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Flexibility of the Carbodiphosphorane, (Ph3P)2C: Structural Characterization of a Linear Form Patrick J. Quinlivan and Gerard Parkin* Department of Chemistry, Columbia University, New York, New York 10027, United States S Supporting Information *

that we have obtained a crystalline form of (Ph3P)2C, from solutions in benzene,19 which exhibits a linear P−C−P moiety (Figure 2),20,21 counter to that expected on the basis of simple VSEPR theory. Interestingly, the linear structure of (Ph3P)2C also differs from the previous structures by the presence of a staggered arrangement of phenyl groups, as illustrated in Figure 3. Thus, the Ph−P···P−Ph torsion angle of linear (Ph3P)2C is 60.0°, whereas the bent forms have average torsion angles of 26.1°, 6.8°, and 6.4° (Figure 3), with the latter two being close to eclipsed.22

ABSTRACT: X-ray diffraction studies demonstrate that crystals of the carbodiphosphorane, (Ph3P)2C, obtained from solutions in benzene, exhibit a linear P−C−P interaction. This observation is in contrast to the highly bent structures that have been previously reported for this molecule, thereby providing experimental evidence that the coordination geometry at zerovalent carbon may be very flexible. Density functional theory calculations support the experimental observations by demonstrating that the energy of (Ph3P)2C varies relatively little over the range 130−180°. arbodiphosphoranes, (R3P)2C,1−4 which were first reported in 1961,1a constitute an interesting class of molecules that feature two-coordinate carbon (Figure 1). In this regard, carbodiphosphoranes are structurally related to carbenes but differ by virtue of the fact that the carbon atoms of carbodiphosphoranes are formally zerovalent with two lone pairs,5,6 whereas the carbon atoms of carbenes are divalent with one lone pair.7,8 The presence of the lone pairs is manifested by carbodiphosphoranes being effective ligands for a variety of metals and nonmetals.5,9−12 Furthermore, since there are two lone pairs, carbodiphosphoranes may coordinate to two metals.13 In addition to providing for the critical mode of reactivity of carbodiphosphoranes, the presence of lone pairs is also associated with the fact that such molecules are typically bent in the solid state.6,7c,14 Therefore, it is significant that we report here the molecular structure of a linear form of a carbodiphosphorane, thereby providing experimental evidence that such molecules are highly flexible.

C

Figure 2. Molecular structure of linear (Ph3P)2C.

Density functional theory (DFT) geometry optimization (B3LYP) of (Ph3P)2C with S6 symmetry reproduces well the experimental linear structure (Figure 2). We have also performed geometry optimization calculations on the various bent forms of (Ph3P)2C that have been reported and, in each case, a bent geometry (137.4−139.5°) is obtained, with a conformation similar to that of the respective experimentally determined geometry (Table 1).23 Interestingly, despite the fact that the P−C−P angles of the conformers investigated vary considerably from 137.4° to 180.0°, the energies of the molecules are similar (Table 1).24 Therefore, to examine the sensitivity of the energy of (Ph3P)2C with respect to the P−C−P bond angle, we performed a series of geometry optimization calculations in which the P−C−P bond angle is varied incrementally (Figure 4).25,26 The data indicate that there is very little energetic penalty associated with either increasing or decreasing the P−C−P bond angle from that corresponding to the lowest-energy structure. For example, the energy of the molecule increases by 0.84 kcal mol−1 upon increasing the angle

Figure 1. Two representations of carbodiphosphoranes, (R3P)2C.

Previous X-ray diffraction studies have demonstrated that carbodiphosphoranes with alkyl or aryl subsitutents are highly bent, with P−C−P bond angles in the range 121.8(3)− 143.8(6)°.13c,15−18 For example, (Ph3P)2C has been structurally characterized in two crystalline forms, which exhibit P−C−P bond angles that range from 130.1(6)° to 143.8(6)°.15 The bent nature of (Ph3P)2C is also affirmed by computational studies, which result in P−C−P bond angles of 135.0° and 136.9° (BP86/SVP and BP86/TZ2P, respectively).3a,b,4a In view of these experimental and computational studies, it is noteworthy © 2017 American Chemical Society

Received: February 10, 2017 Published: April 21, 2017 5493

DOI: 10.1021/acs.inorgchem.7b00381 Inorg. Chem. 2017, 56, 5493−5497

Communication

Inorganic Chemistry

Figure 3. Different crystalline forms of (Ph3P)2C, which illustrate the staggered and eclipsed extremes (hydrogen atoms omitted for clarity): (A) this work; (B) ref 15a; (C1 and C2) ref 15b.

Table 1. Geometry-Optimized (B3LYP) Structures of Different Conformers of (Ph3P)2C P−C−P/deg Ph−P···P−Ph/deg Erel/kcal mol−1 a a

A

B

C1

C2

180.0 60.0 0.00

139.5 19.3 −0.20

138.3 8.4 −0.32

137.4 13.7 −0.55

Erel = E(X) − E(A); X = A, B, C1, C2.

Figure 5. Lone-pair NLMOs of the linear (left) and bent (right) forms of (Ph3P)2C.

carbon component of both lone-pair NLMOs is 2p in character for the linear geometry, but a significant amount of 2s character becomes incorporated in the in-plane (σ-type)23,28 orbital upon bending of the P−C−P bonds (Figure 5);29 this change is also accompanied by a lowering of the energy of the σ-type NLMO (Figure 6). In contrast to these changes, the perpendicular (πtype)23,28 lone-pair orbital incorporates negligible carbon 2s character upon bending of the P−C−P bonds,30 such that there is little variation in the energy of this NLMO (Figure 6). It is well-known that the geometry of simple [AH2]Q± molecules is a compromise resulting from the fact that bending of the H−A−H bonds (i) stabilizes the 2a1 orbital (i.e., a nonbonding np orbital on A in the linear geometry) but (ii) destabilizes the bonding 1b2 orbital (i.e., the in-phase combination of an np orbital on A and the two hydrogen 1s orbitals).31 Depending upon the occupancies of these orbitals, the molecule may be either linear (e.g., BeH2) or bent (e.g., H2O). With respect to (Ph3P)2C, the stabilization of the σ-type lone-pair NLMO upon bending the molecule is offset by the

Figure 4. Variation in the energy of (Ph3P)2C, [(Ph3P)2N]+ and (Ph3Si)2O as a function of bond angle.

from 145.0° to 180.0° and by 0.75 kcal mol−1 upon decreasing the angle to 130.0°. As such, it is evident that packing effects associated with the presence of benzene19 in the solid state would be sufficient to perturb the molecular structure.15a,16 Analysis of the natural localized molecular orbitals (NLMOs) of (Ph3P)2C indicates that the two highest energy orbitals correspond to carbon lone pairs that are delocalized to a small extent onto the adjacent phosphorus atoms (Figure 5).23 For example, the two lone-pair orbitals of linear (Ph3P)2C, which are approximately degenerate, possess 78% carbon and 13% phosphorus character. In addition, the application of natural resonance theory27 demonstrates that while the bonding in (Ph3P)2C is delocalized, 92.4% of the Lewis structures for the linear form possess two lone pairs on carbon; likewise, 91.7% of the bent (139.5°) form possess two lone pairs on carbon. The 5494

DOI: 10.1021/acs.inorgchem.7b00381 Inorg. Chem. 2017, 56, 5493−5497

Communication

Inorganic Chemistry

only 0.62 kcal mol−1 higher than that of the fully geometryoptimized molecule with an angle of 147.2°. Over the range 140−180°, the energy surface for [(Ph3P)2N]+ corresponds closely to that of (Ph3P)2C, but the energy of [(Ph3P)2N]+ increases more rapidly than that for (Ph3P)2C upon decreasing the the angle from 140° (Figure 4). The lone-pair NLMOs for [(Ph3P)2N]+ are qualitatively similar to those of (Ph3P)2C, with the principal distinction being that they are more localized on the central atom of the former, a change that is in accordance with the electronegativity differences.40 For further comparison, we have also investigated the isoelectronic disiloxane, (Ph3Si)2O. Interestingly, and in contrast to the behavior for (Ph3P)2C and [(Ph3P)2N]+, the energy surface for bending of the Si−O−Si bonds does not exhibit a minimum, such that the energy of the molecule increases monotonically as the Si−O−Si angle decreases from 180°.38,41 This observation is in accord with the fact that only a linear structure has been reported to date for (Ph3Si)2O.42,43 The energy surface for bending of the Si−O−Si bonds is, nevertheless, shallow. For example, (Ph3Si)2O with a Si−O−Si angle of 145° is only 0.93 kcal mol−1 higher in energy than that for the linear geometry.44,45 In summary, crystals of (Ph3P)2C obtained from solutions in benzene exhibit a linear P−C−P interaction, thereby contrasting the highly bent structures that have been previously reported. The observation that (Ph3P)2C may exhibit different structures in the solid state provides experimental evidence that the coordination geometry at zerovalent carbon may be very flexible. Furthermore, computational studies demonstrate that (Ph3P)2C is more flexible than isoelectronic [(Ph3P)2N]+ and (Ph3Si)2O.

Figure 6. Variation of the NLMO energies as a function of the P−C−P bond angle.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00381. Experimental details and computational data (PDF) Crystallographic data in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Gerard Parkin: 0000-0003-1925-0547 Figure 7. P−C bonding NLMOs of linear (left) and bent (right) forms of (Ph3P)2C.

Notes

corresponding destabilization of the two P−C bonding NLMOs (Figure 7),32 as illustrated in Figure 6. Therefore, the differential behavior of the lone pair and bonding orbitals upon bending is one component that provides a simple rationalization for the flexibility of (Ph3P)2C. It is pertinent to compare the energy surface for bending of the P−C−P bonds of (Ph3P)2C with that for the P−N−P bonds of the isoelectronic bis(triphenylphosphine)iminium cation, [(Ph3P)2N]+, which is widely employed as a noncoordinating counterion.33,34 [(Ph3P)2N]+ typically exhibits a bent geometry,33,35,36 although a linear geometry has also been observed.37 In this regard, DFT calculations on [(Ph3P)2N]+ indicate that a bent geometry is favored, although the energy surface for bending of the P−N−P bond is also very flat (Figure 4).38,39 For example, the energy of the molecule with an angle of 180.0° is

ACKNOWLEDGMENTS We thank the National Science Foundation (Grant CHE1465095) for support of this research, and Professor Clark Landis is thanked for helpful advice.

The authors declare no competing financial interest.

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REFERENCES

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Inorganic Chemistry (30) For example, the carbon component of the π-type NLMO possesses only 0.13% 2s character when the P−C−P bond angle is 140.0°. (31) Albright, T. A.; Burdett, J. K.; Whangbo, M.-H. Orbital Interactions in Chemistry, 2nd ed.; John Wiley & Sons, Inc.: New York, 2013. (32) As would be expected, the two P−C bonding NLMOs possess more phosphorus character than do the lone pairs. For example, the P− C bonding NLMOs of the linear form possess an average of 37.5% phosphorus character, which increases to 40.7% for a P−C−P angle of 120°. (33) Lewis, G. R.; Dance, I. Crystal supramolecularity. Multiple phenyl embraces by [PPN]+ cations. J. Chem. Soc., Dalton Trans. 2000, 299− 306. (34) Despite the widespread use of [(Ph3P)2N]+ as a counterion, there are examples in which it associates with varying degrees with a metal center and may also undergo reaction. See, for example: (a) Sussman, V. J.; Ellis, J. E. A total loss of innocence: double ortho-metallation of bis(triphenylphosphano)iminium cation, [N(PPh3)2]+, by tris(ηnaphthalene)tantalate(1-). Chem. Commun. 2008, 5642−5644. (b) Bauzá, A.; Frontera, A.; Mooibroek, T. J.; Reedijk, J. The N-atom in [N(PR3)2]+ cations (R = Ph, Me) can act as electron donor for (pseudo) anti-electrostatic interactions. CrystEngComm 2015, 17, 3768−3771. (c) Tilset, M.; Zlota, A. A.; Folting, K.; Caulton, K. G. A loss of innocence? Charge-transfer interactions with Ph3PNPPh3+ as electron-acceptor. J. Am. Chem. Soc. 1993, 115, 4113−4119. (d) Darensbourg, M.; Barros, H.; Borman, C. Extent of association of bis(triphenylphosphine)iminium cation with organometallic anions in tetrahydrofuran solution. J. Am. Chem. Soc. 1977, 99, 1647−1648. (e) Ragaini, F.; Sironi, A.; Fumagalli, A. Deactivation of a [PPN][Rh(CO)4]-based catalytic system [PPN+ = (PPh3)2N+]. The first decomposition reaction of PPN+ and the formation of [Rh10P(CO)22)]3‑. Chem. Commun. 2000, 2117−2118. (f) Bolli, C.; Gellhaar, J.; Jenne, C.; Kessler, M.; Scherer, H.; Seeger, H.; Uzun, R. Bis(triphenyl-λ5-phosphanylidene)ammonium fluoride: a reactive fluoride source to access the hypervalent silicates [MenSiF5‑n]− (n = 0−3). Dalton Trans. 2014, 43, 4326−4334. (g) Carpenter, A. E.; Margulieux, G. W.; Millard, M. D.; Moore, C. E.; Weidemann, N.; Rheingold, A. L.; Figueroa, J. S. Zwitterionic stabilization of a reactive cobalt trisisocyanide monoanion by cation coordination. Angew. Chem., Int. Ed. 2012, 51, 9412−9416. (35) See, for example: Churchill, M. R.; Lake, C. H.; Wang, P.; Atwood, J. D. Crystal-structure of bis(triphenylphosphine)iminium p-tolylsulfonate, [(PPh3)2N+][CH3C6H4SO3−]. J. Chem. Crystallogr. 1994, 24, 473−476 and references cited therein.. (36) For example, the average P−N−P bond angle in [(Ph3P)2N]+ derivatives listed in the Cambridge Structural Database (CSD, version 5.37) is 142.4°, while only 3.2% have a bond angle ≥ 170°. See: Groom, C. R.; Bruno, I. J.; Lightfoot, M. P.; Ward, S. C. The Cambridge Structural Database. Acta Crystallogr., Sect. B: Struct. Sci., Cryst. Eng. Mater. 2016, 72, 171−179. (37) (a) Wilson, R. D.; Bau, R. Linear [(Ph3P)2N]+ cation. Crystalstructure of [(Ph3P)2N]+[V(CO)6]−. J. Am. Chem. Soc. 1974, 96, 7601− 7602. (b) Kirtley, S. W.; Chanton, J. P.; Love, R. A.; Tipton, D. L.; Sorrell, T. N.; Bau, R. Synthesis, Characterization, and CrystalStructures of [(Ph3P)2N]3+{Na[Mo3(CO)6(NO)3(μ2-OCH3)3(μ3O C H 3 ) ] − , a n u n u s u a l o r g a n o m e t a l l i c tr i p l e i o n , a n d [Me4N]+[Mo3(CO)6(NO)3(μ2-OCH3)3(μ3-OCH3)]−. J. Am. Chem. Soc. 1980, 102, 3451−3460. (c) Haiduc, I.; Cea-Olivares, R.; Hernández-Ortega, S.; Silvestru, C. Unexpected linear P-N-P fragment in the anion [SPh 2 PNPPh 2 S] − : Crystal structure of bis(triphenylphosphine)iminium dithiotetraphenylimidodiphosphinate, [Ph3PNPh3]+[SPh2PNPPh2S]−. Polyhedron 1995, 14, 2041−2046. (38) The energy surfaces for [(Ph3P)2N]+ and (Ph3Si)2O correspond to conformations that are similar to those of (Ph3P)2C. (39) Calculations on hypothetical [(H3P)2N]+ also indicate a flat energy surface. See ref 33. (40) For example, the lone-pair orbitals of linear [(Ph3P)2N]+ possess 88.2% nitrogen character compared to 78.0% carbon character for (Ph3P)2C.

(41) Continuing the trend in composition of the NLMOs as a function of the electronegativity difference, the lone-pair orbitals of linear (Ph3Si)2O possess more oxygen character (94.7%) than do the central atoms in [(Ph3P)2N]+ (88.2%) and (Ph3P)2C (78.0%). (42) There are 11 structures for (Ph3Si)2O listed in the CSD, which correspond to either unsolvated or solvated modifications, and each of these are linear at oxygen. See, for example: (a) Hönle, W.; Manríquez, V.; von Schnering, H. G. Structures of solvated hexaphenyldisiloxanes. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1990, 46, 1982−1984. (b) Purdy, A. P.; Smoot, E.; Butcher, R. J.; Kerr, A. Triclinic polymorph of bis(triphenylsilyl) oxide toluene disolvate. Acta Crystallogr., Sect. E: Struct. Rep. Online 2012, 68, o588. (c) Percino, J.; Pacheco, J. A.; Soriano-Moro, G.; Ceron, M.; Castro, M. E.; Chapela, V. M.; BonillaCruz, J.; Lara-Ceniceros, T. E.; Flores-Guerrero, M.; Saldivar-Guerra, E. Synthesis, characterization and theoretical calculations of model compounds of silanols catalyzed by TEMPO to elucidate the presence of Si-O-Si and Si-O-N bonds. RSC Adv. 2015, 5, 79829−79844. (d) Prince, P. D.; Bearpark, M. J.; McGrady, G. S.; Steed, J. W. Hypervalent hydridosilicates: synthesis, structure and hydride bridging. Dalton Trans. 2008, 271−282. (43) It is, nevertheless, worth noting that strongly bent disiloxanes are known with other substituents. For example, [(2,5-Mes2C5H3)Me2Si]2O has a Si−O−Si angle of 141.9°, while (Ph2ButSi)2O has an angle of 152.4°. See: (a) Karle, I. L.; Karle, J. M.; Nielsen, C. J. On the SiOSi angle in 1,2-di-tert-butyl-1,1,2,2-tetraphenyldisiloxane. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1986, 42, 64−67. (b) Pietschnig, R.; Merz, M. Selective formation of functionalized disiloxanes from terphenylfluorosilanes. Organometallics 2004, 23, 1373−1377. (44) Calculations on smaller disiloxanes, (R3Si)2O (R = H, Me), also indicate a flat energy surface for bending the Si−O−Si bonds. See, for example: (a) Cypryk, M.; Gostynski, B. Computational benchmark for calculation of silane and siloxane thermochemistry. J. Mol. Model. 2016, 22, 35. (b) Weinhold, F.; West, R. The nature of the silicon-oxygen bond. Organometallics 2011, 30, 5815−5824. (c) Csonka, G. I.; Reffy, J. Density-functional study of the equilibrium geometry and Si-O-Si potential-energy curve of disiloxane. Chem. Phys. Lett. 1994, 229, 191− 197. (d) Zhang, Y.; Li, Z. H.; Truhlar, D. G. Computational requirements for simulating the structures and proton activity of silicaceous materials. J. Chem. Theory Comput. 2007, 3, 593−604. (e) Shambayati, S.; Blake, J. F.; Wierschke, S. G.; Jorgensen, W. L.; Schreiber, S. L. Structure and basicity of silyl ethers: A crystallographic and ab initio inquiry into the nature of silicon oxygen interactions. J. Am. Chem. Soc. 1990, 112, 697−703. (45) As noted above, the different geometries of simple [AH2]Q± molecules is a consequence of the fact that the energies of the lone pair and bonding orbitals do not vary in the same manner upon changing the H−A−H bond angle. An additional complicating feature for nonhydride [AX2]Q± molecules is that the np orbitals on A can also participate in negative hyperconjugative interactions with the X substituents, in which electron density is formally delocalized into orbitals on X with local σ* character (see, for example, reference 44). In view of these competing factors it is, therefore, not surprising that molecules of this class are rather flexible. It should also be noted that explanations based on d-orbital participation and the existence of A−X multiple bonding are no longer considered valid.

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DOI: 10.1021/acs.inorgchem.7b00381 Inorg. Chem. 2017, 56, 5493−5497