Communication pubs.acs.org/Organometallics
Carbene Complexes of Phosphorus(V) Fluorides by Oxidative Addition of 2,2-Difluorobis(dialkylamines) to Phosphorus(III) Halides Tobias Böttcher,† Oleg Shyshkov,‡ Matthias Bremer,§ Bassem S. Bassil,† and Gerd-Volker Röschenthaler*,† †
School of Engineering and Science, Jacobs University Bremen, Campus-Ring 1, 28759 Bremen, Germany Dyneon GmbH, Industrieparkstraße 1, 84508 Burgkirchen, Germany § Merck KGaA, Frankfurter Straße 250, 64293 Darmstadt, Germany ‡
S Supporting Information *
ABSTRACT: Carbene-stabilized complexes of substituted and unsubstituted phosphorus(V) fluorides were obtained by oxidative addition of 2,2-difluorobis(dialkylamines) to phosphorus(III) halides. Octahedral and hydrolytically stable complexes were obtained in quantitative yields. All compounds were characterized in the solid state by single-crystal X-ray diffraction.
N
-heterocyclic diaminocarbenes (NHC) have attracted a great deal of attention during the past two decades, not only because of scientific curiosity but also for their excellent applicability in numerous chemical, technical, and medicinal processes.1 In the focus are primarily NHC complexes of late transition metals, due to their catalytic activity, the most prominent examples being the second-generation Grubbs catalysts.2,3 In contrast, fewer compounds of main-group elements have been reported, although many of them give insight into the kind of chemistry that was only known for d-block elements.4 Moreover, NHC ligands are used to stabilize main-group elements in high oxidation states to form charged and uncharged stable complexes as Lewis acid and base adducts.5,6 Our workgroup recently used 2,2-difluorobis(dialkylamines) as carbene precursors for oxidative addition to Ge(II) and Sn(II) halides in order to obtain carbenestabilized complexes of Ge(IV) and Sn(IV).7 Here we applied the method mentioned above in order to obtain carbene-coordinated complexes of phosphorus(V) fluorides. The aim was to obtain complexes of carbene ligands which are as small as possible: i.e., without bulky substituents at the N,N′ position (enhancing steric stabilization) and with no aromaticity (causing electronic stabilization). The aim was also to obtain acyclic carbene complexes. Therefore, compounds 1 and 2 were used as sources of carbene ligands (Figure 1). The corresponding free carbene is very reactive for 18 and not known in the case of 2. The smallest known stable, acyclic, and isolated free carbene is bis(diisopropylamino)carbene.9 Compounds 1 and 2 are also prone to halide metathesis, which allowed the use of chlorophosphines as well as fluorophosphines as starting materials (see Scheme 1). All reactions were carried out under Schlenk conditions in diethyl ether at −40 °C for 12 h. The products are shown in Scheme 1. © 2011 American Chemical Society
Figure 1. Cyclic (1) and acyclic (2) 2,2-difluorobis(dialkylamines).
Scheme 1. Synthesis of the Carbene Complexes of Phosphorus(V) Fluorides
The phosphorus in all complexes is hexacoordinated, and all compounds were obtained as colorless solids, which are stable and sparingly soluble in water. All reactions proceeded quantitatively in solution, which was confirmed through monitoring by 31P NMR. For the preparation of compounds 4 and 5, chlorophosphines were used as starting materials (see Scheme 1). In both cases 2-chloroimidazolidinium chloride precipitated as a byproduct, which was easily removed by washing with Special Issue: Fluorine in Organometallic Chemistry Received: October 18, 2011 Published: December 22, 2011 1278
dx.doi.org/10.1021/om2009827 | Organometallics 2012, 31, 1278−1280
Organometallics
Communication
water. The stability of hexacoordinated phosphorus(V) fluorides in water is known and was reported some time ago by our workgroup, involving however a different acid−base adduct of ammonia and phosphorus pentafluoride.10 A plausible mechanism for these reactions is proposed in Scheme 2. In step A a fluoride is transferred from 2 to
Table 1. Selected Bond Lengths (pm) and Angles (deg) for Compounds 3−6 C1−P1 P1−F5 P1−C6 av P1−Fcis N1−C1−N2 F1−P1−F4 F2−P1−F3
Scheme 2. Proposed Mechanism for the Addition of 2 and PF3
3
4
5
6
190.3(2) 159.80(15)
191.56(16)
192.03(16)
192.49(12) 161.04(8)
183.14(18) 164.13(11) 110.40(14) 174.91(5) 175.31(5)
184.38(15) 163.51(11) 110.14(14) 175.89(5) 175.64(5)
160.64(17) 110.9(2) 179.54(9) 179.79(9)
161.03(9) 116.97(11) 179.47(6) 179.31(5)
The most remarkable difference between all the compounds are the bond lengths of the carbene ligand to the phosphorus center. The shortest C1−P1 bond length was found in the cyclic carbene−PF5 adduct 3 (190.3(2) pm) and the longest in the acyclic carbene−PF5 adduct 6 (192.49(12) pm), having in between the two trans-substituted derivatives 4 and 5 with respective similar bond lengths of 191.56(16) and 192.03(16) pm (Figure 2). The average N−C−N angle is 110.5(2)° for the cyclic complexes and increases in the acyclic complex to 116.97(11)°. The distance of the phosphorus to the carbon atom in the position trans to the NHC ligand is 179.82(7) pm for the methyl group in 4 and 184.38(15) pm for the ipso carbon of the phenyl substituent in 5. Furthermore, the imidazolidine ring is planar and is in a staggered conformation to the cis fluorine atoms. Moreover, the phenyl ring in 5 is in an eclipsed conformation with the imidazolidine ring (Figure 2). Derivatives of 3 and 5 were reported earlier by Arduengo and co-workers by addition of free carbene to PF5 and PhPF4, yielding compounds 7 and 8 (Figure 3).5,6 Complex 7 is isostructural
phosphorus(III) trifluoride to give the respective phosphoranide and 2-fluoro-amidinium ions. In step B the carbon in the 2-position of the 2-fluoro-amidinium cation under goes nucleophlic attack by the phosphoranide anion and a carbon−phosphorus bond is formed. In step C the final redoxrearrangement takes place, where the electron-poor and highly Lewis-acidic phosphorus(III) abstracts the second fluoride from the carbon in the 2-position and is oxidized to give phosphorus(V), whereas carbon(IV) is reduced to yield the carbenic carbon. Low temperature NMR measurements were performed in order to confirm the existence of the tetrafluorophosphoranide in solution.11 However, the reaction was too fast and spontaneously resulted in formation of the final product at −94 °C. Crystals of all products were obtained by slow diffusion of diethyl ether into a solution of the compounds in acetonitrile. X-ray diffraction analysis confirmed the structure of the four octahedral complexes 3−6 of phosphorus(V), as shown in Figure 2. Selected bond lengths and angles are shown in Table 1. All products were fully characterized by 1H, 13C, 19F, and 31P NMR spectroscopy and high-resolution mass spectrometry (see the Supporting Information).
Figure 3. Earlier reported NHC−phosphorus(V) fluoride complexes.
with 3. Although 7 is coordinated by a very different NHC ligand (aromatic ring with bulky substituents in N,N′ positions), the bond lengths and angles differ only slightly between the two compounds (the carbenic carbon−phosphorus bond length is 0.5 pm larger in 3, whereas P−Ftrans is 0.4 pm and P−Fcis is 1.1 pm larger).
Figure 2. (top row) Solid-state molecular structures of 3−6 with thermal ellipsoids set at the 50% probability level (H atoms are omitted for clarity). (bottom row) Views along the C1−P1 bond. 1279
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(10) Storzer, W.; Schomburg, D.; Röschenthaler, G.-V. Chem. Ber. 1983, 116, 367−374. (11) Christe, K. O.; Dixon, D. A.; Mercier, H. P. A.; Sanders, J. C. P.; Schrobilgen, G. J.; Wilson, W. W. J. Am. Chem. Soc. 1994, 2850−2858.
In compound 8, on the other hand, the phenyl substituent is twisted by 90° and is therefore in the staggered conformation to the imidazolidine ring, whereas the phenyl ring in 5 is in the eclipsed conformation. In analogy with 3 and 7, the bond lengths and angles are only slightly affected by the different NHC ligand used in 8 (the carbenic C−P bond length is 1.0 pm shorter in 5, whereas the average P−Fcis bond length is 2.3 pm longer, and the C−P bond to the ipso carbon of the phenyl substituent is equal within the standard deviation to that of 8). In summary, we have used 2,2-difluorobis(dialkylamines) as precursors for carbene-coordinated complexes of substituted and unsubstituted phosphorus(V) fluorides and were able to obtain four different compounds, the crystal structure of each being determined by single-crystal XRD.
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ASSOCIATED CONTENT
S Supporting Information *
Text, figures, tables, and CIF files giving experimental details for synthesis of compounds 3−6 and X-ray crystallographic and NMR spectroscopic data. 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:
[email protected]. ACKNOWLEDGMENTS G.-V.R. and T.B. acknowledge the financial support of the Deutsche Forschungsgemeinschaft (Grant Ro 362/47/1-0). REFERENCES
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