Synthesis of multiply-bonded dichromium complexes with a variety of formamidinate ligands

Article abstract of DOI:10.1016/S0277-5387(98)00362-3

A series of dichromium tetraformamidinate complexes with (ArNCHNAr)^- where Ar is X₂C₆H₃ or XC₆H₄ and X is the remote substituent 3,5-Cl₂, 3,4-Cl₂, p-Cl, p-CF₃, m-CF₃, p-OCH₃ or m-OCH₃ has been synthesized, and the structures of four of the compounds determined by X-ray crystallography. The Cr-Cr bond distances of the complexes structurally characterized are 1.9072(10) A [Cr₂((p-ClC₆H₄N)₂CH)₄], 1.9162(10) A [Cr₂((3,5-Cl₂C₆H₃N) ₂CH)₄], 1.9018((8) A [Cr₂((m-CF₃C₆H₄N) ₂CH)₄ ], and 1.9178(11) A [Cr₂((m-OCH₃C₆H₄N) ₂CH)₄]. ¹H NMR spectroscopy and electrochemistry were performed on the series. The diamagnetic anisotropy of the Cr-Cr bond was calculated from X-ray crystallographic and ¹H NMR data and correlates with the Hammett constant of the formamidine ligands.

Full text of DOI:10.1016/S0277-5387(98)00362-3

Polyhedron 18 (1999) 817–824
Synthesis of multiply-bonded dichromium complexes with a variety of
formamidinate ligands
b
a
c
Kathryn M. Carlson-Day, Judith L. Eglina, , Chun Lin , Laura T. Smith , Richard J. Staples ,
*
David O. Wipfa
aDepartment of Chemistry, Mississippi State University, Mississippi State, MS 39762, USA
bDepartment of Chemistry, Florida Institute of Technology, Melbourne, FL 32901, USA
cDepartment of Chemistry, Harvard University, Cambridge, MA 02138, USA
Received 12 July 1998; accepted 11 September 1998
Abstract
A series of dichromium tetraformamidinate complexes with (ArNCHNAr)2 where Ar is X2C6H3 or XC6H4 and X is the remote
substituent 3,5-Cl2, 3,4-Cl2, p-Cl, p-CF3, m-CF3, p-OCH3 or m-OCH3 has been synthesized, and the structures of four of the
˚
compounds determined by X-ray crystallography. The Cr–Cr bond distances of the complexes structurally characterized are 1.9072(10) A
˚
˚
˚
[Cr2(( p-ClC6H4N)2CH)4], 1.9162(10) A [Cr2((3,5-Cl2C6H3N)2CH)4], 1.9018((8) A [Cr2((m-CF3C6H4N)2CH)4 ], and 1.9178(11) A
[Cr2((m-OCH3C6H4N)2CH)4]. 1H NMR spectroscopy and electrochemistry were performed on the series. The diamagnetic anisotropy of
the Cr–Cr bond was calculated from X-ray crystallographic and 1H NMR data and correlates with the Hammett constant of the
formamidine ligands.
1999 Elsevier Science Ltd. All rights reserved.
Keywords: Dichromium; Formamidinate; Multiply-bonded
1. Introduction
fundamental geometry of the complex. A series of di-
molybdenum tetraformamidinate complexes has been syn-
thesized and the electronic, structural, and spectroscopic
properties systematically investigated [9]. For the
Mo2(form)4 series, a correlation between the Hammett
constant (s) of the remote substituents of the formamidi-
nate ligands and the redox potentials determined electro-
chemically demonstrates a methodology to fine tune
properties via the linear free energy relationship (LFER)
[16]. An extension of the Mo2(form)4 study to the
Cr2(form)4 series allows the comparison of group 6
transition metal complexes of the general type M2(form)4
with 3d or 4d electrons.
The versatility of the formamidinate ligand (form) is
reflected in the wide range of transition metal complexes
[Pt, V, Fe, Co, Ni, Cu, Tc, Ru, Rh, Pd, Re, and Os]
containing a variety of bond orders that have been previ-
ously synthesized [1–8]. Analogous to a carboxylate
ligand, the formamidinate ligand allows the synthesis of
homologous series of the group 6 transition metal com-
plexes in order to systematically study the nature of the
quadruple bond [9–14]. To date, the number of dich-
romium tetraformamidinate complexes structurally char-
acterized is limited to Cr2(( p-CH3C6H3N)2CH)4 [10],
Cr2[HC(N-3,5-xylyl)2]4 [14], and Cr2(CH3NC(C6H5)-
NCH3)4 [15].
The synthesis of a series of quadruply-bonded di-
chromium complexes, Cr2(( p-ClC6H4N)2CH)4 (1),
Cr2((3,5-Cl2C6H3N)2CH)4 (2), Cr2((3,4-Cl2C6H3N)2-
CH)4 (3), Cr2(( p-CF3C6H4N)2CH)4 (4), Cr2((m-CF3C6-
H4N)2CH)4 (5), Cr2(( p-OCH3C6H4N)2CH)4 (6), and
Cr2((m-OCH3C6H4N)2CH)4 (7), is presented with struc-
By varying the electron donating or withdrawing ability
of the remote substituents on the phenyl rings of the
formamidinate ligand, the resultant electron density at the
transition metal core can be altered without a change in the
1
tural information provided for 1, 2, 5, and 7. The H NMR
spectroscopy and electrochemistry of the series of
Cr2(form)4 complexes are reported and compared to the
*
Corresponding author.
0277-5387/99/$ – see front matter
PII: S0277-5387(98)00362-3
1999 Elsevier Science Ltd. All rights reserved.

818
K.M. Carlson-Day et al. / Polyhedron 18 (1999) 817–824
Hammett constants of the remote substituents of the
formamidinate ligands.
of Cr2(( p-ClC6H4N)2CH)4 were grown from a CH2Cl2
solution of the compound layered with hexanes. 1H NMR
(C6D6, ppm): 6.02 (d, 8.0 Hz), 6.83 (d, 8.0 Hz) and 8.54
(s).
2. Experimental
2.3. Preparation of Cr2((3,5-Cl2C6H3N)2CH)4 (2)
2.1. General considerations
A Schlenk flask was charged with 2.7 g (9.0 mmol) of
(3,5-Cl2C6H3N)2HCH and 10 ml of THF. After cooling to
08C, 5.7 ml (8.0 mmol) of CH3Li was added. Once all gas
evolution had occurred, the pale brown solution was
transferred via cannula to a flask containing a 10 ml THF
solution of CrCl2 (0.49 g, 4.0 mmol). The reaction was
stirred at room temperature overnight then all solvent
removed under vacuum. The yellow product was dissolved
in toluene and toluene/hexanes mixtures added to precipi-
tate the product. The resultant yellow solid was washed
using 30 ml aliquots of hexanes and all solvent removed to
yield Cr2((3,5-Cl2C6H3N)2CH)4 (2.56 g, 89.1%). Crystals
of Cr2((3,5-Cl2C6H3N)2CH)4 were grown from a benzene
solution of the compound layered with hexanes. 1H NMR
(C6D6, ppm): 5.95 (s), 6.89 (s) and 7.82 (s).
Standard Schlenk, drybox, and vacuum line techniques
were used under an argon atmosphere. Commercial grade
tetrahydrofuran (THF), toluene, and hexanes were dried
over potassium/sodium benzophenone ketyl. Methylene
chloride was dried over P2O5. All solvents were freshly
distilled under argon atmosphere prior to use. The starting
material CrCl2 was purchased from Strem Chemicals and
used as received. Triethylorthoformate, p-chloroaniline,
3,5-dichloroaniline, 3,4-dichloroaniline, m-methoxyaniline,
m-(trifluoromethyl)aniline, p-(trifluoromethyl)aniline, and
p-methoxyaniline were purchased from ACROS Chemi-
cal Company. All of the ligands, ( p-ClC6H4N)2HCH,
(3,5-Cl2C6H3N)2HCH, (3,4-Cl2C6H3N)2HCH, ( p-CF3C6-
H4N)2HCH, (m-CF3C6H4N)2HCH, ( p-CH3OC6H4N)2-
HCH, and (m-CH3OC6H4N)HCH, were synthesized by the
previously reported method [17]. Methyllithium was pur-
chased as a 1.4 M solution in diethylether from Aldrich
Chemical Company in a Sure-Seal bottle and used as
received.
2.4. Preparation of Cr2((3,4-Cl2C6H3N)2CH)4 (3)
A Schlenk flask was charged with 2.7 g (9.0 mmol) of
(3,4-Cl2C6H3N)2HCH and 10 ml of THF, then cooled to
08C and methyllithium (5.7 ml, 8.0 mmol) added. Once all
gas evolution had ceased, the pale orange brown solution
was transferred via cannula to a second flask containing a
10 ml THF solution of CrCl2 (0.49 g, 4.0 mmol). The
reaction was stirred at room temperature for several hours
to result in the formation of a yellow solid. The solid was
washed several times with 30 ml aliquots of hexanes and
all solvent removed to yield the product, Cr2((3,4-
Cl2C6H3N)2CH)4 (2.51 g, 87.4%).
1H NMR spectra (400 MHz) were recorded in 5 mm
NMR tubes on a General Electric Omega instrument using
a 10 mm broad band probe. The proton chemical shifts (d )
were referenced to residual C6H6 in the deuterated solvent
C6D6.
Cyclic voltammetry measurements of the dichromium
tetraformamidinate compounds 1, 2, and 4–7 were per-
formed in a drybox under an argon atmosphere. Measure-
ments were made in 0.1 M (n-Bu4N)PF6 electrolyte and
0.6 mM Cr2(form)4 THF solutions using a Solartron S1
1287 Electrochemical Interface with a 1.6 mm diameter Au
disk working electrode and a Ag wire quasi-reference
electrode. Au potentials are reported versus the ferrocene/
ferrocenium1 E1 / 2 value.
2.5. Preparation of Cr2((p-CF3C6H4N)2CH)4 (4)
A Schlenk flask was charged with 2.6 g (9.0 mmol) of
( p-CF3C6H4N)2HCH and 10 ml of THF. After cooling the
solution in an ice bath, CH3Li (5.7 ml, 8.0 mmol) was
added. Once all gas evolution had ceased, the solution was
transferred via cannula to a second flask containing a 10
ml THF solution of 0.49 g of CrCl2 (4.0 mmol). The
reaction mixture was allowed to stir at room temperature
overnight. The resultant orange solid was washed with
hexanes several times and all solvent removed to yield
the product, Cr2(( p-CF3C6H4N)2CH)4 (2.59 g, 90.6%). 1H
NMR (C6D6, ppm): 6.02 (d, 8.0 Hz), 7.34 (d, 8.4 Hz) and
8.28 (s).
2.2. Preparation of Cr2((p-ClC6H4N)2CH)4 (1)
To a Schlenk flask charged with 1.2 g (4.5 mmol) of
( p-ClC6H4N)2HCH and 10 ml of THF cooled in an ice
bath, 2.9 ml of methyllithium (4.1 mmol) was added. Once
all gas evolution had ceased, the resultant pale yellow
solution of the lithium formamidinate salt was transferred
via cannula to a flask containing a 10 ml THF solution of
CrCl2 (0.25 g, 2.0 mmol). The reaction was stirred at room
temperature overnight and filtered through Celite. The
yellow product precipitated from solution upon addition of
50 ml of hexanes. Subsequent washing with 30 ml aliquots
of hexanes followed by solvent removal yielded the yellow
powder Cr2(( p-ClC6H4N)2CH)4 (0.88 g, 76%). Crystals
2.6. Preparation of Cr2((m-CF3C6H4N)2CH)4 (5)
A Schlenk flask was charged with 2.6 g (9.0 mmol) of
(m-CF3C6H4N)2HCH and 10 ml of THF, then cooled in

K.M. Carlson-Day et al. / Polyhedron 18 (1999) 817–824
819
an ice bath. CH3Li (5.7 ml, 8.0 mmol) was added to the
chilled solution. After all gas evolution ceased, the solution
was transferred via cannula to a second flask containing a
10 ml THF solution of CrCl2 (0.49 g, 4.0 mmol). The
reaction mixture was allowed to stir at room temperature
overnight and all solvent removed under vacuum. The
solid product was dissolved in toluene and toluene/hex-
anes solvent mixtures added to precipitate the yellow
powder. The powder was washed three times with hexanes.
Solvent removal yielded the product Cr2((m-
CF3C6H4N)2CH)4 (2.35 g, 82.2%). Crystals of Cr2((m-
CF3C6H4N)2CH)4 were grown from a toluene solution of
using a Siemens SMART CCD (charge coupled device)
based diffractometer equipped with an LT-2 low-tempera-
ture apparatus operating at 213 K. Suitable crystals of each
of the respective compounds were mounted on a glass fiber
using grease. Data were measured using omega scans of
0.38 per frame for 30 s such that a hemisphere was
collected for 1, 2, 5, and 7. A total of 1271 frames were
˚
collected with a final resolution of 0.75 A for Cr2(( p-
˚
ClC6H4N)2CH)4, 0.90 A for Cr2((3,5-Cl2C6H3N)2CH)4,
˚
˚
0.85 A for Cr2((m-CF3C6H4N)2CH)4, and 0.85 A for
Cr2((m-OCH3C6H4N)2CH)4. The first 50 frames were
recollected for 1, 2, 5, and 7 at the end of data collection to
monitor for decay, but no decomposition was observed
during data collection for any of the crystals. Cell parame-
ters were retrieved using SMART [18] software and
refined using SAINT on all observed reflections. Data
reduction was performed using the SAINT software [19]
which corrects for Lp and decay. Absorption corrections
were applied using SADABS [20] supplied by George
Sheldrick. The structures were solved by the direct method
using the SHELXL-90 [21] program and refined by least
squares method on F2, SHELXL-93 [22], incorporated in
SHELXTL-PC V 5.03 [23].
1
the compound layered with hexanes. H NMR (C6D6,
ppm): 6.24 (d, 8.0 Hz), 6.27 (s), 6.73 (t, 7.8 Hz), 7.04 (d,
7.6 Hz) and 8.30 (s).
2.7. Preparation of Cr2((p-OCH3C6H4N)2CH)4 (6)
A Schlenk flask was charged with 2.0 g (7.7 mmol) of
( p-CH3OC6H4N)2HCH and 10 ml THF, cooled in an ice
bath, and CH3Li (5.7 ml, 8.0 mmol) added. Once all gas
evolution had occurred, the solution was transferred via
cannula to a flask containing a 10 ml THF solution of
CrCl2 (0.49 g, 4.0 mmol). The reaction mixture was stirred
at room temperature overnight and all solvent removed
under dynamic vacuum. The crude product was dissolved
in THF, and a yellow solid precipitated upon the addition
of 50 ml of hexanes. The complex was washed several
times with 30 ml aliquots of hexanes, and all solvent
removed to yield Cr2(( p-OCH3C6H4N)2CH)4 (1.09 g,
48.4%). 1H NMR (C6D6, ppm): 3.22 (s), 6.40 (d, 8.5 Hz),
6.65 (d, 8.5 Hz) and 8.70 (s).
The structures of Cr2(( p-ClC6H4N)2CH)4, Cr2((3,5-
Cl2C6H3N)2CH)4, Cr2((m-CF3C6H4N)2CH)4, and Cr2-
((m-OCH3C6H4N)2CH)4 were solved in the space group
¯
P1 ([2). All non-hydrogen atoms were refined aniso-
tropically. Hydrogen atoms for 1, 2, 5, and 7 were
calculated by geometrical methods and refined as riding
models. In the structure of Cr2((3,5-Cl2C6H3N)2CH)4, 3.5
molecules of C6H6 are present in the asymmetric unit as
interstitial solvent.
2.8. Preparation of Cr2((m-OCH3C6H4N)2CH)4 (7)
3. Results and discussion
A Schlenk flask was charged with 2.0 g (7.7 mmol) of
(m-CH3OC6H4N)HCH and 10 ml THF, then cooled to
08C. CH3Li (5.7 ml, 8.0 mmol) was added. Once all gas
evolution ceased, the solution was transferred via cannula
to a second flask containing a 10 ml THF solution of CrCl2
(0.49 g, 4.0 mmol). The reaction mixture was stirred
overnight at room temperature and then all solvent re-
moved. The compound was dissolved in THF, filtered
through Celite, and the yellow solid precipitated upon
addition of 50 ml of hexanes. The resultant powder was
washed several times with 30 ml aliquots of hexanes then
all solvent removed to yield Cr2((m-OCH3C6H4N)2CH)4
(1.20 g, 53.3%). Crystals of Cr2((m-OCH3C6H4N)2CH)4
were grown from a THF solution of the compound layered
3.1. Preparation and general properties
As shown in Scheme 1 [1], the preparative method for
Cr2(( p-CH3C6H3N)2CH)4 [10] using the starting material
CrCl2 can be generalized to other substituted formamidi-
nate ligands. Compounds 1–7 are very air-sensitive, and
the brilliant yellow color of the complexes changes to
green within a few minutes upon exposure to air.
3.2. Molecular structures
The crystallographic data for compounds 1, 2, 5, and 7
are provided in Table 1 with selected geometric parameters
in Table 2. Similar to the dimolybdenum tetraformamidi-
nate complexes with the analogous paddlewheel geometry,
the space groups for these complexes were found to be of
1
with hexanes. H NMR (C6D6, ppm): 3.14 (s), 5.95 (s),
6.28 (d, 8.3 Hz), 6.51 (d, 8.3 Hz), 6.84 (t, 7.93 Hz), and
8.78 (s).
¯
2.9. Crystallographic studies
lower symmetry (P1) than for Cr2(( p-CH3C6H3N)2CH)4
(Pbnb) [10]. Changes in the space group for the Cr2(form)4
Crystallographic data were collected for 1, 2, 5, and 7
analogs can be attributed to the choice of solvents used in

820
K.M. Carlson-Day et al. / Polyhedron 18 (1999) 817–824
Scheme 1.
the crystallization process and the different substituents on
the phenyl rings of the formamidinate ligands to result in
variations in crystalline packing.
bond distance for the dichromium tetracarboxylates [24–
28]. The average Cr–Cr bond distance for the series of
dichromium tetraformamidinate complexes structurally
characterized in this study is 1.911 A, between 0.3 to 0.6 A
shorter in comparison to either the shortest Cr–Cr bond
˚
˚
Inspection of the ORTEP drawings in Figs. 1–4 of
Cr2(( p-ClC6H4N)2CH)4, Cr2((3,5-Cl2C6H3N)2CH)4, Cr2-
((m-CF3C6H4N)2CH)4, and Cr2((m-OCH3C6H4N)2CH)4,
respectively, confirms the paddlewheel geometry is re-
tained about the dichromium core, analogous to the
dimolybdenum complexes [9]. The metal–metal bond
distances for the dichromium complexes, 1, 2, 5, and 7, are
distance determined for
Cr2(O2CCH3)4(C4H4N2) (2.295(5) A) [29] or the longest
Cr–Cr bond distance found in the carboxylate complex,
a
carboxylate complex,
˚
˚
Cr2(O2CCF3)4[O(C2H5)2]2 (2.541(1) A) [28]. Of note in
the structures of the dichromium formamidinate complexes
is the absence of axial coordination of either solvent
molecules such as the THF used in the preparative method
or a nitrogen atom of a neighboring formamidinate ligand.
In contrast, dichromium tetracarboxylate complexes con-
tain axial ligands of either infinite chains of Cr2(O2CR)4
molecules or coordinated solvents [26,28] such as
˚
1.9072(10), 1.9162(10), 1.9018(8), and 1.9178(11) A,
respectively. As in the case of Cr2(( p-CH3C6H3N)2CH)4
[10] and Cr2[HC(N-3,5-xylyl)2]4 [14], the Cr–Cr bond
distances fall short of the value for a chromium ‘super-
˚
short’ quadruple bond of 1.90 A. A plot of the Cr–Cr bond
distance versus the Hammett constant indicates no correla-
tion and minimal variation in the Cr–Cr bond distances
˚
Cr2(O2CO-t-Bu)4(THF)2
(2.367(3)
A)
[30]
or
˚
˚
(1.90 to 1.92 A) [9,12,16].
Cr2(O2Cbiph)4(THF)2 (2.316(3) A) where biphCO2H is
2-phenylbenzoic acid [27].
The dichromium tetraformamidinate complexes contain
significantly shorter Cr–Cr bond distances than the Cr–Cr
The torsional angles for the crystallographically char-
Table 1
X-ray crystallographic data for compounds 1, 2, 5, 7
1
2
5
7
Formula
C52H36Cl8Cr2N8
1160.49
Triclinic
¯
P1
C72.50H48.50Cl16Cr2N8
1702.89
Triclinic
¯
P1
C60H36Cr2F24N8
1428.97
Triclinic
¯
P1
C60H60Cr2N8O8
1125.16
Triclinic
¯
P1
Formula weight
Cryst. syst.
Space group
˚
a, A
10.166(3)
11.319(3)
12.481(4)
106.38(1)
91.64(1)
114.46(2)
1236.7(6)
1
1.558
0.919
0.71073
1.72–28.29
213(2)
15.4591(4)
15.7105(3)
16.8096(3)
71.775(1)
89.002(1)
88.787(1)
3876.6(1)
2
1.459
0.879
0.71073
1.28–22.50
213(2)
10.8581(1)
12.0770(2)
13.4575(2)
66.582(1)
80.206(1)
64.066(1)
1456.28(4)
1
1.629
0.499
0.71073
1.65–24.68
213(2)
10.5569(1)
11.7164(3)
12.0171(3)
101.810(1)
104.931(1)
105.504(1)
1322.86(5)
1
1.412
0.477
0.71073
1.83–25.00
213(2)
˚
b, A
˚
c, A
a, deg
b, deg
g, deg
3
˚
Volume, A
Z
rcalc, mg/mm3
m, mm21
˚
l, A
u range, deg
T, K
R1
0.0409
0.0538
0.0430
0.0554
wR2
0.1000
1.015
0.1049
1.035
0.0959
1.059
0.1054
1.085
Goodness of fit on F2

K.M. Carlson-Day et al. / Polyhedron 18 (1999) 817–824
821
Table 2
Selected geometric parameters for compounds 1, 2, 5, 7
1
2
5
7
Cr(1)–Cr(1A)
Cr(1)–N(1)
Cr(1)–N(2)
Cr(1)–N(3)
Cr(1)–N(4)
N(1)–C(1)
N(3A)–C(1)
N(2)–C(8)
1.9072(10)
2.063(3)
2.064(2)
2.034(3)
2.056(2)
1.329(4)
1.325(4)
1.322(3)
1.315(4)
93.70(7)
97.08(7)
96.82(8)
93.41(7)
93.14(10)
88.16(10)
169.31(9)
168.63(9)
86.25(10)
93.14(10)
115.8(2)
116.2(2)
118.7(3)
120.5(3)
Cr(1)–Cr(2)
Cr(1)–N(1)
Cr(1)–N(7)
Cr(1)–N(3)
Cr(1)–N(5)
N(1)–C(1)
N(2)–C(1)
N(7)–C(4)
1.9162(10)
2.036(4)
2.066(4)
2.047(4)
2.067(4)
1.332(6)
1.328(6)
1.317(6)
1.336(6)
95.43(11)
94.39(12)
96.42(11)
94.67(11)
89.8(2)
Cr(1)–Cr(1A)
Cr(1)–N(1)
Cr(1)–N(2)
Cr(1)–N(3)
Cr(1)–N(4)
N(1)–C(1)
N(3A)–C(1)
N(2)–C(2)
1.9018(8)
2.060(2)
2.061(2)
2.064(2)
2.053(2)
1.322(3)
1.320(3)
1.324(4)
1.327(3)
93.58(7)
97.59(7)
97.22(7)
92.90(6)
90.06(9)
91.84(9)
168.94(8)
169.20(8)
91.84(9)
90.01(9)
115.9(2)
113.2(2)
120.3(3)
118.7(2)
Cr(1)–Cr(1A)
Cr(1)–N(1)
Cr(1)–N(2)
Cr(1)–N(3)
Cr(1)–N(4)
N(1)–C(1)
N(2A)–C(1)
N(3)–C(2)
1.9178(11)
2.068(3)
2.055(3)
2.071(3)
2.050(3)
1.325(5)
1.323(5)
1.324(5)
1.322(4)
94.53(9)
95.75(9)
96.89(9)
93.82(9)
88.83(12)
89.07(12)
169.68(12)
169.21(12)
91.62(12)
88.58(12)
115.3(2)
112.1(2)
119.5(3)
121.0(3)
N(4A)–C(8)
N(8)–C(4)
N(4A)–C(2)
N(4A)–C(2)
Cr(1)–Cr(1A)–N(1)
Cr(1)–Cr(1A)–N(2)
Cr(1)–Cr(1A)–N(3)
Cr(1)–Cr(1A)–N(4)
N(4)–Cr(1)–N(1)
N(4)Cr(1)–N(3)
N(1)–Cr(1)–N(3)
N(2)–Cr(1)–N(4)
N(3)–Cr(1)–N(2)
N(4)–Cr(1)–N(1)
C(1)–N(1)–Cr(1)
C(8A)–N(4)–Cr(1)
N(1)–C(1)–N(3A)
N(2)–C(8)–N(4A)
Cr(1)–Cr(2)–N(1)
Cr(1)–Cr(2)–N(7)
Cr(1)–Cr(2)–N(3)
Cr(1)–Cr(2)–N(5)
N(1)–Cr(1)–N(5)
N(1)–Cr(1)–N(7)
N(1)–Cr(1)–N(3)
N(5)–Cr(1)–N(7)
N(5)–Cr(1)–N(3)
N(1)–Cr(1)–N(7)
C(1)–N(1)–Cr(1)
C(4)–N(7)–Cr(1)
N(1)–C(1)–N(2)
N(7)–C(4)–N(8)
Cr(1)–Cr(1A)–N(1)
Cr(1)–Cr(1A)–N(2)
Cr(1)–Cr(1A)–N(3)
Cr(1)–Cr(1A)–N(4)
N(1)–Cr(1)–N(2)
N(1)–Cr(1)–N(4)
N(1)–Cr(1)–N(3)
N(2)–Cr(1)–N(4)
N(1)–Cr(1)–N(4)
N(3)–Cr(1)–N(4)
C(1)–N(1)–Cr(1)
C(2)–N(2)–Cr(1)
N(1)–C(1)–N(3A)
N(2)–C(2)–N(4A)
Cr(1A)–Cr(1)–N(1)
Cr(1A)–Cr(1)–N(2)
Cr(1A)–Cr(1)–N(3)
Cr(1A)–Cr(1)–N(4)
N(3)–Cr(1)–N(1)
N(3)–Cr(1)–N(2)
N(1)–Cr(1)–N(2)
N(3)–Cr(1)–N(4)
N(1)–Cr(1)–N(4)
N(2)–Cr(1)–N(4)
C(1)–N(1)–Cr(1)
C(2)–N(3)–Cr(1)
N(1)–C(1)–N(2A)
N(4A)–C(2)–N(3)
89.2(2)
168.1(2)
170.9(2)
89.6(2)
89.2(2)
116.2(3)
115.6(3)
118.3(4)
119.2(5)
acterized dichromium complexes are as follows: N(3)–
Cr(1)–Cr(1A)–N(1A) (1.98) and N(4)–Cr(1)–Cr(1A)–
N(2A) (4.48) for 1; N(1)–Cr(1)–Cr(2)–N(2) (5.18), N(3)–
Cr(1)–Cr(2)–N(4) (4.08), N(5)–Cr(1)–Cr(2)–N(6) (4.58),
and N(7)–Cr(1)–Cr(2)–N(8) (4.78) for 2; N(4)–Cr(1)–
Cr(1A)–N(2A) (2.68) and N(3)–Cr(1)–Cr(1A)–N(1A)
(2.48) for 5; and N(3)–Cr(1)–Cr(1A)–N(4A) (1.38) and
N(1)–Cr(1)–Cr(1A)–N(2A) (0.98) for 7 with minimal
distortion from the ideal paddlewheel geometry. A plot of
the Cr–N bond distances versus the Hammett constant
indicates no correlation.
95.248 for the p-Cl, 3,5-Cl2, m-CF3 and m-OCH3
Cr2(form)4 analogs are slightly larger than the Mo2(form)4
complexes with Mo–Mo–N bond angles of 92.75, 92.79,
92.71, and 92.668 for the p-OCH3, H, m-Cl and 3,5-Cl2
analogs [9]. The trend, shorter M–M bond distances
resulting in larger M–M–N bond angles, is observed for
the M2(form)4 series and includes Cr, Mo, and W com-
plexes [9,10,12]. As expected from significant shortening
of one of the bonds (Cr–Cr) in a five-membered ring
system, the average N–C(H)–N bond angles are more
acute for the Cr2(form)4 series than the Mo2(form)4 series.
In order to compare the metal–metal bond distances of
Cr, Mo, and W complexes, the ‘formal shortness ratio’
(FSR) is used to account for the difference in size of the
As a result of shorter Cr–Cr bond distances in com-
parison to the Mo–Mo bond distances previously reported,
the average Cr–Cr–N angles of 95.25, 95.23, 95.32, and
Fig. 1. ORTEP drawing of Cr2(( p-ClC6H4N)2CH)4 at 50% probability
level. Hydrogens atoms are omitted for clarity.
Fig. 2. ORTEP drawing of Cr2((3,5-Cl2C6H3N)2CH)4 at 50% probability
level. Hydrogens atoms are omitted for clarity.

822
K.M. Carlson-Day et al. / Polyhedron 18 (1999) 817–824
nificantly larger than either the dichromium or di-
molybdenum tetraformamidinate compounds, but consis-
˚
tent with that fact that the W2 bond distance is 0.10 A
longer than the Mo2 bond distance [10,31,32].
3.3. 1H NMR spectroscopy
1H NMR spectroscopy was performed for the
Cr2(form)4 series with the exception of the poorly soluble
Cr2((3,4-Cl2C6H3N)2CH)4 (3). Simple 1H NMR spectra
are characteristic of the high symmetry (D4) of the
paddlewheel geometry and confirm the structures of 3, 4,
and 6 where X-ray crystallographic data were not obtained.
1
The H NMR spectra of the Cr2(form)4 complexes con-
tained only peaks for the coordinated formamidinate
ligands and the deuterated solvent to reflect the high purity
of the compounds.
The 1H NMR chemical shifts of the methine protons,
N–C(H)–N, of the Cr2(form)4 complexes are shifted
downfield in comparison to the free ligand with the
exception of Cr2((3,5-Cl2C6H3N)2CH)4. The degree of
diamagnetic anisotropy, the effect of a secondary magnetic
field on the methine proton created by the metal–metal
multiple bond, was calculated for each of the complexes
where crystallographic information was available. The
calculations were carried out using Eq. (2)
Fig. 3. ORTEP drawing of Cr2((m-CF3C6H4N)2CH)4 at 50% probability
level. Hydrogens atoms are omitted for clarity.
metal atoms. The equation to determine the FSR for a bond
A–B is shown in Eq. (1)
FSRAB 5 DA2B /RA1 1 RB1
(1)
where DA–B is the distance between the atoms, RA1 is the
radius of atom A, and RB1 is the radius of atom B [1]. The
FSR was calculated for the Cr2(form)4 complexes using
the average Cr–Cr bond distance for the p-Cl, 3,5-Cl2,
Dx 5 (Dd )(4p)/[(1 2 3 cos2u )/(3r3)]
(2)
where Dx is the diamagnetic anisotropy, Dd is the change
in the chemical shift of the proton relative to that of the
dinickel analogs with no M–M bond [5], r is the distance
of the proton nucleus from the center of the Cr–Cr bond,
and u is the angle between the r vector and the center of
the Cr–Cr axis [33].
As the s value increases, the diamagnetic anisotropy
decreases, as shown in Table 3. With a decrease in electron
donating ability of the remote substituents of the for-
mamidinate ligand, a weakening of the ring current
resulting from the multiple bond between the two metal
centers occurs, although no significant difference between
the Cr–Cr bond distances of 1, 2, 5 and 7 is observed [34].
A plot of the Hammett constant versus the diamagnetic
anisotropy for the crystallographically characterized
Cr2(form)4 complexes is shown in Fig. 5, and a high
correlation constant [0.991] is obtained. A linear least-
squares fit to the data, diamagnetic anisotropy versus 8s,
generated Eq. (3)
˚
m-CF3, and m-OCH3 analogs (1.911 A) and a FSR of
0.806 obtained. For the dimolybdenum series with an
˚
average Mo–Mo bond distance of 2.096 A for the p-
OCH3, H, m-Cl and 3,5-Cl2, the FSR is 0.809 [5]. The
ditungsten complexes W2(DFM)4 and W2(DCPF)4, where
DCPF is the 3,5-dichlorophenylformamidinate ligand, have
FSRs of 0.838 and 0.840, respectively [10,31]. The values
of the FSR for the two W2(II, II) compounds are sig-
Dx 5 4838.9 3 10236 m3 /molecule 2 8sr
(3)
with a r value of 405.71310236 m3 /molecule to reflect
the significant dependence of the diamagnetic anisotropy
on the Hammett constant. With zero additional electron
donating or withdrawing ability of the formamidinate
ligand due to remote substituents and s equal to zero, the
diamagnetic anisotropy of a Cr2(form)4 compound is
Fig. 4. ORTEP drawing of Cr2((m-OCH3C6H4N)2CH)4 at 50% probabili-
ty level. Hydrogens atoms are omitted for clarity.

K.M. Carlson-Day et al. / Polyhedron 18 (1999) 817–824
823
Table 3
1
Hammett constant, diagmagnetic anisotropy, H NMR data for the methine proton, and electrochemical data for the formamidinate complexes of the
general type: Cr2(form)4
Compound
s
Dx
(10236 m3 /molecule)
Methine
proton (d )
Irresversible
oxidation (mV)
1 ( p-Cl)
0.23
0.74
0.54
0.43
20.27
0.12
4313
2341
8.54 ppm (s)
7.82 ppm (s)
8.28 ppm (s)
8.30 ppm (s)
8.70 ppm (s)
8.78 ppm (s)
613
789
644
849
380
513
2 (3,5-Cl2)
4 ( p-CF3)
5 (m-CF3)
6 ( p-OCH3)
7 (m-OCH3)
3407
4522
4838.9310236 m3 /molecule, reflected in the intercept of
Eq. (3).
ated in order to allow a general comparison to the
previously observed trend [9].
A similar plot was generated for the Mo2(form)4 series
synthesized by Ren and coworkers based on the published
values of the diamagnetic anisotropy [9]. The correlation
coefficient is poor [0.866] for a plot of the diamagnetic
anisotropy versus Hammett constant for the data of Ren.
However, the diamagnetic anisotropy for the Mo2(form)4
series changes with different ligand substituents in the
same manner as the Cr2(form)4 series, although the degree
is significantly smaller (r538.991310236 m3 /molecule).
The trend observed for the Mo2(form)4 series, an
anionic shift with an increase in the Hammett constant of
the remote substituents of the form ligands, also occurs for
the Cr2(form)4 series [9]. Due to the irreversibility of the
oxidation wave in the Cr2(form)4 compounds, the correla-
tion coefficient of a linear fit to the data is poor [0.867] and
only qualitative not quantitative comparisons can be made.
The reactivity constant, r, is approximately 40% smaller
(52.8 mV) than the reactivity constant of the Mo2(form)4
series (87.2 mV) [9].
3.4. Electrochemistry
3.5. Conclusions
Electrochemical studies of the Cr2(form)4 series were
performed with the exception of the poorly soluble
Cr2((3,4-Cl2C6H3N)2CH)4. Data were obtained at a scan
rate of 100 mV/s, and the potential of the oxidation wave
ranged from 350 to 750 mV for the Cr2(form)4 series. The
oxidation is not reversible at either 100 or 200 mV/s. A
plot of the Hammett constant versus the first oxidation of
the dichromium core for the Cr2(form)4 series was gener-
In the series of Cr2(form)4 complexes synthesized with
variations in the Hammett constant of the remote sub-
stituents of the formamidinate ligands, no correlation
between the Cr–Cr bond distances or Cr–N bond distance
and s is observed. The diamagnetic anisotropy of the
Cr–Cr bond and electrochemical studies of the oxidation
wave correlate with the Hammett constant of the for-
mamidine ligands. In contrast to previous studies of the
Mo2(form)4 series [9], only qualitative data were obtained
in the electrochemical studies of the Cr2(form)4 complexes
due to the irreversibility of the oxidation wave, even at
higher scan speeds. While the oxidation potential of the
Cr2(form)4 complexes has a smaller dependence on the
Hammett constant in the electrochemical studies than the
Mo2(form)4 complexes [9], the dependence of the diamag-
netic anisotropy of the Cr2(form)4 complexes on the
Hammett constant is significantly stronger than the
Mo2(form)4 complexes [9]. Studies of a series of
W2(form)4 complexes are in process to determine the
dependence of the electrochemical processes and diamag-
netic anisotropy on the Hammett constant of the ligand
substituents.
Acknowledgements
K.M. Carlson-Day, J.L. Eglin and L.T. Smith would like
to acknowledge the support of the National Science
Foundation EPSCoR program [Grant No. EHR 9108767]
Fig. 5. Dependence of diamagnetic anisotropy on the Hammett constant
(8s) for the Cr2(form)4 series with the hollow circles for the calculated
values and the solid line as the least squares fit to the data.

824
K.M. Carlson-Day et al. / Polyhedron 18 (1999) 817–824
[12] C. Lin, J.D. Protasiewicz, E.T. Smith, T. Ren, J. Chem. Soc., Chem.
Commun. (1995) 2257.
[13] F.A. Cotton, T. Inglis, M. Kilner, T.R. Webb, Inorg. Chem. 14
(1975) 2023.
and the ARI Program [Grant No. CHE 92-14521]. Author
J. Eglin would like to acknowledge Dr. T. Ren for helpful
discussions.
[14] W.H. De Roode, K. Vrieze, J. Organomet. Chem. 135 (1977) 183.
[15] A. Bino, F.A. Cotton, W. Kaim, Inorg. Chem. 18 (1979) 3566.
[16] L.P. Hammett, Physical Organic Chemistry, McGraw-Hill, New
York, 1970.
[17] W. Bradley, I. Wright, J. Chem. Soc. (1956) 640.
[18] SMART V 4.043, Software for the CCD detector system, Siemens
Analytical Instruments Division, Madison, WI, 1996.
[19] SAINT V 4.035, Software for the CCD detector system, Siemens
Analytical Instruments Division, Madison, WI, 1995.
[20] SADABS, Program for absorption corrections using Siemens CCD
based on the method of Bob Blessing, Acta Crystallogr. A51 (1995)
33.
[21] G.M. Sheldrick, SHELXS-90, Program for the solution of crystal
structures, University of Gottingen, Germany, 1986.
[22] G.M. Sheldrick, SHELXL-93, Program for the refinement of crystal
structures, University of Gottingen, Germany, 1993.
[23] SHELXTL 5.03 (PC-Version), Program library for structure solution
and molecular graphics, Siemens Analytical Instruments Division,
Madison, WI, 1995.
Supporting Information Available
Complete tables of crystal data, positional and isotropic
equivalent thermal parameters, anisotropic thermal param-
eters, bond distances, and bond angles for the molecules of
Cr2(( p-ClC6H4N)2CH)4, Cr2((3,5-Cl2C6H3N)2CH)4, Cr2-
((m-CF3C6H4N)2CH)4, and Cr2((m-OCH3C6H4 N)2CH)4
(34) have been deposited at Cambridge Crystallographic
Data Centre, number 102155 for C52H36Cl8Cr2N8,
102156 for C72.5H48.5Cl16Cr2N8, 102157 for C60H36-
Cr2F24N8, and 102158 for C60H60Cr2N8O8. The above
and a listing of observed and calculated structure factors
(55) are available from author J.L.E. upon request.
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