Stepwise reduction of dinitrogen occurring on a divanadium model compound: A synthetic, structural, magnetic, electrochemical, and theoretical investigation on the [V=N=N=N](n+) [n = 4-6] based complexes

Article abstract of DOI:10.1021/ja971229q

This report details an extensive investigation on vanadium-dinitrogen complexes containing the [V(μ-N₂)V](n+) skeleton with a variable oxidation state of the metal and reduction degree of dinitrogen, n varying from 6 → 5 → 4. This skeleton can be associated to alkali cations in ion-separated or ion-pair tight forms. The reaction of [(Mes)₃V(thf)], 1 [Mes = 2,4,6-Me₃C₆H₂], with Lewis acids [AlPh₃, B(C₆F₅)₃] removed the THF molecule leading under nitrogen to the formation of the diamagnetic complex [(Mes)₃ V (μ-N₂) V (Mes)₃], 4. The reaction of 1 with N₂ also occurs under reducing conditions using Na or K metals. As supported by the electrochemical study, the preliminary stages is in both cases the formation of the vanadium(II) desolvated form [V(mes)₃]^-, 5. The reaction of 5 with N₂ in the case of potassium and in the presence of 1 leads to the compound {[(mes)₃V(μ-N₂)V(Mes)₃]^-[K(digly)₃]⁺}, 11, which, depending on the reaction conditions, undergoes a further reduction to [{K(digly)₃(μ-Mes)₂(Mes)V}₂(μ-N₂)], 12 [digly = diethylene glycol dimethyl ether]. Similarly, the reduction of 1 with sodium gave, depending on the workup model, {[(Mes)₃V(μ-N₂)V(Mes)₃]^2-[Na(digly)₂]⁺₂}, 13 or {[(Mes)₃V(μ-N₂)(μ-N₂(μ-Na)V(Mes)₃]^-[Na(digly)₂]⁺}, 14, both containing the dianion [(Mes)₃V(μ-N₂)V(Mes)₃]^2-, 8. Complex 4 release upon protonation exclusively N₂, while complexes 11-14 release N₂, N₂H₄, and NH₃ in amounts depending on the degree of reduction of the dinitrogen and the structure of the complex. The electrochemical reduction of 1 shed light on the intermediate formation of 5 and its conversion to 8, which was oxidized to the monoanion [(Mes)₃V(μ-N₂)V(Mes)₃]^-, 7 (which is the precursor of 8 in the chemical reduction). The magnetic studies coupled with the theoretical interpretation are in agreement with the presence of d²-d² couple in 4, a d¹-d¹ in 12-14, and a d¹-d⁰ in 11. This, along with the structural studies on 11-14, suggest a cumulenic structure [V=N=N=V](+n) [n = 5, 4] for 11-14.

Full text of DOI:10.1021/ja971229q

10104
J. Am. Chem. Soc. 1997, 119, 10104-10115
Stepwise Reduction of Dinitrogen Occurring on a Divanadium
Model Compound: A Synthetic, Structural, Magnetic,
Electrochemical, and Theoretical Investigation on the
[VdNdNdV]n+ [n ) 4-6] Based Complexes
Richard Ferguson,†,‡ Euro Solari,‡ Carlo Floriani,*,‡ Domenico Osella,§
Mauro Ravera,§ Nazzareno Re, Angiola Chiesi-Villa,| and Corrado Rizzoli|
Contribution from the Institut de Chimie Mine´rale et Analytique, BCH, UniVersite´ de Lausanne,
CH-1015 Lausanne, Switzerland, Dipartimento di Chimica Inorganica,
Chimica Fisica e Chimica dei Materiali, UniVersita` di Torino, I-10125 Torino, Italy,
Dipartimento di Chimica, UniVersita` di Perugia, I-06100 Perugia, Italy,
and Dipartimento di Chimica, UniVersita` di Parma, I-43100 Parma, Italy
ReceiVed April 17, 1997X
Abstract: This report details an extensive investigation on vanadium-dinitrogen complexes containing the [V(µ-
N2)V]n+ skeleton with a variable oxidation state of the metal and reduction degree of dinitrogen, n varying from 6
f 5 f 4. This skeleton can be associated to alkali cations in ion-separated or ion-pair tight forms. The reaction
of [(Mes)3V(thf)], 1 [Mes ) 2,4,6-Me3C6H2], with Lewis acids [AlPh3, B(C6F5)3] removed the THF molecule leading
under nitrogen to the formation of the diamagnetic complex [(Mes)3V(µ-N2)V(Mes)3], 4. The reaction of 1 with N2
also occurs under reducing conditions using Na or K metals. As supported by the electrochemical study, the preliminary
stage is in both cases the formation of the vanadium(II) desolvated form [V(Mes)3]-, 5. The reaction of 5 with N2
in the case of potassium and in the presence of 1 leads to the compound {[(Mes)3V(µ-N2)V(Mes)3]-[K(digly)3]+},
11, which, depending on the reaction conditions, undergoes a further reduction to [{K(digly)3(µ-Mes)2(Mes)V}2(µ-
N2)], 12 [digly ) diethylene glycol dimethyl ether]. Similarly, the reduction of 1 with sodium gave, depending on
the workup mode, {[(Mes)3V(µ-N2)V(Mes)3]2-[Na(digly)2]+ }, 13, or {[(Mes)3V(µ-N2)(µ-Na)V(Mes)3]-[Na(digly)2]+},
2
14, both containing the dianion [(Mes)3V(µ-N2)V(Mes)3]2-, 8. Complex 4 releases upon protonation exclusively
N2, while complexes 11-14 release N2, N2H4, and NH3 in amounts depending on the degree of reduction of the
dinitrogen and the structure of the complex. The electrochemical reduction of 1 shed light on the intermediate
formation of 5 and its conversion to 8, which was oxidized to the monoanion [(Mes)3V(µ-N2)V(Mes)3]-, 7 (which
is the precursor of 8 in the chemical reduction). The magnetic studies coupled with the theoretical interpretation are
in agreement with the presence of a d2-d2 couple in 4, a d1-d1 in 12-14, and a d1-d0 in 11. This, along with the
structural studies on 11-14, suggest a cumulenic structure [VdNdNdV]+n [n ) 5, 4] for 11-14.
Introduction
the stepwise reduction of N2 and the incorporation of N2 into
organic substrates.1f An important feature of this chemistry is
that N2 must be polarized before being attacked by an appropri-
ate nucleophile or electrophile. We felt that this polarization
of N2 might be achieved by using either strongly reducing metal
or bifunctional systems containing both an electron rich (transi-
tion metal ion) and a Lewis acid center, i.e., an alkali cation.
The latter can be introduced in the reduction step of the transition
metal bound N2 using alkali metals. The first widely known
bifunctional system for converting N2 into NH3 and N2H4 is
based on the vanadium(II)-magnesium(II) couple.1g,3 Such a
bifunctional approach along with the recent discovery of a
vanadium-containing nitrogenase4 prompted us to chose vana-
dium; also, some rare simple examples of vanadium compounds
fixing dinitrogen are actually known.5
The metal-assisted activation of dinitrogen is a challenging
topic in synthetic chemistry and catalysis.1 At present, N2
complexes are known for many transition metals; however, those
which promote the reduction of dinitrogen or react in a well-
defined manner to give nitrogen-containing compounds are
few.1,2 Metal-mediated N2 chemistry has proven to be much
less manageable and characterizable than the metal-assisted
chemistry of the isoelectronic carbon monoxide molecule. Most
current research centers on understanding the mechanism for
* To whom correspondence should be addressed.
† Present address: Department of Chemistry and Physics, Wagner
College, Staten Island, NY 10301.
‡ Universite´ de Lausanne.
§ Universita` di Torino.
Universita` di Perugia.
| Universita` di Parma.
(3) Luneva, N. P.; Nikonova, L. A.; Shilov, A. E. Kinet. Katal. 1980,
21, 1041. Shilov, A. E.; Denisov, D. N.; Efimov, O. N.; Shuvalov, V. F.;
Shuvalova, N. I.; Shilova, A. K. Nature (London) 1971, 231, 460. Denisov,
N. T.; Efimov, O. N.; Shuvalova, N. I.; Shilova, A. K.; Shilov, A. E. Zh.
Fiz. Khim. 1970, 44, 2694.
(4) George, G. N.; Coyle, C. L.; Hales, B. J.; Cramer, S. P. J. Am. Chem.
Soc. 1988, 110, 4057. Morningstar, J. E.; Hales, B. J. J. Am. Chem. Soc.
1987, 109, 6854. Robson, R. L.; Eady, R. R.; Richardson, T. H.; Miller, R.
W.; Hawkins, M.; Postgate, J. R. Nature (London) 1986, 322, 388. Arber,
J. M.; Dobson, B. R.; Eady, R. R.; Stevens, P.; Hasnain, S. S.; Garner, C.
D.; Smith, B. E. Nature (London) 1986, 325, 372. Bishop, P. E.; Jarlenski,
D. M. L.; Hetherington, D. R. J. Bacteriol. 1982, 150, 1244.
X Abstract published in AdVance ACS Abstracts, October 1, 1997.
(1) (a) Allen, A. D.; Harris, R. O.; Loescher, B. R.; Stevens, J. R.;
Whiteley, R. N. Chem. ReV. 1973, 73, 11. (b) Chatt, J.; Dilworth, J. R.;
Richards, R. L. Chem. ReV. 1978, 78, 589. (c) Henderson, R. A.; Leigh, G.
J.; Pickett, C. J. AdV. Inorg. Chem. Radiochem. 1983, 27, 197. (d) Leigh,
G. J. Acc. Chem. Res. 1992, 25, 177. (e) Leigh, G. J. New J. Chem. 1994,
18, 157. (f) Hidai, M.; Mizobe, Y. Chem. ReV. 1995, 95, 1115. (g)
Bazhenova, T. A.; Shilov, A. E. Coord. Chem. ReV. 1995, 144, 69.
(2) Laplaza, C. E.; Johnson, M. J. A.; Peters, J. C.; Odom, A. L.; Kim,
E.; Cummins, C. C.; George, G. N.; Pickering, I. J. J. Am. Chem. Soc.
1996, 118, 8623-8638 and the exhaustive list of references therein.
S0002-7863(97)01229-8 CCC: $14.00 © 1997 American Chemical Society

Stepwise Reduction of Dinitrogen on a DiVanadium
J. Am. Chem. Soc., Vol. 119, No. 42, 1997 10105
0.1 M HCl(aq). A 250 mL two-neck round bottom flask was charged
with 100 mL of 0.1 M HCl (NH3 free) and degassed under N2. A thin
glass vial containing the sample to be analyzed (0.600-1.000 g) was
placed in the flask, and the flask was connected to a device for
measuring gas evolution. The temperature of the system was adjusted
to 15-22 °C. After thermodynamic equilibrium was reached, the vial
was broken by a stirring and shaking action, and the volume of N2
evolved was measured.
The solution containing decomposition products (typically light blue)
was filtered and a stock solution (250 mL) prepared with doubly distilled
water. A sample was removed, diluted, and on addition of Kovac’s
reagent, [p-(dimethylamino)benzaldehyde],11 analyzed spectrophoto-
metrically (424, 482 nm) for NH2NH2 (note: sample solution must
contain between 0.1 and 1 mg/L of hydrazine). The amount of
hydrazine was determined from a calibration curve prepared from
known hydrazine hydrate concentrations.
A sample, typically 50-100 cm3, of the stock solution was
transferred to a 250 mL distillation apparatus containing a thin glass
vial of KOH (ca. 1.5 g connected to a receiver flask containing HCl
(0.1 M, 100 mL, NH3 free) (N.B. care was taken to ensure that the
receiver arm was submerged beneath the surface of the 0.1 M HCl
solution). On reflux the KOH vial was broken by vigorous stirring
and the solvent removed to near dryness. The distillate was transferred
to a volumetric flask (100 mL) and the NH3 analyzed spectrophoto-
metrically at 524 nm using Nessler’s reagent (N.B. the sample must
contain between 0.1 and 1 mg/L of NH3). Concentrations were
determined from a calibration curve prepared from an NH4Cl stock
solution.
Synthesis of 3. A toluene (300 mL) suspension of 1 (5.66 g, 11.7
mmol) was added to a toluene suspension of AlPh3 (3.0 g, 11.7 mmol)
at -30 °C. The suspension was warmed up to room temperature to
give a brown solution. Then the solution was evaporated to dryness
and the solid collected and washed several times with n-hexane (90%).
Anal. Calcd for 3, [V(Mes)3]n, C27H33V: C, 79.41; H, 8.08. Found:
C, 78.97; H, 8.27. µeff ) 2.25 BM at 298 K.
We focused our attention on vanadium-aryl compounds,
because aryl groups are particularly appropriate as ancillary
ligands since they are almost exclusively σ-donor in character,
being unable to accommodate electron density from the metal.
This strategy has precedents in the area of dinitrogen reduction
with low-valent homoleptic aryl-iron compounds. No dini-
trogen-iron species have yet been clearly identified.6
A second important perspective in using metal-alkyl or -aryl
derivatives is the expectation that the polarized -N2- molecule
would participate in migratory insertion reactions like CO,
namely, as far as concerns early transition metals.7 Our model
compound [VMes3(THF)]8 (Mes ) 2,4,6-Me3C6H2) undergoes
stepwise reduction to give three different dinitrogen complexes
containing the [VsNsNsV]n+ skeleton [n ) 6, 5, 4]; the
degree of N2 reduction depends on the oxidation state of the
metal ion. The electrochemical, theoretical, and magnetic
studies will be presented in addition to the synthetic and
structural data; such information gives us a better understanding
of the electronic configuration of the [VsNsNsV]n+ species,
which have been isolated in both ion-separated and ion-pair
forms. Our initial results have already been communicated.5d
Experimental Section
All operations were carried out under an atmosphere of purified
nitrogen. All solvents were purified by standard methods and freshly
distilled prior to use. Magnetic susceptibility measurements were made
on a MPMS5 SQUID susceptometer (Quantum Design Inc.) operating
at a magnetic field strength of 1 kOe. Corrections were applied for
diamagnetism calculated from Pascal constants.9 Effective magnetic
moments were calculated by the equation µeff ) 2.828(øT)1/2 where ø
is the magnetic susceptibility per vanadium. Fitting of the magnetic
data to the theoretical expression were performed by minimizing the
agreement factor, defined as
Synthesis of 4, Method A. To a toluene (200 mL) solution of 1
(2.05 g, 4.26 mmol) at -60 °C was added B(C6F5)3 (2.20 g, 4.30 mmol).
The temperature was raised to 0 °C and the mixture stirred for 2 days
under 1-2 atm of N2. The resulting orange microcrystalline product
(pyrophoric) collected by filtration at -10 °C and dried in Vacuo (65%).
Anal. Calcd for 4, C54H66N2V2: C, 76.76; H, 7.82; N, 3.32. Found:
C, 77.13; H, 7.88; N, 3.88. Reaction of 4 with THF liberates N2 to
give the blue color of 1. Complex 4 is diamagnetic.
Synthesis of 4, Method B. To a toluene (400 mL) suspension of 1
(4.96 g, 10.3 mmol) at -30 °C was added AlPh3 (2.69 g, 10.4 mmol).
The suspension was stirred at room temperature for 12 h to give a
solution from which 4 precipitated as a brown microcrystalline solid
(75%). Anal. Calcd for (Mes)3VNNV(Mes)3, C54H66N2V2: C, 76.76;
H, 7.82; N, 3.32. Found: C, 76.92; H, 7.89; N, 3.18.
Synthesis of 9. A 500 mL two-neck round bottom flask was charged
with 200 mL of diglyme and degassed under Ar. 1 (4.09 g, 8.49 mmol)
was added, followed by a large excess (0.43 g, 18.7 mmol) of sodium
sand. The mixture was stirred vigorously at room temperature for 2 h
under Ar. The color changed from blue to violet to brown. The mixture
was filtered to eliminate excess Na (no other solid was found in the
filter, in contrast to the reaction under N2). The filtrate was concentrated
under vacuum down to 50 mL. At room temperature, a microcrystalline
material precipitated and was collected by filtration at 0 °C. The
product was washed with Et2O (50 mL), yielding a light purple,
microcrystalline solid (air/H2O sensitive) (50%). The second product,
10, “V(Mes)2”, was not isolated or characterized. Anal. Calcd for 9,
C48H72NaO6V: C, 70.41; H, 8.80. Found: C, 70.23; H, 8.37.
Synthesis of 11. Diglyme (100 mL) and 1 (4.37 g, 9.09 mmol)
were combined in a 1000 mL two-neck round bottom flask. The
temperature was lowered to 0 °C, and then a ball of potassium metal
(0.35 g, 8.95 mmolsexcess) was added. The partial N2 pressure was
raised to 1-2 atm and the mixture stirred vigorously for 18 h, resulting
in a black color. Filtration of the product mixture at -5 °C yielded a
microcrystalline black solid and any excess K, which was removed.
[øoi bsdTi - øci alcdTi]2
F )
∑
(øoi bsdTi)2
i
through a Levenberg-Marquardt routine.10
The synthesis of [V(Mes)3(THF)], 1, has been performed as
reported.8a
Analysis for N2, N2H4, and NH3 for Dinitrogen-Containing
Complexes.11,12 Reaction of Nitrogen-Containing Compounds with
(5) (a) Song, J.-I.; Berno, P.; Gambarotta, S. J. Am. Chem. Soc. 1994,
116, 6927. (b) Berno, P.; Hao, S.; Minhas, R.; Gambarotta, S. J. Am. Chem.
Soc. 1994, 116, 7417. (c) Buijink, J.-K. F.; Meetsma, A.; Teuben, J. H.
Organometallics 1993, 12, 2004. (d) Ferguson, R.; Solari, E.; Floriani, C.;
Chiesi-Villa, A.; Rizzoli, C. Angew. Chem., Int. Ed. Engl. 1993, 32, 396.
(e) Rehder, D.; Woitha, C.; Priebsch, W.; Gailus, H. J. Chem. Soc., Chem.
Commun. 1992, 364. (f) Woitha, C.; Rehder, D. Angew. Chem., Int. Ed.
Engl. 1990, 29, 1438. (g) Leigh, G. J.; Prieto-Alco´n, R.; Sanders, J. R. J.
Chem. Soc., Chem. Commun. 1991, 921. (h) Edema, J. J. H.; Meetsma, A.;
Gambarotta, S. J. Am. Chem. Soc. 1989, 111, 6878.
(6) Some significant precedents in the use of low-valent homoleptic iron-
aryl compounds in dinitrogen reduction exist, though no iron-dinitrogen
species has been yet clearly identified: (a) Shilov, A. E. New J. Chem.
1992, 16, 213 and references therein. (b) Bazhenova, T. A.; Kachapina, L.
M.; Shilov, A. E. J. Organomet. Chem., 1992, 428, 107. (c) Shilov A. E.
In PerspectiVes in Coordination Chemistry; Williams, A. F., Floriani, C.,
Merbach, A., Eds.; VCH, Weinheim: 1992; p 233.
(7) Durfee, L. D.; Rothwell, I. P. Chem. ReV. 1988, 88, 1059 and
references therein.
(8) (a) Vivanco, M.; Ruiz, J.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C.
Organometallics 1993, 12, 1794. (b) Vivanco, M.; Ruiz, J.; Floriani, C.;
Chiesi-Villa, A.; Rizzoli, C. Organometallics 1993, 12, 1802. (c) Ruiz, J.;
Vivanco, M.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Organometallics
1993, 12, 1811.
(9) Boudreaux, E. A.; Mulay, L. N. In Theory and application of
Molecular paramagnetism; Wiley: New York, 1976; pp 491-495.
(10) Press, W. H., Flannery, B. P.; Teukolsky, S. A.; Vetterling, W. T.
Numerical Recipes; Cambridge University Press: Cambridge, 1989.
(11) Thomas, L. C.; Chamberlain, G. J. Colorimetric Chemical Analytical
Methods, 9th ed.; Wiley: New York, 1980; p 193.
(12) Vogel’s Textbook of QuantitatiVe Chemical Analysis, 5th ed.;
Longman: London, 1989; p 679.

10106 J. Am. Chem. Soc., Vol. 119, No. 42, 1997
Ferguson et al.
Table 1. Experimental Data for the X-ray Diffraction Studies on Crystalline Complexes 11-14a
compd
11
12
13
14
formula
C54H66N2V2‚C18H42KO9 C66H94K2N2O6V2‚0.6667C4H10O C54H66N2V2‚2C12H28NaO6‚0.5C4H8O C54H66N2NaV2‚C12H28NaO6‚C6H6
a )
b )
c )
R )
â )
γ )
V )
Z )
fw
23.322(8)
13.250(3)
25.944(9)
90
116.50(1)
90
40.222(2)
40.222(2)
14.348(4)
90
90
120
16.376(4)
21.782(5)
25.472(6)
90
92.18(2)
90
11.823(4)
21.651(2)
28.696(6)
90
90
90
7175(4)
4
1286.6
20153(6)
9
1241.0
R3 (No. 148)
22
1.54178
0.920
29.06
9079(4)
4
1463.7
P21/n (No. 14)
22
1.54189
1.071
22.37
7346(3)
4
1237.5
C2221 (No. 20)
22
1.54178
1.119
26.24
space group P21/n (No. 14)
t )
λ )
Fcalc )
µ )
transmission 0.985-1.000
coeff
R )
wR2 )
GOF )
-97
0.71069
1.191
3.61
0.902-1.000
0.562-1.000
0.861-1.000
0.048
0.101
1.047
0.081
0.272
0.897
0.085
0.272
0.878
0.045 [0.060]a
0.138 [0.168]
0.779
2
2
a R ) Σ|∆F|/Σ|Fo|. wR2 ) [Σ(w(∆F2)2/Σ(wFo )2]1/2. GOF ) [Σw|∆F| /(NO - NV)]1/2. Values in square brackets refer to the “inverted structure”.
The product was dried in Vacuo (89%) and stored at 4 °C. X-ray quality
crystals of 11 were obtained as follows: a solution of 11 (5.20 g) in
benzene (50 mL) was filtered at 5 °C and dried in Vacuo. This purified
product (1.65 g) was then redissolved in benzene (45 mL) and filtered
at 40 °C, and the filtrate was cooled to 5 °C over 3 days. The resulting
black crystals were collected by filtration at 5 °C. Anal. Calcd for
11, C72H108KN2O9V2: C, 67.21; H, 8.46; N, 2.18. Found: C, 67.74;
H, 8.53; N, 2.58.
vigorously for 2.5 days at ca. 20 °C, resulting in a color change from
blue to black-brown. Filtration of the product mixture yielded a shiny
black solid, which was washed with benzene (100 mL) at room
temperature and dried in Vacuo (96%) (note that Na is the limiting
reagent to avoid contamination of product by Na). The product was
stored at 4 °C. Making Crystals Suitable for X-ray Structure
Determination. 14 (2.65 g) and benzene (100 mL) were combined in
a 250 mL two-neck round bottom flask. The mixture was briefly
shaken, and then the bottom quarter of the flask was immersed in an
oil bath at ca. 65 °C; the mixture was left to convect for 2 h. The
heating element was disengaged, and when the oil bath had reached
room temperature, the black crystals were collected by decantation.
The reaction gave the same product (plus unconsumed 1) when carried
out with 0.5 equiv of Na. Anal. Calcd for 14, C66H94N2Na2O6V2: C,
68.38; H, 8.17; N, 2.42. Found: C, 68.15; H, 8.01; N, 2.71.
Synthesis of 12. To a toluene (200 mL) solution of 1 (4.32 g, 8.99
mmol) at 0 °C was added a ball of K (0.35 g, 8.98 mmol), and the
partial N2 pressure was raised to 1-2 atm. After 18 h of vigorous
stirring, the color had changed from blue to orange-brown. Diglyme
(3 mL) was added to the product mixture, and stirring was continued
for 10 min at room temperature. The reaction mixture was concentrated
in Vacuo to 10 mL, and then diethyl ether (150 mL) was added. The
flask was briefly shaken and left for 3 h, resulting in a black crystalline
product. This was then collected by filtration and dried in Vacuo (69%)
and the product stored in the refrigerator. Making Crystals Suitable
for X-ray Structure Determination. During the preparation, after
addition of the diglyme, the solution was concentrated to 30 mL (instead
of 10 mL). Diethyl ether (200 mL) was then added, the flask was
shaken and left to stand for 1 h; then the mixture was filtered at room
temperature. The filtrate was left in a refrigerator at 4 °C for 2 days.
The resulting black crystals were collected by filtration at 4 °C. The
reaction yields the same product (plus unconsumed [V(Mes)3])
when carried out with 0.5 equiv of K. Anal. Calcd for 12·0.7Et2O,
C66H94K2N2O6V2·0.7C4H10O: C, 66.53; H, 7.95; N, 2.35. Found: C,
66.54; H, 8.13; N, 2.48.
Synthesis of 13. To a THF (100 mL) solution of 1 (2.91 g, 6.05
mmol) at 0 °C were added four equally sized pieces of Na (0.14 g,
6.05 mmol). The partial N2 pressure was raised to 1-2 atm by pinching
the N2 bubbler line before closing the stopcock of the flask. The
mixture was stirred vigorously for 10 h, and the color changed from
blue to brown. Diglyme (50 mL) was added to the mixture, resulting
in the formation of a black microcrystalline solid. This was collected
by filtration, washed with diethyl ether (50 mL), dried in Vacuo (88%),
and stored at 4 °C. Making Crystals Suitable for X-ray Structure
Determination. Approximately 6.0 g of product was combined with
THF (45 mL) in a 250 mL two-neck round bottom flask. The mixture
was filtered at room temperature, and the filtrate was slowly cooled to
-35 °C over 1 week. The black crystals were collected by filtration
at -35 °C. Anal. Calcd for 13·0.5THF, C80H126N2Na2O0.5V2: C, 65.62;
H, 8.61; N, 1.96. Found: C, 65.34; H, 8.64; N, 1.49.
X-ray Crystallography for Complexes 11-14. Crystal data and
details associated with data collection are given in Tables 1 and S1.
Data were collected on a single-crystal diffractometer (Enraf-Nonius
CAD4 for 11, Rigaku AFC6S for 12-14) at 178 K for 11 and at 295
K for 12-14. For intensities and background, individual reflection
profiles13 were analyzed. The structure amplitudes were obtained after
the usual Lorentz and polarization corrections, and the absolute scale
was established by the Wilson method.14 The crystal quality was tested
by ψ scans showing that crystal absorption effects could not be
neglected. The data were corrected for absorption using a semiempirical
method15 for all the complexes. The function minimized during the
least-squares refinement was ∆w(∆F2)2. Weights were applied ac-
2
2
cording to the scheme w ) 1/[σ2(Fo ) + (aP)2] (P ) (Fo +2Fc2)/3 and
a ) 0.0548, 0.1516, 0.1169, and 0.0623, for complexes 11-14,
respectively. Anomalous scattering corrections were included in all
structure factor calculations.16b Scattering factors for neutral atoms were
taken from ref 16a for non-hydrogen atoms and from ref 17 for H.
Among the low-angle reflections no correction for secondary extinction
was deemed necessary.
All calculations were carried out on an IBM PS2/80 personal
computer and on an ENCORE 91 computer. The structures were solved
by the heavy-atom method starting from three-dimensional Patterson
maps using the observed reflections. Structure refinements were carried
(13) Lehmann, M. S.; Larsen, F. K. Acta Crystallogr. Sect. A: Cryst.
Phys., Diffr., Theor. Gen. Crystallogr. 1974, A30, 580.
(14) Wilson, A. J. C. Nature 1942, 150, 151.
(15) North, A. C. T.; Phillips, D. C. Mathews, F. S. Acta Crystallogr.
Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1968, A24, 351.
(16) (a) International Tables for X-ray Crystallography; Kynoch Press:
Birmingham, England, 1974; Vol IV, p 99. (b) Reference 16a, p 149.
(17) Stewart, R. F.; Davidson, E. R.; Simpson, W. T. J. Chem. Phys.
1965, 42, 3175.
Synthesis of 14. To a diglyme (100 mL) solution of 1 (2.80 g,
5.82 mmol) was added a sodium ball (0.12 g, 5.22 mmol). The partial
N2 pressure was raised to 1-2 atm by pinching the N2 bubbler line
before closing the stopcock of the flask. The mixture was stirred

Stepwise Reduction of Dinitrogen on a DiVanadium
J. Am. Chem. Soc., Vol. 119, No. 42, 1997 10107
out using SHELXL9218 and based on the unique observed data for 11
and on the unique total data for 12-14.
Scheme 1
During the refinements the aromatic rings of complexes 12-14 were
constrained to be regular hexagons (C-C 1.39(1) Å), while the C-C
and C-O bond distances of the diglyme molecule in 12 and of the
THF molecule of crystallization in 13 were constrained to be 1.54(1)
and 1.48(1) Å, respectively. The hydrogen atoms, except those related
to the solvent molecules of crystallization in 12 and 13, which were
ignored, were located from difference Fourier maps for 11 and put in
geometrically calculated positions for 12-14. They were introduced
in the subsequent refinements as fixed atom contribution with U’s fixed
at 0.05 for 11, 0.12 for 14, and 0.15 Å2 for 12 and 13. The final
difference maps showed no unusual feature, with no significant peak
above the general background. The crystal chirality of complex 14,
which crystallizes in a polar space group, was tested by inverting all
the coordinates (x, y, z | -x, -y, -z) and refining to convergence once
again. The resulting R values quoted in Table 1 indicated that the
original choice should be considered the correct one.
Scheme 2
Crystal data are listed in Table S1 and final atomic coordinates in
Tables S2-S5 for non-H atoms and in Tables S6-S9 for hydrogens.
Thermal parameters are given in Tables S10-S13 and bond distances
and angles in Tables S14-S17.
Electrochemistry. Electrochemical measurements were performed
using an EG&G PAR 273 electrochemical analyzer interfaced to a
personal computer, employing PAR M270 Electrochemical Software.
A standard three-electrode cell was designed to allow the tip of the
reference electrode to closely approach the working electrode. The
cell was enclosed in a drybox. All measurements were carried out
under Ar or N2 in anhydrous deoxygenated THF. Solutions were 1 ×
10-3 M with respect to the compounds under study and 2 × 10-1
M
with respect to the supporting electrolyte, NaBPh4 × 2triglyme. The
working electrode was a Pt disk (diameter ) 1 mm) sealed in soft
glass which was polished with alumina. The reference electrode was
a AgCl-coated silver wire dipped in a 2 × 10-1 M solution of NaBPh4
× 2 triglyme in THF and separated from the cell solution by a fine
frit. At the end of each experiment, the potential of the Fc/Fc+ couple
was measured Vs the pseudo-reference electrode and then Vs an aqueous
SCE, to which all data are referred. Under the actual experimental
conditions the Fc/Fc+ couple is located at +0.405 V Vs SCE. Since
the resistence of such a THF solution was exceptionally high, albeit
positive feedback iR compensation, some potential distortions were
observed. This led to higher values of the peak-to-peak separation,
∆Ep. Therefore in all experiments we compared the ∆Ep of the peak
couples under study to that of Fc/Fc+, added as internal standard. Any
deviation from the ∆Ep value of 70 mV, assumed to be the constant
value of Fc/Fc+ couple at all scan rates, is taken as an indication of
uncompensed iR drop and computated as a factor of correction.
Therefore, the statement Nernstian throughout this work indicates a
redox process as reversible as the oxidation of Fc is.19
were active, the most effective and less time consuming from
a synthetic point of view were the trialkyl or triaryl aluminum
species; AlPh3 gave the best yield of 4. The effectiveness of
the Lewis acid is not dependent on its intrinsic Lewis acidity
but on its solubility. This is understandable if we compare the
very low yield of 4 when using AlR3 [R ) Me or Et] with the
higher yield when using AlPh3. The slight solubility of AlPh3
allows a slow delivery of the active species [V(Mes)3], 2
(Scheme 2), compatible with the low concentration of N2 in
toluene, though some amount of 2 untrapped by N2 evolves to
give 3. Complex 3 does not react with N2. Formation of 3
becomes predominant when the reaction in Scheme 1 is carried
out using AlR3 [R ) Me, Et]. Both of these Lewis acids are
soluble in toluene, quickly giving rise to a high concentration
of 2, which cannot be stabilized as 4 by the low concentration
of N2. The generation of an N2-active species should occur
roughly at the same rate as dinitrogen is supplied to the solution.
Carrying out reaction 1 under Ar resulted only in 3. In the
absence of any X-ray analysis, we are not in the position to
propose a plausible structure for 3, whose formulation corre-
sponds to [V(Mes)3]n, without any solvent. The protonation of
4 with HCl, a standard test for the presence of reduced forms
of dinitrogen (NH3, N2H4), gave exclusively N2 (see Experi-
mental Section). This was not entirely surprising since no
reduced N2 was found in the literature complex {[Me3-
CCH2)3V]2(µ-N2)} either.5c Complex 4 releases N2 when treated
with THF, while the oligomeric form 3 does not go back to 1
under the same conditions.
The number of electrons transferred (n) in a particular redox process
was determined by controlled potential coulometry at a Pt gauze; all
experiments were done in duplicate.
Results and Discussion
A. Synthesis. Electron-rich vanadium(III) derivatives are
good candidates for dinitrogen fixation, provided the appropriate
choice of reaction conditions is made. The parent compound
[V(Mes)3(thf)], 1, we examine here is completely unreactive
toward N2; however, when the THF molecule is removed using
an appropriate Lewis acid, the metal center reacts by either
oligomerizing (under Ar), 3, or binding N2 (under nitrogen), 4
(Scheme 1).
The access to reduced, activated forms of dinitrogen followed
two synthetic routes, using either 1 or 4 as starting materials.
The two pathways A and B are shown in Scheme 2.
In both pathways we must remove the THF on vanadium.
This was done by using a Lewis acid (pathway A) or reducing
the metal (pathway B). Although pathway A appears more
logical on paper, it is pathway B which is more practical, since
the transformation of 1 into 2, and then 4, using a Lewis acid
The nature of the Lewis acid is crucial for the occurrence of
the reaction and largely determines the relative yields of products
3 and 4. Although some boron derivatives, such as [B(C6F5)3],
(18) Sheldrick, G. SHELXL92: Program for Crystal Structure Refine-
ment; University of Go¨ttingen: Go¨ttingen, Germany, 1992.
(19) (a) Connelly, N. G.; Geiger, W. E. AdV. Organomet. Chem. 1984,
23, 1. (b) Geiger, W. E. In Organometallic Radical Processes; Trogler, W.
C., Ed.; Elsevier: Amsterdam, 1990.

10108 J. Am. Chem. Soc., Vol. 119, No. 42, 1997
Ferguson et al.
Table 2
Scheme 3
yield of N2,
%
yield of N2H4,
%
yield of NH3,
total,
%
compd
%
4
11
12
13
14
98
66
42
53
49
98
93
101
101
96
20
38
27
36
7
21
21
11
chemical test for the presence of reduced N2 is the reactivity of
the metal-bonded N2 with electrophiles1a such as H+ in the
protonolysis reaction.1d We used the protonolysis reaction to
test our complexes, but we note that the figures we obtained
undoubtedly underestimate the reduction degree of dinitrogen
due to the presence of the proton sensitive aryl group at the
metal.
Scheme 4
Protonation of 4 and 11-14 was carried out with aqueous
HCl (0.1 M) at 15-22 °C, and the amounts of N2, N2H4, and
NH3 obtained (Table 2) were analyzed as reported in the
Experimental Section. Protonation of 11-14 under nonaqueous
conditions in THF with dry HCl gave the same results. Some
interesting conclusions can be drawn from these data if one
assumes that the principal route to free N2 is Via protonation of
the V-C bonds and to N2H4 and NH3 by protonation of N2.
The yield of N2 is 100% in the case of 4 due to the low electron
density on the unreduced nitrogen atoms; protonation of the
more basic carbon of the V-C bond is preferred. In the case
of 11, protonation of both carbon and nitrogen atoms occurs.
The N/C protonation ratio increases with compounds 11 through
14, because of the presence of reduced dinitrogen. Within this
sequence we can make some interesting observations. The
yields of N2 from 13 and 14 are roughly the same for these
equally reduced species; on the other hand, the N2H4/NH3 ratio
is greater for 14, possibly because the µ-N2-Na moiety in 14
acts as a template for the production of N2H4. The yield of N2
is the least for 12, possibly because the two K+ partially block
the protonation site of the V-C bonds, thereby further favoring
the protonation of N2 to give N2H4 and NH3.
Although the protonation of dinitrogen in 11-14 has as a
competing reaction the protonation of the mesityl groups, the
yield of NH3 and N2H4, with respect to 12 and 13, is higher
than expected if one considers the number of electrons exceeding
the vanadium(III) oxidation state. This fact is particularly
relevant and in agreement with the evidence that, as is true for
11-14, we are not dealing anymore with [V(III)-N2-V(III)]-n
[n ) 1, 2] systems but, when n extra electrons are added to 4,
with vanadium(IV) systems (Vide infra). This implies that the
protonation of 11-14 should produce not only vanadium(III)
but also some vanadium(IV) in order to account for the excess
of NH3 and N2H4 derived from 12 and 13.
requires two steps instead of one. The electron-rich vanadium-
(II) in 5 loses THF easily, and a coordination site becomes
available for N2, to give 6. The reduction is carried out in the
presence of N2, which acts as the stabilizing agent for 5. In
the absence of N2 (reaction under Ar) 5 is not stable and reacts
with 1, resulting in a ligand disproportionation reaction (Scheme
3).
The enhanced basicity of metal-bonded N2 in 6 (Scheme 2)
allows the displacement of THF from 1 to give 7, which is
further reduced to 8. Although the sequences (pathways A and
B) in Scheme 2 show all the probable intermediates, the
syntheses of compounds 4, 7, and 8 were all carried out starting
from 1, and not following the sequence 4 f 7 f 8. Interest-
ingly, the degree to which 1 was reduced to 7 or 8 depended
only on the nature of the alkali cation and the solvent conditions,
not on how much alkali metal was used.
The reaction of 1 with potassium metal in a V/K ) 1 molar
ratio in diglyme gave 11 (Scheme 4), containing the monoanion
7, and [K(digly)3]+ as countercation. When the reaction was
carried out in THF followed by the crystallization from diglyme,
we obtained 12, containing the dianion 8, and two [K(digly)]+
cations in a tight ion-pair form. Thus, regardless of the V:K
stoichiometric ratio, the solvent helps to determine both the
degree of N2 reduction in the dinuclear skeleton and the
occurrence of 11 as an ion-separated molecule, or 12 as an ion-
pair form. Such solvent structure dependence was also observed
in the case of Na metal, though only the dielectronic reduction
of the [V(III)-N2-V(III)] skeleton occurred. The reduction
in THF, followed by the addition of diglyme, gave the ion-
separated form 13 (Scheme 4). When the reaction was carried
out in diglyme the ion-pair 14 was isolated (Scheme 4).
The main question arising at this point was how much is the
dinitrogen reduced in complexes 4 and 11-14, namely, in the
anions 7 and 8, as a function of the degree of oxidation of the
[V-N2-V]n+ skeleton [n ) 6, 5, 4]. The most common
B. Structural Studies. First, we will describe the overall
structure of each complex in the series 11-14, followed by a
conclusive comparison of the structural parameters related to
the common V-N-N-V skeleton.
The structure of 11 is determined by the packing of anions 7
(Figure 1) and [K(digly)3]+ cations in the molar ratio 1/1. A
selection of bond lengths and angles is reported in Table 3. The
dihedral angles formed by the V1-N1 and V2-N2 lines with
the normal to the mean plane through the σ-bonded carbon
atoms around V1 and V2 are 11.0(1) and 6.8(2)°, respectively.
The vanadium atoms are displaced from their coordination basal
planes by 0.543(1) and 0.416(1) Å for V1 and V2, respectively.
The V-C bond distances are affected by the charge of the
complex, being significantly shorter [average: 2.085(12) Å] than
those found in complex 13 [average: 2.189(11) Å] (Table 3).

Stepwise Reduction of Dinitrogen on a DiVanadium
J. Am. Chem. Soc., Vol. 119, No. 42, 1997 10109
Figure 2. ORTEP drawing of complex 12 (20% probability ellipsoids).
Prime denotes a transformation of 1 - x, -y, 1 - z.
Figure 1. ORTEP drawing of the anion in complex 11 (80%
probability ellipsoids).
Table 4. Selected Bond Distances (Å) and Angles (deg) for
Complexes 12 and 14
Table 3. Selected Bond Distances (Å) and Angles (deg) for
Complexes 11 and 13
(a) Within the [{V(Mes)3}2-µ-(N2)] Units
11
13
12
14
V1-N1
1.744(6)
2.139(11)
2.110(11)
2.12(2)
1.233(8)
118.5(5)
122.1(4)
96.8(5)
99.7(4)
110.4(4)
109.5(4)
178.3(8)
1.757(6)
2.123(9)
2.172(9)
2.193(9)
1.271(8)
V1-N1
V1-C1
V1-C11
V1-C21
V2-N2
V2-C31
V2-C41
V2-C51
N1-N2
1.770(3)
2.117(3)
2.076(3)
2.106(3)
1.741(3)
2.112(3)
2.051(3)
2.087(3)
1.222(4)
1.775(5)
2.185(5)
2.195(5)
2.171(5)
1.750(6)
2.164(5)
2.238(5)
2.175(6)
1.225(7)
V1-C1
V1-C11
V1-C21
N1-N1′
C11-V1-C21
C1-V1-C21
C1-V1-C11
N1-V1-C21
N1-V1-C11
N1-V1-C1
V1-N1-N1′
118.9(4)
113.7(4)
105.4(4)
92.4(3)
119.5(4)
106.7(5)
174.7(10)
C11-V1 -C21
C1-V1 -C21
C1-V1-C11
N1-V1-C21
N1-V1-C11
N1-V1-C1
C41-V2-C51
C31-V2-C51
C31-V2-C41
N2-V2-C51
N2-V2-C41
N2-V2-C31
114.4(1)
123.5(2)
120.7(1)
105.3(1)
95.4(1)
112.0(3)
102.6(2)
102.0(2)
110.2(2)
105.3(2)
101.2(2)
113.2(2)
122.0(2)
104.8(3)
115.1(3)
99.7(2)
(b) Within the K1 Coordination Sphere in Complex 12
105.0(1)
115.0(1)
105.9(1)
125.2(1)
117.0(1)
104.9(1)
106.0(1)
94.6(1)
K1-O1
K1-O2
K1-O3
K1-C1
K1-C2
K1-C3
K1-C4
K1-C5
2.703(12)
2.717(20)
2.805(20)
3.178(12)
3.362(12)
3.535(9)
K1-C6
3.175(8)
K1-C11
K1-C12
K1-C13
K1-C14
K1-C15
K1-C16
3.218(15)
3.179(14)
3.300(15)
3.454(11)
3.491(18)
3.376(20)
3.534(7)
3.358(10)
(c) Within the Na1 Coordination Sphere in Complex 14
Na1-N1
Na1-N1′
Na1-C21
Na1-C26
2.513(12)
2.607(12)
2.585(17)
2.492(17)
Na1-C21′
Na1-C22′
Na1-C23′
2.570(17)
2.388(17)
2.601(16)
The structure of 12 is shown in Figure 2, and a selection of
bond lengths and angles is reported in Table 4. The structure
consists of [{K(digly)3(µ-Mes)2(Mes)V}2(µ-N2)] centrosym-
metric molecules and Et2O solvent molecules of crystallization
in the molar ratio 3/2. The complex molecule possesses a
crystallographically imposed inversion center at the midpoint
of the N-N bond (Figure 2). A comparison with complexes
11 and 13 shows that the C1‚‚‚C6 and C11‚‚‚C16 mesityl rings
are slightly rearranged, giving rise to a cavity which accom-
modates the potassium cation. The two rings are in an
“eclipsed” conformation as seen from the torsion angles quoted
in Table 5. The potassium cation completes its coordination
through three oxygen atoms from a diglyme molecule.
and 9.1(2)°, respectively. The V-C bond distances are in
agreement with those observed in complex 14 (Tables 3 and
4).
The structure of 14 consists of dimeric [(Mes)3V(µ-N2)(µ-
Na)V(Mes)3]- anions (Figure 4), [Na(digly)2]+ cations, and
benzene solvent molecules of crystallization in the molar ratio
of 1/1/1. A selection of bond lengths and angles is reported in
Table 4.
The anion has an imposed crystallographic C2 symmetry
(Figure 4), a dinitrogen molecule linearly bridging two adjacent
[V(Mes)3] units. [V1-N1-N1′, 174.7(10)°]. Coordination
around vanadium can be described as a flattened trigonal
pyramid with vanadium lying 0.597(3) Å from the plane of the
three σ-bonded carbon atoms toward the nitrogen atom (Table
5). The dihedral angle formed by the V1-N1 line with the
The structure of 13 consists of dianion 8 (Figure 3), [Na-
(digly)2]+ cations, and THF solvent molecules of crystallization
in the molar ratio 1/2/0.5. A selection of bond lengths and
angles is reported in Table 3. The dihedral angles formed by
the V1-N1 and V2-N2 lines with the normal to the mean plane
through the corresponding σ-bonded carbon atoms are 4.6(2)

10110 J. Am. Chem. Soc., Vol. 119, No. 42, 1997
Ferguson et al.
Table 5. Comparison of Structural Parameters within the V(Mes)3 Units for Complexes 11-14a
11
12
13
14
(a) Within each V(Mes)3 Unit
Distances of Vanadium from the Mesityl Rings (Å)
dist of V1 from C1..C6 plane
dist of V1 from C11..C16 plane
dist of V1 from C21..C26 plane
dist of V2 from C31..C36 plane
dist of V2 from C41..C46 plane
dist of V2 from C51..C56 plane
0.333(1)
0.033(1)
0.038(1)
0.405(1)
0.056(1)
0.113(1)
0.217(3)
0.324(4)
0.182(2)
0.028(1)
0.139(1)
0.047(1)
0.035(1)
0.045(1)
0.107(1)
0.126(2)
0.067(3)
0.637(3)
Dihedral Angles between Mesityl Rings (deg)
C1..C6 ∧ C11..C16
C1..C6 ∧ C21..C26
C11..C16 ∧ C21..C26
C31..C36 ∧ C41..C46
C31..C36 ∧ C51..C56
C41..C46 ∧ C51..C56
80.6(1)
66.4(1)
71.2(1)
82.2(1)
67.3(1)
89.5(1)
82.1(5)
67.0(3)
70.5(4)
85.2(2)
66.7(2)
53.0(2)
62.7(2)
67.8(2)
88.2(2)
83.5(3)
81.6(3)
70.5(3)
Dihedral Angles between the Mesityl Rings and the Basal Coordination Plane (deg)
C1..C6 ∧ C1,C11,C21
36.0(1)
79.5(1)
38.0(2)
85.4(2)
43.5(1)
16.9(1)
75.3(4)
75.7(5)
11.3(4)
84.4(2)
34.4(2)
24.2(2)
25.8(2)
63.6(2)
18.4(2)
57.8(3)
65.2(3)
24.5(3)
C11..C16 ∧ C1,C11,C21
C21..C26 ∧ C1,C11,C21
C31..C36 ∧ C31,C41,C51
C41..C46 ∧ C31,C41,C51
C51..C56 ∧ C31,C41,C51
Relevant Torsion Angles between Mesityl Rings (deg)
C6-C1..C11-C12
C2-C1..C11-C16
C46-C41..C51-C52
C42-C41..C51-C56
30.9(4)
27.4(4)
28.0(4)
29.2(5)
-0.1(13)
2.0(13)
-22.2(7)
-28.1(7)
-17.7(7)
-21.2(7)
-18.0(11)
-24.2(9)
(b) Within the Dimeric [{V(Mes)3}2-µ-(N2)] Unit
Relevant Torsion Angles (deg)
C21-V1..V1′-C21′
C21-V1..V2..C31
180.0(-)
-28.7(4)
168.3(2)
-160.8(2)
Relevant Dihedral Angles (deg)
C21..C26 ∧ C21′..C26′
180.0(-)
24.5(3)
C21..C26 ∧ C31..C36
38.0(2)
24.2(2)
a Prime denotes a transformation of 1 - x, -y, 1 - z and -x, y, -0.5 - z for 12 and 14, respectively.
Figure 4. ORTEP drawing of the anion in complex 14 (25%
probability ellipsoids). Prime denotes a transformation of -x, y, 0.5 -
z. The disorder involving Na1 has been omitted for clarity.
Figure 3. ORTEP drawing of the anion in complex 13 (25%
probability ellipsoids).
normal to the mean basal plane is 15.1(3)°. Two symmetry
related mesityl ligands (C21‚‚‚C26) in a dimer face each other
[dihedral angle 24.5(3)°] to provide a cavity which accom-
modates a sodium cation (disordered over two positions about
the 2-fold axis) which in turn gives rise to a sandwich-type
compound. The trend of the Na1‚‚‚C distances involving the
carbon atoms suggests a η2 sodium-mesityl and η3 sodium-
mesityl for the C21-C26 and C21′-C26′ mesityl rings,
respectively (and Vice Versa for the Na1′ cation) (Table 4). Other
Na1···C contacts are longer than 2.93 Å.21 In addition the alkali
metal η2 interacts with the dinitrogen molecule [Na1-N1, 2.513-
(12); Na1-N1′, 2.607(12) Å] probably giving rise to a
significant lengthening of the N-N bond distance with respect
to the values found in 11, 12, and 13 where this kind of

Stepwise Reduction of Dinitrogen on a DiVanadium
J. Am. Chem. Soc., Vol. 119, No. 42, 1997 10111
interacion is not present. The V-C bond distances are longer
than those observed in 18a as a consequence of the presence of
the V-N multiple bond. They are in agreement with those
found in complex 13. The large displacement of vanadium from
the plane of the C21‚‚‚C26 aromatic ring is consistent with the
pyramidal nature of the C21 carbon atom induced by the
interaction with the sodium cation. The distance of C21 from
the plane through the three bonded atoms is 0.156(9) Å. The
mutual orientation of the mesityl rings in the two [V(Mes)3]
units is mainly determined by the inclusion of the Na+ cation,
as can be seen from the appropriate conformational parameters
properly given in Table 3. The two [V(Mes)3] units in a dimer
are arranged in such a way as to put two rings approximately
parallel to each other: C21‚‚‚C26 and C21′‚‚‚C26′ for 12 and
14; C21‚‚‚C26 and C31‚‚‚C36 for 11 and 13. The two parallel
rings are cis-oriented in complex 14 and trans-oriented in
complexes 11-13 with respect to the V‚‚‚V line, as indicated
by the values of the C-V‚‚‚V-C torsion angles (Table 5).
Within each V(Mes)3 unit the orientation of the mesityl rings
is given by the dihedral angles between mesityl rings and
between the mesityl rings and the coordination plane (Table
5). In spite of the rather low accuracy of the X-ray analysis
we can make the following observations: (i) the geometry of
the molecule is similar to that observed in complexes 11 and
13; the values of the V-C bond distances [average 2.124(9)
Å] are midway between those observed in complexes 11, 13,
and 14 and are in agreement with those found in the neutral
[V(Mes)3(THF)]8a starting molecule (Table 5); (ii) the range of
the K-Cmes distances in complex 12 suggest η6 interactions
involving the potassium cation and the two eclipsed mesityl
rings.20 Such an interaction is almost usual. In Table 5 we
report the comparison between the most important conforma-
tional parameters related to complexes 11-14. These data show
that the interaction of the alkali cation with the dinuclear
skeleton is almost ineffective for the general conformation of
the dimeric complexes.
The structural parameters for the V-N-N-V skeleton
(Tables 3-5) are extremely close in all four complexes
regardless of the overall charge of the complex, the oxidation
state of the metal or the reduction degree of N2, and the binding
(or not) of the alkali cation within the dimeric framework. The
V-N-N-V skeleton is linear, with a V-N bond distance
ranging from 1.744(6) to 1.770(3) Å and a N-N length varying
from 1.222(4) to 1.271(8) Å.5 These data, while not providing
any criteria for the reduction degree of N2 or its activation
toward electrophiles, allow us to draw the [VdNdNdV]
cumulene structure for the V2N2 skeleton.
C. Electrochemistry. This section reports the electrochemi-
cal behavior of 1 under argon and dinitrogen. The numbering
scheme is that shown in Schemes 2, 3, and 5. We will assume
that, since the solvent is THF, the naked anions 7 and 8 shown
in Scheme 2 form from 11-14 dissolved in THF.
Scheme 5. Proposed Mechanism for the Electrochemical
Reduction of 1 under N2
data are consistent with a Nernstian (electrochemically revers-
ible) process); (iii) ipc(B)/ipa(A) ratio increases from 0.83 to 0.93
as the sweep rate increases; furthermore Ep(A) vs log V plot is
linear with a slope of 5 mV. These features are consistent with
a chemically almost reversible 1e oxidation process.21
This behavior can be easily interpreted as a metal-centered
oxidation from [V(Mes)3] to [V(Mes)3]+, 16 (see Scheme 5),
though the process is certainly not involved in the N2-reduction
pattern.
The cathodic sweep shows a well-behaved peak couple (C/
D) at E°′ ) -2.09 V. The electrochemical parameters, in the
scan rate range 0.05-50 V s-1 are the following: (i) ip vs V1/2
plots for both peaks are linear through the origin; (ii) ∆Ep varies
from 83 to 140 mV, even when corrected by comparison with
the behavior of the Fc/Fc+ couple; moreover (Ep - Ep/2) values
of both peaks increase from 70 to 120 mV as the sweep rate
increases; (iii) ipa(D)/ipc(C) ratio is unity, and Ep(C) vs log V
plot is linear with a slope of 25 mV.
All these data are consistent with a 1e electrochemically
semireversible and chemically reversible reduction process from
1 to [V(Mes)3]-, 5 (Schemes 2 and 5). Some geometrical
rearrangements should occur in 5, if one attaches structural
significance to the moderate rate of the observed charge
transfer.19 However, there is no evidence of [V(Mes)4]- anion
formation during such experiments. Indeed, an authentic sample
of 9 (Figure 5a) shows a fully reversible oxidation process (peak
couple A′/B′) at E°′ ) +0.68 V, very far from the A/B couple
of 1. Therefore, in the short CV time scale the evolution from
5 to [V(Mes)4]-, 9, does not occur. This would require, as in
the much slower chemical reduction, the presence of unreacted
1 (see Scheme 3).
If N2, instead of Ar, is bubbled, the cathodic scenario modifies
as shown in Figure 5b. A new peak system E/F appears at a
more cathodic potential (E°′ ) -2.40 V), together with two
reoxidation peaks G (Ep) -1.79 V) and H (Ep) -0.95 V) (both
evaluated at 0.2 V s-1 scan rate). Peaks G and H are
unambiguously related to the C/D system: their appearance
corresponds to a decrease of the peak D current, and when the
sweep is reversed at Eλ) -2.30 V (so eliminating the electro-
generation of couple E/F), both are still present in similar
intensity. Moreover, if we impose a vertex delay of 5 s at Eλ)
-2.30 V, peaks G and H both increase with respect to peak D.
Reoxidation peaks G and H as well as the peak couple E and
F disappear at scan rates as high as 5 V s-1. Therefore, the
increase of scan rates (which quenches ensuing chemical
reactions) restores a CV response identical to that obtained under
the Ar atmosphere. These extra peaks represent electrochemical
processes corresponding to different species slowly generated
from 5. A multiscan CV experiment, traversing peak H, shows
Figure 5a shows the cyclic voltammetric (CV) response of a
THF solution of 1 at a Pt electrode under argon. A single-
oxidation process is displayed in the anodic window (peak
couple A/B) at E°′ ) +0.16 V vs SCE. The electrochemical
parameters for the A/B couple in the scan rate (V) range 0.05-
50 V s-1 are the following: (i) ip vs V1/2 plots for both peaks
are linear through the origin, indicating diffusion-controlled and
adsorption-free processes;21 (ii) ∆Ep is constantly 70 mV when
corrected by comparison with the behavior of the ferrocene/
ferrocenium (Fc/Fc+) couple (added as an internal standard);
moreover (Ep - Ep/2) values of both peaks are constantly 65
mV, near the theoretical expectation for a 1e Erev process21 (these
(21) (a) Bard, A. J.; Faulkner, L. L. Electrochemical Methods; Wiley:
New York, 1980. (b) Brown, E. R.; Sandifer, J. R. In Physical Methods of
Chemistry; Rossiter, B. W., Hamilton, G. F., Eds; Wiley: New York, 1986;
Vol. II, Chapter IV. (c) Adams, R. N. Electrochemistry at Solid Electrodes;
Marcel Dekker: New York, 1971. (d) Heardridge, J. B. Electrochemical
Techniques for Inorganic Chemists; Academic: London, 1969.
(20) Weiss, E. Angew. Chem., Int. Ed. Engl. 1993, 32, 1501.

10112 J. Am. Chem. Soc., Vol. 119, No. 42, 1997
Ferguson et al.
Figure 6. (a) CV responses of a THF solution of parent complex 1
(solid line) and of the resulting solution after exhaustive electrolysis at
Ew ) -2.30 V under N2, V ) 0.2 V s-1. (b) CV response of a THF
solution of 7 (11) under N2, V ) 0.2 V s-1, (b) starting potential.
similar to that of Figure 5c. Noteworthy, the intensity of peak
I is about half (47%) of the initial peak C, suggesting that it is
related to a dimer.
Variations of Ep(C) have been examined both as a function
of scan rate and concentration (c) of 1. Logarithmic analysis
of Ep(C) as a function of V have been performed in concentra-
tions of 1 ranging from 1 to 4 mM. An average value of 24
mV/decade is obtained for the slope of δEp/δ(log V), whereas
a negligible value is obtained for the slope of δEp/δ(log c). The
observed variation of Ep(C) with log V is in agreement with a
dimerization mechanism, but the very small variation of Ep(C)
with log c apparently contradicts this interpretation.21 However,
on the basis of the chemical results and the following tests on
the model compound 7, N2 must plays an important role in such
a dimerization process. Its concentration after saturation of THF
solvent is very low, and this can represent a limiting factor. In
conclusion (see Scheme 5), the primary reduction product 5
reacts with nitrogen to afford [(Mes)3V-N2-V(mes)3]2-, 8. This
complex can be reversibly reduced in CV time scale to 15.
However, controlled-potential coulometry at a platinum gauze
(Ew) -2.70 V) causes total decomposition of the complex as
can be seen by the disappearance of CV peaks.
Further evidence of the original dimerization is provided by
comparing the previous electrochemical results with that of an
authentic sample of [(Mes)3V(µ-N2)V(Mes)3]-, 7, in the form
of 11. Two reduction processes (Figure 6b) are present in the
cathodic sweep corresponding to the peak couples I/G and E/F.
The peak-current ratio of these reductions, namely, [ipc(I)/ipc-
(E)], is 1:1. When the anodic sweep is analyzed, the passage
through the oxidation peak H (having the same current intensity
as peaks I and F) causes the regeneration of 1, as previously
shown. 1 can be further oxidized at peak A; interestingly the
ratio of the previously observed peaks (namely, I, E, and H)
with respect to this 1e oxidation is 1:2. This feature unambigu-
Figure 5. (a) CV responses of THF solutions of 1 (solid line) and 9
(dashed line), containing NaBPh4 (0.2 M) as supporting electrolyte at
a Pt electrode, under Ar, V ) 0.2 V s-1, (b) starting potentials. (b) CV
responses of a THF solution of 1 under Ar (solid line) and under N2
(dashed line), V ) 0.2 V s-1, (b) starting potential. (c) Multiscan CV
response of a THF solutions of 1 under N2, V ) 0.2 V s-1, (b) starting
potential.
no difference in the number of peaks and relative intensities in
the successive cathodic sweeps. This seems to indicate that
only the reoxidation process at peak H regenerates neutral 1 in
almost quantitative yield, in spite of the cascade of electro-
chemically induced transformations. On the contrary, if the
reoxidation sweep is stopped before traversing peak H (Eλ )
-1.55 V), a new peak I (clearly related to peak G) appears at
Ep ) -1.88 V (0.2 V s-1), and the intensities of peak C
considerably decrease (Figure 5c), indicating that part of 1 has
been subtracted to the diffusion layer. In these experimental
conditions one can observe three reversible peak systems at E°′
) -1.84 V (G/I), -2.10 V (C/D) and -2.40 V (E/F) (Figure
5c).
Controlled-potential coulometry at a platinum gauze (Ew )
-2.30 V) consumes 1e per molecule of 1, with a concomitant
color change from blue to dark brown. The CV response of
the exhaustively reduced solution (Figure 6a) gives a response

Stepwise Reduction of Dinitrogen on a DiVanadium
J. Am. Chem. Soc., Vol. 119, No. 42, 1997 10113
Table 6. Redox Potentials (V vs SCE) for the Complexes under
Study in THF Solution (0.2 M NaBPh4) at a Pt Electrode
peaks
compd
A/Ba C/Da
E/Fa
G/Ia
Hb
1
+0.16 -2.09 -2.40 -1.84 -0.95
-2.40 -1.80 -0.94
solution after reduction of 1
7 (11)
+0.16
-2.37 -1.79 -0.92
aE°′ ) (Epa + Epc)/2. b Ep evaluated at 0.2 V s-1
.
ously demonstrates that all the species associated at the peak
couple I/G, E/F, and H/J are in dimeric forms. The electro-
chemical parameters for the I/G and E/F couples are consistent
with diffusion-controlled, chemically and electrochemically
reversible 1e/molecule reduction processes. On the contrary,
the [ipc(J)/ipa(H)] ratio is always less than unity (namely, it varies
from 0.3 to 0.7 in the scan rate range 0.05-50 V s-1) showing
that the [(Mes)3V(µ-N2)V(Mes)3], 4, (Scheme 5) releases N2
back to 1 in the presence of THF.
Table 6 summarizes the electrochemical data.
The proposed mechanism of the electroreduction of 1 in the
presence of nitrogen is summarized in Scheme 5. The intimate
mechanism of the dimerization process (a key step for N2-
activation) has not been completely elucidated on the basis of
the electrochemical experiments. The paramount step seems
to be the coupling between two 5 anions.
The distinctive characterstics of the chemical and electro-
chemical reduction of 1 in the presence or absence of N2 should
be commented on at this stage. The striking difference between
the two is the much faster electrochemical reduction, compared
with the chemical one, of [V(Mes)3(THF)], 1, to [V(Mes)3]-,
5. Remember that both the ligand disproportionation reaction
(Scheme 3) and the formation of 7 require the simultaneous
presence of 1 and 5. It is for this reason that the electrochemical
reduction of 1 under Ar does not result in the formation of
[V(Mes)4]-. However, while under N2 the primary compound
formed is not 7, but 8, a consequence of the reaction of 6 with
5. In the much slower chemical reduction the formation of 7
accounts for the presence of 1 reacting with 6.
D. Magnetic Properties and Theoretical Calculations.
Magnetic susceptibilities data for complexes 11-13 and 4 were
collected with a SQUID magnetometer in the temperature range
1.9-300 K and are displayed in Figures 7 and 8. The data for
14 are not reported as they are practically coincident with those
for 13.
We see that for 11-14 the magnetic moment per vanadium
is essentially constant down to low temperatures, with a value
of about 1.7 µB for complexes 12-14 and 1.3 µB for complex
11. In complex 11 there is only one spin center per dimer so
that the magnetic moment can be simply fitted with a Curie
spin-only formula22 giving g ) 2.09. For complexes 13 and
14 the magnetic data are accounted for with two independent S
) 1/2 spin centers on the two metals (giving g ) 1.96 and g )
1.95, respectively) while for 12 the data can be fitted with the
simple Bleany-Bowers equation23 obtained, assuming that the
interaction between the two spin centers were described by the
Heisenberg spin Hamiltonian H ) -2JS1‚S2, considering a pair
of exchange-coupled ions with S1 ) S2 ) 1/2. The best fit to
the collected data yielded g ) 2.06, J ) -0.65 cm-1. These
results indicate a negligible or very small antiferromagnetic
coupling between the two S)1/2 spin centers through the N-N
bridge.
Figure 7. Magnetic susceptibility (O; 10-3 cm3 mol-1) and effective
magnetic moments (b; BM) per vanadium Vs temperature for complex
11 (a) and complex 12 (b).
degenerate singlet and triplet, for complex 12 the ground state
is a singlet with a triplet only 1.3 cm-1 above, and for complex
11 the ground state is a well-isolated doublet.
A completely different behavior has been observed for 4, for
which the magnetic moment shows a steady decrease from 1.2
µB at 300 K to 0.2 µB at 1.9 K (Figure 8b). A careful analysis
of the temperature dependence of the magnetic susceptibility
reveals that such behavior is essentially due to a large temper-
ature independent paramagnetism (TIP) and a good fit is actually
obtained for NR ) 1.2 × 10-3 including the presence of an
almost negligible amount, <0.1%, of monomeric V(III) impuri-
ties. Such magnetic behavior indicates an isolated nonmagnetic
singlet ground state, the large TIP being due to the presence of
a low-lying (but thermally nonpopulated) triplet state (presum-
ably around 1000-2000 cm-1 or higher).24
Such observed magnetic behaviors are inconsistent with pairs
of d3-d3 (complexes 12-14) or d2-d3 (complex 11) ions,
corresponding to V(II)-V(II) and V(III)-V(II) formal oxidation
states of the metals, respectively. Rather they indicate pairs of
d1-d1 or d0-d1 ions, suggesting a change in the oxidation states
of vanadium atoms, respectively, to V(IV)-V(IV) or V(V)-
V(IV), with a corresponding four-electron reduction of the
dinitrogen moiety. The magnetic behavior of 4 could, instead,
be interpreted as due to a very strong antiferromagnetic
interaction within a pair of d2-d2 ions, which leaves an isolated
singlet state.
In terms of the energy level pattern the results above indicate
that for complexes 13-14 the lowest states are two perfectly
(22) Carlin, R. L. Magnetochemistry; Springer: Berlin, Germany, 1986.
(23) O’Connor, C. J. Progr. Inorg. Chem. 1982, 29, 203.
(24) Kahn, O. Molecular Magnetism; VCH: Weinheim, Germany, 1983.

10114 J. Am. Chem. Soc., Vol. 119, No. 42, 1997
Ferguson et al.
Scheme 7
molecular orbitals and which are designated 1e, 2e, 3e, 4e. The
scheme is completed by four δ type orbitals (usually designated
as 1δ -4δ and lying between 2e and 4e), built by the bonding
and antibonding combinations of the dδ metal orbitals which
do not interact with any dinitrogen orbital. These orbitals are
to be filled with four electrons from the N2 moiety and n + m
electrons from the two metal fragments (assumed with formal
dn and dm configurations). The strengths of the M-N and N-N
bonds are determined by the occupancies of the 2e and 3e levels.
The 2e level has π*(N-N) character and its population results
in a relevant decrease of the N-N bond order while the 3e
orbital has a π(N-N) character and its population results in an
increase of the N-N bond order. Most of the qualitative26-29
and semiempirical30-32 descriptions of the metal dinitrogen
bonding in dinuclear N2-bridged complexes have been per-
formed on the basis of this model. Most of them generally agree
that the best electron count for dinitrogen activation corresponds
to two d2 metal fragments.
Figure 8. Magnetic susceptibility (O; 10-3 cm3 mol-1) and effective
magnetic moments (b; BM) per vanadium Vs temperature for complex
13 (a) and complex 4 (b).
We have recently performed ab initio calculations on a series
of [{VH3}2(µ-N2)]q (q ) 0, -1, -2) complexes of C2V symmetry
as simplified models of 4 and 11-14,33 interpreting the results
in terms of the four-center model. Some of those results will
now be used to interpret the magnetic behavior and the extent
of dinitrogen activation in such complexes. The calculated
energy ordering and nature of the main frontier molecular
orbitals is shown in Scheme 7, where we report the occupancy
for 4. There are two sets of almost doubly degenerate orbitals.
The two highest occupied orbitals correspond to the 2e level
while the two lowest unoccupied orbitals, about 1 eV higher
in energy, are mostly metal orbitals of dδ character. The 3e
level has been found to be far higher in energy, while the 1e
level is low-lying and essentially of π(N-N) character. In 4,
the 2e set is filled by two d electrons from each V(Mes)3
metal fragment (formally d2) and a singlet ground state is
found. However, the gap between the 2e and 1-2 δ sets of
Scheme 6
These magnetic behaviors can be better rationalized on the
basis of the overall electronic structure of the considered
dinuclear complexes. A qualitative description of bonding in
metal dinitrogen complexes is based on the four-center four-
orbital interaction scheme for the MNNM linear moiety reported
in Scheme 6.25
There are two sets of these four-center molecular orbitals
which are obtained by the linear combinations of the two sets
of mutually orthogonal dπ (M) and pπ (N) atomic orbitals. As
an overall result, there will be four doubly degenerate energy
levels with energies increasing with the nodal number of the
(26) Sunner, R. D.; Manriquez, J. M.; Marsh, R. E.; Bercaw, J. E. J.
Am. Chem. Soc. 1976, 98, 8351.
(27) Sunner, R. D.; Duggan, D. M.; McKenzie, T. C.; Marsh, R. E.;
Bercaw, J. E. J. Am. Chem. Soc. 1976, 98, 8358.
(28) O’Regan, M. B.; Liu, A. H., Finch, W. C.; Schrock, R. R.; Davis,
W. M. J. Am. Chem. Soc. 1990, 112, 4331.
(29) Kol, M.; Schrock, R. R.; Kempe, R.; Davis, W. M. J. Am. Chem.
Soc. 1994, 116, 4382.
(30) Pelican, P.; Boca, L. Coord. Chem. ReV. 1984, 55, 55.
(31) Powell, C. B.; Hall, M. B. Inorg. Chem. 1984, 23, 4619.
(32) Goldberg, K. I.; Hoffmann, D. F.; Hoffmann, R. Inorg. Chem. 1982,
21, 3863.
(25) Chatt, J.; Fay, R. C.; Richards, R. L. J. Chem. Soc. A 1971, 2399.
Treitel, J. M.; Flood, M. T.; March, R. E.; Gray, H. B. J. Am. Chem. Soc.
1969, 91, 6512. Treitel, J. M.; Flood, M. T.; March, R. E.; Gray, H. B. J.
Angew. Chem., Int. Ed. Engl. 1974, 13, 639.
(33) Re, N.; Rosi, M.; Sgamellotti, A.; Floriani, C.; Solari, E. Inorg.
Chem. 1994, 33, 4390. Re, N.; Rosi, M.; Sgamellotti, A.; Floriani, C. Inorg.
Chem. 1995, 34, 3410.

Stepwise Reduction of Dinitrogen on a DiVanadium
J. Am. Chem. Soc., Vol. 119, No. 42, 1997 10115
Conclusions
orbitals is not very high so that the triplet excited state (although
not populated) is sufficiently low to give a high second-order
contribution to magnetic susceptibility. In 11, one more electron
must be put in the δ orbital and the ground state is a well-
isolated doublet. In 12-13, two more electrons occupy the
two degenerate δ orbitals. Taking into account that the δ
orbitals have no contribution from the bridging dinitrogen
ligand, such a (1δ)1(2δ)1 configuration physically corresponds
to a neglegible antiferromagnetic interaction between two
formally d1 metal centers consistent with the observed mag-
netic behavior. Our previous calculations33 gave a triplet ground
state. However, we should consider that an SCF approac
h is inappropriate for this situation24 and favors the high spin
states.
This is a rather unique case in which the dinitrogen molecule
bridging two metal centers has been subjected to a controlled
stepwise chemical and electrochemical reduction going from
[(Mes)3V(µ-N2)V(Mes)3], 4, to [(Mes)3V(µ-N2)V(Mes)3]-, 7,
and then to [(Mes)3V(µ-N2)V(Mes)3]2-, 8, that is from an
inactive to reactive reduced form. We have to emphasize a quite
remarkable effect as a consequence of the addition of a single
electron to 4. This does not cause a monoelectronic reduction
of N2, but instead induces the dimetallic unit to promote a
multielectronic reduction of dinitrogen in 7, and consequently
in 8. This has been expressed by writing down the cumulene
structures for 7 and 8, as supported by the structural analysis
on 11-14. In addition, the magnetic and theoretical studies
support the formulation of a d2-d2 couple in the case of 4, a
d1-d0 and a d1-d1 in the case of 7 and 8 , respectively, with
the effective reduction of the dinitrogen molecule. This has
been chemically revealed in the protonation reaction leading to
NH3 and N2H4 in the case of 7 and 8.
On the basis of this orbital scheme some considerations can
be made on the nature of the metal-dinitrogen bonding. In all
the considered complexes, the 2e level is occupied and the 3e
level is empty, so that a marked activation of dinitrogen would
be expected. However, a much lower activation is observed
for 4. This can be explained by taking into account that
dinitrogen activation depends not only on the occupancy of the
2e and 3e levels but also on the extent of metal-nitrogen mixing
in these orbitals. In particular, the mixing in the 2e level is
determined by the energy difference between the dπ orbitals of
the metal fragments and the empty π* orbitals of N2. The higher
the metal orbitals are, the higher π*(N-N) character the 2e
level has, i.e., a higher reduction of the N-N bond order is
expected. The d orbital energies in [VII(Mes)3]- metal frag-
ments are considerably higher than those in [VIII(Mes)3]
fragments (V(II) is a much better π donor than V(III)) so that
a higher dinitrogen activation is expected for 12-14 with respect
to 4, as observed. An intermediate situation is expected and
found for the mixed valence species 11. The ab initio
calculations of ref 33 confirmed these forecasts; a small π*-
(N-N) contribution was calculated for the 2e orbitals of 4 and
increasingly higher contribution for 11 and 12-14.
Acknowledgment. We thank the Fonds National Suisse de
la Recherche Scientifique (FNS, Bern, Switzerland, Grant No.
20-40268.94), Ciba Specialty Chemicals (Basel, Switzerland),
and the Centro Nazionale delle Ricerche (CNR, Rome, Progetto
Bilaterale Italia/Svizzera) for financial support; Dr. Lubomir
Pospisil (Academy of Science of the Czech Republic, Praha) is
also acknowledged for stimulating discussion.
Supporting Information Available: Supplementary draw-
ings (Figures S1-S4), tables of experimental details associated
with data collection and structure refinement (Table S1), atomic
coordinates (S2-S9), anisotropic thermal parameters (S10-
S13), bond lengths and bond angles (S14-S17), and details of
X-ray crystallography for 11-14 (34 pages). Ordering informa-
tion is given on any current masthead page.
JA971229Q