Metallacrown ethers with trans-tetracarbonylmolybdenum(0) centers. X-ray crystal structures of trans-Mo(CO)4{Ph2P(CH2CH2O)nCH2CH2PPh2-P,P′} (n=3, 5) and trans-Mo(CO)4{Ph2P(CH2CH2O)2-1-C6H4-2-(OCH2CH2)2PPh2-P,P′}

Photolysis of the cis-Mo(CO)4{Ph2P(CH2CH2O)nCH2CH2PPh2-P,P′}, (1, n=3; 2, n=4; 3, n=5) metallacrown ethers in tetrahydrofuran under nitrogen gives moderate yields of the corresponding trans-Mo(CO)4{Ph2P(CH2CH2O)nCH2CH2PPh2-P,P′} (4, n=3; 5, n=4; 6, n=5) metallacrown ethers. The trans-metallacrown ethers are also obtained when catalytic amounts of HgCl2 are added to chloroform solutions of the cis-metallacrown ethers. The 1⇄4 and 2⇄5 equilibria are established within 3 min at ambient temperature and approximately equal amounts of the cis- and trans-metallacrown ethers are present in the equilibrium mixtures. The 3⇄6 equilibrium is more complicated because HgCl2 complexation by 3 also occurs in these solutions. Addition of excess HgCl2 to the equilibrium mixture of 3 and 6 results in the formation of the previously reported cis-Mo(CO)4{μ-Ph2P(CH2CH2O)5CH2CH2PPh2-P,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)2PPh2-P,P′} (7) indicates that both ring size and flexibility affect the conformations of the metallacrown ether rings. The larger oxygen–oxygen distances in the trans-metallacrown ethers, compared with those in the corresponding cis-metallacrown ethers 1, 3 and cis-Mo(CO)4{Ph2P(CH2CH2O)2-1-C6H4-2-(OCH2CH2)2PPh2-P,P′} (8), may explain why the trans-metallacrown ethers have not been observed to bind strongly the hard metal cations.