Di- and trinuclear complexes with the mono- and dianion of 2,6-bis(phenylamino)pyridine: High-field displacement of chemical shifts due to the magnetic anisotropy of quadruple bonds

Article abstract of DOI:10.1021/ic001376g

The monoanion of 2,6-bis(phenylamino)pyridine (HBPAP^-) has been found to support quadruply bonded Cr₂⁴⁺ and Mo₂⁴⁺ units in Cr₂(HBPAP)₄ (1) and Mo₂(HBPAP)₄ (2). The corresponding dianion BPAP^2- was able to stabilize the trinuclear complexes, (TBA)₂Cr₃(BPAP)₄ (3) and (TBA)₂Ni₃(BPAP)₄ (4), where TBA is the tetrabutylammonium cation. The dinuclear complexes have the typical paddlewheel configuration with Cr-Cr distances of about 1.87 A and a Mo-Mo distance of 2.0813(5) A and exhibit a high-field displacement of the corresponding N-H signals caused by the magnetic anisotropy of the quadruple bonds. For the trinuclear complexes, 3 has a linear chain of three chromium atoms arranged in an unsymmetrical fashion with two chromium atoms paired to give a quadruply bonded unit (Cr-Cr distance: 1.904(3) A and an isolated, square planar Cr^II unit at 2.589(3) A from the dimetal unit. On the other hand, the three nickel atoms in 4 are evenly spaced, having Ni···Ni distances of 2.3682(8) A. The trinuclear compounds show a twisted conformation with an overall torsion angle of about 30°.

Full text of DOI:10.1021/ic001376g

2778
Inorg. Chem. 2001, 40, 2778-2784
Di- and Trinuclear Complexes with the Mono- and Dianion of
2,6-Bis(phenylamino)pyridine: High-Field Displacement of Chemical Shifts Due to the
Magnetic Anisotropy of Quadruple Bonds
F. Albert Cotton,*,† Lee M. Daniels,† Peng Lei,† Carlos A. Murillo,*,†,‡ and Xiaoping Wang†
Department of Chemistry and Laboratory for Molecular Structure and Bonding, P.O. Box 30012,
Texas A&M University, College Station, Texas 77842-3012, and Department of Chemistry,
University of Costa Rica, Ciudad Universitaria, Costa Rica
ReceiVed December 6, 2000
The monoanion of 2,6-bis(phenylamino)pyridine (HBPAP-) has been found to support quadruply bonded Cr2
4+
4+
and Mo2 units in Cr2(HBPAP)4 (1) and Mo2(HBPAP)4 (2). The corresponding dianion BPAP2- was able to
stabilize the trinuclear complexes, (TBA)2Cr3(BPAP)4 (3) and (TBA)2Ni3(BPAP)4 (4), where TBA is the
tetrabutylammonium cation. The dinuclear complexes have the typical paddlewheel configuration with Cr-Cr
distances of about 1.87 Å and a Mo-Mo distance of 2.0813(5) Å and exhibit a high-field displacement of the
corresponding N-H signals caused by the magnetic anisotropy of the quadruple bonds. For the trinuclear complexes,
3 has a linear chain of three chromium atoms arranged in an unsymmetrical fashion with two chromium atoms
paired to give a quadruply bonded unit (Cr-Cr distance: 1.904(3) Å) and an isolated, square planar CrII unit at
2.589(3) Å from the dimetal unit. On the other hand, the three nickel atoms in 4 are evenly spaced, having
Ni‚‚‚Ni distances of 2.3682(8) Å. The trinuclear compounds show a twisted conformation with an overall torsion
angle of about 30°.
Introduction
unsymmetrical chains can occur1,4 and interpretation of the
magnetic behavior of certain complexes such as Co3(dpa)4XY
Growing attention has been given to the preparation and study
of compounds having linear chains of metal atoms surrounded
by four ligands, both in our laboratory1 and elsewhere.2 So far,
linear chains with up to nine atoms have been reported.3 Studies
from our group on the trinuclear dpa complexes,1,4 where dpa
stands for the anion of di(2-pyridyl)amine, have shown that the
behavior of the trinuclear systems is far more complicated than
one might have expected. Specifically, both symmetrical and
has been a challenging task;1,4 here X and Y are mononuclear
- 1h
anions such as BF4-,5 CN-,1h N(CN)2
,
Cl-,1e Br-,1g and
SCN-.1h We believe that only after a better understanding of
the simplest trinuclear systems is obtained could we hope to
make progress in the preparation of molecular wires, an
important goal of the study of linear metal chain systems.
In all known M3(dpa)4X2 compounds (M ) Cr,1d Co,1e Ni,1f,2e
Cu,2e Ru,2d and Rh2d), the dpa anion is not planar because of
the repulsion of two hydrogen atoms on different, but adjacent
pyridine rings shown in Scheme 1a. This leads a considerable
torsion angle and a helical arrangement of the four dpa ligands
around the trimetal unit as shown in Scheme 2. The configu-
ration can be of either right- or left-handed helicity; chiral
crystals containing only one of the two configurations have been
isolated.6 To avoid the H‚‚‚H repulsion and thus obtain a flat
ligand that will not demand a helical conformation in the
product, another tridentate ligand having free rotation of two
terminal aromatic rings was considered desirable. We have
prepared one such ligand, namely, 2,6-bis(phenylamino)pyridine,
H2BPAP, shown in Scheme 1b. Moreover, since a recent study
in our group had shown that electron-donating atoms in
appended groups near the axial positions of a dichromium core
can greatly affect the Cr-Cr distance,7 we felt that preparing
dinuclear complexes with this tridentate ligand could also be
of interest. Finally, since the BPAP ligand is a dianion, the
resulting trimetal complexes, M3(BPAP)42-, must be anionic
* To whom correspondence should be addressed. E-mail: cotton@
tamu.edu; murillo@tamu.edu.
† Texas A&M University.
‡ University of Costa Rica.
(1) See, for example: (a) Cotton, F. A.; Daniels, L. M.; Murillo, C. A.;
Wang, X. Chem. Commun. 1998, 39. (b) Cle´rac, R.; Cotton, F. A.;
Daniels, L. M.; Dunbar, K. R.; Murillo, C. A.; Pascual, I. Inorg. Chem.
2000, 39, 752. (c) Cle´rac, R.; Cotton, F. A.; Dunbar, K. R.; Lu, T.;
Murillo, C. A.; Wang, X. J. Am. Chem. Soc. 2000, 122, 2272. (d)
Cotton, F. A.; Daniels, L. M.; Murillo, C. A.; Pascual, I. J. Am. Chem.
Soc. 1997, 119, 10223. (e) Cotton, F. A.; Daniels, L. M.; Jordan, G.
T., IV. Chem. Commun. 1997, 421. (f) Cle´rac, R.; Cotton, F. A.;
Dunbar, K. M.; Murillo, C. A.; Pascual, I.; Wang, X. Inorg. Chem.
1999, 38, 2655. (g) Cle´rac, R.; Cotton, F. A.; Daniels, L. M.; Dunbar,
K. M.; Murillo, C. A.; Wang, X. J. Chem. Soc., Dalton Trans. 2001,
386. (h) Cle´rac, R.; Cotton, F. A.; Jeffery, S. P.; Murillo, C. A.; Wang,
X. Inorg. Chem. 2001, 40, 1265.
(2) (a) Shieh, S. J.; Chou, C. C.; Lee, G. H.; Wang, C. C.; Peng, S. M.
Angew. Chem., Int. Ed. Engl. 1997, 36, 56. (b) Wang, C. C.; Lo, W.
C.; Chou, C. C.; Lee, G. H.; Chen, J. M.; Peng, S. M. Inorg. Chem.
1998, 37, 4059. (c) Pyrka, G. J.; El-Mekki, M.; Pinkerton, A. A. J.
Chem. Soc., Chem. Commun. 1991, 84. (d) Sheu, J. T.; Lin, C. C.;
Chao, I.; Wang, C. C.; Peng, S. M. Chem. Commun. 1996, 315. (e)
Adulchecha, S.; Hathaway, B. J. Chem. Soc., Dalton Trans. 1991,
993.
(5) Cotton, F. A.; Daniels, L. M.; Jordan IV, G. T.; Murillo, C. A. J. Am.
Chem. Soc. 1997, 119, 10377.
(6) Cle´rac, R.; Cotton, F. A.; Dunbar, K. R.; Lu, T.; Murillo, C. A.; Wang,
X. Inorg. Chem. 2000, 39, 3065.
(7) Cotton, F. A.; Daniels, L. M.; Murillo, C. A.; Pascual, I.; Zhou, H. J.
Am. Chem. Soc. 1999, 121, 6856.
(3) (a) Chen, Y. H.; Lee, C. C.; Wang, C. C.; Lee, G. H.; Lai, S. Y.; Li,
F. Y.; Mou, C. Y.; Peng, S. M. Chem. Commun. 1999, 1667. (b) Peng,
S. M.; Wang, C. C.; Jang, Y. L.; Chen, Y. H.; Li, F. Y.; Mou, C. Y.;
Leung, M. K. J. Magn. Magn. Mater. 2000, 209, 80.
(4) Cotton, F. A. Inorg. Chem. 1998, 37, 5710.
10.1021/ic001376g CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/01/2001

Magnetic Anisotropy of Quadruple Bonds
Inorganic Chemistry, Vol. 40, No. 12, 2001 2779
room temperature, the volume was reduced to ca. 7 mL and some
yellow precipitate formed. Addition of 35 mL of hexanes increased
the amount of solid. This was collected by filtration, washed with 10
mL of hexanes, and then dried under vacuum. Yield: 0.256 g (95%).
1H NMR (300 MHz, CD2Cl2, δ): 7.38 (d, 4H), 7.29 (m, 5H), 7.02 (t,
2H), 6.44 (broad, 1H), 6.31 (d, 2H).
Scheme 1
Preparation of 1‚toluene and 1‚C6H14. The compound H2DPAP
(0.522 g, 2.00 mmol) was dissolved in 15 mL of THF and then
deprotonated by adding MeLi (2.1 mmol) at -78 °C. The THF solution
was allowed to return slowly to room temperature and transferred into
a flask containing CrCl2 (0.135 g, 1.10 mmol). After the mixture was
stirred at room temperature overnight, a red orange solid formed and
was separated by filtration. This solid was found to be slightly soluble
in toluene and dichloromethane. To the solid was added 20 mL of
toluene, and the mixture was filtered. Then a layer of hexanes was
added to the toluene solution. After 2 weeks, crystals of 1‚toluene
suitable for X-ray studies formed. The filtrate from the original mixture
was dried under vacuum. The solid was washed with ether, 2 × 20
mL; it was dissolved in THF, and the solution was layered with hexanes.
Red crystals of 1‚C6H14 suitable for X-ray study were obtained after
Scheme 2
when M is a divalent ion and would not therefore be expected
to have axial anionic ligands. We hoped, therefore, to see if
the absence of axial ligands would have a significant effect on
the electronic structure of the trimetal chain.
1
9 days. The overall yield was 0.430 g (87%). H NMR (300 MHz,
CD2Cl2, δ): 7.23 (m, 12H), 7.02 (m, 12H), 6.75 (m, 12H), 5.95 (d,
4H), 6.78 (t, 12H), 3.4 (d, 4H). Anal. Calcd for C68H56N12Cr2: C, 71.32;
H, 4.92; N, 14.68. Found: C, 70.75; H, 5.11; N, 14.44. CV: one
quasireversible wave at 0.32 V in the scan range 0.00 to +0.06 V and
two irreversible waves at 0.38 and 0.86 V in the scan range 0.00 to
+1.00 V.
Preparation of 2‚toluene.. The ligand precursor H2BPAP (0.522 g,
2.00 mmol) was dissolved in 15 mL of THF and deprotonated with 1
equiv of MeLi at -78 °C. After the temperature of the solution had
slowly risen to room temperature, the solution was added to a flask
containing Mo2(O2CCF3)4 (0.32 g, 0.50 mmol). The reaction mixture
was stirred overnight, and the solvent was then removed under vacuum.
The remaining solid was washed with 2 × 20 mL of hexanes, then
dissolved in toluene, and layered with hexanes. Red crystals suitable
for X-ray study were obtained after 2 weeks. Yield: 0.320 g (78%).
1H NMR (300 MHz, CD2Cl2, δ): 7.43 (d, 16H), 7.40 (t, 4H), 7.35 (t,
16H), 7.05 (t, 8H), 6,34 (d, 8H), 3.04 (broad, 4H). Anal. Calcd for
C68H56N12Mo2: C, 66.23; H, 4.58; N, 13.63. Found: C, 66.18; H, 4.33;
N, 13.32. CV: one quasireversible wave at 0.56 V in the scan range
0.00 to +1.00 V.
Here we report the preparation and structural characterization
of four species, two dinuclear and two trinuclear. They are Cr2-
(HBPAP)4 (1), Mo2(HBPAP)4 (2), (Bun N)2Cr3(BPAP)4 (3), and
4
(Bun N)2Ni3(BPAP)4 (4).
4
Experimental Section
General Procedures. All manipulations were carried out under
nitrogen using standard Schlenk techniques. Solvents were dried by
conventional methods and were freshly distilled under nitrogen before
use. Anhydrous CrCl2 and NiCl2, as well as 2,6-dibromopyridine, were
purchased from Strem Chemicals and stored in a N2 drybox; MeLi,
Bun NBr, and aniline hydrochloride were purchased from Aldrich and
4
used as received; Mo2(O2CCF3)4 was prepared as reported earlier.8 The
1H NMR spectra were recorded on a Varian XL-300 instrument at 300
MHz. Cyclic voltammetry was performed in dichloromethane for
compound 1 and in THF for compound 3 with a BAS model 100
scanning potentiostat using Pt working and auxiliary electrodes and
0.1 M TBAH (Bun NPF6) as the supporting electrolyte. Potentials are
Preparation of 3‚2THF. The compound H2DPAP (0.267 g, 1.00
mmol) was dissolved in 15 mL of THF and deprotonated at -78 °C
by the addition of 2 equiv of MeLi (1.5 mL of 1.4 M MeLi in ether.)
After the THF solution had slowly warmed to room temperature, it
was transferred to a flask containing a mixture of CrCl2 (0.10 g, 0.85
mmol) and tetrabutylammonium bromide (0.16 g, 0.50 mmol). The
reaction mixture was stirred for 2 h at room temperature and then
refluxed overnight. After cooling, the solvent was removed under
vacuum and the remaining solid was washed with 2 × 20 mL of ether
and 20 mL of a 10:1 ether-THF solution. The remaining solid was
redissolved in THF and layered with hexanes. Dark red crystals formed
after 6 days. Yield: 0.144 g (25%) Anal. Calcd for C100H126N14Cr3:
C, 71.40; H, 7.67; N, 11.66. Found: C, 71.81; H, 7.62; N, 11.46. CV:
one quasireversible wave at 0.03 V with scanning between -0.50 and
0.40 V, which is likely to correspond to oxidation of the separated CrII
atom (see structure) to CrIII. In the extended scan range -0.5 to 1.4 V,
three irreversible peaks were seen at +0.03, 0.69, and 1.08 V.
Preparation of 4‚2THF. The compound H2DPAP (0.267 g, 1.00
mmol) was dissolved in 15 mL of THF and deprotonated at -78 °C
with 2 equivalents of MeLi (1.5 mL of 1.4 M MeLi in ether.) After
the THF solution had slowly warmed to room temperature, it was
transferred into a flask which contained NiCl2 (0.11 g, 0.85 mmol)
and tetrabutylammonium bromide (0.16 g, 0.50 mmol). The reaction
mixture was stirred for 2 h at room temperature and then refluxed
overnight. After cooling, the solvent was removed under vacuum. The
dark red solid was washed with 2 × 20 mL of ether and 20 mL of a
10:1 ether-THF solution and then dissolved in THF and layered with
hexanes. Dark-red crystals were obtained after 5 days. Yield: 0.268 g
4
referenced to the ferrocene/ferrocenium (Fc/Fc+) couple, which occurs
at E1/2 ) +0.44 V versus Ag/AgCl. The values of E1/2 were taken as
(Epa + Epc)/2, where Epa and Epc are the anodic and cathodic peak
potentials, respectively, for the quasireversible peaks. Elemental
analyses were performed by Canadian Microanalytical Service Ltd.
Preparation of 2,6-Bis(phenylamino)pyridine. This was prepared
according to the literature9 with some modifications: 2,6-dibromopy-
ridine (12.5 g, 50.0 mmol) and aniline hydrochloride (23.5 g, 250 mmol)
were added to a 500 mL three-neck flask fitted with a stirring bar and
a condenser. The mixture was heated to 200 °C. As the mixture melted,
it turned dark orange. After 4 h of stirring under N2, 100 mL of aqueous
10% sodium carbonate solution was added to the mixture to neutralize
the acid. Chloroform (2 × 150 mL) was added to extract the product
from the aqueous solution. The volume was reduced to ca. 40 mL under
reduced pressure, and then 100 mL of ethanol was added. Further
addition of a large amount of water into the EtOH/CHCl3 solution
precipitated a yellow product, which was recrystallized twice from
water-ethanol and then from an ethyl acetate-hexanes mixture.
Yield: 16.5 g (65%, based on 2,6-dibromopyridine). Anal. Calcd for
C17N3H15: C, 78.16; H, 5.75; N, 16.09. Found: C, 78.11; H, 5.70; N,
1
16.02. H NMR (300 MHz, CD2Cl2, δ): 8.04 (broad, 2H), 7.63 (d,
4H), 7.45 (t, 1H), 7.20 (t, 4H), 6.88 (t, 2H), 6.28 (d, 2H).
Preparation of Li(HBPAP). Methyllithium (1.05 mmol) was added
to a 15 mL THF solution containing H2BPAP (0.267 g, 1.00 mmol) at
-78 °C. After the THF solution was allowed to warm up slowly to
(8) Cotton, F. A.; Norman, J. G., Jr. J. Coord. Chem. 1971, 1, 161.
(9) Klinga, M.; Polama M.; Leskela, M. Acta Crystallogr. 1994, C50,
2051.
1
(56%). H NMR (300 MHz, CD3CN, δ): 8.54 (d, 16H), 8.39 (t, 4H),
8.26 (t, 16), 7.96 (t, 8H), 7.24 (d, 8H), 2.92 (t, 16H), 2.55(q, 16H),

2780 Inorganic Chemistry, Vol. 40, No. 12, 2001
Table 1. Crystal Data and Structure Refinement
Cotton et al.
1‚toluene
1‚C6H14
2‚toluene
3‚2THF
4‚2THF
empirical formula
fw
space group
a, Å
b, Å
C75H64Cr2N12
1237.38
P21/n
10.460(1)
32.32(4)
18.798(1)
105.94(2)
6111(7)
4
C74H70Cr2N12
1231.42
C2/c
14.521(5)
25.003(5)
17.664(4)
108.03(3)
6099(3)
4
C75H64Mo2N12
1325.26
P21/n
10.601(2)
32.432(2)
18.809(4)
106.149(9)
6212(2)
C108H140Cr3N14O2
1822.34
C2/c
31.483(6)
14.080(2)
26.842(4)
123.385(3)
9935(3)
C108H140N14Ni3O2
1842.47
C2/c
31.729(2)
13.7505(8
27.167(2)
125.104(1)
9696(1)
c, Å
â, deg
V, (Å3)
Z
4
4
4
T, K
213(2)
1.345
0.061/0.078
0.136/0.155
213(2)
1.341
0.080/0.110
0.203/0.243
213(2)
1.417
0.048/0.060
0.120/0.138
213(2)
1.218
0.076/0.109
0.187/0.216
213(2)
1.262
0.112/0.174
0.239/0.279
F
calc, g/cm-3
R1a,c/R1a,d
wR2b,c/wR2b,d
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑[w(Fo2 - Fc2)2]/∑[w(Fo )2]]1/2; w ) 1/[2(Fo ) + (aP)2 + bP], P ) [max(Fo2 or 0) + 2(Fc2)]/3. c Denotes
2
2
the value of the residual considering only the reflections with I > 2σ(I). d Denotes the value of the residual considering all the reflections.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
2.36 (m, 16H), 2.01 (t, 24H). Anal. Calcd for C100H128N14Ni3: C, 70.56;
H, 7.58; N, 11.52. Found: C, 70.20; H, 7.68; N,11.86. CV: no
Cr2(HDPAP)4·toluenea
Cr(1)-Cr(2)
Cr(1)-N(11)
Cr(1)-N(9)
Cr(2)-N(8)
Cr(2)-N(1)
Cr(1)‚‚‚N(12)
Cr(2)‚‚‚N(4)
1.873(2)
2.043(4)
2.065(4)
2.045(4)
2.088(4)
3.136(4)
3.110(4)
Cr(1)‚‚‚N(7)
Cr(1)-N(6)
Cr(2)-N(5)
Cr(2)-N(10)
Cr(1)-N(2)
Cr(1)‚‚‚N(3)
3.148(5)
2.063(3)
2.039(3)
2.071(4)
2.027(4)
3.066(5)
reversible or quasireversible waves between +0.1 and +1.2 V.
X-ray Crystallography. Crystals of 1‚toluene, 1‚C6H14, and 2‚
toluene were used for data collection at 213 K on a Nonius FAST area
detector system, utilizing the software MADNES.10 Each crystal was
mounted on a quartz fiber with a small amount of silicone grease and
transferred to a goniometer head. Cell parameters for FAST data were
obtained from an autoindexing routine and were refined using 250
strong reflections. Cell dimensions and Laue symmetry were confirmed
by axial photography. All data were corrected for Lorentz and
polarization effects.
Data for 3‚2THF and 4‚2THF were collected on a Bruker SMART
1000 CCD area detector system. Cell parameters were obtained using
SMART11 software. Data were corrected for Lorentz and polarization
effects using the program SAINTPLUS.12 Absorption corrections were
applied using SADABS.13
Cr(2)-Cr(1)-N(2)
N(2)-Cr(1)-N(11) 161.4(1) Cr(2)-Cr(1)-N(6)
99.2(1) Cr(2)-Cr(1)-N(11)
99.3(1)
92.8(1)
89.1(1)
87.1(1)
174.2(1)
98.6(1)
92.2(1)
89.8(1)
89.1(1)
175.6(1)
124.1(3)
N(2)-Cr(1)-N(6)
Cr(2)-Cr(1)-N(9)
N(11)-Cr(1)-N(9)
Cr(1)-Cr(2)-N(5)
N(5)-Cr(2)-N(8)
N(5)-Cr(2)-N(10)
Cr(1)-Cr(2)-N(1)
N(8)-Cr(2)-N(1)
C(7)-N(1)-Cr(2)
90.4(1) N(11)-Cr(1)-N(6)
92.8(1) N(2)-Cr(1)-N(9)
91.6(1) N(6)-Cr(1)-N(9)
98.7(1) Cr(1)-Cr(2)-N(8)
162.8(1) Cr(1)-Cr(2)-N(10)
89.1(1) N(8)-Cr(2)-N(10)
92.1(1) N(5)-Cr(2)-N(1)
90.7(1) N(10)-Cr(2)-N(1)
118.9(3) C(1)-N(1)-Cr(2)
The positions of the heavy atoms were found by the direct methods
program in SHELXS-97.14 Subsequent cycles of least-squares refine-
ment followed by difference Fourier syntheses revealed the positions
of the non-hydrogen atoms. All aromatic hydrogen atoms were added
in idealized positions. The hydrogen atoms attached to N were refined
for Cr2(HDPAP)4 and Mo2(HDPAP)4.
a The Cr-Cr distances are 1.875(2) in 1‚C6H14. All other interatomic
distances are also similar to those for 1‚toluene.
satisfactory. The ligand is fairly soluble in organic solvents,
such as ether and hexane.
Results and Discussion
The compound H2BPAP has two N-H groups, each of which
can be sequentially deprotonated by methyllithium to give the
corresponding mono- and dianion. The second Ka is very small,
and weaker deprotonating agents such as K+OBu- will not
eliminate the second proton. For example, when 2 equiv of
K+OBu- was added to 1 equiv of H2BPAP along with CrCl2
and TBA+ in THF, under reflux conditions, the red orange
dinuclear compound 1 was the only isolated product. The
formation of the trinuclear chromium compound 3 required the
use of MeLi; 3 is extremely sensitive to moisture, and it is
slowly decomposed by CH2Cl2. Single crystals containing the
trinuclear anion were obtained only after the addition of the
large tetrabutylammonium counterion.
Structures of the Dinuclear Compounds. Two crystal forms
of compound 1, namely, 1‚toluene and 1‚C6H14, were obtained
by slow diffusion of hexanes into the corresponding solutions
of 1 in toluene and THF. Crystal data and structure refinement
parameters are listed in Table 1, and some important bond
distances and angles are shown in Table 2. The idealized D2d
core structure in the two crystals is essentially the same, having
a metal-metal bonded dichromium unit with each chromium
atom surrounded by nitrogen atoms. If the phenyl groups are
neglected, the structure is essentially eclipsed as shown in
Figures 1 and 2. The Cr-Cr distances of 1.873(2) Å in 1‚toluene
and 1.875(2) Å in 1‚C6H14 are similar to that, 1.870(3) Å, in
Synthetic Considerations. The ligand precursor, H2BPAP,
can be made easily according to
2,6-dibromopyridine + 2PhNH2‚HCl f
2,6-bis(phenylamino)pyridine + 2HCl + 2HBr
Aniline hydrochloride must be present in great excess in order
to obtain a satisfactory yield because it readily sublimes during
the reaction. The analytically pure compound can be obtained
by recrystallization of the crude product from an ethanol-water
mixture. An additional recrystallization from a mixture of ethyl
acetate and hexanes is necessary to obtain anhydrous crystals.
1
Elemental analysis and H NMR indicated that the purity was
(10) Pflugrath, J.; Messerschmitt, A. MADNES, Munich Area Detecor (New
EEC) System, Version EEC 11/1/89, with enhancement by Enraf-
Nonius Corp., Delft, The Netherlands. A description of MADNES
appears: Messerschmitt, A.; Pflugrath, J. J. Appl. Crystallogr. 1987,
20, 306.
(11) SMART V5.05 Software for the CCD Detector System; Bruker
Analytical X-ray Systems, Inc.: Madison, WI, 1998.
(12) SAINTPLUS V5.05 Software for the CCD Detector System; Bruker
Analytical X-ray Systems, Inc.: Madison, WI, 1998.
(13) SADABS. Program for absorption correction using SMART CCD based
on the method of Blessing: Blessing, R. H. Acta Crystallogr. 1995,
A51, 33.
(14) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467.

Magnetic Anisotropy of Quadruple Bonds
Inorganic Chemistry, Vol. 40, No. 12, 2001 2781
Figure 1. Perspective view of Cr2(HBPAP)4 in 1‚toluene. Atoms are
drawn at the 40% probability level, and aromatic hydrogen atoms are
omitted for clarity.
Figure 3. Perspective view of Mo2(HBPAP)4 in 2‚toluene. Atoms are
drawn at the 40% probability level, and aromatic hydrogen atoms are
omitted for clarity.
Table 3. Selected Bond Lengths (Å) and Angles (deg) for
Mo2(HDPAP)4·toluene
Mo(1)-Mo(2)
Mo(1)-N(5)
Mo(1)-N(11)
Mo(1)-N(1)
Mo(1)-N(7)
2.0813(5)
2.137(4)
2.148(4)
2.174(4)
2.178(4)
Mo(2)-N(10)
Mo(2)-N(8)
Mo(2)-N(2)
Mo(2)-N(4)
2.153(4)
2.137(4)
2.143(4)
2.144(4)
Mo(2)-Mo(1)-N(5)
94.1(1) N(5)-Mo(1)-H(7N)
55.1(1)
Mo(2)-Mo(1)-N(11) 94.01(9) N(11)-Mo(1)-H(7N) 117.4(1)
N(5)-Mo(1)-N(11) 171.9(2) N(1)-Mo(1)-H(7N) 105.6(1)
Mo(2)-Mo(1)-N(1)
N(5)-Mo(1)-N(1)
N(11)-Mo(1)-N(1)
Mo(2)-Mo(1)-N(7)
N(5)-Mo(1)-N(7)
N(11)-Mo(1)-N(7)
N(1)-Mo(1)-N(7)
90.8(1) N(7)-Mo(1)-H(7N)
72.0(1)
89.7(2) Mo(2)-Mo(1)-H(9N) 145.1(1)
89.8(2) N(5)-Mo(1)-H(9N) 115.4(1)
91.3(1) N(11)-Mo(1)-H(9N) 56.8(1)
88.9(2) N(1)-Mo(1)-H(9N)
91.2(2) N(7)-Mo(1)-H(9N) 106.6(1)
177.5(1) Mo(1)-Mo(2)-N(8) 94.5(1)
72.2(1)
3). The Mo-Mo and Mo-N distances and the angles in the
metal core are very similar to those found in Mo2(PhNPy)4,17
where PhNPy stands for the anion of 2-anilinopyridine.
1H NMR Studies of Cr2(HDPAP)4 and Mo2(HDPAP)4. The
1H NMR spectra showed that the purity of 1 and 2 was good.
All signals corresponding to the aromatic C-H protons were
sharp, as expected from a diamagnetic species. However, the
signals associated with the amine hydrogen atoms on each of
the pendent N atoms were slightly broader and were found at
3.46 ppm for 1 and 3.04 ppm for 2. The corresponding signal
in the neutral H2BPAP is at 8.04 ppm, and for the lithium salt
of HBPAP- it is at 6.44 ppm. This displacement is significantly
larger than that of the aromatic hydrogen atoms which change
by less than 1 ppm from the neutral molecule to the monoanionic
species to those of the metal-metal bound complexes. The small
downfield displacement, 1.6 ppm, of the signals corresponding
to the amine hydrogen atoms of the monoanionic species
compared to that of the neutral one is not surprising and can be
attibuted to the negative charge on the monoanion as well as
possible pairing of ions by formation of intermolecular hydrogen
bonds between N-H units and deprotonated nitrogen atoms in
the monoanionic species, as shown in Scheme 3. The much
greater displacement of the N-H signals of 1 and 2 toward
higher field is quite remarkable. When compared to those of
LiHBPAP, these signals are displaced by 2.98 and 3.40 ppm
upfield for 1 and 2, respectively. There are many previous
examples of relatively large shifts in the opposite direction. For
example, the methine hydrogen atoms of bridging formamidinate
groups, ArN-C(H)-NAr, in M2(DArF)4 complexes are well
Figure 2. A view down the Cr-Cr vector in 1‚toluene.
Cr2(6-Me-2-NHC6H3N)4, where the ligand is very similar to the
coordinated part of HBPAP.15 However, the Cr-Cr distance is
very short when compared with those in other dichromium
compounds with somewhat similar tridentate nitrogen ligands.
For example, the distances between chromium atoms in
Cr2(DPhIP)‚2THF (DPhIP is the anion of 2,6-di(phenylimino)-
piperidine) and Cr2(dpa)4 are 2.155(1) Å and 1.943(2) Å,
respectively.7 While the average distance of 3.114[4] Å16 from
a pendent N to a metal atom in 1 is not much greater than the
corresponding distances of 2.901[3] Å16 and 2.920[6] Å16 in
Cr2(DPhIP)4‚2THF and Cr2(dpa)4, respectively,7 there is a
significant chemical difference between these ligands. Because
of the sp3 hybridization and the proton present in the anilido
nitrogen atom in Cr2(HBPAP)4, the donor ability of the pendent
N atom from the anilido group is far inferior to that of the sp2
N atoms found at the axial positions of Cr2(DPhIP)4 and
Cr2(dpa)4. Thus the dichromium distance in Cr2(HDPAP)4 falls
into the typical range of dichromium quadruple bonds.
The dimolybdenum complex, 2‚toluene, is isomorphous to
the dichromium complex, 1‚toluene, and, as shown in Figure
3, the molecular structure is very similar. The Mo-Mo distance
of 2.0813(5) Å is typical of a quadruply bonded Mo2 unit (Table
(15) Cotton, F. A.; Niswander, R. H.; Sekutowski, J. C. Inorg. Chem. 1978,
17, 3541.
(16) Numbers within square brackets are associated with average values.
(17) Chakravarty, A. R.; Cotton, F. A.; Shamshoum, E. S. Inorg. Chem.
1984, 23, 4216.

2782 Inorganic Chemistry, Vol. 40, No. 12, 2001
Cotton et al.
Scheme 3
documented18 for compounds containing M-M multiple bonds.
These signals are found at ca. 8.53 ppm in quadruply bonded
dimolybdenum complexes and at ca. 6.28 ppm in the corre-
sponding nonmetal-metal bound dinickel complexes.18 Familiar
also are the differences of ca. 1 ppm observed for proximal
and distal protons in complexes of the type M2Cl4(PR)4,19 M2-
21
Cl4(diphos)2,20 or M2(NR2)6 where again there are metal-
metal bonds.
High- and low-field displacements share a common origin.
In a semiclassical way, the angular momentum of electrons in
the M-M multiple bond about the bond axis induces a magnetic
field which is highly anisotropic. This magnetic anisotropy has
also been found in many organic compounds and is mainly
responsible for the significantly different shifts found in benzene
and alkyne systems. For example, in acetylene the protons have
signals at δ 2.35 ppm, more shielded than those in ethylene (δ
4.60 ppm).22 In acetylene, the triple bond is along the molecular
axis. When this axis is aligned with the applied magnetic field,
the circulation of the π-electrons of the bond gives rise to an
opposing magnetic field and the NMR peak is found further
upfield than that predicted from electronegativity considerations
alone. Clearly, similar behavior must occur for triple and
quadruple bonds between metal atoms.
For metal-metal multiply bound systems, the theoretical
considerations of McConnell23 can be used to explain long-
range shielding or deshielding. This simple mathematical model
relates the chemical shift change, ∆δ, and the molar diamagnetic
anisotropy, ø. The magnitude of the magnetic anisotropy, given
by the difference between the parallel and perpendicular
components along the metal-metal bond, has been demon-
strated18 to be correlated to the chemical shift of a given
hydrogen atom by the following equation:
12πR3∆δ
1 - 3 cos2 θ
Figure 4. Shielding and deshielding caused by magnetic anisotropy
of the M-M multiple bond in dinuclear paddlewheel compounds.
Positive signs (+) indicate shielding zones, while negative signs (-)
represent deshielding zones.
∆ø ) ø| - ø )
where R is the length of the vector from the center of the bond
to the proton in question (e.g., the N-H proton on the hanging
N atom of a ligand in 1 and 2) and θ is the angle subtended by
R and the M-M axis. The value ∆δ is the displacement of the
resonance from where it would be in the absence of the
anisotropy. Because of the angular dependence of this relation-
ship, the value of the function for an axially symmetrical bond
changes sign at 55° 44′. Thus one would expect protons above
the M-M bond to be deshielded (e.g., the methine protons of
a formamidinate ligand) and thus shifted downfield, or shielded
if they are more or less along the M-M axis, as shown
schematically in Figure 4 and thus shifted upfield. Since the
amine protons in 1 and 2 fall in the positive shielding zone,
one would expect an upfield displacement, relative to that of
the noncoordinated ligands, exactly as observed here. To our
knowledge, this is the first time an upfield displacement has
been documented in the chemistry of multiply bonded com-
plexes.
(18) See, for example: (a) Cotton, F. A.; Ren, T. J. Am. Chem. Soc. 1992,
114, 2237. (b) Lin, C.; Protasiewicz, J. D.; Smith, E. T.; Ren, T. Inorg.
Chem. 1996, 35, 6422. (c) Cotton, F. A.; Daniels, L. M.; Murillo, C.
A. Angew. Chem., Int. Ed. Engl. 1992, 31, 737.
(19) San Filippo, J., Jr. Inorg. Chem. 1972, 11, 3140.
(20) See, for example: (a) Cotton, F. A.; Kitagawa, S. Polyhedron 1988,
18, 1673. (b) Cotton, F. A.; Kitagawa, S. Inorg. Chem. 1987, 26, 3463.
(21) See, for example: Chisholm, M. H.; Cotton, F. A.; Frenz, B. A.;
Reichert, W. W.; Shive, L. W.; Stults, B. R. J. Am. Chem. Soc. 1976,
98, 4469.
(22) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric
Identification of Organic Compounds, 4th ed.; John Wiley & Sons:
New York, 1981.
Using the above equation, many calculations have been
performed18 using the methine protons as probes for metal-
metal bonded paddlewheel compounds having four formamidi-
nate ligands. For these rigid groups, the angle θ is essentially
90° and the distance R is easily calculated using the structural
parameters. Such calculations have, therefore, been very suc-
cessful in correlating the 1H NMR chemical shifts with structure
(23) McConnell, H. N. J. Chem. Phys. 1957, 27, 226. See also refs 17-20.

Magnetic Anisotropy of Quadruple Bonds
Inorganic Chemistry, Vol. 40, No. 12, 2001 2783
Table 4. Selected Bond Lengths (Å) and Angles (deg) for
(TBA)2Cr3(BPAP)4·THFa,b
Cr(1)-Cr(2)
Cr(1)-N(1A)
Cr(1)-N(4)
Cr(1)-N(6)
Cr(1)-N(1)
Cr(2)-N(3A)
Cr(2)-N(5A)
1.904(3)
2.050(4)
2.059(4)
2.062(4)
2.082(4)
2.029(6)
2.036(4)
Cr(2′)‚‚‚Cr(1)
Cr(2′)-N(2A)
Cr(2′)-N(7)
Cr(2′)-N(3)
Cr(2′)-N(5)
Cr(2)-N(2)
Cr(2)-N(7A)
2.589(2)
2.031(6)
2.044(4)
2.071(6)
2.076(4)
2.071(6)
2.079(4)
Cr(2)-Cr(1)-N(1A)
Cr(2)-Cr(1)-N(4)
N(1A)-Cr(1)-N(4)
Cr(2)-Cr(1)-N(6)
N(1A)-Cr(1)-N(6)
N(4)-Cr(1)-N(6)
Cr(2)-Cr(1)-N(1)
N(1A)-Cr(1)-N(1)
N(4)-Cr(1)-N(1)
94.0(2) N(6)-Cr(1)-N(1)
93.47(8) Cr(1)-Cr(2)-N(3A)
90.7(1) Cr(1)-Cr(2)-N(5A)
88.8(1)
95.6(2)
95.9(1)
94.00(8) N(3A)-Cr(2)-N(5A) 90.9(2)
89.7(1) Cr(1)-Cr(2)-N(2)
172.7(1) N(3A)-Cr(2)-N(2)
93.4(2) N(5A)-Cr(2)-N(2)
172.5(1) Cr(1)-Cr(2)-N(7A)
95.8(2)
168.4(2)
89.7(2)
95.0(1)
89.8(1) N(3A)-Cr(2)-N(7A) 89.6(2)
N(5A)-Cr(2)-N(7A) 169.0(2) N(3)-Cr(2′)-N(5)
88.7(2)
89.6(2)
N(2)-Cr(2)-N(7A)
N(2A)-Cr(2′)-N(5)
N(7)-Cr(2′)-N(5)
87.6(2) N(2A)-Cr(2′)-N(7)
2-
Figure 5. Perspective view of Cr3(BPAP)4 in 3‚THF. Atoms are
89.7(2) N(2A)-Cr(2′)-N(3) 168.1(2)
drawn at the 40% probability level, and hydrogen atoms are omitted
for clarity. One set of disordered chromium atoms is represented by
broken circles.
167.6(2) N(7)-Cr(2′)-N(3) 89.4(2)
a Atomic labels with the prime symbol (′) denote the disordered atom
counterparts of the unprimed atoms. b Atomic labels with “A” denote
symmetry-related atoms.
for the formamidinate compounds, and quantitative estimates
of the diamagnetic anisotropies have been made. Unfortunately
for the M2(HBPAP)4 compounds 1 and 2, this type of calculation
is not reliable. One could obtain all the necessary values from
the solid state structure, but since the N-H groups of the
pendent HBPAP ligand are not restrained to fixed positions in
solution,24 no correlation between the solid state and the solution
structures, in this respect, is possible.
Structures of Compounds 3 and 4. Compound 3 crystallized
in the space group C2/c with two interstitial THF molecules.
Here, a linear chain of three chromium atoms is surrounded by
four BPAP dianions, as shown in Figure 5. The trinuclear anion
resides on a crystallographic site of 2-fold symmetry with the
2-fold axis perpendicular to the Cr-Cr-Cr chain. However,
the trinuclear chromium chain is actually very unsymmetrical.
The initial appearance of equality of the Cr-Cr distances is
due to crystallographic disorder. This type of disorder has been
encountered also in other species containing linear Cr36+ cores.1b
Full anisotropic refinement of the chromium atoms in the chain
was not possible because of disorder, but the two Cr to Cr
distances were found to be quite different, namely, 1.904(3) Å
for Cr(1)-Cr(2) and 2.589(3) Å for Cr(1)-Cr(2′) (Table 4).
Therefore, the bonding in this trinuclear chromium chain should
be considered to consist of a Cr-Cr quadruple bond and an
isolated CrII unit, with the latter having approximately square
Figure 6. A view looking down the Cr3 unit in 3‚THF.
planar geometry. The countercations, Bun N+, reside on general
The anion therefore has crystallographic symmetry C2, and in
this case the metal atom chain is symmetrical; there is no
disorder. Again, there is no axial ligation and there is an overall
twist of about 30°. The structure is depicted in Figure 7, and
metrical parameters are listed in Table 5. The end nickel atoms
as well as the central one are coordinated by an essentially
square planar array of nitrogen atoms, and the Ni-N distances
to the end nickel atoms, with an average value of 1.917[6] Å,16
are scarcely different from those to the central nickel atom,
1.906[6] Å.16 From this structure it would be expected that all
three nickel atoms would be in a low-spin (i.e., diamagnetic)
4
positions in the unit cell.
Figure 6 shows an end view of the trinuclear anion of 3. The
torsion angle N(2)‚‚‚Cr(2)‚‚‚Cr(1)‚‚‚(N1) is 15.3°; the total
torsion angle in the molecule is 30.6°. This is smaller than that
in the similar trinuclear dpa complex (45.4°).1e Thus, the idea
that simply eliminating H‚‚‚H interactions such as those in the
dpa anion (Scheme 1a) would produce a trinuclear species
without a torsion angle did not prove correct. The reason for
this is not readily evident. We hope to address this problem in
the future by employing molecular mechanics calculations.
The trinuclear nickel compound, 4‚2THF, is isomorphous
with 3‚2THF, crystallizing in space group C2/c, with Z ) 4.
1
state, and this is indeed the case as shown by the H NMR
spectrum.
The two Ni‚‚‚Ni separations are 2.368(1) Å, which is much
shorter than those in Ni3(dpa)4Cl2,1f 2.433[8] Å.16 However, in
Ni3(dpa)4Cl2 as well as in all nickel compounds in this class
with more extended chains,25 i.e., Ni5, Ni7, Ni9, the nickel atoms
at the ends are different from those inside the chain. The end
(24) Furthermore, the value of ∆δ for M2(formamidinate)4 complexes is
typically obtained from the chemical shifts of the metal-metal bound
complex and the analogous nonmetal-metal bound Ni2(formamidinate)4
complex. So far we have been unable to prepare a nickel analogue of
the M2(HBPAP)4, M ) Cr and Mo.

2784 Inorganic Chemistry, Vol. 40, No. 12, 2001
Cotton et al.
Scheme 4
nickel atoms being square-planar, low-spin atoms that allows
for their closer approach to the central nickel atom.
Compound 4 is not the first compound with a chain of nickel
atoms having no axial ligation. The compound shown in Scheme
4 was reported in 1999.25b It is diamagnetic, like 4, but has
Ni‚‚‚Ni separations that are appreciably shorter than those in
4, namely, 2.3269[6], 2.3010[6], and 2.3280[6] Å,16 as compared
to 2.368(1) Å in 4. The Ni-N distances in both this compound
and 4 are quite similar, all being in the range 1.90-1.93 Å. It
seems clear that unless there are axial ligands present, chains
of nickel(II) atoms simply consist of stacked square, diamagnetic
NiN4 units. In both compounds the stacks are twisted, however.
2-
Figure 7. Perspective view of the anion Ni3(BPAP)4 in 4‚THF.
Atoms are drawn at the 40% probability level, and aromatic hydrogen
atoms are omitted for clarity.
Table 5. Selected Interatomic Separations (Å) and Angles (deg) for
(TBA)2Ni3(BPAP)4·THFa
Conclusions
Ni(2)‚‚‚Ni(1)
Ni(2)-N(6)
Ni(2)-N(4)
Ni(2)-N(2)
Ni(2)-N(2A)
2.368(1)
1.897(8)
1.907(7)
1.910(5)
1.910(5)
It has been shown that the potentially tridentate ligands,
BPAP2-, can do two things: (1) remain as HBPAP- and bind
only a pair of metal ions (Cr24+, Mo22+) locked into a strong
quadruply bonded pair, or (2) support a chain of three metal
ions (Cr36+, Ni36+). In the first case, what distinguishes
M2(HBPAP)4 compounds from previous cases where potentially
tridentate ligands have formed only binuclear compounds (for
example, Cr2(DPhIP)4 and Cr2(dpa)4) is that in the M2(HBPAP)4
compounds there are no significant axial interactions to cause
lengthening of the M2 bond. Importantly, an upfield displace-
ment caused by the magnetic anisotropy of the quadruple bonds
was observed for this first time in the NMR spectra of
compounds 1 and 2. In the case of the M3(BPAP)42- ions, there
are, again, no axial interactions because these anionic species
do not attract ligands. This results in their having different
properties from the M3(dpa)4X2 molecules. Most notably, the
Ni(1)-N(3A)
Ni(1)-N(7)
Ni(1)-N(5)
Ni(1)-N(1)
1.916(5)
1.917(5)
1.919(6)
1.919(5)
N(6)-Ni(2)-N(4)
N(6)-Ni(2)-N(2)
N(4)-Ni(2)-N(2)
N(6)-Ni(2)-N(2A)
N(4)-Ni(2)-N(2A)
N(2)-Ni(2)-N(2A)
N(6)-Ni(2)‚‚‚Ni(1A)
N(4)-Ni(2)‚‚‚Ni(1A)
N(2)-Ni(2)‚‚‚Ni(1A)
N(2A)-Ni(2)‚‚‚Ni(1A) 89.9(2) N(7)-Ni(1)‚‚‚Ni(2)
N(6)-Ni(2)‚‚‚Ni(1)
N(4)-Ni(2)‚‚‚Ni(1)
a Atomic label with “A” denotes the symmetry-related atom unit.
180.00(1) N(2A)-Ni(2)‚‚‚Ni(1) 90.1(2)
89.8(2) Ni(1A)-Ni(2)‚‚‚Ni(1) 179.93(7)
90.2(2) N(3A)-Ni(1)-N(7)
89.8(2) N(3A)-Ni(1)-N(5)
90.2(2) N(7)-Ni(1)-N(5)
90.8(2)
88.9(2)
173.0(2)
179.5(3) N(3A)-Ni(1)-N(1) 173.1(2)
90.04(4) N(7)-Ni(1)-N(1)
89.96(4) N(5)-Ni(1)-N(1)
89.0(2)
90.5(2)
90.1(2) N(3A)-Ni(1)‚‚‚Ni(2) 86.6(2)
86.4(2)
86.5(2)
86.5(2)
90.04(4) N(5)-Ni(1)‚‚‚Ni(2)
89.96(4) N(1)-Ni(1)‚‚‚Ni(2)
2-
Ni3(BPAP)4 ion is diamagnetic and all three nickel ions can
nickel atoms have a fifth ligand atom (e.g., Cl-, NCS-) and
are high spin. Thus, while the inner nickel atom(s) is (are)
square-planar, low-spin atoms with relatively short (ca. 1.90
Å) Ni-N bonds, those on the ends are five-coordinate, high-
spin atoms, with longer Ni-N bonds (ca. 2.1 Å) and the Ni‚‚‚Ni
separations to the terminal nickel atoms are long. It is the
absence of axial ligation in the present case leading to the end
be described as square-planar, low-spin NiII centers.
Efforts are underway to make compounds containing
2-
M3(BPAP)4 ions with other metal atoms.
Acknowledgment. We gratefully acknowledge financial
support from the National Science Foundation.
Supporting Information Available: X-ray crystallographic files
in CIF format for compounds 1-4. This material is available free of
charge via the Internet at http://pubs.acs.org.
(25) (a) Wang, C. C.; Lo, W. C.; Chou, C. C.; Lee, G. H.; Chen, J. M.;
Peng, S. M. Inorg. Chem. 1998, 37, 4059. (b) Lai, S. Y.; Lin, T. W.;
Chen, Y. H.; Wang, C. C.; Lee, G. H.; Yang, M. K.; Leung, M. K.;
Peng, S. M. J. Am. Chem. Soc. 1999, 121, 250.
IC001376G