recently achieved and is the first and only other characterized example to date.11 Similarly, the reaction of 2 with KH and 18C6 (18crown-6 ether) in THF yielded a cherry red solution of 4, [U(COTTIPS2)Cp*(NH)][K(18C6)] (see Scheme 2); 4 was obtained as a pink powder in 32% yield after removal of solvent and washing with tBuOMe. IR spectroscopic data for 4 showed no vibration attributable to νN−H, nor was it observed for [U(TrenTIPS)(NH)][K(15C5)].11 Poor solubility in common NMR solvents frustrated the collection of NMR data and made complete assignment of the 1H NMR spectra of 4, which exhibited broad paramagnetically shifted resonances, problematic. Attempts to isolate crystalline material suitable for X-ray diffraction studies only yielded red needles, from which lowquality data were obtained. Extensive disorder in the 18C6 moiety and TIPS groups could not be sufficiently well modeled; however, the connectivity and gross structure could be established and revealed a significant interaction between the K cation and the terminal NH group (see the Supporting Information for a graphic). Further attempts to deprotonate 2 using other conditions (2.2.2-cryptand or 12-crown-4 as encapsulating agents, with KH or NaH), were unsuccessful.
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AUTHOR INFORMATION
Corresponding Author
*E-mail for F.G.N.C.:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the European Research Council and the University of Sussex for financial support. Thanks go to the National Crystallography Service for assistance with collecting crystallographic data for compound 3 and to Dr. A. Abdul-Sada (University of Sussex) for mass spectrometry.
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DEDICATION Dedicated to the memory of Mike Lappert, a distinguished pioneer of organometallic and metal amide chemistry. REFERENCES
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