| A heteropentadienyl ligand is a molecule in which the terminal carbon of a pentadienyl ligand has been replaced with a heteroatom (O, PR, NR, S, SiR2). The study of heteropentadienyl-transition metal complexes has been an active area of research in the last decade, and has recently received significant attention in the chemical literature. These molecules can now be synthesized by generalized methods, allowing their unique reactivity to be the focus of current research. These molecules have shown the ability to adopt and shift between a variety of bonding modes, opening coordination sites at the metal center and showing promise for use as homogeneous catalysts.;This work has focused on the synthesis and spectroscopy of a new class of heteropentadienyl-transition metal-phosphine complexes. Specifically, thiapentadienyl-cobalt- and oxapentadienyl-cobalt-phosphine complexes were synthesized for comparison to the existing heteropentadienyl-iridium and --rhodium systems. Though some similarities in initial bonding modes were seen, the thiapentadienyl system showed distinctive dimeric and trimeric ground state bonding modes as a result of the small size of the cobalt metal center relative to other metals in the same group. Treatment of ClCo(PMe3)3 with potassium thiapentadienide produced the dimer [Co(PMe3)2(thiapentadienyl)] 2. This dimer was reactive toward the small two-electron donor ligand, CO, forming the products (5-eta1-cis-5-thiapentadienyl)Co(PMe 3)2(CO)2 and (5-eta1- trans-5-thiapentadienyl)Co(PMe3)2(CO) 2. Treatment of ClCo(PMe3)3 with lithium 2,3-dimethyl-5-thiapentadienide yielded the trimer [Co(PMe3)2(eta4-2,3-dimethylthiapentadienyl)] 2[mu-Co(2,3-dimethylthiapentadienyl)2]. This trimer was also reactive toward the two-electron donor, CO, forming (5-eta1 -trans-2,3-dimethyl-5-thiapentadienyl)Co(PMe 3)2(CO)2, upon reaction. Some similarities to previously reported systems were seen in the initial bonding modes, though one new bonding mode was seen: mu2-eta4,eta 1-bonding mode where one cobalt center bonds to the butadiene moiety in an eta4-fashion while a second cobalt coordinates the anionic sulfur atom.;When ClCo(PMe3)3 was treated with potassium oxapentadienide, the monomeric product, (1,2,3-eta-oxapentadienyl)Co(PMe3) 3 was formed. The oxapentadienyl-cobalt-phosphine system showed a remarkable stability of the all carbon eta3 bonding mode, losing a phosphine ligand to form (1,2,3-eta-oxapentadienyl)Co(PMe3)2(CO) upon exposure to carbon monoxide. The addition of methyl groups to the oxapentadienyl ligand resulted in no change to the initial reactivity. Treatment of ClCo(PMe3)3 with potassium 2,4-dimethyloxapentadienide again afforded the monomeric, eta3 product: (1,2,3-eta-2,4-dimethyloxapentadienyl)Co(PMe 3)3. However, the additional steric bulk of the methyl groups, along with their electron donating properties, did affect the reaction of (1,2,3-eta-2,4-dimethyloxapentadienyl)Co(PMe3)3 with CO. In this case, two phosphine ligands were lost to form (1,2,3-eta-2,4-dimethyloxapentadienyl)Co(PMe 3)(CO)2. The compounds (1,2,3-eta-oxapentadienyl)Co(PMe 3)3 and (1,2,3-eta-oxapentadienyl)Co(PMe3) 2(CO) were reactive toward small electrophiles, H+ and Me+, at the ligand oxygen forming stable eta4-butadienol-cobalt or eta4-butadienyl methyl ether-cobalt complexes. The compounds (1,2,3-eta-2,4-dimethyl oxapentadienyl)Co(PMe3)3 and (1,2,3-eta-2,4-dimethyl oxapentadienyl)Co(PMe3)(CO)2 were also reactive exclusively at the ligand oxygen; however, the initially formed products were not stable, resulting in the formation of Co(PMe 3)4+O3SCF3- in situ which was then converted to Co(PMe3)3(CO) 2+O3SCF3- by exposure to CO. |
