Quadruply bonded dichromium complexes with variously fluorinated formamidinate ligands

Complexes of chromium with various amidinate anions of N,N'-di(biphenyl)for mamidine (DbiPhF), N,N'-di(pentafluorophenyl)formamidine ((DPh5F)-F-F), N,N '-di(p-fluorophenyl)formamidine (DPhp-FF), N,N'-di(o-fluorophenyl)formamidi ne (DPho-FE), N,N'-di(3,5-fluorophenyl)formamidine (Dph(3,5-F)F), and N,N'- di(m-fluorophenyl)formamidine (DPhm-FF) have been synthesized and structura lly characterized to study the response of the M-M multiple bond to the don or capacity of the ligand by varying the substituents of the aromatic rings . All six of these dinuclear fluorinated amidinate derivatives of the chrom ium(II) compounds have the paddlewheel configuration. The synthetic route i nvolves the reaction of CrCl2 with the corresponding lithium salt of the li gands, LiDArF (where Ar = o-C6H4F, m-C6H4F, p-C6H4F, p-C6H4C6H5, C6F5, or 3 ,5-C6H3F2) Compounds [Cr-2(DPhP-FF)(4)] (1), [Cr-2(DPhm-FF)(4)] (2), [Cr-2( DPh3,5-FF)(4)]. C6H14 (3 . C6K14), and [Cr-2(DbiPhF)(4)]. 0.7CH(2)Cl(2) (4. 0.7CH(2)Cl(2)) show no variation in the Cr-Cr quadruple bond length, even t hough the ligands have very different basicities. In the solid state, [Cr-2 ((DPh5F)-F-F)(4)] (5) shows close axial contacts between the o-F atoms and the chromium metal centers. The F-19 NMR show an unresolved and broad signa l for all o-F atoms that cannot be resolved even at very low temperature. T o assess the efficiency of the contacts, [Cr-2(DPho-FF)(4)] (6) was prepare d. The crystal structure shows the same kind of Cr ... F interactions as in 5, and an elongation of the Cr-Cr quadruple bond, compared with the values for the complexes 1, 2, 3, and 4. These new complexes reveal that the elec tronic contribution of the ligand basicity to the M-M bond is smaller and l ess important than the axial interactions of the chromium centers. Crystal data: for 1, orthorhombic, space group Fddd with a=25.25(7) Angstrom, b = 2 6.752(12) Angstrom, 28.57(4) Angstrom, alpha = beta = gamma = 90 degrees, a nd Z = 16; for 2, triclinic, space group P (1) over bar with a 9.606(12) An gstrom, b=9.727(10) Angstrom, c = 13.249(11) Angstrom, alpha=69.24(1)degree s, beta = 73.84(2)degrees, gamma = 84.24(2)degrees, and Z = 1; for 3 . C6H1 4, triclinic, P (1) over bar with a=10.8274(10) Angstrom, b = 13.739(2) Ang strom, c=18.152(4) Angstrom, alpha=83.25(1)degrees, beta= 75.61(2)degrees, gamma=70.246(10)degrees, and Z = 2; for 4 . 0.7CH(2)Cl(2) triclinic P1 with a = 9.689(2) Angstrom, b = 13.7088(3) degrees, c 16.844(3) Angstrom, alpha =69.90(3)degrees, beta= 87.10(3)degrees, gamma = 70.13(3)degrees, and Z = 1 ; for 5, monoclinic, C2/c with a=19.114(3) Angstrom, b = 18.957(3) Angstrom , c 29.923(6) Angstrom, beta = 97.27(1)degrees, and Z = 4; for 6, triclinic , P (1) over bar with a =10.225(2) Angstrom, b = 11.312(2)Angstrom, c=11.79 7(3) Angstrom, alpha = 117.08(1)degrees, beta = 96.432(2)degrees, gamma = 1 07.52(2)degrees, and Z = 1.