Organometallics 1996, 15, 2657-2659
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Notes Use of Chelating Diphosphines To Prepare New Phosphido Clusters of Aluminum and Gallium David A. Atwood,† Alan H. Cowley,* Richard A. Jones,* Ronald J. Powell, and Christine M. Nunn Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712 Received February 16, 1996X Summary: Novel phosphido cluster complexes of formula (Me2M)4(µ-(1,2-PR)2C6H4)2 (M ) Al, R ) Me (1); M ) Ga, R ) t-Bu (2)) result when the chelating diphosphine ligand 1,2-bis(alkylphosphino)benzene is mixed with trialkylaluminum and -gallium reagents. When the potentially chelating ligand 1,2-bis(tert-butylphosphino)ethane is mixed with 2 equiv of AlMe3 under the same conditions, the secondary phosphine bis-adduct complex Me3Al‚PH(t-Bu)CH2CH2(t-Bu)HP‚AlMe3 (3) is formed. Dimeric, trimeric, and tetrameric complexes featuring bonding between the group 13/15 elements are well established in the literature.1 These types of complexes generally involve a group 15 moiety of general formula -ER1 or -ER2. Except for the well-documented reactions of trialkylaluminum and -gallium species with open chain multidentate amines,2 the use of multidentate group 15 ligands in cluster formation represents a relatively unexplored area of research. In this publication we describe the synthesis and characterization of tetranuclear M4P4 cluster compounds (M ) Al, Ga). The synthetic approach involves the use of the 1,2-bis(alkylphosphino)benzene ligand to prepare cluster compounds of formula (Me2M)4(µ(PR)2C6H4)2 (M ) Al, R ) Me (1); M ) Ga, R ) t-Bu (2)). In addition, the synthesis and characterization of the open chain secondary phosphine adduct, Me3Al‚PH(t-Bu)CH2CH2(t-Bu)HP‚AlMe3 (3), is also reported. The solid-state structures of compounds 2 and 3 have been determined by single-crystal X-ray diffraction studies. Experimental Section Synthesis. All manipulations were performed under a dry, oxygen-free, dinitrogen or argon atmosphere using standard Schlenk techniques or in a Vacuum Atmospheres HE-493 drybox. The solvents toluene and hexane were freshly distilled from sodium/benzophenone prior to use. 1H and 31P NMR spectra were recorded on a General Electric QE 300 instrument operating at 300.17 MHz (1H) and 32.38 MHz (31P) and are referenced to Me4Si and 85% aqueous H3PO4 (δ 0.0 ppm). Mass spectra (EI) were recorded on a Bell and Howell 21-491 instrument, and elemental analyses were determined on a Perkin-Elmer 2400 analyzer. 1,2-Bis(alkylphosphino)ben† Present address: Department of Chemistry, North Dakota State University, Fargo, ND, 58105. X Abstract published in Advance ACS Abstracts, April 1, 1996. (1) For reviews see: (a) Cowley, A. H.; Jones, R. A. Angew. Chem., Int. Ed. Engl. 1989, 28, 1208. (b) Wells, R. L. Coord. Chem. Rev. 1992, 112, 273. (2) Robinson, G. H. In Coordination Chemistry of Aluminum; Robinson, G. H., Ed.; VCH Publishers, Inc.: New York, 1993; p 57.
S0276-7333(96)00122-7 CCC: $12.00
zenes3 and 1,2-bis(tert-butylphosphino)ethane4 were prepared according to the literature methods. Al2Me6 and GaMe3 were purchased commercially (Aldrich). Preparation of (Me2Al)4(µ-(PMe)2C6H4)2 (1). A solution of 1,2-bis(methylphosphino)benzene (0.28 g, 1.6 mmol) in toluene (15 mL) was treated with AlMe3 (0.8 mL, 8 mmol) at 25 °C, heated to reflux for 50 min, and then allowed to cool to 25 °C. It was then stored at -25 °C for 2 days where upon colorless crystals of 1 formed (0.13 g, 86% yield). Mp: 278281 °C (dec). 1H NMR (C6D6, 25 °C): δ 7.40 (m, 4H), 7.02 (m, 4H), C6H4, 1.26 (s, 12H), PMe, -0.22 (s, 12H) chelated AlMe, -0.57 (s, 12H) bridging Al-Me. 31P{1H} NMR (C6D6, 25 °C): δ -101.76 (s). EIMS: m/z 549 amu (M+ - Me). Anal. Calcd for C24H44Al4P4: C, 51.07; H, 7.86; P, 21.95. Found: C, 51.12; H, 7.91; P, 21.43. Preparation of (Me2Ga)4(µ-(t-BuP)2C6H4)2 (2). A solution of 1,2-bis(tert-butylphosphino)benzene (0.24 g, 1.3 mmol) in toluene (15 mL) was treated with a toluene solution of GaMe3 (0.37 M, 2.6 mmol, 7.0 mL) and then heated to reflux for 3 h. The mixture was then cooled, and the volatiles were removed under vacuum to yield a white solid (0.38 g, 91%). The solid was extracted with toluene (20 mL) and then cooled to -25 °C for 2 days to yield colorless crystals of 2 (mp 207208 °C (dec)). 1H NMR (C6D6, 25 °C): δ 7.44 (m, 4H), 6.94 (m, 4H) C6H4, 1.22 (s, 36H) P-t-Bu, -0.24 (s, 12H) chelated GaMe, -0.57 (s, 12H) bridging GaMe. 31P{1H} NMR (C6D6, 25 °C): δ -31.4 (s). EIMS: m/z 888 amu (M+ - Me). Anal. Calcd for C36H68Ga4P4: C, 47.85; H, 7.58; P, 13.71. Found: C, 47.55; H, 7.45; P, 13.70. Preparation of Me3Al‚PH‚t-Bu(CH2CH2(t-Bu)HP‚AlMe3 (3). A solution of 1,2-bis(tert-butylphosphino)ethane (0.22 g, 1.1 mmol) in hexane (10 mL) was treated with AlMe3 (0.26 mL, 2.7 mmol) at 0 °C, resulting in the immediate formation of a white precipitate. The mixture was filtered and the precipitate dissolved in warm toluene (20 mL). The solution was then cooled to -25 °C for 4 days to yield colorless crystals of 3 (0.18 g, 49%). Mp: 115-118 °C (dec). 1H NMR (C6D6, 25 °C): δ 3.52 (t, 4H) CH2CH2, 2.54 (t, 2H) P-H, 2.00 (s, 18H) P-t-Bu, -0.33 (s, 18H) AlMe3. 31P{1H} NMR (C6D6, 25 °C): -14.85 (s). EIMS: m/z 350 amu (M+). Anal Calcd for C16H42Al2P2: C, 52.84; H, 12.08; P, 17.68. Found: C, 54.80; H, 12.11; P, 17.34.
Results and Discussion The reaction of 1,2-bis(alkylphosphino)benzene3 with MMe3 in refluxing toluene yields colorless 1 (M ) Al, (3) Kyba, E. P.; Liu, S. T.; Harris, R. L. Organometallics 1983, 2, 1877. 1,2-Bis(tert-butylphosphino)benzene: Kyba, E. P.; Kerby, M. C.; Rines, S. P. Organometallics 1986, 5, 1189. (4) 1,2-bis(tert-butylphosphino) ethane; Weisheit, V. R.; Stendel, R.; Messbauer, B.; Langer, C.; Wather, B. Z. Anorg. Allg. Chem. 1983, 504, 147.
© 1996 American Chemical Society
2658 Organometallics, Vol. 15, No. 11, 1996
alkyl ) Me) and 2 (M ) Ga, alkyl ) t-Bu) in high yields. Reactions conducted at 25 °C lead to initial adduct formation followed by slow conversion (days) to the phosphido derivatives 1 and 2. Attempts to isolate pure samples of the adducts were unsuccessful. In contrast, however, the reaction of 1,2-bis(tert-butylphosphino)ethane4 with 2 equiv of AlMe3 without reflux leads to the isolation of the bis-adduct complex 3. The stability of the adduct species 3, in comparison to 1 and 2, may be due to the reduced acidity of an alkylphosphine compared to an aromatic phosphine. The mass spectral data for 1 and 2 were consistent with the tetranuclear formulations. NMR spectroscopic data for 2 are consistent with the solid-state structure as determined by X-ray crystallography. Thus the 1H NMR spectrum for 2 at room temperature consists of three sharp singlets at δ 1.22 (s), -0.24 (s), and -0.44 (s). We propose that 1 has a structure similar to that of 2 on the basis of spectroscopic evidence. Thus the 1H NMR spectrum for 1 showed resonances for three different methyl groups at δ 1.26 (s), -0.22 (s), and, -0.57 (s) assigned to Me-P, bridging Me-Al, and chelated Me-Al, respectively. Unfortunately, X-rayquality crystals of 1 could not be obtained. The 31P NMR spectra for 1 and 2 consist of singlets at δ -101.7 and -31.4, respectively. The mass spectral data for compound 3 confirms the presence of the bis adduct (m/z ) 350 amu, M+). In the 1H NMR spectrum, the ethane protons (-PCH2CH2P-) appear as a triplet at δ 3.52, the PH group appears as a triplet at δ 2.54, the t-Bu group appears as a singlet at δ 2.00, and the AlMe3 group appears as a singlet at δ -0.33. The 31P NMR resonance for 3 at δ -14.85 (s) is downfield relative to that observed for 1 and 2 as expected for donor compared to σ bonding. As with previously reported complexes of bis(diphenylphosphino)methane with trialkylaluminum and -gallium reagents,5 there is no evidence for the solution-state dissociation of 3. Details of the X-ray data collection and structure refinement for compounds 2 and 3 are presented in Table 1. The molecular structure and atom-numbering schemes for 2 and 3 are given in Figures 1 and 2, respectively. Selected bond distances and angles are given in the figure captions. Compound 2 has an interesting framework structure in which the central core can be viewed as an expanded six-membered ring system with the P‚‚‚P atoms of each (1,2-t-BuP)2C6H4 group connected by two carbons of the phenyl ring. Two gallium atoms (Ga1) form a bridge between pairs of phosphorus atoms located on different ligands, while the second set of unique gallium atoms (Ga2) are chelated by the two phosphorus atoms of each ligand. There is a center of inversion present in 2 which equates the two halves of the molecule containing a diphosphido unit (C6H4(t-BuP)2), a chelated GaMe2 group (Ga2), and bridging GaMe2 group (Ga1). The chelated gallium atom displays a P1-Ga2-P2 angle of 87.33(7)°. While there is little precedent for this type of geometry in main group complexes, this P-M-P “bite” angle is in the same range as that observed in the transition metal complexes 1,2-(i-Pr2P)2C6H4‚MCl2, where M ) Ni (89.41(5) Schmidbaur, H.; Lauteschlaeger, S.; Mueller, G. J. Organomet. Chem. 1985, 281, 25.
Notes Table 1. Crystal Data, Details of Intensity Measurement, and Structure Refinement for 2 and 3 compd formula fw cryst dimens (mm) cryst system space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Mcalc (cm-1) scan technique 2θ range (deg) tot. reflcns tot. obsd no. params R Rw
2
3
C36H68Ga4P4 902.79 0.15 × 0.30 × 0.34 triclinic P1 h 9.829(6) 10.348(1) 12.068(3) 107.06(2) 100.38(2) 108.23(1) 1063.4(5) 2 26.7 θ/2θ 3-55 4879 3422 199 0.052 0.076
C16H42Al4P4 387.98 0.4 × 0.4 × 0.4 monoclinic P21/c 9.541(1) 11.021(2) 12.313(3) 90 109.72(1) 90 1218(6) 4 4.806 θ/2θ 2-40 1972 1423 95 0.0852 0.1011
Figure 1. Selected bond lengths (Å) and angles (deg) for 2: Ga1-P1, 2.464(2); Ga1-P2, 2.472(3); Ga1-C7, 2.01(1); Ga1-C8, 1.99(1); Ga2-P1, 2.421(2); Ga2-P2, 2.429(2); Ga2-C9, 2.01(1); Ga2-C10, 1.98(1); P1-C1, 1.842(7); P1C11, 1.91(1); P1-C2, 1.842(9); P2-C21, 1.922(7); P1-Ga1P2, 113.06(7); P1-Ga1, C7, 109.9(3); P1-Ga1-C8, 101.1(3); P2-Ga1-C7, 103.1(4); P2-Ga1-C8, 108.3(4); C7Ga1-C8, 121.4(5); P1-Ga2-P2, 87.33(7); P1-Ga2-C9, 116.6(4); P1-Ga2-C10, 113.1(3); P2-Ga2-C9, 115.6(2); P2-Ga2-C10, 112.8(3); C9-Ga2-C10, 109.9(4); Ga1-P1Ga2, 118.86(9); Ga1-P1-C1, 112.8(3); Ga1-P1-C11, 108.4(2); Ga2-P1-C1, 102.8(3); Ga2-P1-C11, 108.0(3); C1P1-C11, 105.0(4); Ga1-P2-Ga2, 122.39(9); Ga1-P2-C2, 109.3(3); Ga1-P2-C21, 106.6(3); Ga2-P2-C2, 102.5(2); Ga2-P2-C21, 108.8(3); Ga2-P2-C21, 106.6(3); Ga2-P2C2, 122.2(7); C2-P2-C21, 106.1(4).
(5)°), Pd (87.79(4)°), and Pt (88.62(9)°).6 In contrast, the bridging units (P2-Ga1-P1) display wider angles of 113.06(7)°. Despite this angle strain, however, 2 apparently does not dissociate in toluene solution. This strain is also evident in the Ga-P bonds which display relatively longer distances for the bridging Me2Ga1 (6) Liu, S. T.; Chen, J. T.; Peng, S. M.; Hsiao, Y. L.; Cheng, C. M. Inorg. Chem. 1990, 29, 1169.
Notes
Organometallics, Vol. 15, No. 11, 1996 2659
ligands such as diphos, trisphos, and tertraphos.7 Compound 3 is a rare example of a structurally characterized secondary phosphine adduct complex. In the structure the two tetrahedral aluminum centers are linked across a C2 axis of symmetry by the bridging phosphine ligand. The Al-P distance in 3 (2.533(3) Å) is only marginally shorter than the corresponding distance found in the (AlMe3)2‚diphos adduct (2.544(4) Å)9 and may be attributed to a reduction in steric hindrance on going from a tertiary to a secondary phosphine.
Figure 2. Selected bond lengths (Å) and angles (deg ) for 3: P1-Al, 2.533(3); P1-C1, 1.875(8); P1-C8, 1.856(7); AlC5, 1.98(1); Al-C7, 2.007(9); Al-P1-C1, 118.3(3); Al-P1C8, 116.3(3); C1-P1-C8, 104.6(4); P1-Al-C5, 100.5(3); P1-Al-C6, 105.2(3); C5-Al-C6, 114.2(5); P1-Al-C7, 102.8(3); C5-Al-C7, 117.4(6); C6-Al-C7, 114.0(5).
groups (2.464(2) and 2.472(3) Å) versus the chelated groups (2.421(2) and 2.429(2) Å). These distances are, however, in the range seen for monodentate gallium phosphido complexes.1 There are a large number of group 13 adduct complexes reported for tertiary multidentate phosphine
Acknowledgment. We wish to thank the Robert A. Welch Foundation, the Army Research Office, the American Chemical Society Petroleum Research Fund, and the Science and Technology Program of the National Science Foundation (NSF Grant CHE-8921120). Supporting Information Available: Tables of positional and thermal parameters and complete bond lengths and angles (7 pages). Ordering information is given on any current masthead page. OM960122T (7) Bradley, D. C.; Chudzynska, H.; Faktor, M. M.; Frigo, D. M.; Hursthouse, M. B.; Hussain, B.; Smith, L. M. Polyhedron 1988, 7, 1289.
