Quadruply Bonded Dichromium Complexes with Variously Fluorinated Formamidinate Ligands

Complexes of chromium with various amidinate anions of N,N‘-di(biphenyl)formamidine (DbiPhF), N,N‘-di(pentafluorophenyl)formamidine (DPhF5F), N,N‘-di(p-fluorophenyl)formamidine (DPhp-FF), N,N‘-di(o-fluorophenyl)formamidine (DPho-FF), N,N‘-di(3,5-fluorophenyl)formamidine (DPh3,5-FF), and N,N‘-di(m-fluorophenyl)formamidine (DPhm-FF) have been synthesized and structurally characterized to study the response of the M−M
multiple bond to the donor capacity of the ligand by varying the substituents of the aromatic rings. All six of
these dinuclear fluorinated amidinate derivatives of the chromium(II) compounds have the paddlewheel
configuration. The synthetic route involves the reaction of CrCl2 with the corresponding lithium salt of the ligands,
LiDArF (where Ar = o-C6H4F, m-C6H4F, p-C6H4F, p-C6H4C6H5, C6F5, or 3,5-C6H3F2). Compounds [Cr2(DPhp-FF)4]
(1), [Cr2(DPhm-FF)4] (2), [Cr2(DPh3,5-FF)4]·C6H14 (3·C6H14), and [Cr2(DbiPhF)4]·0.7CH2Cl2 (4·0.7CH2Cl2) show
no variation in the Cr−Cr quadruple bond length, even though the ligands have very different basicities. In the
solid state, [Cr2(DPhF5F)4] (5) shows close axial contacts between the o-F atoms and the chromium metal centers.
The 19F NMR show an unresolved and broad signal for all o-F atoms that cannot be resolved even at very low
temperature. To assess the efficiency of the contacts, [Cr2(DPho-FF)4] (6) was prepared.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 electronic contribution of
the ligand basicity to the M−M bond is smaller and less important than the axial interactions of the chromium
centers. Crystal data:  for 1, orthorhombic, space group Fddd with a = 25.25(7) Å, b = 26.752(12) Å, c =
28.57(4) Å, α = β = γ = 90°, and Z = 16; for 2, triclinic, space group P1̄ with a = 9.606(12) Å, b = 9.727(10)
Å, c = 13.249(11) Å, α = 69.24(1)°, β = 73.84(2)°, γ = 84.24(2)°, and Z = 1; for 3·C6H14, triclinic, P1̄ with
a = 10.8274(10) Å, b = 13.739(2) Å, c = 18.152(4) Å, α = 83.25(1)°, β = 75.61(2)°, γ = 70.246(10)°, and Z
= 2; for 4·0.7CH2Cl2, triclinic P1 with a = 9.689(2) Å, b = 13.7088(3) Å, c = 16.844(3) Å, α = 69.90(3)°, β
= 87.10(3)°, γ = 70.13(3)°, and Z = 1; for 5, monoclinic, C2/c with a = 19.114(3) Å, b = 18.957(3) Å, c =
29.923(6) Å, β = 97.27(1)°, and Z = 4; for 6, triclinic, P1̄ with a =10.225(2) Å, b = 11.312(2) Å, c = 11.797(3)
Å, α = 117.08(1)°, β = 96.432(2)°, γ = 107.52(2)°, and Z = 1.