Anionic and Mononuclear Phosphinidene and Imide Complexes of

Communication pubs.acs.org/Organometallics

Anionic and Mononuclear Phosphinidene and Imide Complexes of Niobium Keith Searles, Patrick J. Carroll, and Daniel J. Mindiola* Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia Pennsylvania 19104, United States S Supporting Information *

ABSTRACT: The reactivity of a terminal methylidene complex of niobium, [(ArO)2NbCH2(CH3)(CH2PPh3)] (1), with the primary phosphine PhPH2 results in formation of the anionic phosphinidene complex [H3CPPh3][(ArO)2NbPPh(CH3)2] (2), where both the methylidene and ylide ligands of the precursor experience protonation. Multinuclear NMR spectroscopy and X-ray diffraction studies indicate formation of a bent phosphinidene ligand. Similar reactivity is also observed with primary amines, specifically AdNH2 (Ad = 1-adamantyl) and MesNH2 (Mes = 2,4,6trimethylphenyl), resulting in formation of the corresponding imide complexes [H3CPPh3][(ArO)2NbNR(CH3)2] (R = Mes, (3), Ad, (4)). Whereas 2 has a bent phosphinidene ligand and pseudo trigonal bipyramidal structure, the solid-state structure determination of complex 4 reveals a linear imide ligand and square-pyramidal geometry, where the imide enjoys significant Nb− NR multiple-bond character.

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arly-transition-metal complexes possessing imide1 (RN2−) and phosphinidene2 (RP2−) ligands have emerged as useful reagents for both stoichiometric and catalytic chemical reactions. More recently, terminal imide complexes of niobium have found application as reagents for metal−organic chemical vapor deposition, stoichiometric and catalytic C−F bond activation reactions, and selective semihydrogenation of alkynes.3 Although numerous imide complexes of niobium have been reported in the literature, terminal phosphinidenes of niobium have been far less explored and are limited, thus far, to the activation of white phosphorus,4 transmetalation of anionic phosphides,5 or the treatment of low-valent niobium precursors with either PH3 or PMe3.6 We communicate herein a synthetic pathway for the synthesis of anioninc phosphinidene and imide complexes of niobium employing a previously reported niobium methylidene complex, [(ArO)2 NbCH2(CH3) (CH2PPh3)] (1; −OAr = 2,6-bis(diphenylmethyl)-4-tert-butylphenoxide),7 as a precursor for double-deprotonation reactions of primary phosphine and amine substrates. The methylene and the ylide are both proton acceptors, therefore yielding rare examples of anionic forms of phosphinidene and imides. Treatment of the previously reported methylidene complex 17b dissolved in toluene with a stoichiometric amount of PhPH2 resulted in gradual formation of a dark red solution over the duration of ca. 10 min. After the mixture was stirred for an additional 5 h, all volatiles were removed under reduced pressure and addition of pentane resulted in formation of a brown solid, which was isolated in 68% yield. The most notable NMR features of the new product are two resonances found in the 31P{1H} NMR spectrum, obtained in C6D6, centered at 393.21 and 22.30 ppm. The downfield resonance, 393.21 ppm, © XXXX American Chemical Society

is similar to those of the other few group 5 complexes possessing a terminal and bent phosphinidene ligand,4−6,8 while the upfield resonance, 22.30 ppm, occurs at a chemical shift similar to those reported for phosphonium salts.9 Formation of a triphenylmethylphosphonium cation, [H3CPPh3]+, is also evident from an HSQC NMR experiment where a doublet in the 1H NMR spectrum centered at 2.03 ppm (3H, 2JHP = 13.01 Hz) correlates to an upfield doublet in the 13C{1H} NMR spectrum centered at 9.13 ppm (1JCP = 55.11 Hz). An additional resonance in the 1H NMR spectrum is a singlet at 0.51 ppm (6H), which does not display any coupling. This resonance is indicative of the complex retaining two methyl groups, which was further supported by monitoring the reaction via 1H NMR spectroscopy. No significant amount of methane10 was detected during the progression of the reaction, suggesting that σ-bond metathesis involving a methyl ligand or α-hydrogen abstraction involving a methyl ligand is not occurring and that the new molecule most likely retains both methyl residues (one derived from protonation of the methylidene). In all, the 1H, 13C{1H}, and 31P{1H} NMR spectroscopic studies suggest formation of an anionic phosphinidene with a chemical formula of [H3CPPh3][(ArO)2NbPPh(CH3)2] (2) (Scheme 1). To conclusively establish the identity and degree of aggregation in 2, X-ray diffraction studies were performed on a dark red crystal, which has been solved in the monoclinic Special Issue: Gregory Hillhouse Issue Received: June 15, 2015

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DOI: 10.1021/acs.organomet.5b00518 Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics

phenyl) for the synthesis of anionic imide complexes of niobium (Scheme 1). Accordingly, treatment of complex 1 dissolved in benzene with a stoichiometric amount of either AdNH2 or MesNH2 ultimately resulted in the formation of yellow and yellow-orange solutions, respectively. Analysis of the products via multinuclear NMR spectroscopy indicated formation of the imide salts [H3CPPh3][(ArO)2NbNR(CH3)2] (3, R = 2,4,6-Me3C6H2 (Mes); 4, R = 1-adamantyl (Ad)). Complexes 3 and 4 both display diagnostic chemical shifts in their respective 31P{1H} NMR spectra at 20.59 and 20.55 ppm, revealing the presence of a phosphonium cation, akin to complex 2 (vide supra). Additionally, upfield doublets observed in the 13C{1H} and 1H NMR spectra, representing the methyl of [H3CPPh3]+, display the appropriate one- and two-bond-order coupling to phosphorus, respectively. The imide substituent for both complexes 3 and 4 displays wellresolved and readily identifiable spectroscopic features in the 1 H and 13C{1H} NMR spectra.11 Finally, monitoring the formation of complex 3 and 4 via 1H NMR spectroscopy indicates formation of methane but in very small quantities, which is similar to the formation of complex 2 and ultimately supports formation of an anionic imide. To further corroborate the formation of the imide complexes, X-ray diffraction studies were performed on a yellow single crystal of complex 4, which confirmed the expected composition (Figure 1). However, there are several notable structural differences in comparison to complex 2. Notably, the coordination environment of complex 4 is best described as a distorted-square-pyramidal geometry (τ5 = 0.29)12 with mutually cis aryloxide and cis methyl ligands occupying the equatorial positions, while the imide ligand resides in the lone axial position. The short Nb1−N1 distance (1.772(4) Å) and nearly linear angle Nb1−N1−C73ipso (174.4(4)°) tentatively suggests a high degree of multiplebond character between the imide nitrogen and the niobium center. The structural dissimilarities observed between complexes 2 and 4 do not appear to be a result of steric demand and are most likely electronic in origin. Formation of 2−4 mostly proceeds via a species such as [(ArO)2Nb(CH3)2(XHR) (CH2PPh3)] (X = P, R = Ph; X = N, R = Mes, Ad) followed by α-hydrogen abstraction to yield the phosphinidene or imide ligand. We propose protonation of the methylidene ligand, since addition of a weak acid to 1 results in formation of [(ArO)2Nb(CH3)2(CH2PPh3)]+ without any evidence of phosphonium formation.7b In conclusion, we have communicated the synthesis of anionic and terminal phosphinidene and imide complexes of niobium. This synthetic strategy employs a double-deprotonation reaction that takes advantage of a nucleophilic methylidene, as well as an ylide ligand of the niobium methylidene precursor. Surprisingly, we do not see methane loss as the primary pathway to formation of these terminally bound ligands. Presently, we are examining the reactivity of complexes 2−4 as well as the bonding phenomena between Nb and P or N.

Scheme 1. Synthesis of Anionic and Terminal Phosphinidene and Imide Complexes of Niobium

space group P21/c (Figure 1). Despite the reasonable atomic displacement parameters upon anisotropic refinements, a

Figure 1. Molecular structures of 2 (top) and 4 (bottom) displaying thermal ellipsoids at the 50% probability level. Carbons of the −tBu and −CHPh2 groups as well as all hydrogens have been omitted for clarity.

statistically suitable model for the structure of complex 2 has proven problematic.11 Regardless, the solid-state structure provides a means of unequivocally confirming the monomeric nature and chemical identity of 2. The overall geometry of complex 2 is best described as a distorted-trigonal-bipyramidal geometry (τ5 = 0.60),12 where the two methyl ligands and terminal phosphinidene ligand comprise the equatorial positions, leaving the mutually trans aryloxide ligands in the axial positions. The short Nb1−P1 distance (2.370(4) Å) and acute Nb1−P1−C73ipso angle (110.1(5)°) are consistent with formation of a terminal phosphinidene4−6,8 and are in agreement with solution 31P{1H} NMR measurements (vide supra). The same synthetic strategy was also employed using AdNH2 (Ad = 1-adamantyl) and MesNH2 (Mes = 2,4,6-trimethyl-

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00518. B

DOI: 10.1021/acs.organomet.5b00518 Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics

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Pink, M.; Urnezius, E.; Protasiewicz, J. D.; Mindiola, D. J. Chem. Commun. 2009, 4521−4523. (9) For representative papers of organometallic complexes with phosphonium cations see: (a) Tonzetich, Z. J.; Schrock, R. R.; Müller, P. Organometallics 2006, 25, 4301−4306. (b) Sarkar, S.; McGowan, K. P.; Kuppuswamy, S.; Ghiviriga, I.; Abboud, K. A.; Veige, A. S. J. Am. Chem. Soc. 2012, 134, 4509−4512. (c) O’Reilly, M. E.; Ghiviriga, I.; Abboud, K. A.; Veige, A. S. J. Am. Chem. Soc. 2012, 134, 11185−11195. (10) A small amount of methane is observed in the reaction mixture leading to complexes 2−4. Heating samples of 2−4 at 45 °C for 10 h also resulted in formation of methane. Unfortunately, we have been unable to fully characterize the complexes resulting from methane elimination. (11) See the Supporting Information. We have been unable to provide a rationalization for the poor structure refinement of 2. The Xray data are, perhaps, “weak” but are certainly not out of the ordinary. We investigated twinning, both pseudomerohedral and nonmerohedral, by use of the programs ROTAX and CELL_NOW and alternative space groups P21 and Pc. However, none of these attempts led to an explanation for the poor refinement. An examination of the simulated precession photos also does not reveal anything suspicious. Numerous crystals have been tested (grown under differing conditions) to improve the overall data. Unfortunately this has not yielded better refinement data. (12) (a) τ is defined as (A − B)/60, with A and B being the largest and smallest transoid ligand−metal−ligand angles in the base of an approximate square-pyramidal geometry, where τ = 0 for SP and τ = 1 for TBP.. (b) Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor, G. C. J. Chem. Soc., Dalton Trans. 1984, 1349−1356.

Experimental details, crystallographic parameters, and NMR spectra (PDF) Crystallographic data (CIF) Crystallographic data (CIF)

AUTHOR INFORMATION

Corresponding Author

*E-mail for D.J.M.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is dedicated to the memory of Professor Gregory Lee Hillhouse, an artist, scientist, mentor, and friend. For funding, we thank the University of Pennsylvania and the National Science Foundation (CHE-0848248 and CHE1152123).

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REFERENCES

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DOI: 10.1021/acs.organomet.5b00518 Organometallics XXXX, XXX, XXX−XXX