2.1 Synthesis and structure of catalysts

Fig. 1 shows the two Schiff-base ligands used in this study, (cyc)salenH₂ (1a) and (et)salenH₂ (1b) ((cyc)salenH₂ = N,N'-bis-(3,5-di-tert-butylsalicylidene)-1,2-cyclohexyldiimine and (et)salenH₂ = N,N′-bis-(3,5-di-tert-butylsalicylidene)-1,2-ethylenediimine). The gallium complexes 2a and 2b were synthesized from the reaction of GaCl₃ with the potassium salt of the ligands in THF solution as illustrated in Fig. 2.

X-ray quality crystals of 2a and 2b were grown by slow evaporation of a concentrated CH₂Cl₂ solution of the respective complex into neat toluene. The crystals of 2a are a CH₂Cl₂ solvate with three disordered solvent molecules present. Crystal and structure refinement data are listed in Table 1.

A thermal ellipsoid plot of 2a is shown in Fig. 3. The complex possesses slightly distorted square pyramidal geometry. The basal plane has a random mean square (RMS) deviation of 0.0876 from the best-fit plane amongst O(1), O(2), N(1), and N(2) with the gallium ion lying 0.4858(10) Å above the plane. The angles O(2)–Ga(1)–O(1), O(1)–Ga(1)–N(1), and O(1)–Ga(1)–Cl(2) are all very near 90° at 87.5(3)°, 90.5(3)°, and 102.4(2)°, respectively. Further illustration of the distortion is demonstrated by the angles O(2)–Ga(1)–N(1) and O(1)–Ga(1)–N(2), which are fairly obtuse at 144.4(3)° and 155.9(3)°, respectively. The average Ga–O bond distance is 1.867 Å and the average Ga–N distance is 2.016 Å. The Ga–Cl distance of the apical chloride ligand is 2.214(3) Å.

These distances are in close agreement with those of a similar structure reported by Atwood containing a phenyl backbone as opposed to the cyclohexyl reported here [14]. A thermal ellipsoid plot of 2b is shown in Fig. 4. The structure also displays a greater distortion of square planar geometry with an RMS deviation from planarity by the basal atoms of 0.2755 Å and the Ga atom displaced by 0.4560(32) Å. The bond angles around the basal plane are comparable to 2a with O(1)–Ga(1)–O(2) = 91.7(3)°, O(1)–Ga(1)–N(1) = 89.6(3)°, and O(1)–Ga(1)–Cl(2) = 115.11(19)°. A similar structure was reported by Atwood, in which the apical ligand is an ethyl group as opposed to a chloride [14]. Our average Ga–O distance of 1.877Å is shorter than in Atwood’s structure, while the average Ga–N distance of 2.112 Å is longer. The Ga(1)–Cl(2) distance is 2.213 Å, which is almost identical to that seen in 2a. Fig. 5 depicts an overlay of the coordination spheres in complexes 2a and 2b with the gallium centers fixed in the same position. As is apparent from this representation, there are no significant differences in the structural details of the coordination spheres between salen ligands with the cyclohexyl vs more flexible ethylene backbones.

In order to study the electronic effects of the apical ligand on the catalytic activity of the gallium complexes we replaced the chloride ligand with the more nucleophilic azide group. Complexes 3a and 3b were synthesized from 2a and 2b via reaction with AgBF₄ and NaN₃, as shown in Fig. 6. The chromium equivalent of 3a has previously been reported by Jacobsen [9], but the synthesis of gallium(III) azide species have not been previously communicated. Unlike the Cr complex, 3a is readily soluble in common non-coordinating solvents such as methylene chloride and toluene, but not in hydrocarbons. A thermal ellipsoid plot of 3a obtained from X-ray quality crystals is displayed in Fig. 7. Again, the five-coordinate gallium structure displays a distorted square pyramidal geometry (RMS deviation = 0.1145Å) with the gallium atom displaced 0.4869(5)Å above the basal plane formed by the coordinated nitrogen atoms N1 and N2, as well as O1 and O2. The Ga-azide bond distance (Ga(1)–N(3)) is 1.918(7)Å and the azide ligand is tilted away from the z-axis (N(4)–N(3)–Ga(1) = 120.0(6)°). Bond distances within the azide are 1.228(10)Å (N(3)–N(4)) and 1.149(10)Å (N(4) – N(5)). Likewise X-ray-quality crystals of 3b were obtained and a thermal ellipsoid plot is given in Fig. 8.

As seen in 2b, the increased flexibility of the ethylene backbone over the cyclohexane backbone results in a greater distortion of the square pyramidal geometry with the RMS deviation of N(1),N(2), O(1), and O(2) from the best-fit plane equal to 0.2834Å and the gallium atom lying 0.4985(16)Å above the plane. The Ga–azide bond distance is comparable to that of 3a at 1.897(4)Å with a slightly more pronounced tilt to the coordinated azide ligand (N(4)–N(3)–Ga(1) = 124.0(3)°). Bond distances within the azide are comparable to 3a at 1.203(5)Å (N(3)–N(4) ) and 1.178(6)Å (N(4)–N(5)).

Jacobsen theorizes that the insolubility of the chromium azide complex is due to intermolecular contacts between the azide of one molecule and the open chromium coordination site on a neighboring molecule creating a network that is broken up by coordinating solvent filling the sixth coordination site [9]. It is interesting to note that the unit cell illustrated in Fig. 9 shows no Ga–azide–Ga intermolecular contacts. The molecules, in fact, pack in such a way as to orient the azides away from neighboring molecules.

2.2 Epoxide/CO₂ copolymerization studies

Copolymerization studies were conducted using 2a and 2b as catalysts for formation of polycarbonates from cyclohexene oxide and CO₂. No polymer was observed with either 2a or 2b in the presence of N-methylimidazole. The characteristic infrared stretch of the carbonate found in polycyclohexylcarbonate at ~1750 cm^–1 was not observed in the reaction solution. We have previously proposed, in the case of (cyc)salenCr(III)Cl, that the copolymerization is initiated through a second order process in which the nucleophile (Cl) on one chromium center ring-opens a bound epoxide on another chromium center [10,11]. Presumably, in the case of gallium, the imidazole binds to the metal center well enough to preclude displacement by an epoxide even when it is in large excess, and therefore prevents the initiation step. This is not observed in the chromium case as the presence of the Lewis base increases the activity of the catalyst and decreases the percentage of polyether linkages found in the resultant copolymer. Conversely, for an identical copolymerization reaction carried out in the absence of imidazole, a white precipitate forms upon addition of methanol to the reaction solution. A carbonate stretch at 1750 cm^–1 was not observed in infrared spectroscopic analysis, which is indicative of the formation of polyether as opposed to polycarbonate. This was confirmed by ¹H NMR of the polymer, which shows a broad resonance at 3.6–3.2 ppm, typical of the homopolymer. Concomitantly, no resonance in the region 4.8–4.4 ppm is seen as would be observed in polycyclohexylcarbonate.

An attempt was made to enhance the quantity of copolymer produced by adding only one-half an equivalent of imidazole to the reaction mixture. Presumably, this would leave one-half the Ga(III) complexes available for activating an epoxide. Copolymerization studies with 2b and one-half an equivalent of imidazole produced no observable copolymer. Further attempts were made to enhance copolymer production by replacing the imidazole with a more donating co-catalyst such as triphenylphosphine. Copolymerization studies with 2b and one equivalent of PPh₃ as well yielded no observable copolymer.

Polymerization runs conducted utilizing 3a in the absence of a co-catalyst produced no copolymer. In the presence of two equivalents of co-catalyst, in this case a donating phosphine (tricyclohexylphosphine) to further enhance the nucleophilicity of the azide, trace amounts of copolymer were observed in the infrared spectrum of the reaction mixture, but quantifiable amounts of polycarbonate eluded us.

4.1 Synthesis of (salen)GaCl

A 100-ml Schlenk flask was charged with KH (0.045 g; 1.12 mmol) and stirred in 20 ml THF. To this slurry a 20 ml THF solution of (cyc)salenH₂ (0.300 g; 0.55 mmol) was added. The resulting mixture was allowed to stir overnight. A three-necked round bottom flask fitted with a condensor was charged with GaCl₃ (0.097 g; 0.55 mmol) in 15 ml of THF. The salen solution was added to the solution of GaCl₃ and the resultant solution was refluxed during 6 h, cooled, and filtered. The solvent was removed under reduced pressure and the yellow solid washed with hexanes (2 × 20 ml). Yields 0.286 g (80%) of yellow solid. ¹H NMR(C₆D₆) δ = 1.41 [s, 18H, C(CH₃)], 1.92 [s, 18H, C(CH₃)], 3.31 [t, 2H, N(CH(CH₂CH₂CH₂)CH)N], 7.08 [t, 2H, Ar–H], 7.60,7.77 [m, 2H, Ar–H], 7.82 [t,2H,Ar–CH]. Elemental analysis: calculated for C: 66.52%; H: 8.06%; N: 4.31%; found: C: 66.34%; H: 8.63%; N: 3.93%.

4.2 Synthesis of (salen)GaN₃

A 50-ml Schlenk flask was charged with AgBF₄ (0.195 g; 1.0 mmol) and stirred in 10 ml CH₃CN. A separate 50-ml Schlenk flask was charged with [(et)salen]GaCl (0.550 g; 0.92 mmol) and dissolved in 30 ml CH₃CN. The salen solution was added to the AgBF₄ suspension via canula and the resultant mixture was stirred overnight. The mixture was filtered through celite and added to a 20-ml CH₃CN suspension of NaN₃ (0.065 g; 1.0 mmol). The new mixture was stirred overnight, filtered, and the solvent removed under reduced pressure. Yields 0.542 g (90%) of a yellow solid. ¹H NMR (C₆D₆): δ = 1.37 [s, 18H, C(CH₃)], 1.84 [s, 18H, C(CH₃)], 2.49 [q, 2H, N(CH₂CH₂)N], 2.99 [q, 2H, N(CH₂CH₂)N], 6.81[d, 2H, Ar–H], 7.34[s, 2H, Ar–H], 7.77[d, 2H,Ar–CH] IR(toluene): ν(N₃) = 2095 cm^–1.

4.3 X-ray diffraction studies

Yellow–green crystals of 2a, 2b, 3a, and 3b were grown by slow evaporation of a concentrated CH₂Cl₂ solution of the respective complex into neat toluene. Crystals were coated with mineral oil and mounted on a glass fiber at room temperature. A hemisphere of data was collected on a Bruker Smart CCD diffractometer at 100 K [16,17]. Structures were solved via a combination of Patterson maps and direct methods using SHELXS-97 [18]. Solutions were refined utilizing SHELXL as found in the SHELXTL software package [19]. Molecular diagrams and publication materials were produced with programs found in SHELXTL [19]. Crystal and refinement data are listed in Table 1.

4.4 Copolymerizaton studies

0.075 g of catalyst and 2.5 equiv of appropriate co-catalyst (i.e. N-methylimidazole via microsyringe, tricyclohexyl phosphine, triphenyl phosphine) were dissolved in 20 ml cyclohexene oxide and injected via inlet port into a Parr autoclave. The reactor was subsequently charged to 500 psi with bone-dry CO₂ and stirred at 80 °C for 24 h. After this time, the CO₂ was bled off, the reactor opened, and its contents analyzed via infrared spectroscopy. Methanol was then added to the reactor contents to precipitate any copolymer, which was then analyzed via ¹H NMR spectroscopy.