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We have structurally characterized a number of lithiated calix[4]arenes, where the bridge in the calix[4]arene is thia (-S-, L S H 4 ), sulfinyl (-SO-, L SO H 4 ), sulfonyl (-SO 2 -, L SO2 H 4 ), dimethyleneoxa (-CH 2 OCH 2 -, L COC H 4 ) or methylene (-CH 2 -, LH 4 ). In the case of L 4S H 4 , interaction with LiO t Bu led to the isolation of the complex [Li 8 (L 4S ) 2 (THF) 4 ]·5THF (1·5THF), whilst similar interaction of L 4SO H 4 led to the isolation of [Li 6 (L 4SO H) 2 (THF) 2 ]·5(THF) (2·5THF). Interestingly, the mixed sulfinyl/sulfonyl complexes [Li 8 (calix[4]arene(SO)(SO 2 )(SO 1.68 ) 2 ) 2 (THF) 6 ]·8(THF) (3·8THF) and [Li 5 Na(L SO/3SO2 H) 2 (THF) 5 ]·7.5(THF) (4·7.5(THF) have also been characterized. Interaction of LiO t Bu with L SO2 H 4 and L COC H 4 afforded [Li 5 L 4SO2 (OH)(THF) 4 ]·2THF (5·2THF) and [Li 6 (L COC ) 2 (HO t Bu) 2 ]·0.78THF·1.22hexane (6·0.78THF·1.22hexane), respectively. In the case of LH 4 , reaction with LiO t Bu in THF afforded a monoclinic polymorph [LH 2 Li 2 (thf)(OH 2 ) 2 ]·3THF (7·3THF) of a known triclinic form of the complex, whilst reaction of the de-butylated analogue of LH 4 , namely de-BuLH 4 , afforded a polymeric chain structure {[Li 5 (de-BuL)(OH)(NCMe) 3 ]·2MeCN}  n  (8·2MeCN). For comparative catalytic studies, the complex [Li 6 (L Pr ) 2 (H 2 O) 2 ]·hexane (9 hexane), where L Pr2 H 2 = 1,3-di- n -propyloxycalix[4]areneH 2 , was also prepared. The molecular crystal structures of 1-9 are reported, and their ability to act as catalysts for the ring opening (co-)/polymerization (ROP) of the cyclic esters ε-caprolactone, δ-valerolactone, and rac -lactide has been investigated. In most of the cases, complex 6 outperformed the other systems, allowing for higher conversions and/or greated polymer M  n .

Lithium calix[4]arenes: structural studies and use in the ring opening polymerization of cyclic esters