Dinitrogen coordination and cleavage promoted by a vanadium complex of a σ,π,σ-donor ligand

Article abstract of DOI:10.1021/ic701219h

The deprotonation of the tripyrrole MeTPH₂ [MeTPH₂ = 2,5-[(2-pyrrolyl)(C₆H₅)₂C] ₂(MeNC₄H₂)], containing one N-methylated pyrrolyl ring, was carried out with 2 equiv of KH. The corresponding dipotassium salt reacted with VCl₃(THF)₃ to afford the complex [(MeTP)VCl(THF)]·THF (1). While the two lateral pyrrolide rings are σ-bonded, the central one is perpendicularly oriented in a sort of π-fashion. However, the bond distances clearly indicated that only the quaternized N atom is forming a bonding contact. Subsequent reduction of 1 with Na yielded the corresponding divalent complex [(MeTP)V(THF)]·(C ₇H₈)_0.5 (2) where the central N-methylated ring adopted a more regular π-orientation. When treated with a strong Lewis acid (AlMe₃), THF was extracted from the vanadium coordination sphere, forming the dinuclear dinitrogen complex [(MeTP)V(μ-N₂)] ₂·(C₇H₈)_2.9 (3). Reduction of 3 with potassium graphite gave cleavage of dinitrogen, affording the mixed-valent nitride-bridged complex [(MeTP)V(μ-N)]₂·(THF) (4).

Full text of DOI:10.1021/ic701219h

Inorg. Chem. 2007, 46, 8836−8842
Dinitrogen Coordination and Cleavage Promoted by a Vanadium
Complex of a -Donor Ligand
σ,π,σ
Indu Vidyaratne, Patrick Crewdson, Emeric Lefebvre, and Sandro Gambarotta*
Department of Chemistry, UniVersity of Ottawa, Ottawa, Ontario K1N 6N5, Canada
Received June 21, 2007
The deprotonation of the tripyrrole MeTPH2 [MeTPH2
N-methylated pyrrolyl ring, was carried out with 2 equiv of KH. The corresponding dipotassium salt reacted with
VCl3(THF)3 to afford the complex [(MeTP)VCl(THF)] THF (1). While the two lateral pyrrolide rings are -bonded,
the central one is perpendicularly oriented in a sort of -fashion. However, the bond distances clearly indicated
that only the quaternized N atom is forming a bonding contact. Subsequent reduction of 1 with Na yielded the
corresponding divalent complex [(MeTP)V(THF)] (C7H8)0.5 (2) where the central N-methylated ring adopted a more
regular -orientation. When treated with a strong Lewis acid (AlMe3), THF was extracted from the vanadium
coordination sphere, forming the dinuclear dinitrogen complex [(MeTP)V( -N2)]2 (C7H8)2.9 (3). Reduction of 3
with potassium graphite gave cleavage of dinitrogen, affording the mixed-valent nitride-bridged complex
[(MeTP)V( -N)]2 (THF) (4).
) 2,5-[(2-pyrrolyl)(C6H5)2C]2(MeNC4H2)], containing one
‚
σ
π
‚
π
µ
‚
µ
‚
Introduction
the research activity sparked by these questions, a few cases
of vanadium dinitrogen complexes3 have been discovered,
as well as formation of nitrides via the 6-electron reductive
cleavage of the coordinated dinitrogen unit.1b,3j,4
Pyrrole-based ligand systems and especially di- and cyclic
tetrapyrroles have shown a particular versatility in enhancing
and supporting the reactivity of metals toward dinitrogen
fixation and activation.5 Their resilience to strongly reducing
Among the diversified transformations promoted by high-
spin, octahedral d3 complexes of vanadium, dinitrogen
activation has traditionally provided the strongest motivation
to justify synthetic efforts for developing the chemistry of
the divalent state.1 The discovery of the presence of vanadium
as key element in some of the nitrogenase enzymes2 still
poses challenging questions about the role of this element
in the reduction of dinitrogen to ammonia. As a result of
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* To whom correspondence should be addrssed. E-mail:
sgambaro@uottawa.ca.
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8836 Inorganic Chemistry, Vol. 46, No. 21, 2007
10.1021/ic701219h CCC: $37.00
© 2007 American Chemical Society
Published on Web 09/21/2007

Dinitrogen Coordination and CleaWage
Scheme 1
metals is most likely the basis of this useful behavior.
However, ligand denticity also seems to be a rather critical
factor. For example, simple monodentate pyrrolides, which
adopt either the π- or σ-bonding modes depending on the
nature of the metal,6 to the best of our knowledge, have never
produced a case of dinitrogen activation/fixation with any
metal. On the contrary, the polypyrrolide ligand systems
almost invariably adopt the dual σ- and π-bonding mode,7
while favoring the assembly of cluster structures capable of
cooperative dinitrogen reduction.8
In this first paper, we describe the synthesis and charac-
terization of the V(III) and V(II) complexes of the tripyrrolide
dianion of MeTPH2, the formation of the corresponding
dinitrogen vanadium complex and further cleavage to a
dinuclear linearly bonded mononitride species.
While in combination with pyrrolide-based ligand systems,
the behavior of divalent vanadium has been so far disap-
pointing, having yielded standard σ-bonded complexes9 with
no sign of possible reactivity with dinitrogen. Recently, by
using strong Lewis acids (such as AlR3) capable of locking
the pyrrolide nitrogen sp2 orbital, a complex containing two
π-bonded pyrrolide rings with an overall vanadocene-type
of structure has been obtained.10 However, no dinitrogen
activation has been observed in this particular species in spite
of the divalent state and the hemi-lability and dynamism of
the monodentate pyrrolide ligand.10 Hence, our design
of the tripyrrolide 2,5-{2-[(C6H5)2C]pyrrole]2(N-Me-pyrrole)
(MeTPH2) ligand system used in this work for targeting low-
valent vanadium. The empirical idea behind its design was
to have a ligand in which the two terminally positioned
pyrrolide rings provide two σ-bonded anionic N atoms.
Indeed, the chemistry of the anionic amide complexes of
both di- and trivalent vanadium has demonstrated the ability
of anionic nitrogen donor atom to promote dinitrogen
fixation.3f-h,j The central methylated pyrrolide ring was
expected to provide the metal with a π-interaction11 which,
as suggested by the chemistry of low-valent lanthanides,5,7,8
seems also beneficial to enhance and promote reactivity
toward dinitrogen.
Results and Discussion
In order to prepare the tripyrrole ligand MeTPH2, the
classical direct condensation between pyrrole and carbonyl
derivatives cannot be used, as it tends to lead to either
dipyrrole or porphyrinogen-type structures with only small
amounts of tripyrrole being produced.12 Thus, its synthesis
(see Scheme 1) was obtained via double lithiation of the two
1-methyl pyrrole R-positions13 followed by the subsequent
addition of benzophenone and quenching. In a second step,
the resulting diol 2,5-[(C6H5)2C(OH)]2(N-Me-pyrrole) was
condensed with neat pyrrole, acting as both a solvent and
reagent, affording MeTPH2 in good yield (83%) of analyti-
cally pure solid. Deprotonation of the two terminal pyrroles
and formation of the corresponding dipotassium salt MeTPK2
was conveniently carried out in situ via standard treatment
with KH in THF.
Reaction of MeTPK2 with VCl3(THF)3 in THF formed
dark red crystals of the corresponding [(MeTP)VCl(THF)]‚
THF (1) in moderate yield (Scheme 2).
The connectivity has been confirmed by an X-ray crystal
structure. The only unexpected feature in the structure (Figure
1) is that the central N-methylated ring does not adopt the
expected η5-coordination mode, but formed instead a direct
V-N σ-bond via quaternization of the nitrogen atom. The
geometry around the metal center can be described in terms
of a distorted trigonal bipyramidal geometry with the apical
positions occupied by the N atom of the N-methylated central
ring [V1-N2 ) 2.232(3) Å] and the oxygen atom of one
coordinated THF [V1-O1 ) 2.107(2) Å]. The equatorial
plane is comprised of two σ-bound pyrrolides N atoms [V1-
N1 ) 1.957(3) Å and V1-N3 ) 1.958(3) Å] and the
chloride [V1-Cl1 ) 2.2526(11) Å]. The axial vector deviates
a little from the linearity [N2-V1-O1 ) 172.59(9)°], and
a minor distortion is present in the equatorial plane [Cl1-
V1-N3 ) 119.98(8), Cl-V1-N1 ) 117.44(8)°, N1-V1-
N3 ) 122.35(11)°].
(5) (a) Jubb, J.; Gambarotta, S. J. Am. Chem. Soc. 1994, 116, 4477. (b)
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Dube, T.; Ganesan, M.; Conoci, S.; Gambarotta, S.; Yap, G. P. A.
Organometallics 2000, 19, 3716. (e) Korobkov, I.; Gambarotta, S.;
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Hoveyda, A. H. J. Am. Chem. Soc. 2006, 128, 16373.
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Organometallics 2001, 20, 2443. (b) Ganesan, M.; Berube, C. D.;
Gambarotta, S.; Yap, G. P. A. Organometallics 2002, 21, 1707. (c)
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Gambarotta, S.; Yap, G. P. A. Organometallics 2003, 22, 3742. (e)
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Chem. 2003, 42, 8579.
The magnetic moment was as expected for the high-spin
d2 electronic configuration of mononuclear trivalent vana-
dium.
When complex 1 was reacted with metallic sodium in
THF, a new species was isolated as dark purple crystals in
moderate yield (Scheme 2). The formulation of this complex
(8) (a) Ganesan, M.; Gambarotta, S.; Yap, G. P. A. Angew. Chem., Int.
Ed. 2001, 40, 766.
(9) Edema, J. J. H.; Gambarotta, S.; Meetsma, A.; Spek, A. L.; Veldman,
N. Inorg. Chem. 1991, 30, 2062.
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Int. Ed. In press.
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1999, 1, 73. (b) Ka, J.-W.; Lee, C.-H. Tet. Lett. 2000, 41, 4609.
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887.
(11) Huttner, G.; Mills, O. S. Chem. Ber. 1972, 105, 301.
Inorganic Chemistry, Vol. 46, No. 21, 2007 8837

Vidyaratne et al.
Scheme 2
as the divalent [(MeTP)V(THF)]‚(C7H8)0.5 (2) was again
determined by an X-ray structure (Figure 2). In comparison
to 1, complex 2 has simply lost the chlorine atom as a result
of the reduction to the divalent state. In the process, however,
the N-methylated ring adopts a η5-bonding mode although
not perfectly centered over the V atom [V-centroid ) 2.010-
(4) Å, V1-N2 ) 2.193(3) Å, V1-C6 ) 2.293(4) Å, V1-
C7 ) 2.430(5) Å]. Overall, the coordination geometry of
the metal center can be described as distorted pseudo-
tetrahedral [N1-V1-N3 ) 106.68(14)°, N1-V1-O1 )
97.83(14)°, N1-V1-centroid ) 107.59°] and is comprised
by the centroid of the η5-bound ring, the two N atoms of
the two σ-bonded pyrrolides rings [V1-N1 ) 2.067(4) Å
and V1-N3 ) 2.075(4) Å], and the O atom of one
coordinated molecule of THF [V1-O1 ) 2.115(3) Å]. Even
in this case the magnetism did not show any unexpected
feature.
The lack of dinitrogen fixation in this complex containing
divalent vanadium can certainly be ascribed to the coordina-
tion of one THF molecule. The exchange of THF for a
weakly ligated N2 was previously observed in the chemistry
of divalent vanadium amidinates.3h Thus, when the coordi-
nated molecule of THF was extracted by a strong Lewis acid
such as AlMe3, coordination of dinitrogen was obtained with
formation of the dinuclear dinitrogen-bridged complex
[(MeTP)V(µ-N2)]2‚(C7H8)2.9 (3), which crystallized from
toluene in moderate yield.
The structure (Figure 3) shows the complex containing
two metal centers, each surrounded by one tripyrrolide ligand
system and bridged by one dinitrogen unit in an almost-linear
V-N-N-V array [V1-N7-N8 ) 177.7(2)°, N7-N8-V2
) 176.5(2)°]. Similar to complex 2, each V center is in a
pseudo-tetrahedral coordination geometry comprised of the
η5-bound ring [V1-centroid ) 2.023(6) Å], the two σ-bond-
ed pyrrolides [V1-N1 ) 2.011(5) Å, V1-N3 ) 2.024(6)
Å, V1-N7 ) 1.752(6) Å, N1-V1-N3 ) 100.1(2)°, N1-
V1-N7 ) 106.3(2)°, N1-V1-centroid ) 103.8(2)°] and
one N atom of the bridging N2 unit. The slightly elongated
dinitrogen bond [N7-N8 ) 1.248(5) Å vs 1.0968 Å for free
Figure 1. Crystal structure of 1 with thermal ellipsoids drawn at the 30%
probability level.
Figure 2. Crystal structure of 2 with thermal ellipsoids drawn at the 30%
Figure 3. Crystal structure of 3 with thermal ellipsoids drawn at the 30%
probability level.
probability level.
8838 Inorganic Chemistry, Vol. 46, No. 21, 2007

Dinitrogen Coordination and CleaWage
µB] is in agreement for the nitride-bridged mixed-valent
V(III)-V(IV) dimer with an overall one unpaired electron
configuration.
The formation of the mono-nitrido-bridged species 4 from
dinitrogen cleavage is of some interest and displays a few
aspects of novelty. The apparent stability of the dinitrogen-
bridged 3 in the absence of coordinating solvents implies
that the N-N cleavage is not performed by the two V(II) d3
centers, but instead by some other reduced species generated
by the attack of KC8. This behavior is in contrast with that
of a divalent tris-amide vanadium complexes where the
possibility of V(II) cleaving coordinated dinitrogen into
nitride-bridged di-vanadium complexes has been clearly
demonstrated by the pioneering work of Cloke.3j In that case,
DFT calculations have pointed out that the sideways
coordination of a transient V(II) species to dinitrogen is
followed by further attack by a second unit, ultimately
leading to the six-electron reduction and consequent N-N
triple bond cleavage.4c In the case of the formation of 4,
reduction instead occurs by funneling electrons into the end-
on bridged dinitrogen complex 3. It is tempting to speculate
that this reductive pathway might in fact follow a pathway
not dissimilar from that proposed by Schrock for his
vanadium system.3a The formation of 4 from 3 as reported
in Scheme 1 is a nonbalanced equation. Therefore, intriguing
questions arise about the formation and the nature of possible
byproducts. At this stage, we did not succeed in isolating or
obtaining any evidence for the presence of other vanadium-
containing complexes. We speculate that perhaps some K3N,
undetected in the graphitic residue, may accompany the
formation of 4.
Figure 4. Crystal structure of 4 with thermal ellipsoids drawing at the
30% probability level.
N2] and short V-N2 distance [1.752(6) Å] along with the
nearly linear arrangement of the V1-N2-V2 atoms [V1-
N7-N8 ) 177.7(2)° and N7-N8-V2 ) 176.5(2)°] would
tend to indicate significant double-bond character and thus
a two-electron reduction. However, similar to the case of
other dinitrogen-bridged vanadium complexes13 the magnetic
moment (µeff ) 3.52 µB per vanadium atom) indicates that
the extent of reduction is rather minimal and that the metal
center is predominantly in the divalent state. Accordingly,
treatment of 3 with THF instantly reformed the purple 2.
Further reduction of 3 in toluene in the presence of
KC8.8 afforded the nitride-bridged, mixed-valence complex
[(MeTP)V]2(µ-N)‚(THF) (4) which was crystallized from
THF in moderate yield. As in the previous cases, the central
pyrrole ring is interacting with the metal center in the η5-
mode [V1-centroid ) 2.040(7) Å] (Figure 4). Each V center
assumes a slightly distorted tetrahedral coordination geometry
comprised of the η5-bonded ring, the two pyrrolides, and
the bridging nitride [V1-N1 ) 2.020(7) Å, V1-N2 )
2.001(7) Å, V1-N4 ) 1.7680(15) Å, N1-V1-N3 ) 103.2-
(2)°, N1-V1-N4 ) 108.4(2)°, N1-V1-centroid ) 103.8-
(2)°]. The short V-N bond length and perfectly linear
arrangement of the V-N-V bond [180.0(2)°] are strong
indicators of multiple bond character between the nitride and
the metal centers.
In order to unequivocally identify the bridging atom as a
nitride, a degradation experiment was carried out by simply
treating complex 4 with a diluted HCl solution. The resulting
solution was reacted with NaBPh4, affording a white
precipitate of NH4BPh4 in good yield. The solid was analyzed
via electrospray mass spectrometry and by comparison of
the spectroscopic properties with those of an analytically pure
sample. Finally, when the preparation of 3 and its further
reduction were carried out under 15N2 atmosphere and the
corresponding complex 4 degraded in the same manner, a
sample of 15NH4BPh4 was obtained which showed the
expected pattern in the mass spectrum. This not only
conclusively identified the bridging atom as nitrogen but also
demonstrated that its origin is from atmospheric dinitrogen.
The room-temperature magnetic moment of 4 [µeff ) 1.74
To the best of our knowledge no conclusive information
is available in the literature about the possibility of actually
protonating the nitride atoms bonded to vanadium when
formed from dinitrogen in the way it has been documented
in detail for molybdenum.14,15 In fact, the vanadium ana-
logues of the versatile Mo-nitride species have proven to
be rather elusive. A mononuclear, terminally bonded vana-
dium dinitrogen complex, isolelectronic with the catalytically
active Mo complex, has been obtained.3a The nitride, the
imido, and the ammonia complexes have also been prepared.3a
It was also conclusively established that these species are
interlinked in an overall cycle for ammonia formation.3a,16
However, these important intermediates so far have not been
prepared from dinitrogen itself, and the simple protonation
of the V-nitride as obtained from dinitrogen cleavage has
+
so far escaped characterization. Protonation of 4 to NH4
was found to be very facile, although obviously the vanadium
(14) (a) Laplaza, C. E.; Cummins, C. C. Science 1995, 268, 861. (c) Laplaza,
C. E.; Odom, A. L.; Davis, W. M.; Cummins, C. C. J. Am. Chem.
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C.; Odom, A. L.; Kim, E.; Cummins, C. C.; George, G. N.; Pickering,
I. J. J. Am. Chem. Soc. 1996, 118, 8623. (e) Yandulov, D. V.; Schrock,
R. R. J. Am. Chem. Soc. 2002, 124, 6252.
(15) (a) Yandulov, D. V.; Schrock, R. R.; Rheingold, A. L.; Ceccarelli,
C.; Davis, W. M. Inorg. Chem. 2003, 42, 796. (b) Yandulov, D. V.;
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Inorganic Chemistry, Vol. 46, No. 21, 2007 8839

Vidyaratne et al.
combined, resulting in the formation of a deep blue solution. This
solution was then stirred for 12 h, after which time the solvent
was removed under vacuum. Hexane (200 mL) was then added
along with 4 equiv of water (8 mL, 0.452 mol), affording an off-
white suspension. This suspension was then filtered, and the solid
washed with hexanes (4 × 50 mL). The product was extracted using
several portions of CH2Cl2 (5 × 40 mL) and dried under vacuum,
yielding the final product as an off-white solid. Yield 26.68 g
(53.1%). Anal. Calcd (Found) for C31H27O2N: C 83.57 (83.52); H
6.11 (6.05); N 3.14 (3.11). [M+] 445, found 445. 1H NMR (CDCl3,
500 MHz) δ 2.86 (br, 2H, OH), 3.13 (s, 3H, N-CH3), 5.29 (s, 2H,
central pyrrole) 7.27 (m, 20H, phenyl). 13C NMR (CDCl3, 500
MHz) δ 35.00 (N-CH3), 78.72 (C-OH), 110.06 (pyrrole), 126.90
(phenyl), δ 127.13 (phenyl), δ 127.74 (phenyl), δ 145.55 (ipso-
phenyl). IR (KBr pellet, cm-1) ν 3484 s, br (OH); 3084, 3058,
3025, 2946 s; 1597, m; 1490, 1447, s; 1411, w; 1329, 1293,
s; 1234, w; 1204, m; 1153, s; 1092, s; 1061, w; 1018, 1001, s;
924, 899, m; 867, m; 792, w; 755, 721, 701, s; 670 s; 628, 612, m;
559 w.
moiety is completely hydrolyzed in this case, therefore
preventing the possibility of obtaining a step by step catalytic
cycle similar to that designed for Mo.15b
In conclusion, in this work we have shown that the
tripyrrolide ligand system is capable of supporting dinitrogen
fixation and activation in vanadium chemistry. The ligand
displayed some rather interesting flexibility in terms of
interchanging from σ- to π-bonding mode of the central
alkylated ring depending on the metal oxidation state. While
the trivalent complex prefers to adopt a σ-type of bonding
with the simple quaternization of the N-Me atom, the softer
divalent centers (both as THF or as N2 complexes) display
the expected symmetrical π-bonding mode. It is tempting
to speculate that, similar to the case of lanthanides, the
simultaneous presence of both σ- and π-bonded pyrrolides
is indeed central to the occurrence of dinitrogen activation.
As stated above, V(II) complexes of pyrrolide ligands solely
having either of the two types of bonding9,10 do not display
reactivity with N2. Finally, in the cleavage of the coordinated
N2 unit, rather than a dinuclear complex with two bridging
nitrides as previously reported in a few instances in the
chemistry of vanadium dinitrogen, 3j,4b-d for the first time a
simple linearly bonded hydrolyzable nitride has been ob-
tained from dinitrogen.
Preparation of 2,5-[(2-pyrrolyl) (C6H5)2C]2(MeNC4H2)
(MeTPH2). Neat 2,5-[(C6H5)2C(OH)]2(N-Me-pyrrole) (15.00 g,
0.034 mol) was dissolved in hot (∼80 °C) pyrrole (60 mL). The
addition of 6 drops of methanesulfonic acid resulted in the formation
of a dark red suspension. After 72 h of stirring and heating, the
suspension was cooled to room temperature. The solid was filtered
and washed with cold methanol (3 × 30 mL), yielding the title
compound as a white solid. The product was dried under vacuum
and used without need for further purification (15.27 g, 0.021 mol,
63.4%). Anal. Calcd (Found) for C39H33N3: C 86.15 (86.27), H
6.12 (6.17), N 7.73 (7.79). [M+] 543, found 543. 1H NMR (CDCl3,
500 MHz) δ 2.14 (s, 3H, N-CH3), 5.46 (s, 2H, central pyrrole),
5.98 (m, 2H, pyrrole), 6.05 (m, 2H, pyrrole), 6.68 (m, 2H, pyrrole),
7.07 (m, 8H, phenyl), 7.13 (m, 4H, phenyl), 7.18 (m, 8H, phenyl),
7.69 (br s, 2H, N-H). 13C NMR (CDCl3, 500 MHz) δ 34.80 (N-
CH3), 56.19 (quatenary), 108.10 (pyrrole), 109.37 (pyrrole), 110.35
(pyrrole), 117.26 (phenyl), 126.73 (phenyl), 127.93 (phenyl), 129.66
(ipso-pyrrole), 139.06 (ipso-central pyrrole), 145.52 (ipso-phenyl).
IRneat (cm-1): 3426, m; 3085, 3065, 3026, 2954, m; 1955, 1899,
1816, w; 1595, 1550, m; 1490, 1443, s; 1410, 1394, m; 1233, 1299,
1267, 1231, 1183 m; 1115, 1093, 1042, 1031, m; 1002, w; 962,
930, w; 903, 885, 847, m; 799, 761, 747, 722, 704, s; 622, w; 568,
548 s.
Experimental Section
General Experimental Details. All reactions were carried out
under a dry nitrogen atmosphere unless otherwise stated. Solvents
were dried using an aluminum oxide solvent purification system.
VCl3(THF)3 was prepared via standard procedure.17 A solution of
n-BuLi and benzophenone was purchased from Aldrich and used
with no further purification. Reagent grade pyrrole was also
purchased from Alrich and used after distillation under reduced
pressure (∼50 Torr). Infrared spectra were recorded on an ABB
Bomem FTIR instrument from Nujol mulls prepared in a drybox,
except in the case of air-stable products. Samples for magnetic
susceptibility were preweighed inside a drybox equipped with an
analytical balance and measured on a Johnson Matthey magnetic
susceptibility balance. Elemental analysis was carried out with a
Perkin-Elmer 2400 CHN analyzer. NMR data was collected on a
Varian INOVA 500 spectrometer and referenced to SiMe4. Data
for X-ray crystal structure determination were obtained with a
Bruker diffractometer equipped with a 1 K Smart CCD area
detector. Electrospray mass spectrometry was performed by a
Micromass Quattro-LC electrospray-triple quadrupole mass spec-
trometer in a dilute THF solution.
Preparation of 2,5-[(C6H5)2C(OH)]2(N-Me-pyrrole). A solution
of N-Methylpyrrole (10.0 mL, 0.113 mol) in hexane (150 mL) was
treated with neat TMEDA (51.0 mL, 0.338 mol) and then cooled
to 0 °C in an ice bath. A solution of n-BuLi in hexane (0.338 mol)
was added dropwise while stirring. The reaction mixture was
warmed up to room temperature and refluxed for 4 h. The resulting
off-white suspension was filtered and the solid portion was washed
with hexane (3 × 50 mL), yielding a white product. The solid was
dried under vacuum (1 h) and redissolved in THF (100 mL), and
the resulting solution cooled to -20 °C. In a separate flask, 2.0
equiv of benzophenone (41.18 g, 0.226 mol) were dissolved in THF
(100 mL) and cooled to -20 °C. The two solutions were then
Preparation of [(MeTP)VCl(THF)]‚THF (1). Solid MeTPH2
(0.200 g, 0.368 mmol) was dissolved in 10 mL of THF and treated
with KH (0.030 g, 0.754 mmol). After stirring for 4 h, VCl3(THF)3
(0.137 g, 0.368 mmol) was added to the solution, which turned
brick-red. After stirring overnight, the solution was centrifuged to
remove the small amount of insoluble material and layered with
hexanes. After several days, dark red X-ray quality crystals of 1
were formed (0.123 g, 0.159 mmol, 43.3%). Anal. Calcd (Found)
for C47H47ClN3O2V: C 73.09 (73.21); H 6.13 (6.17); N 5.44 (5.50).
µeff ) 2.85 µB.
Preparation of [(MeTPH2)V(THF)]‚(C7H8)0.5 (2). A dark red
solution of 1 (0.410 g, 0.586 mmol) in THF (20 mL) was stirred
with Na (0.015 g, 0.655 mmol) for about 4 h until all of the Na
was consumed. The solvent was then removed under vacuum. The
dry residue was redissolved in fresh THF (10 mL) and the
suspension centrifuged to remove a small amount of insoluble
material. Toluene (2 mL) was added to reduce the solubility, and
the resulting dark brown mixture was cooled to -35 °C. Dark purple
crystals of 2 suitable for X-ray diffraction were obtained (0.164 g,
0.231 mmol, 39.4%). Anal. Calcd (Found) for C46.50H43N3OV: C
78.57 (78.21), H 6.10 (6.07), N 5.91 (5.60). µeff ) 3.88 µB.
(17) Manzer, L. E. Inorg. Synth. 1982, 21, 138.
8840 Inorganic Chemistry, Vol. 46, No. 21, 2007

Dinitrogen Coordination and CleaWage
Table 1. Crystal Data and Structure Analysis Results for 1-4
1
2
3
4
formula
fw
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
C47H47ClN3O2V
772.27
C46.50H43N3OV
710.78
C98.13H85N8V2
1478.12
monoclinic, Cc
30.376(3)
14.4526(16)
24.201(3)
90
126.7260(10)
90
8515.7(16)
4
C86H78N7V2O
1343.44
triclinic, P1h
11.872(5)
13.197(5)
14.158(5)
68.906(6)
86.868(6)
67.678(6)
1905.9(13)
2
monoclinic, P2(1)/c
13.551(4)
10.846(3)
26.981(8)
90
100.833(5)
90
3895(2)
4
triclinic, P1h
11.7083(18)
13.427(2)
13.625(2)
93.545(3)
92.316(3)
113.775(3)
1951.4(5)
2
radiation (KR, Å)
0.71073
209(2)
1.317
0.367
1624
0.71073
213(2)
1.210
0.292
748
0.71073
201(2)
1.153
0.270
3103
0.71073
208(2)
1.170
0.296
705
T (K)
D
µ
calcd (g cm-3
)
calcd (mm-1
)
F000
R, Rw
GOF
a
0.0571, 0.1245
1.002
0.0605, 0.1535
1.054
0.0712, 0.1232
1.040
0.0885, 0.2109
1.016
a R ) ∑|Fo| - |Fc|/∑|F|. Rw ) [∑(|Fo| - |Fc|)2/∑wFo ]1/2
.
2
Table 2. Selected Bond Distances and Angles
1
2
3
4
V1-N1 ) 1.957(3)
V1-N1 ) 2.067(4)
V1-N1 ) 2.011(5)
V1-N3 ) 2.024(6)
V1-centroid ) 2.023(6)
V1-N7 ) 1.752(6)
N7-N8 ) 1.248(5)
V1-N1 ) 2.018(7)
V1-N3 ) 1.958(3)
V1-N3 ) 2.075(4)
V1-N3 ) 2.002(7)
V1-O1 ) 2.107(2)
V1-N2 ) 2.232(3)
V1-O1 ) 2.115(3)
V1-centroid ) 2.040(7)
V1-N4 ) 1.7680(15)
V1-N2 ) 2.193(3)
V1-Cl1 ) 2.2526(11)
N3-V1-N1 ) 122.35(11)
N3 -V1-O1 ) 94.87(10)
N1 -V1-O1 ) 92.40(10)
N3 -V1-N2 ) 83.50(10)
N1 -V1-N2 ) 82.46(10)
O1 -V1-N2 ) 172.59(9)
N3 -V1-Cl1 ) 119.98(8)
N1 -V1-Cl1 ) 117.44(8)
O1 -V1-Cl1 ) 87.24(7)
N2 -V1-Cl1 ) 99.85(7)
V1-C6 ) 2.293(4)
V1-C9 ) 2.309(4)
N1 -V1-N3 ) 103.2(2)
N1 -V1-N4 ) 108.4(2)
N3 -V1-N4 ) 104.9(2)
centroid-V1-N1 ) 75.6(3)
centroid-V1-N3 ) 120.5(3)
centroid-V1-N4 ) 132.5(2)
V1-N4-V1a ) 180.0(2)
V1-C7 ) 2.430(4)
N1 -V1-N3 ) 100.1(2)
N1 -V1-N7 ) 106.3(2)
N3 -V1-N7 ) 105.7(2)
centroid-V1-N1 ) 103.8(2)
centroid-V1-N3 ) 105.02
centroid-V1-N7 ) 131.7(2)
V1-N7-N8 ) 177.7(2)
N7-N8-V2 ) 176.5(2)
V1-C8 ) 2.454(4)
N1-V1-N3 ) 106.68(14)
N1-V1-O1 ) 97.83(14)
N3-V1-O1 ) 97.72(14)
N1-V1-N2 ) 87.71(13)
N3-V1-N2 ) 89.13(13)
O1-V1-N2 ) 169.51(13)
Preparation of [(MeTP)V(µ-N2)]‚(C7H8)2.9 (3). A solution of
Me3Al in toluene (0.300 mL, 2 M, 0.597 mmol) was added to a
solution of complex 2 (0.425 g, 0.597 mmol) in the same solvent
(15 mL) at -35 °C. The color of the mixture immediately changed
from dark purple to dark black-brown. The resulting suspension
was centrifuged, and the dark brown solution afforded dark brown
crystals of 3 upon standing 2 days at -35 °C (0.688 g, 0.466 mmol,
78%yield). Anal. Calcd (Found) for C98.13H85N8V2: C 79.73 (79.66),
H 5.80 (5.72), N 7.58 (7.44). µeff ) 3.52 µB (per vanadium atom).
Addition of an equivalent amount of THF to a dark brown solution
of 3 in toluene instantly turns the color to purple. Microcrystalline
2 can then be separated upon freezing (78%).
Preparation of [(MeTP)V]2(µ-N)]‚(THF) (4). A suspension of
3 (0.510 g, 0.345 mmol) in toluene (20 mL) was stirred at room
temperature for ∼7 days in the presence of KC8.8 (0.010 g, 0.690
mmol). During this reaction, the color of the mixture gradually
changed from purple to dark green. After 7 days of stirring, the
dark green solution was centrifuged to remove a small amount of
precipitate and concentrated to ∼12 mL. Dark green crystals of 4
were formed after standing at -35 °C for 3 days (0.120 g, 0.179
mmol, 52%). Anal. Calcd (Found) for C86H78N7O2V2: C 76.88
(76.84), H 5.85 (5.81), N 7.30(7.27). µeff ) 1.74 µB
The degradation test was also performed on the 15N adduct of
complex 4. The IR displays the characteristic isotopic shift of NH
stretching band from 3218 to 3248 cm-1. The MS spectrum also
exhibits the ammonium ion peak at 19 m/z compared to the 18 m/z
peak for the non-isotopically enriched sample. The 15N NMR
resonance was located at 340.1 ppm (referenced to CH315NO2).18
X-ray Crystallography. A suitable crystal was selected, mounted
on a thin glass fiber using paraffin oil, and cooled to the data
collection temperature. Data was collected on a Bruker AXS
SMART 1K CCD diffractometer using 0.3° ω scans at 0°, 90°,
180°, and 270° in φ for the triclinic cells and 0°, 120°, and 240° in
φ for the monoclinic cells. Initial unit-cell parameters were
determined from 60 data frames collected at different sections of
the Ewald sphere. Semiempirical absorption corrections based on
equivalent reflections were applied.19 The diffraction data and unit-
cell parameters were consistent with the reported space group P1h
for complexes 2 and 4. No symmetry higher than triclinic was
observed, and the solution in the centrosymmetric space group
yielded chemically reasonable and computationally stable results
of refinement. The data for complex 1 were consistent with the
space group P2(1)/c, and those for 3 were consistent with Cc. All
structures were solved by direct methods, completed with difference
Fourier syntheses, and refined with full-matrix least-squares
procedures based on F2. All non-hydrogen atoms were refined with
anisotropic displacement coefficients. All hydrogen atoms were
Degradation of 4 and Isolation of NH4BPh4. An analytically
pure sample of 4 (0.200 g) was treated with a 10% HCl/H2O
solution. After centrifugation of the solution and addition of
NaBPh4, a white solid was collected, washed, and identified by
IR, NMR, and MS spectra as NH4BPh4 (43% isolated product) by
comparison to the spectra of an analytically pure sample (Aldrich).
(18) Emsleay, J. W.; Feeney, J.; Sutcliffe, L. H. High reasolution NMR
Spectroscopy; Pergamon Press: Elmsford, NY, 1966; Vol. 2, p 1034.
(19) Blessing, R. Acta Crystallogr. 1995, A51, 33.
Inorganic Chemistry, Vol. 46, No. 21, 2007 8841

Vidyaratne et al.
treated as idealized contributions. Structure 2 was treated with the
Squeeze routine of PLATON20 due to two lattice toluene molecules
which were disordered over four positions. The anisotropic refine-
ment parameters for a CH2Cl2 lattice molecule were only satisfac-
tory when the occupancy was reduced to 3/4. Structure 3 was also
treated with the Squeeze routine due to four lattice toluene
molecules which were disordered over four positions. The aniso-
tropic refinement parameters for a toluene lattice molecule were
only satisfactory when the occupancy was reduced to 0.72. All
scattering factors are contained in the SHELXTL 6.12 program
library.21 Relevant crystal data and selected bond distances and
angles are reported in Tables 1and 2, respectively.
Acknowledgment. This work was supported by the
Natural Science and Engineering Council of Canada.
Supporting Information Available: Complete crystallographic
data for complexes 1-4 the MeTPH2. This material is available
free of charge via the Internet at http://pubs.acs.org.
IC701219H
(20) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7.
(21) Sheldrick, G. M. SHELXTL 6.12; Bruker AXS: Madison, WI, 2001.
8842 Inorganic Chemistry, Vol. 46, No. 21, 2007