Metallocrown ethers with trans-tetracarbonylmolybdenum(0) centers. X-ray crystal structures of trans-Mo(CO)(4){Ph2P(CH2CH2O)(n)CH(2)Ch(2)PP(2)-P,P '}(n=3, 5) and trans-Mo(CO)(4){Ph2P(CH2CH2O)(2)-1-C6H4-2-(OCH2CH2)(2)PPh2-P,P '}

Photolysis of the cis-Mo(CO)(4){Ph2P(CH2CH2O)(n)CH2CH2PPh2-P,P'}, (1, n = 3 ; 2, b = 4; 3, n = 5) metallacrown ethers in tetrahydrofuran under nitrogen gives moderate yields of the corresponding trans-Mo(CO)(4){Ph2P(CH2CH2O)(n )CH2CH2PPh2-P,P'} (4, n = 3; 5, n = 4; 6, n = 5) metallacrown ethers. The t rans-metallacrown ethers are also obtained when catalytic amounts of HgCl2 are added to chloroform solutions of the cis-metallacrown ethers. The 1 rev ersible arrow 4 and 2 reversible arrow 5 equilibria are established within 3 min at ambient temperature and approximately equal amounts of the cis- an d trans-metallacrown ethers are present in the equilibrium mixtures. The 3 reversible arrow 6 equilibrium is more complicated because HgCl2 complexati on by 3 also occurs in these solutions. Addition of excess HgCl2 to the equ ilibrium mixture of 3 and 6 results in the formation of the previously repo rted cis-Mo(CO)(4){mu -Ph2P(CH2CH2O)(5)CH2CH2PPh2-,P',O,O',O " ,O''',O''''} HgCl2. Comparison of the X-ray crystal structures of the trans-metallacrown ethers 4-6 and trans-Mo(CO)(4){Ph2P(CH2CH2O)(2)-1-C6H4-2-(OCH2CH2)(2)PPh2- P,P'} (7) indicates that both ring size and flexibility affect the conforma tions of the metallacrown ether rings. The larger oxygen-oxygen distances i n the trans-metallacrown ethers, compared with those in the corresponding c is-metallacrown ethers 1, 3 and cis-Mo(CO)(4){Ph2P(CH2CH2O)(2)-1-C6H4-2-(OC H2CH2)(2)PPh2-P,P'} (8), may explain why the trans-metallacrown ethers have not been observed to bind strongly the hard metal cations. (C) 2001 Elsevi er Science B.V. All rights reserved.