NMR and X-ray crystallographic studies of unsymmetrical 25,26;27,28-dibridged para-tert-butyl calix[4]arene bisphosphites with a large "through-space" P-P couplingElectronic supplementary information (ESI) available: Further experimental details and data. CCDC 695456, 695457 and 695459 contain the supplementary crystallographic data for 25,26-bridged phosphites 1, 2 and unsymmetrical 25,26;27,28-dibridged bisphosphite 5, respectively. For ESI and crystallographic data in CIF or other electronic

Three 25,26-bridged para - tert -butyl-calix[4]arene phosphites, 1-3 , have been synthesized by linking two proximal phenolic oxygen atoms of para - tert -butyl-calix[4]arene to a P(OR) [R = 2,6-Bu t 2 -4-Me-C 6 H 2 ( 1 ), 2,6-Pr i 2 -C 6 H 3 ( 2 ) or 2,4-Bu t 2 -C 6 H 3 ( 3 )] moiety. NMR spectroscopic data and X-ray crystallographic studies ( 1 and 2 only) show that the calixarene framework in 1 adopts a cone conformation, whereas in 2 and 3 it assumes a partial cone conformation. Unsymmetrical 25,26;27,28-dibridged para - tert -butyl-calix[4]arene bisphosphites 5-7 have been synthesized by the reaction of 25,26-bridged para - tert -butyl-calix[4]arene phosphite 1 with (R′O)PCl 2 [R′ = 2,4-Bu t 2 -C 6 H 3 ( 5 ), 2,6-Pr i 2 -C 6 H 3 ( 6 ) or (1 R ,2 S ,5 R )-(−)menthyl ( 7 )]. NMR investigations (homonuclear 31 P COSY) revealed an unprecedented "through-space" phosphorus-phosphorus coupling of 223-244 Hz for these unsymmetrical 25,26;27,28-dibridged para - tert -butyl-calix[4]arene bisphosphites. The molecular structure of unsymmetrical 25,26;27,28-bridged bisphosphite 5 was determined by X-ray crystallography; the two phosphorus atoms are in close proximity and the P P distance (3.543(2) Å) is less than the sum of their van der Walls radii. The synthesis and structures of three 25,26-bridged para - tert -butyl-calix[4]arene phosphites and unsymmetrical (chiral and achiral) 25,26;27,28-bridged para - tert -butyl-calix[4]arene bisphosphites are reported.