Abstract
The synthesis, isolation and full characterization of ion pairs between alkaline metal ions (Li+, Na+, K+) and mono-anions and dianions obtained from 5H-dibenzo[a,d]cycloheptenyl (C15H11 = trop) is reported. According to Nuclear Magnetic Resonance (NMR) spectroscopy, single crystal X-ray analysis and Density Functional Theory (DFT) calculations, the trop‒ and trop2−• anions show anti-aromatic properties which are dependent on the counter cation M+ and solvent molecules serving as co-ligands. For comparison, the disodium and dipotassium salt of the dianion of dibenzo[a,e]cyclooctatetraene (C16H12 = dbcot) were prepared, which show classical aromatic character. A d8-Rh(I) complex of trop− was prepared and the structure shows a distortion of the C15H11 ligand into a conjugated 10π -benzo pentadienide unit—to which the Rh(I) center is coordinated—and an aromatic 6π electron benzo group which is non-coordinated. Electron transfer reactions between neutral and anionic trop and dbcot species show that the anti-aromatic compounds obtained from trop are significantly stronger reductants.
Highlights
Transition metal complexes with 5H-dibenzo[a,d]cyclohepten-5-yl units as ligands are well established in the literature [1,2,3,4].Their special properties give rise to complexes with extraordinary catalytic activities
The trop silanes were subsequently treated with various alkali metal tert-butoxides to give the ion pairs Litrop, Natrop and Ktrop respectively, with trop- as the anion, as pure crystalline solids in moderate to good yields ((a) in Scheme 2)
All reagents were used as received from commercial suppliers unless otherwise stated
Summary
Transition metal complexes with 5H-dibenzo[a,d]cyclohepten-5-yl units (trivial name tropylidenyl = trop, see Scheme 1, right) as ligands are well established in the literature [1,2,3,4]. Bis(trop)amine as ligand in d8 -Rh(I) complex A (Scheme 1, top) provokes an unusual butterfly-type structure for tetracoordinated sixteen electron configured transition metal complexes and can act as a cooperating ligand [5]. Both factors contribute to the high activities in the hydrogenation of ketone derivatives [6,7] or the dehydrogenative coupling of alcohols [8,9] under very mild reaction conditions. The olefinic double bond in Molecules 2020, 25, 4742; doi:10.3390/molecules25204742 www.mdpi.com/journal/molecules
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