title: Facilitating access to the most easily ionized molecule: an improved synthesis of the key intermediate, W2(hpp)4Cl2, and related compounds.
abstract: A far superior synthesis is reported for W(2)(hpp)(4)Cl(2), a key intermediate in the synthesis of the most easily ionized closed-shell molecule W(2)(hpp)(4) (hpp = the anion of the bicyclic guanidine compound 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine). At 200 degrees C, the one-pot reaction of the air-stable and commercially available compounds W(CO)(6) and Hhpp in o-dichlorobenzene produces W(2)(hpp)(4)Cl(2) in multigram quantities with isolated yields of over 90%. At lower temperatures, the reaction can lead to other compounds such as W(Hhpp)(2)(CO)(4) or W(2)(mu-CO)(2)(mu-hpp)(2)(eta(2)-hpp)(2), which are isolable in good purity depending upon the specific conditions employed. These compounds provide insight into the reaction pathway to W(2)(hpp)(4)Cl(2) and W(2)(hpp)(4). Two additional derivatives, W(2)(hpp)(4)X(2) where X is PF(6)(-) or the anion tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB), have also been synthesized and structurally characterized. A comparison of the electrode potentials of W(2)(mu-CO)(2)(mu-hpp)(2)(eta(2)-hpp)(2) and the di-p-anisylformamidinate analogue shows that oxidation of the hpp compound is significantly displaced (1.12 V) and shows that the bicyclic guanidinate ligand is considerably better than the formamidinate anion at stabilizing high oxidation states. A differential pulse voltammogram of W(2)(hpp)(4)(TFPB)(2) in THF shows two reduction processes with an E(1/2) of -0.97 V for the first and -1.81 V (vs Ag/AgCl) for the second. DFT calculations on the W(2)(hpp)(4)(2+) units in W(2)(hpp)(4)X(2) compounds show that the metal-metal bonding orbitals are destabilized by the axial ligands, which accounts for significant variations in the W-W distances. The low-energy gas-phase ionizations of W(2)(hpp)(4) are also reported and discussed.
authors.affiliation: Department of Chemistry, Laboratory for Molecular Structure and Bonding, Texas A&M University, College Station, USA. cottom@tamu.edu
authors.forename: F Albert
authors.initials: FA
authors.lastname: Cotton
authors.forename: James P
authors.initials: JP
authors.lastname: Donahue
authors.forename: Nadine E
authors.initials: NE
authors.lastname: Gruhn
authors.forename: Dennis L
authors.initials: DL
authors.lastname: Lichtenberger
authors.forename: Carlos A
authors.initials: CA
authors.lastname: Murillo
authors.forename: Daren J
authors.initials: DJ
authors.lastname: Timmons
authors.forename: Laura O
authors.initials: LO
authors.lastname: Van Dorn
authors.forename: Dino
authors.initials: D
authors.lastname: Villagrán
authors.forename: Xiaoping
authors.initials: X
authors.lastname: Wang
mesh_terms: D002712:Chlorides / Q000737:chemistry; D018360:Crystallography, X-Ray; D007477:Ions / Q000737:chemistry; D008958:Models, Molecular; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000138:chemical synthesis* / Q000737:chemistry; D013237:Stereoisomerism; D014414:Tungsten / Q000737:chemistry*
chemical_list: D002712:Chlorides; D007477:Ions; D009942:Organometallic Compounds; D014414:Tungsten