Stable Alkali-Metal Complexes of Hybrid Disila-Crown Ethers

Abstract

The complexation ability of hybrid disilane and ethylene containing crown ether ring systems was analyzed using 1,2-disila[12]crown-4 (1), 1,2-disila[15]crown-5 (2), 1,2-disila[18]crown-6 (3), and 1,2,7,8-tetrasila[12]crown-4 (7). Alkali-metal complexes (Li(+), Na(+), K(+)) were obtained and analyzed via X-ray diffraction. The complex stability of [Li(1,2-disila[12]crown-4)](+) and [Li(1,2,7,8-tetrasila[12]crown-4)](+) was determined, in relation to the lithium complex of [12]crown-4, by density functional theory (DFT) calculations employing the BP86/def2-TZVP level of theory. In solution, the exchange of lithium cations between pure [12]crown-4 and hybrid [12]crown-4 is on even terms, as has been shown from the relative binding affinity of compounds 1 and 7 by means of dynamic proton nuclear magnetic resonance (NMR) spectroscopy.

Structural Study of Mismatched Disila-Crown Ether Complexes

“…Prior studies have shown that Li + matches well with 1,2-disila [12]crown-4 and Na + with 1,2-disila [15]crown-5 [18], so that the two cations together with 1,2-disila [18]crown-6 are supposed to fulfil the criteria of a mismatch. Reaction of 1,2-disila [18]crown-6 with lithium hexafluorophosphat in a 1:1 stoichiometry yielded a highly viscous oil. After freezing at −196 • C and subsequent storage at −35 • C for 3 days, Compound 1 crystallized in the space group P2 1 /c in the form of colorless planks.…”

“…Consequently, higher comparability between organic crown ethers and cyclosiloxanes can be provided by extension of the -SiMe 2 -unit to -SiMe 2 SiMe 2 -. Recent studies of hybrid [12]crown-4 featuring one or two disilane fragments in a residuary organic crown ether framework revealed an increasing coordination ability towards Li + in the series C-O-C < C-O-Si < Si-O-Si (Scheme 1) [18,19].…”

“…Since particularly Si-based crown ethers show limited flexibility [11][12][13][14][15]18,19], we expected not all O crown atoms to participate in the coordination of the metal center [20][21][22]. The first mismatch structure of a hybrid crown ether was very recently published and is constituted of 1,2-disila [18]crown-6 and Ca(OTf) 2 (OTf = − OSO 2 CF 3 ) [23]. Therein, one of the C-bonded O atoms does not participate in the coordination of Ca 2+ , showing the preference of the metal ion to be coordinated by the Si/C-linked O atoms.…”

“…[18]crown-6 with Li + and Na + The hybrid ligand 1,2-disila [18]crown-6 was synthesized in a single step reaction from 1,2-dichlorodisilane and pentaethylene glycol (Scheme 2). Prior studies have shown that Li + matches well with 1,2-disila [12]crown-4 and Na + with 1,2-disila [15]crown-5 [18], so that the two cations together with 1,2-disila [18]crown-6 are supposed to fulfil the criteria of a mismatch. Reaction of 1,2-disila [18]crown-6 with lithium hexafluorophosphat in a 1:1 stoichiometry yielded a highly viscous oil.…”

“…Compared to the hitherto known lithium complexes of hybrid sila-crown ethers, the Li1-O1 bond length is elongated, which may be the result of the strongly twisted ligand. Typically, the O atoms in sila-crown ethers complexes adopt an approximately planar conformation [13][14][15]18,19]. The disilane fragment in 1 is roughly coplanar to the thereon bonded O atoms O1 and O2, but the organic part of the ligand is strongly twisted and is wrapped around the metal center.…”