Ryan L. Holland

Cobalt 1,3-Diisopropyl-1H-imidazol-2-ylidene Complexes: Synthesis, Solid-State Structures, and Quantum Chemistry Calculations

The reaction of (η5-C5H5)Co(PPh3)2 (1) with 1,3-bis(isopropyl)imidazol-2-ylidene (ImiPr2, 17) leads to the formation of (η5-C5H5)Co(PPh3)(ImiPr2) (5) in 69% yield. N-Heterocyclic carbene 17 also undergoes reaction with (η5-C5H5)Co(CO)2 (9) to give (η5-C5H5)Co(ImiPr2)(CO) (6) in 30% yield. The barrier to rotation about the Co−Ccarbene bond in 6 has been determined by variable-temperature 1H NMR spectroscopy (13.6 kcal/mol) and by computation (13.3 kcal/mol). Complex 5 undergoes reaction with PhSSPh to give the paramagnetic thiolato complex (η5-C5H5)Co(ImiPr2)(SPh) (7), which is oxidized to the metallosulfone complex (η5-C5H5)Co(ImiPr2)(SO2Ph) (8). The solid-state structures of 5−8 were determined by X-ray crystallography. The structural and dynamic properties of 6, (η5-C5H5)Co(ImMe2)(CO) (ImMe2 = 1,3-dimethylimidazol-2-ylidene), and (η5-C5H5)Co(ImAr2)(CO) (ImAr2 = 1,3-dimesityl-2-ylidene) were examined by quantum chemistry calculations.

Reactions of a Metallacyclobutene Complex with Alkenes

The first productive reactions of a characterized metallacyclobutene complex with alkenes are reported. Thus, the metallacyclobutene complex (eta5-C5H5)(PPh3)Co[kappa2-(C,C)-C(SO2Ph) C(Si(CH3)3)CH(CO2CH2CH3)] (2) undergoes reaction with alkenes to give 1,4-diene complexes with a high degree of regio- and stereoselectivity. A mechanism is proposed in which the metallacyclobutene generates a cyclic vinylcarbene intermediate that undergoes [4 + 2]-cycloaddition reactions with activated alkenes. A model of the vinylcarbene intermediate has been examined using quantum mechanical methods.

A Photochemical Metallocene Route to Anionic Enediynes: Synthesis, Solid-State Structures, and ab Initio Computations on Cyclopentadienidoenediynes

The first demonstration of photochemical enediyne liberation from a metal complex has led to a new class of enediynes, the cyclopentadienidoenediynes, which are demonstrated to exist as air-stable solids with low ionization potentials and large dipole moments. NMR and IR spectroscopy, X-ray crystallography, and ab initio computations enable a comparison with the ubiquitous benzoenediynes.