Quadruply bonded dichromium complexes with variously fluorinated formamidinate ligands

Article abstract of DOI:10.1021/ic990007l

Complexes of chromium with various amidinate anions of N,N′-di(biphenyl)formamidine (DbiPhF), N,N′-di(pentafluorophenyl)formamidine (DPh^F5F), N,N′-di(p-fluorophenyl)formamidine (DPh^p-FF), N,N′-di(o-fluorophenyl)formamidine (DPh^o-FF), N,N′-di(3,5-fluorophenyl)formamidine (DPh^3,5-FF), and N,N′-di(m-fluorophenyl)formamidine (DPh^m-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, C₆H₅, or 3,5-C₆H₃F₂). Compounds [Cr₂(DPh^p-FF)₄] (1), [Cr₂(DPh^m-FF)₄] (2), [Cr₂(DPh^3,5-FF)₄]·C₆H ₁₄ (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(DPh^F5F)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(DPh^-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) A?, b = 26.752(12) A?, c = 28.57(4) A?, α = β = γ = 90°, and Z = 16; for 2, triclinic, space group P1? with a = 9.606(12) A?, b = 9.727(10) A?, c = 13.249(11) A?, α = 69.24(1)°, β = 73.84(2)°, γ = 84.24(2)°, and Z = 1; for 3·C₆H₁₄, triclinic, P1? with a = 10.8274(10) A?, b = 13.739(2) A?, c = 18.152(4) A?, 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) A?, b = 13.7088(3) A?, c = 16.844(3) A?, 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) A?, b= 18.957(3) A?, c = 29.923(6) A?, β = 97.27(1)°, and Z = 4; for 6, triclinic, P1? with a = 10.225(2) A?, b = 11.312(2) A?, c = 11.797(3) A?, α = 117.08(1)°, β = 96.432(2)°, γ = 107.52(2)°, and Z = 1.

Full text of DOI:10.1021/ic990007l

2182
Inorg. Chem. 1999, 38, 2182-2187
Quadruply Bonded Dichromium Complexes with Variously Fluorinated Formamidinate
Ligands
F. Albert Cotton,*,† Carlos A. Murillo,*,†,‡ and Isabel Pascual†
Laboratory for Molecular Structure and Bonding, Department of Chemistry, P.O. Box 30012,
Texas A&M University, College Station, Texas 77842-3012, and Escuela de Qu´ımica,
Universidad de Costa Rica, Ciudad Universitaria, Costa Rica
ReceiVed January 6, 1999
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-fluorophen-
yl)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, R ) â ) γ ) 90°, and Z ) 16; for 2, triclinic, space group P1h with a ) 9.606(12) Å, b ) 9.727(10)
Å, c ) 13.249(11) Å, R ) 69.24(1)°, â ) 73.84(2)°, γ ) 84.24(2)°, and Z ) 1; for 3‚C6H14, triclinic, P1h with
a ) 10.8274(10) Å, b ) 13.739(2) Å, c ) 18.152(4) Å, R ) 83.25(1)°, â ) 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, R ) 69.90(3)°, â
) 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, â ) 97.27(1)°, and Z ) 4; for 6, triclinic, P1h with a )10.225(2) Å, b ) 11.312(2) Å, c ) 11.797(3)
Å, R ) 117.08(1)°, â ) 96.432(2)°, γ ) 107.52(2)°, and Z ) 1.
Introduction
The Cr-Cr distances are 1.937(2), 2.023(1), 2.221(3), and 2.354
Å, respectively. Thus, the presence of more axial ligands or
stronger donors in axial positions significantly lengthens the
Cr-Cr bonds.
4+
Complexes containing Cr2 moieties have been the focus
of attention of many studies in the area of multiply bonded
compounds;1 they are still a source of many surprises.2 For the
lantern-type complexes, the Cr-Cr distances vary over a range
of more than 0.7 Å. The shortest is 1.828(2) Å for Cr2(2-
oxyphenyl)4 whereas distances of 2.4 Å or more are commonly
found in dichromium tetracarboxylates of the type Cr2(O2CR)4‚
L2. Generally, Cr-Cr distances of less than 2.0 Å are known
as “supershort”.
Axial ligation typically increases the metal-to-metal bond
length. For example, compounds of the type [Cr2((2-xylyl)-
NCMeO)4]L′L3 have been prepared for L′ ) L ) no ligand; L
) THF, L′ ) no ligand; L ) L′ ) THF and L ) L′ ) pyridine.
However, more subtle variations in the Cr-Cr bonds have
been the subject of much speculation. Several attempts to design
experiments to probe such effects have failed. In a series of
related compounds of Cr2L4, L ) 2-methylhydroxy-6-pyridinate,
methylaminopyridinate, and dimethylhydroxypyridine, the Cr-
Cr distances are 1.889(1), 1.870(3), and 1.903(3) Å, respectively.
However, when L ) chlorohydroxypyridinate the Cr-Cr
distance of 1.955(2) Å is distinctly longer. Two hypotheses have
been considered to explain such elongation in the bond. One is
that lone pairs on the chlorine atoms may interact weakly with
the metal atom orbitals, perhaps placing some electron density
into the π* orbital, thus weakening the Cr-Cr bond. The other
is that the electron-withdrawing effect of the Cl atoms weakens
the donor strength of the ring nitrogen atoms and that this, in a
manner not completely clear, weakens the Cr-Cr bond.
† Texas A&M University.
‡ Universidad de Costa Rica.
(1) Cotton, F. A.; Walton, R. Multiple Bonds Between Metal Atoms, 2nd
ed.; Oxford University Press: Oxford, 1993; Chapter 4.
(2) See, for example: Cotton, F. A.; Murillo, C. A.; Pascual, I. Inorg.
Chem. Commun. 1999, 2, 101.
(3) Cotton, F. A.; Ilsley, W. H.; Kaim, W. J. Am. Chem. Soc. 1980, 102,
3464.
At the time a seemingly straightforward experiment was tried
by preparing the analogous compound with fluorine instead of
10.1021/ic990007l CCC: $18.00 © 1999 American Chemical Society
Published on Web 04/10/1999

Quadruply Bonded Dichromium Complexes
Inorganic Chemistry, Vol. 38, No. 9, 1999 2183
Table 1. Crystallographic Data for Complexes 1-6
1
2
3‚C6H14
4‚0.7CH2Cl2
5
6
formula
fw
space group
a, Å
b, Å
c, Å
Cr2F8C40H24N12
1030.31
Fddd
25.25(7)
26.752(12)
28.57(4)
90
Cr2F8C52H40N8
1020.82
P1h
Cr2F16C55.5H28N12
1214.86
P1h
Cr2C100.82H76Cl1.65N8 Cr2F40C52H4N8
Cr2F8C52H40N8
1030.31
P1h
1562.03
P1
1604.63
C2/c
19.114(3)
18.957(3)
29.923(6)
90
97.27(1)
90
10 755(4)
1.982
9.606(12)
9.727(10)
13.249(11)
69.24(1)
73.84(2)
84.24(2)
1141.9(2)
1.484
10.8274(10)
13.739(2)
18.152(4)
83.25(1)
75.61(2)
70.426(10)
2462.7(7)
1.638
9.689(2)
13.708(3)
16.844(3)
69.90(3)
87.10(3)
70.13(3)
1970.3(7)
1.316
10.225(2)
11.312(2)
11.797(3)
117.08(1)
96.432(2)
107.52(2)
1107.5(4)
1.543
R, deg
â, deg
γ, deg
V, Å3
90
90
19 298(58)
1.417
0.526
D
calc g cm-3
µ, mm-1
0.556
0.552
0.388
0.593
0.573
radiation (λ, Å) Mo KR (0.710 73) Mo KR (0.710 73) Mo KR (0.710 73) Mo KR (0.710 73)
Mo KR (0.710 73) Mo KR (0.710 73)
-60
4
temp (°C)
Z
-60
16
0.094, 0.105
-60
1
0.119, 0.147
0.318, 0.350
-60
2
0.044, 0.050
0.108, 0.114
-60
1
-60
1
0.073, 0.0601
0.1628, 0.1450
R11, R12 a,b
0.095, 0.126
0.220, 0.255
0.050, 0.068
0.119, 0.135
wR21, wR22 b,c 0.247, 0.259
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b Superscript 1 denotes value of the residual considering only the reflections for which I > 2σ(I). Superscript 2
denotes value of the residual for all reflections. c wR2 ) ∑[w(Fo - Fc2)2]/∑[w(Fo2)2]]1/2
.
2
Table 2. Selected Bond Distances (Å) for Compounds 1-6,
otherwise stated. Solvents were purified by conventional methods and
distilled under nitrogen from CaH2 or Na/K, as appropriate. The
formamidine HDPh F was prepared following a published procedure
[Cr2(DArF)4]
F
4
5
compd
ligand
Cr-Cr
Cr-N
Cr-N
Cr-F
but with several modifications: triethyl o-formate and pentafluoroaniline
were mixed in stoichoimetric ratio at room temperature; to the mixture
was added concentrated HCl until there was no further precipitation.
The white precipitate was then filtered and recrystallized from CHCl3/
hexanes. This method increased the yield by up to 20% above that
reported in the literature; HDPho-FF, HDPh3,5-FF, HDPhm-FF, and
HDPhp-FF were prepared following similar procedures. Other chemicals
were purchased from Aldrich and used as received. Infrared spectra
were recorded in the range 4000-400 cm-1 on a Perkin-Elmer 16PC
spectrophotometer using KBr pellets. Nuclear magnetic resonance
spectra were recorded on a Bruker 200 at 200 MHz or on a Unity Plus
300 spectrometer at 300 MHz for 1H and 282 MHz for 19F. Reference
standards were as follows: 1H, CH2Cl2 (internal, 5.34 ppm); 19F, CCl3F
(external, 0.0 ppm). Values for the C, H, and N analyses on all the
samples were satisfactory; these analyses were performed by Canadian
Microanalytical Service.
1a
DPhp-F
F
1.917(6)
2.043(6)
2.083(6)
2.054(6)
2.057(5)
2.038(3)
2.043(3)
2.065(3)
2.084(3)
2.062(7)
2.054(6)
2.066(6)
2.054(6)
1.915(5)
2
3
4
5
6
DPh3,5-F
F
1.9057(7)
2.035(3)
2.046(3)
2.073(3)
2.100(3)
DPhm-F
F
1.916(2)
1.928(2)
DbiPhF
2.041(11) 2.063(12)
2.056(11) 2.063(11)
2.064(11) 2.070(12)
2.084(11) 2.072(10)
F
5
DPh F
2.0121(11) 2.061(4)
2.064(4)
2.048(4)
2.082(4)
2.090(4)
2.465(4)
2.498(3)
Preparation of [Cr2(DPhp-FF)4] (1). A freshly prepared suspension
of LiDPhp-FF (0.36 g of HDPhp-FF and 1.65 mL of MeLi 1 M in 15
mL of THF) was added dropwise to 0.10 g of CrCl2 in THF. The gray
solid immediately reacted to give a yellow-green solution, but it was
stirred for an additional 2 h. The solvent was then removed under
reduced pressure, and, to eliminate the LiCl, the residue was extracted
with warm toluene, filtered, and layered with hexanes. The yellow
product was partially soluble in CH2Cl2, toluene, benzene, and pyridine.
2.078(4)
2.109(4)
2.058(5)
2.062(5)
2.065(5)
2.085(5)
DPho-F
F
1.968(2)
2.651(5)
a Two independent molecules.
1
Yield (in crystals): 72% (0.305 g). H NMR (300 MHz, CD2Cl2, rt)
(ppm): 8.63 (C-H, 1); 6.72 (m-H, 2); 6.17 (o-H, 2). 19F: -121.07
(p-F). IR (cm-1): 1607 (m), 1569 (s), 1496 (vs), 1329 (m), 1314 (s),
1210 (vs), 1155 (m), 1095 (w), 962 (m), 938 (m), 833 (m), 775 (m),
581 (w), 511 (w), 464 (w), 447 (w).
chlorine atoms. However, the structure of the fluorine derivative
differed from that of the chlorine derivative in several ways,
including the presence of an axial ligand. These differences
eliminated the possibility of making an informative comparison.
We now know that formamidinate complexes of the type Cr2-
(ArNC(H)NAr)4 are easily made, and thus, various aryl groups
can be used to probe the effect of the substituents on the Cr-
Cr distance. By designing appropriate formamidinates we have
engineered lantern complexes with ligands having different
electronic donating or withdrawing ability; some contain weak
axial interactions; others do not. Our objective was to shed more
light on the question of which effect has a higher impact on
the Cr-Cr distance: weak axial ligation or the basicity of the
bridging ligands? The results presented in this paper will be
seen to favor the former.
Preparation of [Cr2(DPhm-FF)4] (2). It was prepared similarly to
1 (0.10 g of CrCl2, 0.36 g of HDPhm-FF, 1.6 mL of MeLi, 1 M). The
yellow-orange product was more soluble in organic solvents at room
temperature than the o-F derivative. Yield (in crystals): 67% (0.15 g).
1H NMR (300 MHz, CD2Cl2, rt) (ppm): 8.798 (s, 1, C-H methine),
7.005 (q, 2, Hmeta), 6.73 (t, 2, Hp), 6.08 (m, 4, Hortho). 19F: -113.2 ppm
(Fmeta). IR (cm-1): 1572 (s), 1560 (s), 1331 (m), 1263 (m), 1171 (m),
1149 (m), 11369 (w), 951 (m), 774 (m), 688 (m), 520 (w), 443 (e),
420 (w).
Preparation of [Cr2(DPh3,5-FF)4] (3‚C6H14). It was prepared
similarly to 1 (0.1 g of CrCl2, 0.43 g of HDPh3,5-FF, 1.6 mL of MeLi,
1 M). The yellow-orange product was less soluble in organic solvents,
at room temperature, than the p-F and o-F derivatives. Yield: 63%
(0.29 g). 1H NMR (300 MHz, CD2Cl2, rt) (ppm): 8.81 (C-H, 1), 6.52
(p-H, 1), 5.77 (o-H, 2). 19F NMR (282 MHz, CD2Cl2, rt) (ppm):
Experimental Section
Methods and Materials. All operations were carried out under a
nitrogen atmosphere using standard Schlenk or drybox techniques unless
(4) Saeed, A. A. H.; Selman, S. A. Can. J. Spectrosc. 1982, 27, 123.

2184 Inorganic Chemistry, Vol. 38, No. 9, 1999
Cotton et al.
Figure 1. Drawing of the molecular structure of [Cr2(DPhp-FF)4] (1). Displacement ellipsoids are drawn at their 50% probability level. Hydrogen
atoms have been omitted for clarity.
Figure 2. Displacement ellipsoid plot of [Cr2(DPhm-FF)4] (2). Displacement ellipsoids are drawn at their 50% probability level. Fluorine atoms are
disordered over both meta positions; only one is shown.
F
5
-109.62 (Fmeta). IR (cm-1): 1619 (m), 1593 (s), 1543 (m), 1525 (w),
1509 (w), 1459 (m), 1320 (w), 1161 (m), 1118 (s), 1032 (m), 991 (m),
834 (m), 670 (m).
Preparation of [Cr2(DPh F)4] (5). Freshly distilled toluene (ca.
10 mL) was added to 0.05 g of CrCl2 (0.4 mmol). To this suspension
was added LiDPh F, prepared in 8 mL of toluene from 0.30 g of
HDPh F and 0.81 mL of 1 M MeLi in THF/cumene. The mixture was
F
5
F
5
Preparation of [Cr2(DbiPhF)4] (4‚CH2Cl2). It was prepared
similarly to 1 (0.1 g of CrCl2, 0.57 g of HD(biPh)F), 1.6 mL MeLi, 1
M). The orange-red product is very soluble at room temperature in
nonpolar organic solvents. Yield: 74% (0.60 g). 1H NMR (200 MHz,
CD2Cl2, rt) (ppm): 9.07 (s, 1, C-H amidine), 7.47 (Hmeta, 4) 7.36 (Hmeta
stirred for 3 h at room temperature; the solution became greenish; it
was allowed to react for an additional 6 h and was filtered under
nitrogen. The solvent was pumped off and the residue extracted with
CH2Cl2, yielding a clear light green-yellowish solution, which was then
layered with hexanes. Block-shaped, orange crystals formed within a
few days at room temperature. The product, green-yellowish when
powdered, is also soluble in THF, in which it forms dark mauve
solutions, probably due to the coordination of the solvent to the axial
second ring, tt, 4), 7.32 (Hpara, dt, 2), 7.26 (Hortho, 4, dt), 6.52 (d, Hortho
,
4). IR (cm-1): 1560 (vs), 1515 (s), 1482 (vs), 1320 (br, s), 1227 (s),
1190 (m), 1006 (w), 966 (w), 936 (w), 830 (m), 764 (s), 738 (m), 695
(s), 450 (w).

Quadruply Bonded Dichromium Complexes
Inorganic Chemistry, Vol. 38, No. 9, 1999 2185
Figure 3. Molecular structure of [Cr2(DPh3,5-FF)4] in 3‚C6H14. Displacement ellipsoids are drawn at their 50% probability level. Hydrogen atoms
have been omitted for clarity. Fluorine atoms are represented by their equivalent isotropic displacement thermal parameter.
Figure 4. Plot of [Cr2(DbiPhF)4] in 4‚0.7CH2Cl2. Displacement ellipsoids are drawn at their 50% probability level. Hydrogen atoms have been
omitted for clarity.
1
position. H NMR (300 MHz, CD2Cl2) (ppm): 7.94 (C-H, s). 19F:
green and finally to canary yellow. Lithium chloride was filtered off,
and the solution was layered with hexanes. After 10 days at room
temperature, block-shaped yellow crystals formed. Yield: 0.13 g (62%).
IR (cm-1): 1619 (m), 1611 (m), 1587 (s), 1569 (s), 1493 (s), 1455
(m), 1336 (s), 1293 (w), 1247 (s), 1230 (m), 1194 (w), 1150 (w), 1100
(m), 1032 (m), 970 (m), 942 (w), 865 (w), 852 (w), 802 (m), 763 (m),
766 (m), 745 (s), 561 (m), 443 (m). 1H NMR (200 MHz, CD2Cl2)
(ppm): 6.744 (Hmeta), 6.542 (Hpara), 3.69 (C-H)
-155.42 (Fortho), -162.41 (Fpara), -165.40 (Fmeta).
Preparation of [Cr2(DPho-FF)4] (6). Anhydrous chromium dichlo-
ride (0.05 g, 0.4 mmol) was suspended in 10 mL of freshly distilled
toluene. A stoichiometric amount of LiDPho-FF (mole ratio 1:2)
prepared in situ at room temperature in 10 mL of toluene was added
to the gray suspension. The mixture was allowed to reach room
temperature and was stirred for 18 h. The color changed from gray to

2186 Inorganic Chemistry, Vol. 38, No. 9, 1999
Cotton et al.
F
5
Figure 5. Drawing of [Cr2(DPh F)4] (5) showing two Cr‚‚‚F interactions. Displacement ellipsoids are drawn at their 50% probability level. Hydrogen
atoms have been omitted for clarity.
Figure 6. Drawing of the molecular structure of [Cr2(DPho-FF)4] (6). Ellipsoids are drawn at their 50% probability level. Hydrogen atoms have
been omitted for clarity. The Cr‚‚‚F axial interactions are also observable in 5.
Crystallographic Studies. Crystallographic data for complexes 1-6
were collected in a Nonius FAST diffractometer equipped with a low-
temperature device operating at -60 °C. Crystals were attached with
grease at the tips of quartz fibers. Cell parameters, Laue group, cell
type, and mosaic spread for each crystal were determined using at least
150 reflections obtained by scanning over 110° in ω with tilt on ω of
0.3° per scan frame. Data collection was performed in three different
regions, over more than one hemisphere of the reflection sphere. Frames
were collected at different exposure times for each crystal, depending
on the diffracting power of the crystal. Data sets were reduced with
the MADNES subroutine that accounted for Lorentzian and polarization
corrections. Absorption corrections were applied for 4. All the structures
were solved by direct methods. Non-hydrogen atoms were found on
Fourier maps after successive least-squares refinement cycles. Hydrogen
atoms were found in difference maps, but some of these were restrained
to ride on geometric positions of the parent atoms to increase the data-
to-parameter ratio. All the structures were refined using Shelx92
software packages.
Crystallographic data for 1-6 are given in Table 1. In contrast to
the isomorphic chromium amidinate series recently reported,5 in which
most of the complexes crystallized in space group P1h, these compounds
crystallize in various space groups and lattice types, even though they
were crystallized under similar conditions. Compound 4, [Cr2-
(5) Carlson-Day, K. M.; Eglin, J. L.; Lin, C.; Smith, L. T.; Staples, R. J.;
Wipf, D. O. Polyhedron, in press.

Quadruply Bonded Dichromium Complexes
Inorganic Chemistry, Vol. 38, No. 9, 1999 2187
(DbiPhF)4], crystallizes in the noncentrosymmetric space group P1, with
0.7 molecule of interstitial CH2Cl2. The two enantiomers (δδδδ and
λλλλ) cocrystallize in an 80:20 ratio. Complex 1, [Cr2(DPhp-FF)4],
crystallizes solvent-free and packs in a high-symmetry arrangement,
in space group Fddd. Compounds 3, [Cr2(DPh3,5-FF)4], 2, [Cr2-
(DPhm-FF)4]‚hexane, and 6, [Cr2(DPho-FF)4], pack in the solid state in
the centrosymmetric arrangement of space group P1h, with a whole
one thing that distinguishes the two compounds with the longer
Cr-Cr bonds is that they both have ortho fluorine atoms, while
the other ones do not. To strengthen this distinction, we also
cite the Cr-Cr distances in five other Cr2(DArF)4 compounds
that also do not have ortho fluorine atoms. These compounds
and their Cr-Cr distances are those with the following Ar
groups:
F
5
molecule per asymmetric unit. Complex 5, [Cr2(DPh F)4], crystallizes
p-MeC6H4
m-MeOC6H4
p-ClC6H4
3,5-Cl2C6H3
m-CF3C6H4
1.930(2) Å6a
1.918(1) Å5
1.907(1) Å5
1.916(1) Å5
1.902(1) Å5
in space group C2/c. Selected interatomic distances for all compounds
are provided in Table 2.
Results
The reaction of anhydrous CrCl2 with LiDArF, as described
in eq 1, leads to crystalline yellow to orange materials, which
were characterized by elemental analysis, NMR spectroscopy,
and single-crystal X-ray diffraction as the paddlewheel structures
[Cr2(DArF)4].
It seems clear that the presence of flourine atoms at one or both
ortho positions leads to elongation of the Cr-Cr bond. The
question is why.
The answer seems to be that o-F atoms are in a position to
interact axially with the chromium atoms, and they do so, as
can be seen in Figures 5 and 6. Moreover, in the o-FC6H4 case,
compound 6, there are only two such interactions with, Cr‚‚‚F
distances of 2.651 Å, while in the C6F5 case, compound 5, there
are four Cr‚‚‚F interactions, with distances of 2.465 and 2.488
Å, in accord with the fact that in 6 the lengthening (to 1.97 Å)
is not as great as that in 5 (to 2.01 Å).
It has been recognized that Cr-Cr quadruple bond lengths
are very sensitive to the influence of axial ligands,1 but these
results reinforce the message. One would not expect that these
axial Cr‚‚‚F interactions could be very strong, but nonetheless
they result in easily measurable lengthening of the Cr-Cr bond.
On the other hand, over the nine other compounds that have
been mentioned here the variation of Cr-Cr bond distances is
very small and there is no discernible relationship with the
inductive effect of the Ar groups in weakening the basicity of
the formamidinate ligands.
2CrCl2 + 4LiDArF THF8 Cr2(DArF)4 + 4LiCl
(1)
Spectroscopic Results. All complexes are diamagnetic, as
shown by NMR. 1H NMR data for all complexes are consistent
with their solid state structures. The 19F spectrum of 5 recorded
at room temperature has three sets of signals: a sharp triplet
centered at 32.16 ppm assigned to the p-F atoms, a broad doublet
at 29.17 ppm attributed to the F in the meta position, and a
nonresolved broad signal around 39.08 ppm, due to the F atoms
in the ortho position. The 19F NMR spectra at -20, -40, and
-60 °C show no significant changes in the o-F NMR signals.
We believe that, in solution, the Cr-F interactions are probably
involved in a dynamic equilibrium, with rotation of the phenyl
ring.
Structures. Figures 1-6 present thermal ellipsoid plots for
compounds 1-6, respectively. Table 2 summarizes the critical
distances that we wish to compare for these six molecules.
All six molecules have the expected paddlewheel structure
with short Cr-Cr bonds, and the Cr-N distances, which range
from 2.035 to 2.108 Å, have average values that are not
significantly different from one compound to another. However,
there is one conspicuous discontinuity apparent in Table 2,
namely, that in compounds 1-4 the Cr-Cr distances are all in
a narrow range, 1.906(1)-1.928(2) Å, whereas in compounds
5 and 6 they are well outside of this range, with values of 2.012-
(1) and 1.968(2) Å, respectively.
Acknowledgment. We thank Dr. Judy L. Eglin for providing
a copy of the manuscript cited in ref 5. We are also grateful to
the National Science Foundation and the Welch Foundation for
financial support.
Supporting Information Available: Complete listing of crystal-
lographic data, atomic coordinates, bond distances and angles, and
anisotropic displacement parameters in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
IC990007L
Discussion
(6) (a) Cotton, F. A.; Ren, T.J. Am. Chem. Soc. 1992, 114, 2237. (b)
Roode, W. H. D.; Vrieze, K. J. Organomet. Chem. 1997, 135, 183.
(c) Bino, A.; Cotton, F. A.; Kaim, W. Inorg. Chem. 1979, 18,
3566.
We focus here on the bimodal distribution of the Cr-Cr
distances found in the six new compounds reported here. The