Co‐polymerization of propylene oxide and CO 2 using early transition metal (groups IV and V) metallocalix[ n ]arenes (n = 4, 6, 8)

A number of metallocalix[ n ]arenes, where n = 4, 6, or 8, of titanium and vanadium have been screened for their ability to act as catalysts for the co‐polymerization of propylene oxide and CO 2 to form cyclic/polycarbonates. The vanadium‐containing catalysts, namely [VO(L 1 Me)] (1), [(VO 2 )L 2 H 6 ] (2), [Na(NCMe) 6 ] 2 [(Na[VO] 4 L 2 )(Na(NCMe)) 3 ] 2 (3), [VO(μ‐OH)L 3/ H 2 ] 2 ∙6CH 2 Cl 2 (4), {[VO] 2 (μ‐O)L 4 [Na(NCMe) 2 ] 2 } (5), {[V(N p ‐tolyl)] 2 L 4 } (6) and [V(N p ‐RC 6 H 4 )Cl 3 ] (R = Cl (7), OMe (8), CF 3 (9)), where L 1 H 3 = methylether‐p‐ tert ‐butylcalix[4]areneH 3 , L 2 H 8 = p‐ tert ‐butylcalix[8]areneH 8 , L 3 H 4 = p‐ tert ‐butylthiacalix[4]areneH 4 , L 4 H 6 = p‐ tert ‐butyltetrahomodioxacalix[6]areneH 6 , performed poorly, affording, in the majority of cases, TONs less than 1 at 90°C over 6 h and low molecular weight oligomeric products ( M n  ≤ 1665). In the case of the titanocalix[8]arenes, {(TiX) 2 [TiX(NCMe)] 2 (μ 3 ‐O) 2 (L 2 )} (X = Cl (10), Br (11), I (12)), which all adopt a similar, ladder‐type structure, the activity under the same conditions is somewhat higher (TONs >6) and follows the trend Cl > Br > I; by comparison the non‐calixarene species [TiCl 4 (THF) 2 ] was virtually inactive. In the case of 10, it was observed that the use of PPNCl (bis[triphenylphosphine]iminium chloride) as co‐catalyst significantly improved both the polymer yield and molecular weight ( M n 3515). The molecular structures of the complexes [HNEt 3 ] 2 [VO(μ‐O)L 3 H 2 ] 2 ∙3CH 2 Cl 2 (4∙3CH 2 Cl 2 ), [VO(μ‐OH)L 3/ H 2 ] 2 ∙6CH 2 Cl 2 (4 / ) (where L 3/ H 2 is a partially oxidized form of L 3 H 4 ) and {(TiCl) 2 [TiCl(NCMe)] 2 (μ 3 ‐O) 2 (L 2 )}·6.5MeCN (10·6.5MeCN) are reported.