A new Type of Transition-Metal Dimer Based on a Hexaphosphine Ligand System: Co2(CO)4(eHTP)(2+) (eHTP = (Et2PCH2CH2)2PCH2P(CH2CH2PEt2)2)

Article abstract of DOI:10.1021/ja00311a034

The synthesis and characterization of a new hexa-tert-phosphine ligand system, (Et2PCH2CH2)2PCH2P(CH2CH2PEt2)2, 1, is described.This ligand has the ability to both tris-chelate and bridge two transition metals, either in a closed-mode, M-M bonding geometry, or in an open-mode configuration in which the two metal atoms are 6-7 Angstroem apart.Reaction of 1 with 2 equiv of CoCl2 initially produces, in high yiels, the paramagnetic, binuclear, red-brown Co(II) compound, Co2Cl4(eHTP), 4, where eHTP = 1.An Extended X-ray Absorption Fine Structure (EXAFS) spectrum of 4 shows Co-P and Co-Cl bond distances in the 2.2-2.4 Angstroem region and no Co-Co contact distance less than 4.0 Angstroem, indicating that 4 is in an open-mode geometry.On standing, 4 dissociates two chloride ligands to form the green, paramagnetic Co2Cl2(eHTP)(2+) complex, 5, which precipitates out of the reaction mixture, either as the chloride or, if an extra equivalent of CoCl2 is used, the CoCl4(2-) salt.The EXAFS spectrum on the CoCl4(2-) salt of 5 is very similar to that of 4, indicating that 5 is also in an open-mode geometry.Reaction of 4 or 5 (alone or mixed) with H2/CO (33percent CO) and an extra equivalent of CoCl2 at 50 bar and 80 deg C forms, in high yields, the yellow-green, diamagnetic Co(I) binuclear carbonyl/eHTP complex, , 6a, which can be metathesized with aqueous NaPF6 to give the yellow PF6(1-) salt, 6b. 6 has been charactirized by 1H and 31P NMR, EXAFS, mass spectroscopy, IR, and a crystal structure.Crystals of 6b belong to the monoclinic space group C2/c, with unit cell parameters a = 26.464 (4) Angstroem, b = 12.562 (2) Angstroem, c = 14.380 (3) Angstroem, β = 99.04 (1) deg, V = 4721 (3) Angstroem³, and Z = 4.The structure was refined to give R = 0.070 and R_w = 0.106.The molucule lies on a 2-fold axis, passing through the bridging methylene group of the eHTP ligand.The complex has the central bis(phosphino)methane portion of the eHTP ligand adopting a unique inverted coordination geometry to give an open-mode complex with the two cobalt atoms separated by 6.697 (1) Angstroem.The cobalt atoms have a distorted trigonal-bipyramidal geometry with one carbonyl ligand and two terminal phosphorus atoms of the eHTP ligand in the equatorial plane.