TERT-BUTYLAZAPENTADIENYL IRIDIUM PHOSPHINE CHEMISTRY

Potassium tert-butylazapentadienide reacts with (CI)Ir(PMe3)3 to Produce (syn-(1,2,3-eta)-5-tert-butyl-5-azapentadienyl)Ir(PMe3)3 (1a). Treatment of 1a with acetone leads to attack at the central allylic carbon of the azapentadienyl ligand (C2) and production of a novel iridacyclobutane complex, 2. The X-ray crystal structure of 2 (monoclinic, C2/c, a = 31.117(6) angstrom, b = 11.104(2) angstrom, c = 18.457(5) angstrom, beta = 119.26(2)-degrees, V = 5563(2) angstrom3, z = 8, R = 0.033, R(w) = 0.042) shows the expected trans orientation of the imine and acetone substituents on the four-membered ring. When 1a is stirred in pentane solution, it gradually converts to the thermodynamically favored anti-eta3-azapentadienyl isomer, 1b. The structure of 1b has been confirmed by X-ray crystallography (monoclinic, P2(1)/n, a = 15.709(14) angstrom, b = 26.154(10) angstrom, c = 18.915(7) angstrom, beta = 108.97(3)-degrees, V = 7351(3) angstrom3, Z = 12, R = 0.053, R(w) = 0.069). The relatively short C3-C4 bond distance (1.449(26) angstrom) in the azapentadienyl ligand of 1b may reflect some contribution by an eta4-butadiene resonance structure. Treatment of lb with triflic acid results in clean protonation of the nitrogen center and production of [(eta4-(tert-butylamino)butadiene)Ir(PMe3)3]+O3SCF3-(3). Addition of a second equivalent of triflic acid results in a second protonation at nitrogen, generating the dicationic species 5. Treatment of la with 1 or 2 equiv of triflic acid also leads primarily to formation of 3 and 5, respectively. However, a side reaction (approximately 20%) involving protonation at iridium also occurs, generating (syn-eta3-tert-butylazapentadienyl)Ir(PMe3)3(H)+-O3SCF3-(4) and [(syn-eta3-CH2-CH-CHCH=NHC(CH3)3)lr(PMe3)3(H)]2+ (O3SCF3-)2 (6), respectively. Compounds la, lb and 3 are fluxional in solution, due to facile rotation of the pi ligands with respect to the Ir(PMe3)3 fragment. NMR line-shape analysis has yielded rotational barriers (DELTAG(double daggers)) of 11.5(4), 11.1(4), and 16.4(4) kcal/mol, respectively.