Thioether–iodine charge-transfer complexes. Synthesis and low-temperature single-crystal structures of complexes of penta-, hexa- and octa-dentate homoleptic thioether macrocycles

Charge-transfer complexes 2([15]aneS5)·7I2 1 ([15]aneS5 = 1,4,7,10,13-pentathiacyclopentadecane), [18]aneS6·I2 2, [18]aneS6·4I2 3 ([18]aneS6 = 1,4,7,10,13,16-hexathiacyclooctadecane), [24]aneS8·I2 4 and [24]aneS8·6I2 5 ([24]aneS8 = 1,4,7,10,13,16,19,22-octathiacyclotetracosane) have been prepared and their structures and solution properties investigated. The compounds were prepared by slow evaporation of solutions containing I2 and the appropriate thioether macrocycle in CH2Cl2. The single-crystal structure determination of 1 shows three I2 molecules co-ordinated to three exo oriented S donors [S(1)–I(1) 2.797(3), I(1)–I(1′) 2.798(2), S(4)–I(4) 2.885(4), I(4)–I(4′) 2.764(2), S(7)–I(7) 2.828(3) and I(7)–I(7′) 2.779(2) Å; S(1)–I(1)–I(1′) 178.39(8), S(4)–I(4)–I(4′) 171.12(8), S(7)–I(7)–I(7′) 178.80(8)°]. The fourth I2 molecule (with a site occupancy of 0.5) lies close to S(10) [I(10)–S(10) 2.839(5) Å], the bond distance I(10)–I(10′) 2.674(3) Å being unusually short. Compound 2 is an example of a 1∶1 I2∶macrocycle adduct and shows symmetrically bridging I2 molecules [S(1)–I(1) 3.099(2), I(1)–I(1i) 2.7881(10) Å; S(1)–I(1)–I(1i) 178.68(4)°; i 1 – x, 1 – y, 1 – z] between [18]aneS6 macrocycles. Compound 3 is the first example of an adduct between I2 and a homoleptic thioether macrocycle which shows both exo [S(1)–I(1) 2.838(2), I(1)–I(1′) 2.7875(6) Å; S(1)–I(1)–I(1′) 174.95(4)°] and endo [S(4)–I(4) 2.792(2), I(4)–I(4′) 2.8067(7) Å; S(4)–I(4)–I(4′) 174.43(4)°] co-ordination of I2 molecules. The endo-oriented I2 molecules occupy space above and below the macrocyclic plane with the macrocycle adopting a sigmoid conformation. The single-crystal structure determination of the 1∶1 adduct 4 shows symmetrically bridging I2 molecules [S(1)–I(1) 3.215(2) and I(1)–I(1i) 2.758(2) Å; S(1)–I(1)–I(1i) 172.75(3)°; i –x, –y, –z] which are a characteristic of this stoichiometry. Compound 5 contains endo- and exo-oriented S donors within the same adduct [I(1)–I(1′) 2.7861(8), I(4)–I(4′) 2.7937(8), I(7)–I(7′) 2.8345(8), S(1)–I(1) 2.821(2), S(4)–I(4) 2.815(2), S(7)–I(7) 2.741(2) Å; S(1)–I(1)–I(1′) 170.15(5), S(4)–I(4)–I(4′) 177.41(5), S(7)–I(7)–I(7′) 177.24(5)°]. These results are discussed in the context of the stability and characteristics of thioether crown–iodine charge-transfer complexes, and a qualitative MO diagram is proposed to account for the shorter I–I distances in bridging I2 fragments compared to those in terminally bound I2.