The Main Group Macrocycle [{(PCH2CH2PAlMe2)2}4]⋅4 [AlMe3]

Abstract

Four P4 chains link eight aluminum centers in the [P16Al8] macrocycle [{(PCH2CH2PAlMe2)2}4]⋅4 [AlMe3] (1), which has a unique macrocyclic structure comprised solely of main group elements. Compound 1 is produced by the reaction of [AlMe3] (see scheme) with the catalytically produced organotetraphosphane (PCH2CH2PH)2 (2). Recent reviews in the fields of main group,1 organometallic,2 solid-state,3 and organo–phosphorus chemistry4 have reflected the multidisciplinary interest in compounds containing phosphorus. While organo–phosphorus ligands are ubiquitous in transition metal chemistry, the catenation of phosphorus atoms in organophosphanes and related polyphosphorus anions has drawn attention from inorganic and solid-state chemists.1, 3 The work of Scherer, Driess, Fenske, and others2, 5, 6 concerning the studies of substituent-free phosphorus–metal complexes,7 is one example where studies have crossed these disciplinary lines. These disciplinary boundaries have been further eroded with the studies of phosphanide and phosphanediyl clusters reported by the research groups of Fenske8–11 and Driess.12–14 While these systems employ phosphorus atoms to link metals in complex clusters, we have targeted systems in which chains of phosphorus atoms act as metal-linking units. In targeting such systems we first sought a convenient route to organopolyphosphanes as previously known syntheses were arduous.15, 16 To this end, we developed a catalyst for the dehydrocoupling of primary phosphanes to organopolyphosphanes.17 In a recent demonstration of such catalysis, we employed the catalyst precursor [(CpZrH3Li)3] to produce the macrocyclic P16 ring compound (C6H4P2)8 [Eq. (1)].18, 19 Herein we report, the catalytical synthesis of the organotetraphosphane (PCH2CH2PH)2 (1) and employ it to prepare the unique P16–Al12 compound, [{(PCH2CH2PAlMe2)2}4]⋅4 [AlMe3] 3. This species employs four P4 chains to link eight aluminum centers, forming Al2P3 rings in a unique macrocyclic structure comprised of solely main group elements (see Scheme 1). The quantitative catalytic dehydrocoupling of the primary bidentate phosphane PH2CH2CH2PH2, using the catalyst precursor [(CpZrH3Li)3] proceeds smoothly at room temperature over a three day period to afford the colorless product 1. The 31P{1H} NMR spectrum of 1 consists of two resonances at δ=−22.5 and −65.4, each showing second-order coupling. Simulation of the 31P{1H} NMR spectrum shows that it is consistent with an AA′BB′ pattern with one and two bond P–P coupling constants of −237 Hz, −186 Hz and 20 Hz respectively. This data is in good agreement to that reported for 1, previously prepared by Baudler et al. in 30–55 % via two different reductive coupling strategies.15, 16 Recrystallization of 1 from hexane at −35 °C afforded colorless X-ray quality crystals. An X-ray diffraction study of 1 confirmed the formulation as the product of the dehydrocoupling of two molecules of the starting diphosphane, that is (PCH2CH2PH)2 (Figure 1).20 This molecule comprises two interlinked five-membered rings, in which one phosphorus atom of each of two diphosphane fragments forms P−P bonds with the two phosphorus atoms of the other diphosphane moiety. This results in a chain of four phosphorus atoms where the terminal phosphorus atoms retain a single proton. The average P−P bond length in 1 is 2.2069(14) Å while the P-P-P angles average 97.09(5)°. The two five-membered rings are approximately orthogonal, the angle between the mean P3C2 planes is 94.9(1)°. This constrained orientation directs the terminal protons in essentially orthogonal directions, the torsion angles about the H-P-P-P-P-H chain average 88.9°. This geometry of 1 is analogous to one quarter of the previously reported macrocycle (C6H4P2)8.18 ORTEP drawing of 1, thermal ellipsoids are set at the 50 % level. Carbon-bound hydrogen atoms are omitted for clarity. P1-P2 2.1928(14), P2-P3 2.2217(14), P3-P4 2.2063(14); P1-P2-P3 97.22(5), P4-P3-P2 96.96(5). The catalytic formation of 1 provides convenient access to a corner-type molecular building block containing a P4 chain. In attempts to employ 1 to link metal centers, reactions of 1 with [MMe3] (M=Ga, Al) were undertaken (Scheme 1). The reaction of 1 with [GaMe3] in hexane resulted in the formation of a highly unstable species 2. This short-lived species exhibited a 31P NMR spectrum showing signals at δ=−19.4 and −65.1. The downfield resonance appears similar to that observed for 1 while the upfield resonance has increased multiplicity suggesting coordination to Ga and the formulation [{(PCH2CH2PH)GaMe3}2] (2). The rapid degradation of this product with gas evolution (presumably methane) and the deposition of an insoluble white material, precluded further characterization of 2. The nature of the subsequent P–Ga product(s) remains unknown, despite repeated attempts to obtain pure, crystalline material. In contrast, addition of excess [AlMe3] to a solution of 1 does not give a reaction at 25 °C. However, heating a solution of 1 with excess [AlMe3] in hexane to 80 °C for 1 week in a sealed reaction vessel gives a white solid 3 in 64 % yield. Recrystallization of 3 from a 1:1 mixture of THF and hexane at −35 °C yielded colorless cubic crystals. The 31P{1H} NMR spectra of the crystalline 3 showed four broad resonances at δ=1.9, −50.9, −67.1, and −88.8. The 1H NMR spectrum of 3 showed four distinct resonances attributable to aluminum bound methyl groups. While these data infer P–Al interactions, neither of the NMR spectra provided substantial structural information. This prompted an X-ray crystallographic study which confirmed the formulation of 3 as [{(PCH2CH2PAlMe2)2}4]⋅4 [AlMe3] (Figure 2).20 This centrosymmetric macrocyclic compound comprises four P4C4 units that are bridged by two AlMe2 units. One of the Al centers bridges the terminal P atoms of two adjacent P4C4 units. The second Al center is bound to the terminal phosphorous of one P4 unit and to a central P atom of the second P4 moiety, giving five-membered P3Al2 rings. The result is four such rings linked by P units (Figure 3). The bridging Al−P bonds average 2.477(5) Å while the P−P bonds range from 2.192(4) Å to 2.252(3) Å. The angles at Al within the P3Al2 rings range from 89.46(12) to 99.25(13)° while the corresponding angles at phosphorous span the range from 105.24(14) to 110.73(13)°. One of the P atoms of each P3Al2 ring also binds an additional equivalent of [AlMe3] with an average terminal Al−P bond of 2.586(4) Å. The view of 3 in Figure 3 a shows the main group macrocycle within the molecule. The dimensions of the depicted central cavity of this molecule are indicated by the P3−P6, P3−P3′, and P6−P6′ distances of 3.655(5), 5.071(5), and 5.016(5) Å, respectively. The side view of 3 (Figure 3 b) shows the P3Al2 rings are puckered (maximum deviation from the mean plane of 0.35 Å). In addition the adjacent pairs of P3Al2 rings are approximately perpendicular, with the angles between the mean P3Al2 planes being 90.6 and 91.2°. ORTEP drawing of 3, the thermal ellipsoids are set at 20 %. Hydrogen atoms are omitted for clarity. Phosphorus atoms are depicted in orange, aluminum in blue, and carbon in gray. ORTEP drawings of the heteroatom core of 3. Views a) and b) are approximately orthogonal. Phosphorus atoms are depicted in orange, aluminum in blue. Selected distances [Å] and angles [°] Al1-P1 2.576(4), Al2-P5 2.471(4), Al2-P1 2.484(4), Al3-P5 2.458(4), Al3-P2 2.477(4), Al4-P4 2.469(4), Al4-P8′ 2.495(4), Al5-P4′ 2.459(4), Al5-P7 2.502(4), Al6-P8 2.595(4), P1-P2 2.195(4), P2-P3 2.246(3), P3-P4 2.224(3), P5-P6 2.224(3), P6-P7 2.252(3), P7-P8 2.192(4); P5-Al2-P1 99.25(13), P5-Al3-P2 89.46(12), P4-Al4-P8′ 98.94(14), P4′-Al5-P7 87.88(12), P2-P1-Al2 105.24(14), P2-P1-Al1 117.44(15), Al2-P1-Al1 116.44(14), P1-P2-P3 98.76(13), P1-P2-Al3 110.73(13), P3-P2-Al3 116.21(13), P4-P3-P2 92.86(12), P3-P4-Al5′ 115.28(14), P3-P4-Al4 106.66(14), Al5-P4-Al4 106.49(14), P6-P5-Al3 115.44(14), P6-P5-Al2 106.37(14), Al3-P5-Al2 106.57(13), P5-P6-P7 92.69(12), P8-P7-P6 98.85(13), P8-P7-Al5 111.17(14), P6-P7-Al5 115.97(13), P7-P8-Al4′ 104.46(13), P7-P8-Al6 119.84(15), Al4′-P8-Al6 116.48(14), Al3-P8-Al2 106.49(14). In conclusion, we have described the catalytic synthesis of the organotetraphosphane 1, which reacts with [AlMe3] to give the main group macrocycle 3. This species represents a rare, if not unprecedented example of a compound in which polyphosphorus chains linking metal atoms in a macrocyclic array. As this approach provides entry to compounds of cross-disciplinary interest, we are continuing to develop the chemistry of both catalytically generated organopolyphosphanes and their metal derivatives. 1: [(CpZrH3Li)3]19 (0.1 g, 0.27 mmol) was dissolved in THF (3 mL), PH2CH2CH2PH2 (1.0 g, 10.87 mmol) was added dropwise to the stirred, colorless solution which became green, then aquamarine, and finally blue, over a period of 30 min. The reaction was stirred for 72 h, until it became colorless. Solvent was removed in vacuo, leaving a white solid which was dissolved in hexane, filtered, and placed in a freezer (−35 °C). Colorless, X-ray quality crystals (0.86 g, 87 % yield) precipitated from solution, over 24 h. 1H NMR (C6D6): δ=3.65 (dt, 2 H, 1JP,H=330, 3JH,H=10 Hz), 2.43 (m, 2 H), 1.87 (m, 2 H), 1.14 (m, 2 H), 0.74 (m, 2 H); 13C{1H} NMR (C6D6): δ=38.36, 30.66; 31P NMR (C6D6): δ=−22.5 (P1: 1JP1,P1=−237, 1JP1,P0=−186, 2JP1,P0=20 Hz), −65.4 (P0: 1JP1,P1=−237, 1JP1,P0=−186, 2JP1,P0=20 Hz); elemental analysis (%) calcd for P4C4H10: C 26.39, H 5.54; found: C 26.18, H 5.42. 2: A 10 % by weight solution of [GaMe3] in hexane (378 mg, 0.33 mmol) was added to a vial containing 1 (30 mg, 0.16 mmol), with stirring. A very fine white precipitate was evident in the clear colorless solution after 10 min. An aliquot of the reaction mixture was used immediately for 31P NMR spectroscopic characterization. This compound rapidly degraded on standing. 31P NMR (C6D6): δ=−19.4 (m), −65.1 (m). 3: A 2 M hexane solution of [AlMe3] (0.14 mL, 0.28 mmol) was quickly added to a stirred toluene (1 mL) solution of 1 (20 mg, 0.11 mmol). The reaction mixture was heated to 80 °C in a sealed reaction vessel for 1 week, during which time the solution turned a very pale orange. Solvent was removed in vacuo, and the resulting white solid was dissolved in a 1:1 mixture of THF and hexane, and placed in a freezer (−35 °C) until clear, colorless, cubic crystals of 3 precipitated from solution in 64 % yield. 1H NMR (C7D8): δ=2.3 (br s, 2 H), 1.8 (br s, 2 H), 1.2 (br s, 3 H), 1.1 (br s, 2 H), 0.8 (br s, 3 H), 0.6 (br s, 2 H), −0.2 (br s, 3 H), −0.3 (9 H), −0.7 (br s, 3 H); 31P NMR (C6D6): δ=1.9 (br m), −50.9 (br m), −67.1 (br m) −88.8 (br m); elemental analysis (%) calcd for C11H29P4Al3: C 36.08, H 7.98; found: C 35.88, H 7.78.