Acetonitrile Hydration and Ethyl Acetate Hydrolysis by Pyrazolate-Bridged Cobalt(II) Dimers Containing Hydrogen-Bond Donors

The preparation of new CoII−μ-OH−CoII dimers with the binucleating ligands 3,5-bis{bis[(N‘-R-ureaylato)-N-ethyl]aminomethyl}-1H-pyrazolate ([H4PRbuam]5-, R = tBu, iPr) is described. The molecular structure of the isopropyl
derivative reveals that each CoII center has a trigonal-bipyramidial coordination geometry, with a Co···Co separation
of 3.5857(5) Å. Structural and spectroscopic studies show that there are four hydrogen-bond (H-bond) donors near
the CoII−μ-OH−CoII moiety; however, they are too far away to be form intramolecular H-bonds with the bridging
hydroxo ligand. Treating [CoII2H4PRbuam(μ-OH)]2- with acetonitrile led to the formation of bridging acetamidato
complexes, [CoII2H4PRbuam(μ-1,3-OC(NH)CH3)]2-; in addition, these CoII−μ-OH−CoII dimers hydrolyze ethyl acetate
to form CoII complexes with bridging acetato ligands. The CoII−1,3-μ-X‘−CoII complexes (X‘ = OAc-, [OC(NH)CH3]-)
were prepared independently by reacting [CoII2H3PRbuam]2- with acetamide or [CoII2H4PRbuam]- with acetate.
X-ray diffraction studies show that the orientation of the acetate ligand within the H-bonding cavity depends on the
size of the R substituent appended from the urea groups. The tetradentate ligand 3-{bis[(N‘-tert-butylureaylato)-N-ethyl]aminomethyl}-5-tert-butyl-1H-pyrazolato ([H2PtBuuam]3-) was also developed and its CoII−OH complex
prepared. In the crystalline state, [CoIIH2PtBuuam(OH)]2- contains two intramolecular H-bonds between the urea
groups of [H2PtBuuam]3- and the terminal hydroxo ligand. [nPr4N]2[CoIIH2PtBuuam(OH)] does not hydrate acetonitrile
or hydrolyze ethyl acetate. In contrast, K2[CoIIH2PtBuuam(OH)] does react with ethyl acetate to produce KOAc; this
enhanced reactivity is attributed to the presence of the K+ ions, which can possibly interact with the CoII−OH unit
and ester substrate to assist in hydrolysis. However, K2[CoIIH2PtBuuam(OH)] was still unable to hydrate acetonitrile.