Functionalized diaminophenols, H(N(R1R2)N(R3)O), were investigated as ligands for indium catalysts in the ring-opening polymerization of racemic lactide. Precursor complexes (N(Me2)N(Me)O)InCl(2) (1), (N(Pr2)NO)InCl(2) (2), and (N(Mes)NO)InCl(2) (3) were synthesized and fully characterized by (1)H and (13)C NMR spectroscopy, elemental analysis, and single-crystal X-ray crystallography. Complexes 1 and 2 were used to synthesize alkoxy-bridged complexes [(N(Me2)N(Me)O)InCl](2)(μ-Cl)(μ-OEt) (4) and [(N(Pr2)NO)InCl](2)(μ-Cl)(μ-OEt) (5). These complexes catalysed the polymerization of racemic lactide at different rates, with complex 5 being substantially more active than complex 4. The dissociation behaviour of these catalysts in the presence of lactide was also studied and used to make comparisons with previously reported catalyst systems.

Dimethyl- and diphenylstannylene (SnMe2 and SnPh2, respectively) have been successfully detected and characterized in solution. The stannylenes were generated by photolysis of 1,1,3-trimethyl-4-phenyl- (2) and 3,4-dimethyl-1,1-diphenylstannacyclopent-3-ene (3), respectively, which have been shown to extrude the species cleanly and in high (0.6 < Φ < 0.8) quantum yields through trapping studies using dichlorodimethylstannane (Me2SnCl2) as the stannylene substrate. Laser flash photolysis of 2 and 3 in deoxygenated hexanes affords promptly formed transient absorptions assigned to SnMe2 (λmax = 500 nm; ε500 = 1800 ± 600 M–1 cm–1) and SnPh2 (λmax = 290, 505 nm; ε500 = 2500 ± 600 M–1 cm–1), respectively, which decay with absolute second-order rate constants within a factor of 2 of the diffusional limit in both cases. The decay of the stannylenes is accompanied by the growth of new transient absorptions ascribable to the corresponding dimers, the structures of which are assigned with the aid of DFT and time-dependent (TD) DFT calculations at the (TD)­ωB97XD/6-31+G­(d,p)C,H,O-LANL2DZdpSn level of theory. Dimerization of SnMe2 affords a species exhibiting λmax = 465 nm, which is assigned to the expected SnSn doubly bonded dimer, tetramethyldistannene (Me2SnSnMe2, 16a), in agreement with earlier work. In contrast, the spectrum of the dimer formed from SnPh2 exhibits strong absorptions in the 280–380 nm range and a very weak absorption at 650 nm, on the basis of which it is assigned to phenyl­(triphenylstannyl)­stannylene (17b). The calculations suggest that 17b is formed via ultrafast rearrangement of a novel phenyl-bridged stannylidenestannylene intermediate (20), which can be formed either directly by “endo” dimerization of SnPh2 or by isomerization of the “exo” dimer, tetraphenyldistannene (16b); the predicted barriers for these rearrangements are consistent with the experimental finding that the observed product is formed at close to the diffusion-controlled rate. Absolute rate and equilibrium constants are reported for the reactions of SnMe2 and SnPh2 with Me2SnCl2 and methanol (MeOH), respectively, in hexanes at 25 °C.

The Lewis acid-base complexation chemistry of dimethyl- and diphenylstannylene (SnMe2 and SnPh2, respectively) in hexanes solution has been studied by laser photolysis methods. Complexation of the two stannylenes with a series of nine O-, S-, and N-donors (including cyclic and acyclic dialkyl ethers and sulfides, a primary, secondary, and tertiary amine, ethyl acetate and acetone), two alkenes, and an alkyne proceeds rapidly and reversibly to generate the corresponding stannylene-donor Lewis pairs, which have been detected in each case by time-resolved UV-vis absorption spectroscopy. The complexes exhibit UV-vis absorption maxima in the range of λmax ∼ 310-405 nm depending on the donor and substitution at tin. Bimolecular rate constants for complexation (kC), which could be determined for 14 of the 24 Lewis pairs that were studied, were found to fall within a factor of four of the diffusional limit in all cases, with SnMe2 showing consistently higher reactivity than SnPh2. Equilibrium constants for complexation (KC) could be measured for all but one of the stannylene-π- and O-donor pairs, the values corresponding to (gas phase) binding free energies in the range of +1.1 to -3.9 kcal mol-1. Comparison of the experimental equilibrium constants for complexation of SnMe2 and SnPh2 with methanol and diethyl ether to those measured previously for the homologous silylenes and germylenes indicates that Lewis acidity decreases in the order SiR2 > SnR2 > GeR2 for both the dimethyl- and diphenyltetrylene series, the diphenyl derivatives exhibiting significantly stronger Lewis acidity in all three cases. The experimental trends in the binding energies and UV-vis spectra of the complexes are reproduced well by density functional theory (DFT) calculations, which have been carried out at the (TD)ωB97XD/def2-TZVP level of theory. The experimental data also show evidence of a reaction between tetramethyldistannene (Me2Sn[double bond, length as m-dash]SnMe2, 4a) and amine donors, which is suggested to afford the corresponding amine-stabilized stannylidenestannylene structure. The mechanistic proposal is supported by DFT calculations of the complexation of 4a and SnMe2 with model O-, S- and N-donors.

Di-tert-butylsilylene (Si t Bu 2 ) has been detected directly in solution and its reactivity characterized by laser flash photolysis methods. Laser photolysis of 7,7-di-tert-butyl-7-silabicyclo[4.1.0]heptane (5) affords a transient product that exhibits λ max ≈ 520 nm and decays on the microsecond time scale, concurrent with the growth of a second, much longer-lived species exhibiting λ max = 290, 430 nm. The two species are assigned to di-tert-butylsilylene (Si t Bu 2 ) and its disilene dimer, tetratert-butyldisilene ( t Bu 2 SiSi t Bu 2 , 8), respectively. Transient absorption spectra recorded by laser photolysis of hexa-tert-butylcyclotrisilane ( 7) are consistent with the prompt formation of both Si t Bu 2 and 8, the former appearing as a weak shoulder absorption on the red edge of the intense absorption band due to the disilene. Absolute rate constants were determined for the reactions of Si t Bu 2 with a variety of representative silylene substrates in hexanes at 25 °C, including methanol, triethylsilane, acetic acid, acetone, molecular oxygen, and four aliphatic alkenes (1-hexene, cyclohexene, cis-cyclooctene, and 2,3-dimethyl-2-butene). Rate and(or) equilibrium constants for Lewis acid−base complexation of the silylene with diethyl ether, tetrahydrofuran, tetrahydrothiophene, and diethyl-and triethylamine were also determined, along with the UV−vis absorption spectra of the corresponding Lewis acid−base complexes. Rate constants for the reactions of dimethylsilylene (SiMe 2 ) and dimesitylsilylene (SiMes 2 ) with cis-cyclooctene, 2,3-dimethyl-2-butene, and 1hexene are also reported, enabling a comprehensive assessment to be made of steric effects on the reactivities of simple (transient) dialkyl-and diarylsilylene derivatives in solution. The kinetic data show the reactivity of Si t Bu 2 to be intermediate between that of SiMe 2 and SiMes 2 under similar conditions, with the (2 + 1)-cycloaddition reactions with alkenes exhibiting the largest variations in rate constant as a function of substitution at Si. Absolute rate constants for the reactions of tetra-tertbutyldisilene (8) with O 2 and acetone are also reported.

A series of 9H-fluorenols and 9H, 9′H-bifluorenyls were irradiated in less polar solvents giving photoproducts derived from their corresponding 9H-fluorenyl radicals. These transient species were directly observed by laser flash photolysis and their UV/visible spectra compared with those of their corresponding cations. Theoretical calculations (density functional theory [DFT]) of these intermediates indicate their destabilizing nature in similar fashion to the antiaromatic character of the corresponding cations.