{H(3-(t)Bupz)B(3-(t)Bupz)(2)-eta(2)}AlEt(2) and {H(3-(t)Bupz)B(3-(t)Bupz)(5-(t)Bupz)-eta(2)-}AlEt(2). Structure, dynamic solution behavior, and the 1,2-borotropic shift

HB(3-(t)Bupz)(3) and AlEt(3) in benzene yield {H(3-(t)Bupz)B(3-(t)Bupz)(2)-eta(2)}AlEt(2), 1, as a hydrocarbon-soluble crystalline solid. Compound 1 is also obtained in a related reaction involving ClAlEt(2) via a preferential metathesis of the Al-Cl bond. Crystal data for 1 at -101 degrees C: a = 11.770(3) Angstrom, b = 11.054(3) Angstrom, c = 21.973(6) Angstrom, beta = 95.57(1)degrees, Z = 4, space group P2(1)/a. In 1 the Al center is four-coordinate with Al-C = 1.97(1) Angstrom and Al-N = 1.99(1) Angstrom and with C-Ak-C = 127 degrees and N-Al-N = 101 degrees being the largest and smallest angles, respectively. The average N-B-N angle is 109(1)degrees. In toluene-d(8) and tetrahydrofuran-d(8), 1 shows two types of 3-(t)Bupz groups in the integral ratio 2:1 and two distinct ethyl ligands. At low temperature there is a broadening of the 3-(t)bupz singlet that is assigned to the eta(2)-Bupz ligands. up to +60 degrees C, compound 1 is nonfluxional on the NMR time scale but does isomerize to {H(3-(t)Bupz)B(3-(t)Bupz)(5-(t)Bupz)-eta(2)}AlEt(2), 2. Crystal data for 2 at -172 degrees C: a = 29.235(5) Angstrom, b = 11.298(1) Angstrom, c = 22.033(3) Angstrom, beta = 129.66(1)degrees, Z = 8, space group = C2/c. In 2 there is a pseudotetrahedral Al center with Al-C = 1.97(1) Angstrom (average) and Al-N = 1.95(10 Angstrom (average) and with C-Al-C = 119 degrees and N-Al-N = 98 degrees as the largest and smallest angles, respectively. The average N-B-N angle is 108(1)degrees. In 2 the eta(2)-tris(alkylpyrazolyl)borate ligand isomerizes by a 1,2-borotropic shift to give one 5-(t)Bupz fragment that is part of the eta(2)-n,n' aluminum-bonded ligand. Variable-temperature H-1 NMR spectra of 2 in toluene-dg and THF-d(8) reveal temperature-dependent exchange involving the 3(t)-Bupz moieties, with more rapid site exchange in toluene-d(8) than in THF-d(8). At low temperature there are two ethyl signals, one of which indicates diastereotopic methylene protons, as well as three (t)Bu signals in the ratio 1:1:1. the dynamic behavior of 2 is consistent with an eta(2) reversible arrow eta(3) exchange process as opposed to an eta(2) reversible arrow eta(1) exchange wherein the Al center is transiently three-coordinate. The isomerization of 1 to 2 has been studied in benzene-d(6) (Delta H-double dagger = 21.0(2) dcal/mol, Delta S-double dagger = -15(1) eu) and THF-d(8) (Delta H-double dagger = 18.3(4) kcal/mol, Delta S-double dagger = -15(1) eu) and compared to a related isomerization involving {H2B(3-(t)Bupz)(2)-eta(2)} AlMe(2) reported by Parkin and Looney [Polyhedron 1990, 9, 265] in benzene-d(6) (Delta H-double dagger = 6(2) eu). It is proposed that the rate-determining 1,2-borotropic shift in the 1 --> 2 reaction occurs in a noncoordinating (t)Bupz group and that this is followed by a rapid associative interchange of pz groups wherein the sterically less demanding 5-(t)Bupz moiety remains bound to the metal.