www.elsevier.nl/locate/ica
Inorganica Chimica Acta 305 (2000) 69 – 74
Metalmetal versus metalligand bonding in dimetal compounds
with tridentate ligands
F. Albert Cotton a,*1, Lee M. Daniels a, Carlos A. Murillo a,b,*2, Hong-Cai Zhou a
a
Laboratory for Molecular Structure and Bonding, Department of Chemistry, Texas A&M Uni6ersity, PO Box 30012, College Station,
TX 77842 -3012, USA
b
Escuela de Quı́mica, Uni6ersidad de Costa Rica, Ciudad Uni6ersitaria, Costa Rica
Received 11 February 2000; accepted 11 February 2000
Abstract
Reaction of a ‘VCl2·nTHF’ solution, prepared by the reduction of VCl3(THF)3 with NaBEt3H in THF, and Lidpa (dpa=the
anion of 2,2%-dipyridylamine) in a mixture of THF/toluene at reflux temperature yields the bioctahedral V2(dpa)4·THF (1)
compound. A similar reaction performed in THF at 0°C gave [V2(dpa)3(m-Cl)2Li2(THF)6][BEt3H] (2), in which a dpa ligand
adopts a novel ‘doubly-chelating/bridging’ coordination mode. Compound 2 reacts with CH2Cl2 giving V2(dpa)3Cl2·2CH2Cl2 (3),
a valence delocalized V(II)···V(III) bioctahedral complex. In all three complexes, the formation of four additional metalligand
bonds is favored over the formation of a VV bond. The V···V separations are 3.038(2), 3.024(2) and 3.091(2) A, for 1 –3,
respectively. Crystal data are: compound 1, space group P2/n, a=13.102(2), b= 9.294(2), c=16.510(4) A, , b= 98.98(2)°,
V =1985.7(6) A, 3 and Z=2; compound 2, space group I2/a, a= 19.4674(8), b= 14.390(1), c= 24.219(2) A, , b= 92.954(7)°,
V= 6775.4(7) A, 3 and Z= 2; compound 3, space group P21/c, a= 12.0853(8), b= 18.679(2), c= 16.709(2), b = 109.98(1)°,
V =3544.9(6) A, 3 and Z=4. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Crystal structures; Vanadium complexes; Tridentate ligand complexes; Dimetallic complexes
1. Introduction
Over a period of more than 30 years, M2L24 dinuclear
compounds have been made having four bidentate
bridging ligands, L2, that support the M2 units. The
MM bond orders range from zero to four. A typical
core is shown schematically in I [1 – 4].
ligands L3 [5]. Examples are the linear trinuclear complexes such as M3(dpa)4X2, where M =Cr [5b,5e], Co
[5a,5c,6,7,8], Ni [5h], Cu [9], Ru [10], Rh [10] dpa=the
anion of 2,2%-(dipyridyl)amine, and X is typically a
mononegative anion. Not infrequently, instead of the
desired M3L34 product, we have obtained M2L34 [11]. In
some cases these products have had the structure schematically represented as II, in which the third donor
atom on each L3 ligand has been left dangling, while an
M2 unit of the usual type for the metal concerned is
formed.
More recently, our interest has turned to molecules
with linear M3 units supported by tridentate bridging
1
2
*Corresponding author.
*Corresponding author.
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(00)00115-8
70
F.A. Cotton et al. / Inorganica Chimica Acta 305 (2000) 69–74
An example of structure II is provided by Cr2(dpa)4
[5b]. In this case, it was possible to introduce an
additional metal atom and attain the original synthetic
objective, namely, Cr3(dpa)4Cl2, which has a core structure of type III.
2. Experimental
All syntheses and sample manipulations were carried
out under an atmosphere of nitrogen using standard
Schlenk and glove box techniques. The complex
VCl3(THF)3 was prepared as previously reported [15].
Methyllithium (1.0 M in THF) and sodium triethylborohydride (1.0 M in THF) were obtained from
Aldrich and used as received. IR spectra were recorded
from KBr pellets on a Perkin–Elmer 16 PC FT-IR
spectrometer.
2.1. Preparation of V2(dpa)4 ·THF (1)
Similarly, the compound Cr2(DPhIP)4 (DPhIP is the
anion of 2,6-di(phenylimino)piperidine) was made with
a type II structure [11], and was converted to
[Cr3(DPhIP)4Cl]Cl [12] (type III). Most recently, we
reported the synthesis of another type II structure,
Mo2(DPhIP)4 [13]. In the latter, the introduction of the
third Mo atom has not yet been achieved due to the
lack of a suitable mononuclear Mo2 + source. However,
two Cu(I) atoms were introduced into the two type II
molecules Cr2(DPhIP)4 and Mo2(DPhIP)4, to give heteronuclear chains of four metal atoms [13]. In addition,
the compound Mo2(DPhIP)4 was found to undergo two
one-electron oxidations to form a bioctahedral complex
(type IV) [14]. In the resulting Mo(III)···Mo(III) unit, a
triple bond of the type s2p4 (by oxidation of the s2p4d2
unit in Mo2(DPhIP)4) does not form because of the
presence of four additional MoN bonds.
The complex VCl3(THF)3 (0.38 g, 2.0 mmol) was
dispersed in 10 ml of THF, and cooled in a dry
ice/acetone bath. Sodium triethylborohydride (1.0 M in
THF, 1.0 ml) was added to the stirring suspension, and
a violet solution of ‘VCl2·nTHF’ was obtained. Meanwhile, 2,2%-dipyridylamine (Hdpa, 0.34 g, 2.0 mmol)
was dissolved in 10 ml of toluene and deprotonated
with MeLi (1.0 M in THF) at − 78°C. The resulting
milky suspension was transferred to the VCl2 solution.
The dark purple mixture was refluxed for 3 h. A pale
colored solid (presumably LiCl) precipitated on the
walls of the flask. The solid was filtered off, and the
filtrate was layered with hexanes. Shiny purple crystals
of 1 grew overnight. Yield: 0.17 g (43%). IR (KBr
pellet, cm − 1): 1630 (m), 1584 (s), 1525 (w), 1476 (vs),
1461 (vs), 1433 (vs), 1356 (m), 1312 (w), 1263 (m), 1237
(w), 1152 (m), 1100 (w), 1049 (m), 1009 (m), 915 (w),
879 (w), 842 (w), 769 (m), 736 (w), 674 (w), 642 (w), 534
(w), 437 (w).
2.2. Preparation of [V2(dpa)3(m-Cl)2Li2(THF)6][BEt3H]
(2)
In this report we present several examples of attempts to make compounds of type II which have led to
compounds with structures of type IV or V.
A suspension of VCl3(THF)3 (0.75 g, 2.0 mmol) in 10
ml of THF was reduced with excess NaEt3BH (1.0 M in
THF, 3.0 ml) at −78°C. The resulting solution was
added to a cold suspension of Lidpa (3.0 mmol) in 10
ml of THF. The mixture was allowed to warm in an ice
bath, and stirred at 0°C overnight. A dark purple
crystalline solid of 1 was filtered off, and the filtrate was
layered with hexanes, giving plate-shaped crystals of 2.
Yield: 0.40 g (31%).
2.3. Preparation of V2(dpa)3Cl2 ·2CH2Cl2 (3)
Compound 2 (0.25 g, 0.20 mmol) was dissolved in
CH2Cl2 (10 ml), and a violet solution was obtained. It
was layered with hexanes (25 ml). After one week violet
prisms of 3 appeared on the walls of a Schlenk tube and
a pale-colored solid, presumably LiCl, formed at the
bottom of the tube. The yield, based on 2, was quantitative. IR (KBr pellet, cm − 1): 1685 (w), 1653 (s), 1637
F.A. Cotton et al. / Inorganica Chimica Acta 305 (2000) 69–74
(s), 1584 (s), 1561 (m), 1545 (w), 1527 (m), 1508 (w),
1474 (vs), 1437 (m), 1420 (m), 1384 (w), 1262 (s), 1236
(m), 1160 (m), 1097 (vs), 1021 (s), 971 (w), 905 (w), 870
(w), 802 (vs), 774 (s), 669 (w), 649 (w), 613 (w), 534 (w),
477 (w), 442 (w).
2.4. Crystallographic studies
All data were collected on a Nonius Fast area detector diffractometer with each crystal mounted on the tip
Table 1
Crystal and structure refinement data
Complex
1
Empirical
formula
Formula weight
Space group
a (A, )
b (A, )
c (A, )
a(°)
b (°)
g (°)
V (A, 3)
Z
T (K)
Radiation (A, )
Dcalc (g cm−3)
m(Mo Ka)
(cm−1)
* R1 a/R1 b
**wR2 a/wR2 b
C44H40N12OV C60H88BCl2Li2N9
O6V2
2
862.74
1228.86
P2/n
I2/a
13.102(2)
19.4674(8)
9.294(2)
14.390(1)
16.510(4)
24.219(2)
90
90
98.98(2)
92.954(7)
90
90
1985.7(6)
6775.4(7)
2
4
213(2)
213(2)
0.71073
0.71073
1.205
1.443
5.25
4.06
853.21
P21/c
12.0853(8)
18.679(2)
16.709(2)
90
109.98(1)
90
3544.9(6)
4
213(2)
0.71073
1.591
10.19
0.052/0.060
0.126/0.140
0.069/0.082
0.171/0.185
2
3
0.080/0.094
0.204/0.228
C32H28Cl6N9V2
* R1 = S(Fo−Fc)/SFo.
** wR2 = {S[w(F o2−F c2)2]/S[w(F o2)2]}0.5; w=1/[s 2(F o2)+(aP)2+
bP], P = [max(F o2 or 0)+2(F c2)]/3.
a
Denotes value of the residual considering only the reflections with
I\2s(I).
b
Denotes value of the residual considering all the reflections.
Table 2
Selected interatomic separations (A, ) and bond angles (°) for 1
Bond lengths
V···Vc1
VN(1)
VN(2)
VN(3)c1
VN(4)
i
3.038(2)
2.177(4)
2.186(4)
2.138(4)
2.148(4)
Bond angles
N(3)c1VN(6)c 1 178.6(2)
N(3) c 1VN(4)
94.6(2)
N(6)c1VN(4)
86.5(2)
N(3) c 1VN(1)
84.0(2)
N(6)c 1VN(1)
96.8(2)
N(4)VN(1)
99.5(2)
N(3) c 1VN(2)
91.8(4)
N(6) c 1VN(2)
87.6(2)
VN(5)
VN(6)c 1
Vc 1N(3)
V c 1N(6)
2.205(4)
2.142(4)
2.138(4)
2.142(4)
71
of a glass fiber under a stream of nitrogen at −60°C.
Cell parameters were obtained by least-squares refinement of 250 reflections ranging in 2u from 15 to 41°.
Laue groups and centering conditions were confirmed
by axial images. Data were collected using 0.2° intervals
in f for the range 0B fB 220° and 0.2° intervals in v
for two different regions in the range 0B v B72°. In
this way, nearly a full sphere of data was collected. The
highly redundant data sets were corrected for Lorentz
and polarization effects.
The positions of the vanadium atoms and nitrogen
atoms were determined by direct methods, and refined
by using the program SHELXL-93. All nonhydrogen
atoms were found by successive iterations of leastsquares refinement followed by Fourier syntheses and,
during the final cycles, were refined anisotropically.
Hydrogen atoms were placed in idealized positions in 1
and 2, and displacement parameters were set at 1.2
times that of the attached atom. Hydrogen atoms in 3
were fully refined.
In compound 1, two of the four carbon atoms in
each interstitial THF molecule were disordered and the
THF was modeled as having two orientations; the
occupancy of each orientation was optimized. In compound 2, three THF molecules coordinated to a lithium
atom were also found to be disordered. They were
modeled as having two orientations and the occupancy
of each atom was refined as well. The BEt3H− anion in
2 was also disordered; one of the three ethyl groups has
two equally occupied orientations.
Crystallographic data for 1, 2, and 3 are given in
Table 1. Selected bond distances and angles for 1 are
given in Table 2, and those for compounds 2 and 3 are
found in Table 3.
3. Results and discussion
3.1. Synthetic considerations
One of the most frequently used starting materials
for vanadium chemistry is VCl3(THF)3 [3,16–18] and a
common route for the preparation of a paddlewheel
complex is:
THF
VCl3(THF)3 + NaBEt3H
‘VCl2·nTHF’+NaCl
− 72°C
N(4)VN(2)
N(1)VN(2)
N(3)c 1VN(5)
N(6)c 1VN(5)
N(4)VN(5)
N(1)VN(5)
N(2)VN(5)
158.9(2)
61.2(2)
88.7(2)
90.9(2)
61.1(2)
158.7(2)
139.3(2)
i
Symmetry transformation used to generate equivalent atoms: c1
−x+1/2, y, −z+1/2.
+ 0.5H2 + BEt3(THF)
THF
2VCl2·nTHF+ 4LiL
V2L4 + 4LiCl
ambient temperature
The THF solvent in the reaction mixture is then driven
away under vacuum, and the product V2L4 can be
extracted into hot toluene while the byproduct LiCl is
eliminated by filtration.
However, when this reaction route was applied to
Lidpa, a possible tridentate ligand, compound 2,
[V2(dpa)3(m-Cl)2Li2(THF)6][BEt3H], was isolated as one
F.A. Cotton et al. / Inorganica Chimica Acta 305 (2000) 69–74
72
Table 3
Selected interatomic separations (A, ) and bond angles (°) for 2 and 3
Compound 2
Bond lengths
i
V(1)···V(1)c 1
V(1)N(1)
V(1)N(2)
V(1)N(3)c 1
3.024(2)
2.140(4)
2.203(4)
2.134(4)
V(1)N(4)
V(1)N(5)c 1
V(1)Cl(1)
VN (average)
2.165(4)
2.145(4)
2.485(2)
2.157(4)
Bond angles
N(1)V(1)N(2)
C(1)N(2)C(1)c 1
C(1)N(2)V(1)c1
C(1) c1N(2)V(1)c 1
61.7(2)
122.6(6)
131.9(3)
91.8(2)
C(1)N(2)V(1)
C(1)c 1N(2)V(1)
V(1)c 1N(2)V(1)
91.8(2)
131.8(3)
86.7(2)
Compound 3
Bond lengths
V(1)···V(2)
V(1)N(1)
V(1)N(2)
V(1)N(8)
V(1)N(6)
V(1)N(7)
V(1)Cl(1)
V(1)N (average)
3.091(2)
2.154(5)
2.126(6)
2.119(6)
2.147(5)
2.062(6)
2.387(2)
2.122(6)
V(2)N(3)
V(2)N(2)
V(2)N(4)
V(2)N(9)
V(2)N(5)
V(2)Cl(2)
V(2)N (average)
2.142(6)
2.136(5)
2.135(6)
2.158(5)
2.086(6)
2.416(2)
2.131(6)
Bond angles
N(2)V(1)N(1)
C(5)N(2)C(6)
C(5)N(2)V(2)
C(5)N(2)V(1)
62.8(2)
122.1(6)
127.7(4)
92.9(4)
C(6)N(2)V(2)
C(6)N(2)V(1)
V(1)N(2)V(2)
N(2)V(2)N(3)
92.9(4)
128.3(4)
93.0(2)
62.9(2)
i
Symmetry transformation used to generate equivalent atoms:
c1 −x+1/2, y, −z+1.
In order to prepare compound 1, the reaction temperature was raised to reflux temperature. Unlike
V2(DTolF)4, which is very soluble in hot toluene, compound 1 is only slightly soluble in toluene. The separation of 1 and LiCl cannot be achieved efficiently by the
conventional toluene extraction. Therefore THF/
toluene (1:1 volume) was used. The byproduct LiCl was
found to precipitate, and the separation of 1 was
achieved by a single filtration. Both compounds 1 and 2
are extremely sensitive to air. When a purple solution
of either 1 or 2 in THF was exposed to air, the color
faded away immediately, and a pale yellow solution
was obtained.
Compound 3 was first obtained serendipitously when
CH2Cl2 was used to extract a product thought to
contain V3(dpa)4Cl2 (type III). It was found later that
the reaction of 2 and CH2Cl2 is essentially quantitative,
which is not surprising if one considers the fact that
V(II) is a stronger reductant than Cr(II), and a redox
reaction between Cr(II) and CH2Cl2 is well documented
[19].
As mentioned in the introduction, Cr2(dpa)4 can be
readily converted to Cr3(dpa)4Cl2 by using excess
amount of CrCl2 at reflux temperature. A similar reaction between 1 and VCl2 was tried both at reflux
temperature and higher (in a pressure reactor), but so
far the goal of synthesizing a type III complex of
vanadium has not been accomplished.
3.2. Structural considerations
Fig. 1. A drawing of the molecular structure of V2(dpa)4 in 1.
Ellipsoids are shown at the 50% probability level; hydrogen atoms
have been omitted for clarity.
of the products. The yield and purity of 2 can be
increased by using an excess of NaBEt3H and by performing the reaction at 0°C.
A drawing of the molecular structure of 1 is shown in
Fig. 1. It crystallized in the P2/n space group with the
V2(dpa)4 molecule sitting on a two-fold axis. Each dpa
ligand uses two of its three nitrogen-donor atoms to
chelate to one vanadium atom while the third coordinates to the other vanadium atom. The position of the
third nitrogen atom alternates around the V···V vector
giving an idealized S4 type arrangement, if the spiral
conformation of the dpa ligands is ignored. This conformation is forced by the repulsion between two
closely arranged m-hydrogen atoms [5f]. The V···V
separation of 3.038(2) A, indicates there is no bonding
interaction between the two metal atoms (in the triply
bonded V2(DTolF)4 [3] the VV separation is 1.978(2)
A, ).
The structure of V2(dpa)4 can be compared to that of
Cr2(dpa)4 [5b] where a CrCr distance of 1.943(2) was
found. In compound 1, the formation of four additional VN bonds provides energetic compensation to
avoid the formation of a VV triple bond, whereas in
the chromium counterpart, retention of the CrCr bond
is favored, since breaking a CrCr quadruple bond
requires more energy. It is noteworthy that reaction of
F.A. Cotton et al. / Inorganica Chimica Acta 305 (2000) 69–74
V2(DTolF)4 with pyridine leads to the cleavage of the
VV bond producing two trans-V(DTolF)2(py)2
molecules per dinuclear molecule [20]. Here again the
formation of four additional bonds seems to be the
driving force behind the cleavage. This same process
was rediscovered later on by Hao et al. [18]. More
recently, it has been found that Cr2(DPhIP)4 can be
oxidized to [Cr2(DPhIP)4]2 + [21] with concomitant
cleavage of the CrCr bond leading to a type IV
structure, similar to that of 1. In this case, if the CrCr
bond were to be conserved after two one-electron oxidations, the bond order would have been 3. It is also
worth mentioning that in compounds Mo2(DPhIP)4
and [Mo2(DPhIP)4](PF6), which have MoMo bond
orders of 4 and 3.5 respectively, the MoMo bonds
have been conserved, whereas in [Mo2(DPhIP)4](BF4)2,
which could potentially have a MoMo bond order of
3, the MoMo bond was overwhelmed by the formation of four additional MoN bonds, thus leading to
another type IV structure [14].
The structure of the cation in 2 is given in Fig. 2.
This ionic compound crystallized in the I2/a space
group with both the cation and the anion sitting on
two-fold axes. The major difference between 1 and 2 is
that in 2 there are only three dpa ligands, and in 2 each
V atom also binds to one Cl anion, which is shared by
a lithium atom coordinated by three THF molecules.
The vanadium-containing fragment of 2 can be represented schematically by V, also a bioctahedral type
structure similar to IV. Two of the three dpa ligands in
2 coordinate in a way similar to that of a dpa ligand in
1, but the third dpa ligand adopts a novel coordination
mode. As can be seen in either V or Fig. 2, the middle
N atom of the dpa ligand coordinates to both V atoms
73
giving a doubly chelating and doubly bridging coordination mode. The bridging nitrogen atom (N(2) in Fig.
2) is four-coordinate and has a distorted tetrahedral
environment; the VN(2)V c 1 angle is 86.7(2)° (Table
3). Since the two V atoms in compound 2 have a d 3
configuration, as do the V atoms in 1, and six donor
atoms (five N atoms and one Cl atom) are available for
each V atom, it is not surprising that a bioctahedral
complex was obtained. The V···V separation of 3.024(2)
A, is almost the same as in compound 1.
Compound 3 is the oxidation product of 2. Its structure was solved in the space group P21/c; all atoms are
in general positions. A drawing of the molecular structure is shown in Fig. 3. It has the same framework as
the core in 2; the main difference is that now we are
dealing with a V(II) and V(III) mixed valence situation.
Theoretically a VV bond order of 2.5 could be formed
in 3, but in fact a typical bioctahedral complex with a
V···V separation of 3.091(2) A, was formed.
It is interesting to compare the distances and angles
in 2 and 3. With an increased positive charge on the
two metal atoms, the average bond distances in 3 are
shorter, as expected (Table 3), and the VN(2)V angle
is bigger. However, the bond lengths for V(1) and V(2)
are statistically similar; both are different from those in
2, implying that the increased charge is probably delocalized between the two V atoms. This is in agreement
with the doubly-chelating/bridging coordination mode
adopted in 3, which would be expected to make the
communication between the two V atoms easier.
In conclusion, a novel doubly-chelating/bridging coordination mode for the dpa ligand has been found for
the first time. All the bioctahedral complexes made in
this work, together with our previously reported di-
Fig. 2. A drawing of the structure of [V2(dpa)3(m-Cl)2Li2(THF)6]+ cation in 2. Ellipsoids are drawn at the 50% probability level; carbon and
oxygen atoms are shown at arbitrary scales. Hydrogen atoms are omitted for clarity.
74
F.A. Cotton et al. / Inorganica Chimica Acta 305 (2000) 69–74
References
Fig. 3. The molecular structure of 3. Ellipsoids are shown at the 50%
probability level; hydrogen atoms are omitted for clarity.
molybdenum bioctahedral complex [14], suggest that
when the nominal bond order of a dimetal complex is
three or less, the formation of four metalligand bonds,
in addition to the bonds that are necessary to produce
a paddlewheel (type I) frame, is possible. It appears
that formation of bioctahedral complexes is energetically favored over metalmetal bonding.
4. Supplementary material
Tables of crystallographic data including diffractometer and refinement data, atomic coordinates, bond
lengths, bond angles, and anisotripic displacement
parameters are available from the authors (F.A.C. and
C.A.M.) upon request.
Acknowledgements
We are grateful to the National Science Foundation
for the financial support of this work.
.
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