The chemistry of heteroarylphosphorus compounds. Part 16. Unusual substituent effects on selenium-77 nuclear magnetic resonance chemical shifts of heteroaryl- and aryl-phosphine selenides. X-Ray crystal structure of tri(2-furyl)phosphine selenide

Crystals of PR3Se (R = 2-furyl) are monoclinic, space group Cc, with a= 11.720(6), b= 12.527(9), c= 8.569(5)Å, and Z= 4. The structure was solved using multisolution direct methods and refined by least squares to R 0.038 (R′ 0.041) for 1 030 observed diffractometer data. Phosphorus adopts a distorted tetrahedral geometry with mean Se–P–C and C–P–C angles of 114.9 and 103.4°, respectively. The molecule has almost ideal C3 symmetry. The C–P–C angle is the smallest reported for a tertiary phosphine selenide. The average P–C bond length [1.778(6)Å] and the P–Se bond length (2.094 Å) are both considerably shorter than found in other arylphosphine selenides. The oxygen atoms of the 2-fury1 groups are arranged about the selenium atom, with an average Se ⋯ O distance of 3.577 Å. 77Se N.m.r. studies show that the selenium atom is shielded by 86 p.p.m. compared with that in triphenylphosphine selenide, implying a ‘through-space’ interaction involving the 2-fury1 oxygen atoms. Selenium-77 chemical shifts have also been determined for a range of heteroaryl-and substituted phenyl-phosphine selenides. Whereas the 2-furyl group causes the selenium to become shielded, the related 2-thienyl group causes it to become deshielded. Deshielding of selenium is also experienced on introduction of ortho substituents in arylphosphine selenides. Thus, for example, the selenium atom of tris(2,4,6-trimethoxyphenyl)phosphine selenide is deshielded by 240 p.p.m. compared with that in triphenylphosphine selenide. Possible origins of these effects are considered.