Organometallics 2009, 28, 1995–1997
1995
Formation of an Organometallic Phosphanediide via Main-Group Dehydrocoupling Robert J. Less, Vesal Naseri, and Dominic S. Wright* Chemistry Department, Cambridge UniVersity, Lensfield Road, Cambridge CB2 1EW, U.K. ReceiVed January 14, 2009 Summary: The reaction of FcPH2 [Fc ) (C5H4)Fe(C5H5)] with n BuLi and As(NMe2)3 in tmeda (Me2NCH2CH2NMe2) giVes [(FcP)3(Li · tmeda)2] (1), containing the first example of an organometallic phosphanediide anion, [FcP]32-. There has been considerable interest in recent years in the synthesis and coordination chemistry of phosphanediide anions, of general formula [RP]n2-.1-5 These species were first prepared by the P-P bond cleavage reactions of neutral cyclophosphanes [RP]n with Na metal.6 More recently, however, the stoichiometric coupling reactions of aliphatic and aromatic phosphorus dichlorides (RPCl2) with Na metal in the presence of a Lewis base (L) have been shown to provide targeted access to a range of Na complexes containing [RP]22-,1,5 [RP]32-,1,5 and [RP]42- 1-4 anions (eq 1) and the first structural characterization of representatives of this series. These developments have opened up a rich area of transition-metal and main-group-metal coordination chemistry. L
nRPCl2 + (2n + 2)Na 98 [RP]n·(Na·L)2 + 2nNaCl (1) A noticeable gap in this area concerns phosphanediides containing organometallic functionalities as the R groups; all previous examples contain simple organic substituents such as t Bu, Cy, Ph, etc. This situation is in part due to the general scarcity of organometallic phosphorus dihalide precursors but is very largely due to the highly reducing conditions required for the P-P coupling reactions of the dihalides. A viable alternative to this approach is dehydrocoupling of primary phosphines (RPH2), which occurs using milder conditions.7 Stephan and co-workers have shown that coupling of primary phosphines occurs in the presence of transition-metal catalysts such as [Cp*2ZrH3]- to give the metallacyclic species Cp*2Zr(PR)n, which ultimately eliminate the cyclic phosphanes * To whom correspondence should be addressed. E-mail: dsw1000@ cam.ac.uk. (1) Geier, J.; Ru¨egger, H.; Wo¨rle, M.; Gru¨tzmacher, H. Angew. Chem., Int. Ed. 2003, 42, 3951. (2) Geier, J.; Harmer, J.; Gru¨tzmacher, H. Angew. Chem., Int. Ed. 2004, 43, 4093. (3) Wolf, R.; Hey-Hawkins, E. Z. Anorg. Allg. Chem. 2006, 632, 727. (4) Wolf, R.; Gomez-Ruiz, S.; Reinhold, J.; Bo¨hlmann, W.; HeyHawkins, E. Inorg. Chem. 2006, 45, 9107. (5) Stein, D.; Dransfeld, A.; Flock, M.; Ru¨egger, H.; Gru¨tzmacher, H. Eur. J. Inorg. Chem. 2006, 4157. (6) (a) Issleib, K.; Hoffmann, M. Chem. Ber. 1966, 100, 1320. (b) Issleib, K.; Krech, K. Chem. Ber. 1966, 100, 1311. (c) Baudler, M.; Grunerm, C.; ˝ zer, U. Z. Anorg. Allg. Chem. Fu¨rstenberg, G.; Kloth, B.; Saykowski, F.; O 1978, 446, 169. (d) Hoffmann, P. R.; Caulton, K. G. J. Am. Chem. Soc. 1975, 97, 6370. (7) See for example: Greenberg, S.; Stephan, D. W. Chem. Soc. ReV. 2008, 37, 1482. Clark, T. J.; Lee, J.; Manners, I. Angew. Chem., Int. Ed. 2006, 12, 8634. Waterman, R. Dalton Trans. 2009,b813332h.
[RP]n.8 Our recent studies have illustrated an increasingly close link between the transition-metal-catalyzed systems and those involving main-group reagents.9 For example, the reactions of primary aliphatic phosphines with nBuLi/E(NMe2)3 (E ) As, Sb) give heterocyclic anions of the type [(RP)nE]-, which finally expel cyclic phosphanes.10 There are, however, several important differences between the transition-metal reactions and the main-group counterparts. Most obviously, the main-group reactions are stoichiometric rather than catalytic, with reduction of E(III) occurring to Zintl ions (most commonly E73-) in tandem with P-P bond formation.9 Very recently we found that these Zintl ions have the potential to reductively cleave bonds. This was indicated by the reaction of 1,2-(PH2)2-C6H4 with nBuLi/Sb(NMe2)3, which gives the 1,2,3-triphospholide anion [C6H4P3]- via a mechanism involving C-P bond cleavage (Scheme 1, top).11 This outcome can be compared to the transition-metal-catalyzed dehydrocoupling of 1,2-(PH2)2-C6H4 in which no metal-based redox processes are involved and, as a result, only P-P bond formation occurs (Scheme 1, bottom).8b (8) (a) Etkin, N.; Fermin, M. C.; Stephan, D. W. J. Am. Chem. Soc. 1997, 119, 2954. (b) Hoskin, A. J.; Stephan, D. W. Angew. Chem., Int. Ed. 2001, 40, 1865. (c) Masuda, J. D.; Hoskin, A. J.; Graham, T. W.; Beddie, C.; Fermin, M. C.; Etkin, N.; Stephan, D. W. Chem. Eur. J. 2006, 12, 8696. (9) Hopkins, A. D.; Wood, J. A.; Wright, D. S. Coord. Chem. ReV. 2001, 216, 155. (10) Beswick, M. A.; Choi, N.; Hopkins, A. D.; McPartlin, M.; Mosquera, M. E. G.; Raithby, P. R.; Rothenberger, A.; Stalke, D.; Wheatley, A. E. H.; Wright, D. S. J. Chem. Soc., Chem. Commun. 1998, 2485. Bashall, A.; Beswick, M. A.; Choi, N.; Hopkins, A. D.; Kidd, S. J.; Lawson, Y. G.; Mosquera, M. E. G.; Raithby, P. R.; Wheatley, A. E. H.; Wood, J. A.; Wright, D. S. Dalton Trans. 2000, 479. Bashall, A.; Garcı´a, F.; Hopkins, A. D.; Wood, J. A.; McPartlin, M.; Woods, A. D.; Wright, D. S. Dalton Trans. 2003, 1143. (11) García, F.; Less, R. J.; Naseri, V.; McPartlin, M.; Rawson, J. M.; Sancho Tomas, M.; Wright, D. S. Chem. Commun. 2008, 859. Goodman, J. M.; Less, R. J.; Naseri, V.; McInnes, E. J. L.; Mulvey, R. E.; Wright, D. S. Dalton Trans. 2008, 6454. (12) FcPH2 was prepared by reduction of FcP(dO)(OEt)2 with LiAlH4/ Me3SiCl: Oms, O.; Maurel, F.; Carre´, F.; Le Bideau, J.; Vioux, A.; Leclercq, D. J. Organomet. Chem. 2004, 689, 2654. (13) Synthesis of 1: nBuLi (1.6 M in hexanes, 0.47 mL, 0.75 mmol) was added dropwise to a solution of FcPH2 (1.0 M in toluene, 0.75 mL, 0.75 mmol) in 5 mL of tmeda at room temperature, and the mixture was stirred for 1 h. The resulting red suspension was cooled to-78 °C, As(NMe2)3 (1.0 M in toluene, 0.25 mL, 0.25 mmol) was added dropwise, and this mixture was stirred for 14 h at room temperature, giving a red solution. Storage at 5 °C for 48 h gave dark red block-shaped crystals (66 mg, 7 mmol, 30%). 1H NMR (+25 °C, 400.13 MHz, C6D6): δ 4.73 (br s, 2H, P(1)-C5H4), 4.22 (br s, 4H, P(2,3)-C5H4), 4.09 (s., 5H, Fe(1)-Cp), 3.92 (s, 10H, Fe(2,3)-Cp), 3.90 (br t, 2H, P(1)-C5H4), 3.83 (s, 4H, Fe(2,3)-Cp), 2.27 (s, 8H,-CH2-tmeda), 2.12 (s, 24H, Me2N tmeda) (assignments were confirmed by an HSQC experiment). 31P NMR (+25 °C, 500.20 MHz, C6D6): δ-39.9 (t),-90.4 (d) (1JPP ) 240.8 Hz) ((FcPHPFc)-, δ-77.3 (d), 116.5 (d) (1JPP ) 335 Hz; 1JPH ) 206 Hz; 2JPH ) 14 Hz); FcPH-, δ-160.9 (1JPH ) 161 Hz)). 7Li NMR (194.40 MHz, C4D8O): δ 0.20 (s) (+25 °C); 0.01 (s), 0.57 (s) (-78 °C). Anal. Calcd for 1: C, 56.4; H, 6.6; N, 6.3. Found: C, 56.0; H, 6.7; N, 6.7.
10.1021/om9000296 CCC: $40.75 2009 American Chemical Society Publication on Web 03/11/2009
1996 Organometallics, Vol. 28, No. 7, 2009
Communications
Scheme 1. Contrasting Outcomes of the Reactions of 1,2-(PH2)2-C6H4 with nBuLi/Sb(NMe2)3 (Top) and [Cp*2ZrH3]- (Bottom)
We report here that the reaction of FcPH212 with nBuLi/ As(NMe2)3 in neat tmeda (Me2NCH2CH2NMe2) under mild conditions (e25 °C) gives dark red [{FcP}3(Li · tmeda)2] (1) (in 30% crystalline yield),13 containing the first example of an organometallic phosphanediide anion, [FcP]32- (eq 2). It can be noted also that the dichloride FcPCl2 can only be obtained in low yield from ferrocene (ca. 4%),14 so that dehydrocoupling from FcPH2 (obtained in 50-60% yield) is a significantly more efficient pathway to 1 overall than Na coupling of FcPCl2. In the latter reaction there is also the potential for both P-Cl and (Fc)C-H bond activation using the more extreme conditions. nBuLi/As(NMe
2)3
FcPH2 98 [(FcP)3(Li·tmeda)2]
(2)
tmeda
The formation of 1 in neat tmeda contrasts with that of the cyclic [(RP)nAs]- anions in the reactions of nBuLi/As(NMe2)3 with aliphatic primary phosphines.10 As noted in the introduction, the Zintl ions also generated in this type of reaction can cleave bonds, potentially diverting the normal course of the dehydrocoupling reaction. The formation of the [FcP]32- ion in the current study may well arise from P-As bond cleavage of an intermediate [(FcP)3As]- ion.9 It can be noted in this respect that the cleavage of the C-P bond observed in the reaction of 1,2-(PH2)2-C6H4 with nBuLi/Sb(NMe2)3 requires significantly greater energy (ca. 264 kJ mol-1) than would be required for the cleavage of the P-As bonds of an intermediate [(FcP)3As]ion (ca. 174 kJ mol-1).15 However, attempts to observe the formation of such a [{FcP}3As]- ion in this reaction using (14) The precursor FcPH(dO)OH is obtained via a Friedel-Craft reaction of PCl3/AlCl3 with ferrocene in ca. 4% yield after hydrolysis of the mixture of products containing FcPCl2: Sollott, G. P.; Howard, E., Jr. J. Org. Chem. 1964, 29, 2451. FcPCl2 is then regenerated by reaction of FcPH(dO)OH with excess PCl3 in 53% yield: Frank, A. W. J. Org. Chem. 1961, 26, 850. In contrast, FcPH2 is generated in an overall yield of ca. 50-60% via LiAlH4 reduction of FcP(dO)(OEt)2.12. (15) Huyee, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry, Principles of Structure and ReactiVity, 4th ed.; Prentice Hall: Englewood Cliffs, NJ, 1997. (16) 31P NMR studies in tmeda show the formation of a transient intermediate at ca. δ-74 (br s), which is completely absent after completion of the reaction. Also present at this stage is the lithiate FcPHLi (δ-153.7 (d, J31P-1H ) 197.6 Hz)). (17) Crystal data for 1: empirical formula C42H59Fe3Li2N4P3, FW ) 894.27, triclinic crystal system, space group P1j, a ) 10.0819(2) Å, b ) 11.1426(2) Å, c ) 21.2193(6) Å, R ) 96.0097(9)°, β ) 97.7334(9)°, γ ) 111.6478(13)°, V ) 2164.42(9) Å3, Z ) 2, Fcalcd ) 1.372 Mg m-3, µ(Mo KR) ) 1.137 mm-1, 18 153 reflections collected, 7466 independent reflections (Rint ) 0.076), R1 ) 0.063 (I > 2σ(I)), wR2 ) 0.0189 (all data). Data for 1 were collected on a Nonius KappaCCD diffractometer at 180(2) K, solved by direct methods and refined by full-matrix least squares on F2(Sheldrick, G. M. SHELX-97; University of Go¨ttingen, Go¨ttingen, Germany, 1997).
Figure 1. Structure of 1. H atoms have been omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level. Selected bond lengths (Å) and angles (deg): P(1)-P(2) ) 2.191(2), P(1)-P(3) ) 2.218(2), P(1) · · · Li(1) ) 2.892(8), P(2)-Li(1) ) 2.487(9), P(3)-Li(1) ) 2.519(8), P(3)-Li(2) ) 2.527(9), Li(2) · · · C(1) ) 2.69(1), Fe · · · C range 2.024(5)-2.110(5) (Fe(2) · · · CFc ) 2.106(5), Fe(3) · · · CFc ) 2.110(5)), P(2,3)-CFc ) 1.816(5)-1.819(5), P(1)-CFc ) 1.855(5); P(2)-P(1)-P(3) ) 91.36(6), CFc-P(1)P(2,3) ) 101.8(2)-105.6(2), CFc-P(2,3)-P(1) ) 103.7(2)104.2(2).
variable temperature in situ 31P{1H} NMR spectroscopy have so far proved unsuccessful.16 The reaction mixture obtained after completion of the reaction was found to contain almost no other P-containing products apart from 1, with the 31P{1H} NMR spectrum being dominated by the expected A2B system for the [FcP]32- ion (-39.9 (t), -90.4 (d), 1JPP ) 240.8 Hz). The Fc groups attached to the central and terminal P atoms within the [FcP]32- ion are also visable in the room-temperature 1H NMR spectrum of 1 in thf, showing that the structure and conformation of the anion are retained under these conditions. Clearly, however, the Li+ cations are highly mobile at room temperature, with the two Li environments present in the solid state only being resolved at ca. -78 °C in the 7Li NMR spectrum in thf. The low-temperature X-ray structure of 1 (Figure 1)17 shows that the complex consists of discrete, ion-paired molecules [(FcP)3(Li · tmeda)2], containing a [FcP]32- dianion coordinated by two tmeda-solvated Li+ cations. The two Li+ counterions coordinate the [FcP]32- dianion in a distinctly unsymmetrical manner. While tmeda-solvated Li(1) is chelated by the anionic, terminal P centers P(2) and P(3) (Li(1)-P(2) ) 2.468(9) Å, Li(1)-P(3) ) 2.529(8) Å), with a potential weak interaction with the central P atom (Li(1) · · · P(1)
Communications
) 2.892(8) Å), Li(2) forms only a single bond to P(3) (Li(2)-P(3) ) 2.527 Å). Further π-bonding then occurs to the R-C atom of the Fc unit attached to the central P atom (C(11) · · · Li(2) ) 2.69(1) Å18), which spans the P(1)-P(3) bond and gives Li(2) a pseudotetrahedral geometry overall. The different bonding modes found for the terminal P atoms of the [FcP]32- dianion of 1 have a direct impact on the P3 unit, elongating the P(1)-P(3) bond (2.218(2) Å) relative to P(1)-P(2) (2.191(2) Å). The reason for the unsymmetrical coordination of the Li+ cations in 1 is almost certainly steric in origin. The coordination of Li(2) to the lower face of the [FcP]32- ion with the necessary orientation of the terminal P(2,3)-Fc groups upward results in a highly congested upper anion face for which chelation of Li(2) is unfavorable. However, swiveling of the P(1)-Fc group away from P(3) allows sufficient space to establish bonding to Li(2) while establishing the correct orientation for an Fc π-interaction. Unsymmetrical coordination of the alkali-metal cations has also been observed in the few triphosphanediide complexes structurally characterized so far, all of which contain the [PhP]32- dianion. While both of the Na+ cations in [(PhP)3Na · 3tmeda] are chelated by the terminal P atoms of the [PhP]32- anion,1 one cation is coordinated by two tmeda ligands and the other solvated by only one tmeda and π-bonded in an (18) This interaction is typical of π-C · · · Li interactions: search of the Cambridge Crystallographic Data Base, Dec 2008.
Organometallics, Vol. 28, No. 7, 2009 1997
η2 manner to the central P-Ph ring.5 This π-bonding plays a role similar to that involved in stabilizing Li(2) in 1, but the steric constraints preclude any Li(2)-P(2) interaction in this case. The presence of a very acute angle at the central P atom of the [FcP]32- anion (P(2)-P(1)-P(3) ) 91.36(6)°) results in a short P(2) · · · P(3) separation (3.154(9) Å) which is well below the sum of the van der Waals radii (ca. 3.60 Å15). This feature is similar to that found in the [PhP]32- anion of the previously characterized complex [{PhP}3Na · 3tmeda] (P-P-P ) 91.81°; P · · · P ) 3.146(1) Å).1 In conclusion, we have reported the first example of an organometallic phosphanediide anion via a soft main-group dehydrocoupling route. Further studies will be aimed at the use of this and similar species as ligands to various metals. The broader potential of main-group dehydrocoupling in the synthesis of other phosphorus compounds is also currently under investigation.
Acknowledgment. We gratefully acknowledge the EPSRC (R.J.L., V.N., D.S.W.) for financial support. We also thank Dr. J. E. Davies for collecting X-ray data for 1. Supporting Information Available: A CIF file giving crystallographic data for 1. This material is available free of charge via the Internet at http://pubs.acs.org. OM9000296
