Dinuclear organometallic dinitrogen complexes of niobium

Article abstract of DOI:10.1016/S0020-1693(02)01305-1

The niobium(III) chloride precursors ^R[P₂N₂]NbCl stabilized by the bis-(amidophosphine) macrocycle (where ^R[P₂N₂]=RP(CH₂SiMe ₂NSiMe₂CH₂)₂

Full text of DOI:10.1016/S0020-1693(02)01305-1

Inorganica Chimica Acta 345 (2003) 53ꢁ62
/
www.elsevier.com/locate/ica
Dinuclear organometallic dinitrogen complexes of niobium
Michael D. Fryzuk *, Christopher M. Kozak, Brian O. Patrick 1
Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, Canada V6T 1Z1
Received 29 April 2002
Dedicated to Professor Richard R. Schrock in honour of his contributions to inorganic chemistry, particularly in the field of dinitrogen activation
Abstract
The niobium(III) chloride precursors R[P2N2]NbCl stabilized by the bis-(amidophosphine) macrocycle (where R[P2N2]ꢀ
RP(CH2SiMe2NSiMe2CH2)2PR, Rꢀphenyl, Ph, or cyclohexyl, Cy), react with MeMgCl under argon to form the paramagnetic
R[P2N2]NbMe (Rꢀ
Ph, 1; RꢀCy, 2) complexes. The methyl complexes 1 and 2 can be stabilized by the donor solvent pyridine to
form the paramagnetic adducts R[P2N2]NbMe(py) (Rꢀ
Ph, 3; RꢀCy, 4). Methyl complexes 1 and 2 readily react with N2 to form
the diamagnetic dinuclear dinitrogen compounds, (R[P2N2]NbMe)2(m-N2) (Rꢀ
Ph, 5; RꢀCy, 6), which have been characterized
crystallographically. Dinitrogen complex 5 is fluxional and undergoes enantiomeric inversion of the six-coordinate niobium centres
via axial rotation about the NbꢀN2ꢀNb axis. Complex 6 shows no enantiomeric inversion, likely due to the steric effects of the
/
/
/
/
/
/
/
/
/
/
cyclohexyl substituents. The dinitrogen ligand itself is labile at elevated temperatures and undergoes reversible dissociation; the
dinitrogen complexes are reformed upon cooling to room temperature. Reaction of 5 and 6 with CO displaces the N2 fragment
forming the bridging acyl complexes (R[P2N2]Nb)2(m-COCH3)2 (Rꢀ
/
Ph, 7; RꢀCy, 8) where the carbonyl fragments bond to the
/
metal centres in a h2 fashion. Addition of H2 to 5 and 6 results in the formation of the previously reported paramagnetic dinuclear
dinitrogen complex, (R[P2N2]Nb)2(m-N2).
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Dinitrogen complexes; Niobium; Crystallography; Amidophosphine ligands; Macrocycles; Fluxionality
1. Introduction
of
R[P2N2]NbCl (where R[P2N2]ꢀ
Me2CH2)2PR, RꢀPh) with potassium graphite (KC8)
under dinitrogen generates the bridging dinitrogen
complex (Ph[P2N2]Nb)2(m-N2) [5]. We have also reported
the synthesis of stable monomeric high-spin niobium-
(III) alkyl complexes of the formula R[P2N2]NbR?
a
Nb(III) chloride precursor. Reaction of
/RP(CH2SiMe2NSi-
The activation of molecular nitrogen by transition
metal complexes has been an active area of research for
/
almost four decades [1ꢁ3]. However, the synthesis of
/
complexes that contain coordinated dinitrogen is still
considered difficult and oftentimes fortuitous. The most
common method to synthesize a dinitrogen compound
involves reduction of a precursor derivative in the
presence of N2. However, this technique does not
guarantee dinitrogen complex formation and often
complicated side reactions ensue [4].
(where Rꢀ
/
Ph or Cy, and R?ꢀCH2SiMe3 or
/
CH(SiMe3)2) [6]. If the stability of these alkyl species
is due to the bulky nature of the alkyl groups, then it
was of interest to explore the reactivity of the much less
sterically demanding niobiumꢁmethyl complexes. The
/
We have recently reported the synthesis of a para-
magnetic diniobium dinitrogen compound by reduction
high Lewis acidity demonstrated by niobium(III) species
in concert with the more accessible metal centre allowed
by the small methyl ligand would likely generate much
more reactive species as compared to the bulkier
analogs. In this work we present the syntheses of the
* Corresponding author. Tel.: ꢂ
/
1-604-822 2897; fax: ꢂ1-604-822
/
2847.
paramagnetic niobiumꢁ
/
methyl complexes R[P2N2]Nb-
E-mail address: fryzuk@chem.ubc.ca (M.D. Fryzuk).
1
UBC X-ray Structural Chemistry Laboratory.
Me and their ability to readily bind dinitrogen.
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 1 3 0 5 - 1

54
M.D. Fryzuk et al. / Inorganica Chimica Acta 345 (2003) 53ꢁ62
/
2. Experimental
2.2.2. Cy[P2N2]NbMe (2)
Cy[P2N2]NbMe was synthesized according to the
procedure used for Ph[P2N2]NbMe above using 500 mg
(0.744 mmol) of Cy[P2N2]NbCl and 0.25 ml of MeMgCl.
Yield 412 mg (85%). Anal. Calc. for C25H57N2NbP2Si4:
C, 45.99; H, 8.80; N, 4.29. Found: C, 45.69; H, 8.67; N,
4.69%. MS (EI) m/z (%) 652, (80) [M]ꢂ. meff (Evans
2.1. General
Unless otherwise stated, all manipulations were
performed under an atmosphere of dry oxygen-free
nitrogen or argon by means of standard Schlenk or
glovebox techniques (Vacuum Atmospheres HE-553-2
glovebox equipped with a MO-40-2H purification
method)ꢀ/2.5 mB.
2.2.3. R[P2N2]NbMe(py) (Rꢀ
/
Ph (3) or Cy (4))
system and a ꢃ
/
40 8C freezer). Hexanes and toluene
R[P2N2]NbMe (200 mg) was dissolved in pyridine (5
ml) to give deep indigo and dark purple solutions for 3
and 4, respectively. Removal of excess pyridine under
vacuum allowed quantitative isolation of R[P2N2]Nb-
were purchased anhydrous from Aldrich and further
dried by passage through a tower of alumina and
degassed by passage through a tower of Q-5 catalyst
under positive pressure of nitrogen [7]. Anhydrous
diethyl ether and THF were stored over sieves and
distilled from sodium benzophenone ketyl under argon.
Pyridine was dried by refluxing over CaH2 under
nitrogen. Nitrogen and argon were dried and deoxyge-
nated by passage through a column containing mole-
cular sieves and MnO. Deuterated benzene and toluene
were dried by refluxing over sodium and potassium
alloy in sealed vessels under partial pressure followed by
trap-to-trap distillation. They were degassed under three
Me(py) as paramagnetic solids. For RꢀPh, 3: Anal.
/
Calc. for C30H50N3NbP2Si4: C, 50.05; H, 7.00; N, 5.84.
Found: C, 50.09; H, 6.67; N, 5.69%. MS (EI) m/z (%)
719, (10) [M]ꢂ. meff (Evans method)ꢀ
Cy, 4: Anal. Calc. for C30H62N3NbP2Si4: C, 49.22; H,
8.54; N, 5.74. Found: C, 49.39; H, 8.64; N, 5.89%. MS
/
2.5 mB. For Rꢀ
/
(EI) m/z (%) 731, (10) [M]ꢂ. meff (Evans method)ꢀ
2.6
mB.
/
2.2.4. (Ph[P2N2]NbMe)2(m-N2) (5)
freezeꢁ
/
pumpꢁ
/
thaw cycles. Unless otherwise stated, all
MeMgCl (0.25 ml, 0.76 mmol) was slowly added to a
stirred solution of Ph[P2N2]NbCl (500 mg, 0.76 mmol) in
toluene (25 ml) under nitrogen to produce an immediate
colour change from green to dark purple. Within several
minutes the solution turned dark brown. Alternatively, a
toluene solution of 1 was exposed to an atmosphere of
N2 causing a colour change from purple to brown. After
stirring for a further 6 h, solvents were removed in vacuo
to produce a brown residue. Extraction with toluene,
filtration through Celite and removal of solvent af-
forded a diamagnetic brown solid that was washed with
hexanes to give an analytically pure product. Crystals
suitable for X-ray diffraction were obtained by cooling a
NMR spectra were recorded on a Bruker AMX-500
instrument operating at 500.132 MHz for 1H spectra. 1H
NMR spectra were referenced to residual protons in the
deuterated solvent. FTIR spectra were recorded on a
BOMEM MB 100 spectrophotometer. Elemental ana-
lyses were performed by Mr. P. Borda of this depart-
ment. Mass Spectrometry was performed on a Kratos
MS 50 by Mr. M. Lapawa, also of this department.
MeMgCl was purchased from Aldrich as a 3.0 M
solution in THF and used as received. CO gas was
purchased from Matheson and H2 purchased from
Praxair. Both were used without further purification.
saturated 50:50 toluene/hexanes solution to ꢃ
Yield 420 mg (85%). 1H NMR (C6D6, 500.13 MHz, 300
K): 0.12 (s, 12H, SiCH3CH3), 0.18 (s, 12H,
/
40 8C.
2.2. Synthesis
d
SiCH3CH3), 0.35 (s, 12H, SiCH3CH3), 0.40 (s, 12H,
SiCH3CH3), 1.25 (m, 4H, PCHH), 1.28 (m, 8H,
3
PCHH), 1.37 (m, 4H, PCHH), 1.78 (t, JHP
2.2.1. Ph[P2N2]NbMe (1)
ꢀ3.27 Hz,
/
MeMgCl (0.25 ml, 0.760 mmol) was slowly added to a
stirred solution of Ph[P2N2]NbCl (500 mg, 0.760 mmol)
in toluene (25 ml) under argon to produce an immediate
colour change from green to dark purple. After stirring
for 6 h, solvents were removed in vacuo to produce a
purple residue. The residue was extracted with toluene
and filtered through Celite. Removal of toluene afforded
a paramagnetic purple solid that was washed with
hexanes to give an analytically pure product. Yield 430
mg (88%). Anal. Calc. for C25H45N2NbP2Si4: C, 46.86;
H, 7.08; N, 4.37. Found: C, 46.49; H, 7.17; N, 4.69%.
3H, NbCH3), 7.05 (m, 12H, meta, para-Ph), 7.76 (m,
8H, ortho-Ph). 13C{1H} NMR (C6D6, 125.76 MHz, 300
K) d 5.91 (s, 4C, SiCH3), 6.62 (s, 4C, SiCH3), 6.67 (s,
4C, SiCH3), 6.99 (s, 4C, SiCH3), 20.88 (s, 4C, PCH2),
2
21.51 (s, 4C, PCH2), 88.02 (t, JPC
ꢀ16.2 Hz, 2C,
/
3
NbCH3), 128.69 (d, JPC
ꢀ4.7 Hz, 8C, meta-Ph),
/
2
129.84 (s, 4C, para-Ph), 132.59 (d, JPC
ꢀ
/
6.65 Hz, 8C,
1
ortho-Ph), 139.23 (d, JPC
ꢀ
/
13.3 Hz, 4C, ipso-Ph).
31P{1H} NMR (C6D6, 202.49 MHz, 300 K) d 13.11
(s). Anal. Calc. for C50H90N6Nb2P4Si8: C, 45.85; H,
6.93; N, 6.42. Found: C, 46.25; H, 7.06; N, 6.40%. MS
MS (EI) m/z (%) 640, (80) [M]ꢂ. meff (Evans method)ꢀ
2.5 mB.
/
(EI) m/z (%) 1308, (10) [M]ꢂ; 1293, (10) ([M]ꢂꢃ
/Me);
1278, (100) ([M]ꢂꢃ
/
2ꢄMe).
/

M.D. Fryzuk et al. / Inorganica Chimica Acta 345 (2003) 53ꢁ
/62
55
2.2.5. (Cy[P2N2]NbMe)2(m-N2) (6)
(
tion yielded dark brown crystals that analyzed for
Ph: Yield 234 mg (80%)
Cy[P2N2]NbMe)2(m-N2) was synthesized according to
(R[P2N2]Nb)2(m-N2). For Rꢀ
/
the procedure used for (Ph[P2N2]NbMe)2(m-N2) above
using 500 mg of Cy[P2N2]NbCl and 0.25 ml of MeMgCl.
Alternatively, a toluene solution of 2 was exposed to an
atmosphere of N2 causing a colour change from purple
to brown. Crystals suitable for X-ray diffraction were
obtained by cooling a saturated 50:50 toluene/hexanes
Anal. Calc. for C48H84N6Nb2P4Si8: C, 45.05; H, 6.62; N,
6.57. Found C, 44.86; H, 6.47; N, 4.13%. MS (EI) m/z
(%) 1278, (95) [M]ꢂ. For Rꢀ
Cy: Yield 245 mg (82%).
/
Anal. Calc. for C48H108N6Nb2P4Si8: C, 44.22; H, 8.35;
N, 6.45. Found: C, 44.46; H, 8.47; N, 4.43%. MS (EI) m/
z (%) 1302, (95) [M]ꢂ.
solution to ꢃ
(C6D6, 500.13 MHz, 300 K):
/
40 8C. Yield 400 mg (80%). 1H NMR
0.36 (s, 12H,
2.2.9. N2 exchange with (R[P2N2]NbMe)2(m-N2)
(R[P2N2]NbMe)2(m-N2) (0.075 mmol) was dissolved in
/
d
SiCH3CH3), 0.47 (s, 12H, SiCH3CH3), 0.49 (s, 12H,
SiCH3CH3), 0.56 (s, 12H, SiCH3CH3), 0.97, 1.19, 1.42
(m, 44H, PC6H11), 1.75 (m, 8H, PCHH), 1.94 (m, 4H,
toluene (10 ml) and degassed by three freezeꢁ
/
pumpꢁ
thaw cycles. The thawed solution was then placed under
1 atm of 15N2 and stirred for 48 h. All volatiles were
removed under vacuum. MS (EI) m/z (%) For 5-15N2:
1310, (10) [M]ꢂ. For 6-15N2: 1334, (10) [M]ꢂ.
PCHH), 3.78 (t, 3JHP
ꢀ
3.20 Hz, 3H, NbCH3). 13C{1H}
/
NMR (C6D6, 125.76 MHz, 300 K) d 6.58 (s, 4C,
SiCH3), 6.60 (s, 4C, SiCH3), 6.80 (s, 4C, SiCH3), 6.82
(s, 4C, SiCH3), 13.27 (s, 4C, PCH2), 13.39 (s, 4C,
PCH2), 25.16 (s, 4C, para-Cy), 26.80 (d, 2JPC
ꢀ9.56 Hz,
/
2.2.10. (Ph[P2N2]NbMe)2(m-N2) and
1
8C, ortho-Cy), 27.74 (d, JPC
ꢀ10.56 Hz, 4C, ipso-Cy),
/
(
Cy[P2N2]NbMe)2(m-N2) crossover experiment
(Ph[P2N2]NbMe)2(m-N2) (0.075 mmol) and (Cy[P2N2]-
28.89 (d, 3JPC
14.8 Hz, 2C, NbCH3). 31P{1H} NMR (C6D6, 202.49
MHz, 300 K): d 12.1 (s, w1/2 503 Hz) 21.7 (s w1/2 534
ꢀ
/
4.78 Hz, 8C, meta-Ph), 86.77 (d, 2JPC
ꢀ
/
NbMe)2(m-N2) (0.075 mmol) were dissolved in toluene
(25 ml) and stirred for 48 h. Removal of solvent allowed
isolation of a brown residue. 1H NMR spectroscopy
shows complicated resonances arising from a mixture of
5, 6, and (Ph[P2N2]NbMe)(m-N2)(Cy[P2N2]NbMe). In
addition to resonances arising from 5 and 6, the
31P{1H} NMR spectrum shows two peaks resulting
from (Ph[P2N2]NbMe)(m-N2)(Cy[P2N2]NbMe). Alterna-
tively, the same product mixture can be obtained by
addition of MeMgCl (0.1 ml, 0.33 mmol) to an
equimolar mixture of Ph[P2N2]NbCl (0.15 mmol) and
Cy[P2N2]NbCl (0.15 mmol) in toluene (20 ml) under N2.
31P{1H} NMR (C6D6, 161.98 MHz, 300 K): d 12.9 (s),
16.5 (s). MS (EI) m/z (%) For (Ph[P2N2]NbMe)(m-
N2)(Cy[P2N2]NbMe): 1320, (10) [M]ꢂ.
ꢀ
/
ꢀ
/
Hz). Anal. Calc. for C50H114N6Nb2P4Si8: C, 45.02; H,
8.61; N, 6.30. Found: C, 45.32; H, 8.63; N, 5.98%. MS
(EI) m/z (%) 1332, (30) [M]ꢂ; 1302, (2) ([M]ꢂꢃ
Me).
/
2ꢄ
/
2.2.6. (Ph[P2N2]Nb)2(m-COMe)2 (7)
(Ph[P2N2]NbMe)2(m-N2) (300 mg, 0.229 mmol) was
dissolved in toluene (20 ml) and degassed by three
freezeꢁ
/
pumpꢁthaw cycles. The thawed solution was
/
then placed under 1 atm of CO and stirred for 12 h. All
volatiles were removed under vacuum leaving a dark
brown paramagnetic residue. Slow evaporation of a
hexanes solution yielded dark brown crystals. Yield 260
mg (85%). Anal. Calc. for C52H90N4Nb2O2P4Si8: C,
46.69; H, 6.78; N, 4.19. Found: C, 43.85; H, 6.86; N,
4.22%.
([M]ꢂꢃPh
MS
[P2N2]NbCOMe).
(EI)
m/z
(%)
668,
(95)
2.3. X-ray crystallographic analyses of 5 and 6
/
Suitable crystals were selected and mounted on a glass
2.2.7. (Cy[P2N2]Nb)2(m-COMe)2 (8)
(
fibre using Paratone-N oil and freezing to ꢃ100 8C.
/
Cy[P2N2]Nb)2(m-COMe)2 was synthesized according
Measurements were made on a Rigaku/ADSC CCD
area detector with graphite monochromated Mo Ka
radiation. Crystallographic data appear in Table 1. The
data were processed [8] and corrected for Lorentz and
polarization effects and absorption. Neutral atom
scattering factors for all non-hydrogen atoms were
taken from the International Tables for X-ray Crystal-
lography [9,10]. Structures were solved by direct meth-
ods [11] and expanded using Fourier techniques [12].
Compound 6 crystallizes with one molecule of hexane
included in the asymmetric unit. This, as well as one
disordered phenyl ring was refined isotropically. All
other non-hydrogen atoms were refined anisotropically.
to the procedure used for (Ph[P2N2]Nb)2(m-COMe)2
above using 300 mg (0.230 mmol) of (Cy[P2N2]-
NbMe)2(m-N2). Yield 250 mg (80%). Anal. Calc. for
C52H114N4Nb2O2P4Si8: C, 49.55; H, 8.46; N, 3.98.
Found C, 49.66; H, 8.59; N, 3.90%. MS (EI) m/z (%)
680, (95) ([M]ꢂꢃCy
[P2N2]NbCOMe).
/
2.2.8. Reaction of (R[P2N2]NbMe)2(m-N2) with H2
(R[P2N2]NbMe)2(m-N2) (0.229 mmol) was dissolved in
toluene (20 ml) and degassed by three freezeꢁ
/
pumpꢁ
/
thaw cycles. The thawed solution was then placed under
1 atm of H2 and stirred for 12 h. All volatiles were
removed under vacuum leaving a dark brown para-
magnetic residue. Slow evaporation of a toluene solu-
Hydrogen atoms were included but not refined and were
˚
fixed in calculated positions with Cꢀ
/
Hꢀ0.98 A.
/

56
M.D. Fryzuk et al. / Inorganica Chimica Acta 345 (2003) 53ꢁ62
/
Table 1
Crystallographic dataa
5
6
Empirical formula
Formula weight
Colour, habit
C
1395.87
50H90N6P4Si8Nb2×
/
C6H14 C50H114N6P4Si8Nb2
1333.88
brown, platelet
brown, block
Crystal size (mm)
Crystal system
Space group
0.30ꢄ
/
0.30ꢄ/0.20
0.35ꢄ
/
0.15ꢄ0.07
/
monoclinic
P21/c (#14)
16.033(1)
26.351(2)
18.940(1)
90
monoclinic
P21/c (#14)
12.1090(3)
23.6327(5)
24.8984(7)
90
˚
a (A)
˚
b (A)
˚
c (A)
a (8)
b (8)
g (8)
V (A )
Z
113.352(2)
90
97.468(2)
90
3
˚
Scheme 1.
7346.1(7)
4
7064.7(3)
4
T (8C)
rcalc (g cmꢃ3
F(000)
m (Mo Ka) (cmꢃ1
Transmission fac-
tors
ꢃ
/
1009
/
1
ꢃ
/
1009
/
1
soluble in even the most non-polar solvents, the methyl
complexes 1 and 2 are insoluble in hexanes. Also,
toluene solutions of 1 and 2 show slow decomposition
of the methyl complexes. Decomposition of these
products can be prevented by the addition of a donor
solvent such as pyridine. Thus, the addition of excess
)
1.262
2944.00
) 5.66
1.254
2840.00
5.86
0.4065ꢁ
/
1.0000
0.7815ꢁ1.0000
/
2umax (8)
Total number of
50.2
43 746
57.4
51 422
pyridine to solutions of
R[P2N2]NbMe(py) (Rꢀ
1
and
2
Cy, 4), which are
generates
reflections
Number of unique 12 909
reflections
/
Ph, 3; Rꢀ
/
16 109
0.054
considerably more stable than the methyl complexes
alone. Removal of excess pyridine under vacuum allows
the isolation of adducts 3 and 4 as paramagnetic indigo
and dark purple solids, respectively. Magnetic moment
measurements of 3 and 4 in solution give values of 2.5
and 2.6 mB, respectively, again indicating high spin d2
metal centres.
Rmerge
Number reflections 7450, nꢀ
with I ]ns(I)
0.130
/2
10645, nꢀ3
/
/
Number of vari-
ables
R (F2, all data)
Rw (F2, all data)
643
631
0.112
0.183
0.065
0.162
0.85
0.057
0.088
0.030
0.038
0.99
R (F, I ꢀ/3s(I))
Exposure of toluene solutions of 1 and 2 to 1 atm of
N2 causes a rapid colour change from purple to brown.
Removal of solvent under vacuum yields the brown
diamagnetic dinitrogen complexes (R[P2N2]NbMe)2(m-
Rw (F, I ꢀ/3s(I))
Goodness-of-fit
a
Rigaku/ADSC CCD diffractometer, Rꢀ
/
a½½F/2o
/
½ꢃ
/
½F/2c
/
½½/a½F/2o
/
½; Rwꢀ
/
(aw(½F/2o
/
½ꢃ
/
½F/2c½)2/aw½F/o2½2)1/2
/
/
.
N2) (Rꢀ
/
Ph, 5; RꢀCy, 6) (Eq. (1)). Toluene solutions
/
of the pyridine adducts 3 and 4 indicate displacement of
pyridine under N2 atmosphere to also form the dinitro-
gen complexes 5 and 6, respectively, albeit at much
slower rates. The dinitrogen ligand in these derivatives is
thermally labile. Heating toluene solutions of these
complexes under partially evacuated conditions regen-
erates purple solutions of the paramagnetic
R[P2N2]NbMe complexes.
3. Results and discussion
3.1. Synthesis of niobium(III) methyl derivatives
The niobium(III) chloride precursors R[P2N2]NbCl
(RꢀPh, Cy) react with MeMgCl in toluene to generate
the purple niobium methyl complexes R[P2N2]NbMe
(RꢀPh, 1; RꢀCy, 2) (Scheme 1). The choice of
/
Crystals of 5 and 6 suitable for X-ray diffraction were
obtained by cooling saturated 50:50 toluene/hexanes
/
/
alkylating agent appears significant as the use of MeLi
produces only intractable products [13]. Methyl com-
plexes 1 and 2 are paramagnetic and solution magnetic
susceptibility measurements by the Evans method
[14,15] indicate a magnetic moment of 2.5 mB for each
complex consistent with high spin d2 niobium centres.
This value is slightly lowered from the predicted spin-
only magnetic moment of 2.83 mB due to the large spin-
orbit coupling effects inherent in niobium systems
[16,17]. Unlike the previously reported bulkier mono-
meric niobium alkyls of [P2N2] [6], which are highly
solutions to ꢃ40 8C. The structures of the dinitrogen
/
complexes were determined unequivocally by single
crystal X-ray structural analysis. The structure of
complex 5 is shown in Fig. 1; crystallographic data are
given in Table 1 and selected bond lengths, angles and
torsion angles in Table 2. The phenyl ring attached to
P(2) is disordered and was refined isotropically. The
niobium bound methyl group, C(50), appears to have a
steric interaction with this phenyl group and is itself
affected by this disorder as shown by the large thermal

M.D. Fryzuk et al. / Inorganica Chimica Acta 345 (2003) 53ꢁ
/62
57
from the ideal 1808 angle. The two amide donors, N(3)
and N(4), and the dinitrogen ligand, N(6), reside in
equatorial positions. The sum of the N(3)ꢀ
/
Nb(2)ꢀ
/
N(4),
N(4)ꢀNb(2)ꢀN(6), and N(3)ꢀNb(2)ꢀN(6) bond angles
/
/
/
/
is 359.658 and indicates an ideal trigonal planar
arrangement. The remaining methyl ligand, C(50),
caps the face defined by N(3), P(4), and N(6) and causes
inequivalent phosphorus environments for P(3) and
P(4). The two Nbꢀ
Nb(1)ꢀC(49) is 2.209(6) A while Nb(2)ꢀ
2.338(13) A.
/
C bond lengths are different;
˚
/
/C(50) is
˚
ð1Þ
Fig. 1. Molecular structure (ORTEP) of (Ph[P2N2]NbMe)2(m-N2) (5).
Silyl methyl groups are omitted for clarity and only ipso carbons of
phenyl rings are shown.
˚
ellipsoid; a close contact of 0.975 A is observed between
The structure of complex 6 is shown in Fig. 2;
crystallographic data are given in Table 1 and selected
bond lengths, angles and torsion angles in Table 3. The
crystal structure of 6 is more symmetrical than that of 5,
that is, both niobium centres occupy distorted octahe-
dral geometries. The macrocycle forces the methyl and
dinitrogen ligands into cis orientation as observed in the
coordination environment of Nb(1) in complex 5. The
[P2N2] ligands on each niobium lie skewed from one
H atoms on the phenyl and methyl groups. The
molecular structure clearly indicates the presence of an
end-on bridging N2 unit. The geometries surrounding
each niobium centre are different. Nb(1) occupies a
distorted octahedral geometry with the [P2N2] phos-
phine donors, P(1) and P(2), residing trans to each other
while the amide donors, N(1) and N(2), lie in cis
orientation. The methyl, C(49), and dinitrogen, N(5),
ligands complete the coordination sphere and lie cis to
one another. The coordination environment of Nb(2) is
much more distorted and is better described as a capped
trigonal bipyramid. P(3) and P(4) of the [P2N2] macro-
cycle occupy axial positions. As is observed in other
another along the Nbꢀ
/
N2ꢀ
/
Nb axis. The niobiumꢁ
/
methyl groups, therefore, are rotated approximately
1258 from one another. The binding of the end-on
bridging dinitrogen moiety is slightly bent in both
compounds with Nb(1)ꢀ
N(5) angles of 174.5(4) and 172.6(5)8, respectively, in 5
and 175.4(2)8 and 173.7(2)8, respectively, in 6. The Nbꢀ
/
N(5)ꢀ
/
N(6) and Nb(2)ꢀ
/
N(6)ꢀ
/
early transition metal complexes of [P2N2], the Pꢀ
/
Nbꢀ
/
P
angles of the phosphine donors are slightly pinched back
/
Table 2
Selected bond lengths (A) and angles (8) in (Ph[P2N2]NbMe)2(m-N2) (5)
˚
Bond lengths
Nb(1)ꢀ
Nb(1)ꢀ
Nb(1)ꢀ
N(5)ꢀ
Nb(2)ꢀ
Nb(2)ꢀ
Nb(2)ꢀ
/
N(1)
P(1)
N(5)
2.263(5)
2.5807(17)
1.869(5)
Nb(1)ꢀ
Nb(1)ꢀ
Nb(1)ꢀ
Nb(2)ꢀ
Nb(2)ꢀ
Nb(2)ꢀ
/
N(2)
P(2)
2.153(5)
2.6110(17)
2.209(6)
2.177(6)
1.840(6)
2.5881(17)
/
/
/
/
C(49)
N(3)
N(6)
P(4)
/
N(6)
1.280(7)
/
/N(4)
2.166(6)
/
/P(3)
2.5879(18)
2.338(13)
/
/C(50)
Bond angles
N(1)ꢀNb(1)ꢀ
N(3)ꢀNb(2)ꢀ
Nb(1)ꢀN(5)ꢀ
N(5)ꢀNb(1)ꢀ
N(1)ꢀNb(1)ꢀ
N(3)ꢀNb(2)ꢀ
/
/
N(2)
N(4)
N(6)
C(49)
N(5)
N(6)
93.82(19)
P(1)ꢀ
P(3)ꢀ
Nb(2)ꢀ
N(6)ꢀNb(2)ꢀ
N(2)ꢀNb(1)ꢀ
N(4)ꢀNb(2)ꢀ
/
Nb(1)ꢀ
Nb(2)ꢀ
N(6)ꢀ
/
P(2)
155.85(2)
156.01(6)
172.6(5)
75.0(4)
/
/
105.2(2)
174.5(4)
87.2(2)
/
/P(4)
/
/
/
/
N(5)
C(50)
N(5)
N(6)
/
/
/
/
/
/
166.8(2)
141.1(2)
/
/
97.8(2)
113.4(2)
/
/
/
/
Nb(1)ꢀN(5)ꢀ
/
/
N(6)ꢀ
/
Nb(2)
90(5)
35(4)
C(49)ꢀNb(1)ꢀ
/
/
N(5)ꢀ/N(6)
54(4)
C(50)ꢀNb(2)ꢀ
/
/
N(6)ꢀ
/N(5)
ꢃ/

58
M.D. Fryzuk et al. / Inorganica Chimica Acta 345 (2003) 53ꢁ62
/
˚
1.28 A range and are
Nꢀ
more typical of NÄ
NPh for which the Nꢀ
/
N bond lengths lie in the 1.25ꢁ
/
/
N double bonds (compared to PhNÄ
/
˚
/
N distance is 1.255 A) [22ꢁ25].
/
This suggests a two-electron reduction of N2 resulting in
a formally diazenido (N2)2ꢃ fragment bridging two
Nb(IV) (d1) centres [26]. This description would im-
plicate strong anti-ferromagnetic coupling between the
two d1 metal centres in order to achieve the observed
diamagnetic nature of the complexes [5]. Alternatively,
these complexes may be assigned low spin Nb(III) (d2)
oxidation states with the N2 moiety acting as a neutral,
strong field ligand. The Nꢀ
/
N bond length of the ligand
is considerably elongated from that of free N2 (where a
˚
N bond length of ca. 1.1 A is observed), however, the
Nꢀ
/
labile nature of the N2 fragment under elevated tem-
perature is characteristic of such weakly activated
dinitrogen complexes [1ꢁ
in end-on bonded dinitrogen complexes is not easily
deduced from NꢀN bond length alone [27].
/
3]. The degree of N2 reduction
Fig. 2. Molecular structure (ORTEP) of (Cy[P2N2]NbMe)2(m-N2) (6).
Silyl methyl groups are omitted for clarity and only phosphorus bound
carbons of cyclohexyl rings are shown.
/
C bond lengths in 6 are experimentally identical; Nb(1)ꢀ
/
3.2. Variable-temperature NMR spectroscopy of
(R[P2N2]NbMe)2(m-N2) (Rꢀ
Ph, Cy)
˚
C(25) possesses a bond distance of 2.206(3) A while
/
˚
Nb(2)ꢀ
lent to the Nb(1)ꢀ
gen ligands themselves display Nꢀ
/
C(50) is 2.200(3) A. These distances are equiva-
The relatively simple room temperature 1H and
31P{1H} NMR spectra of 5 suggest a molecule of higher
symmetry than observed in the solid-state structure. The
1H NMR spectrum of 5 at room temperature displays
four silyl methyl resonances and four sets of methylene
proton resonances. The niobium methyl proton reso-
nance appears as a triplet at 1.78 ppm due to coupling to
two equivalent 31P nuclei. This was confirmed by the
1H{31P} spectrum where a singlet is observed for the
methyl resonance. The 13C{1H} chemical shift of the
methyl group is observed as a triplet at 88.02 ppm, again
indicating coupling to two equivalent 31P nuclei with
/
C(49) bond length in 5. The dinitro-
N bond lengths of
1.280(7) A in 5 and 1.250(3) A in 6.
/
˚
˚
Assignment of formal oxidation states to the bridging
dinitrogen unit and the niobium centres is challenging.
The diamagnetic nature of these species can be most
easily reconciled by describing each niobium centre as
formally Nb(V) (d0) whereby the dinitrogen moiety has
undergone a four-electron reduction to form a hydra-
zido (N2)4ꢃ unit. Other niobium dinitrogen complexes
have been reported where Nꢀ
/
N bond lengths of this
magnitude have been described as formally single bond
in nature [5,18ꢁ/21]. However, as indicated above, the
2JPC
ꢀ
16.2 Hz. The 31P{1H} spectrum displays a single
/
Table 3
Selected bond lengths (A) and angles (8) in (Cy[P2N2]NbMe)2(m-N2) (6)
˚
Bond lengths
Nb(1)ꢀ
Nb(1)ꢀ
Nb(1)ꢀ
N(5)ꢀ
Nb(2)ꢀ
Nb(2)ꢀ
Nb(2)ꢀ
/
N(1)
P(1)
N(5)
2.234(2)
2.6571(7)
1.868(2)
1.250(3)
2.189(2)
2.6391(7)
2.200(3)
Nb(1)ꢀ
Nb(1)ꢀ
Nb(1)ꢀ
Nb(2)ꢀ
Nb(2)ꢀ
Nb(2)ꢀ
/
N(2)
2.184(2)
2.5800(7)
2.206(3)
2.214(2)
1.875(2)
2.5853(7)
/
/P(2)
/
/
C(25)
N(3)
N(6)
P(4)
/
N(6)
/
/N(4)
/
/P(3)
/
/C(50)
Bond angles
N(1)ꢀNb(1)ꢀ
N(3)ꢀNb(2)ꢀ
Nb(1)ꢀN(5)ꢀ
N(5)ꢀNb(1)ꢀ
N(1)ꢀNb(1)ꢀ
N(3)ꢀNb(2)ꢀ
/
/
N(2)
N(4)
N(6)
C(25)
N(5)
N(6)
94.65(8)
P(1)ꢀ
P(3)ꢀ
Nb(2)ꢀ
N(6)ꢀNb(2)ꢀ
N(2)ꢀNb(1)ꢀ
N(4)ꢀNb(2)ꢀ
/
Nb(1)ꢀ
Nb(2)ꢀ
N(6)ꢀ
/
P(2)
160.69(2)
/
/
93.59(6)
175.4(2)
89.5(1)
/
/P(4)
159.26(2)
173.7(2)
88.7(1)
/
/
/
/
N(5)
C(50)
N(5)
N(6)
/
/
/
/
/
/
164.26(8)
164.42(8)
/
/
98.05(8)
99.07(8)
/
/
/
/
Nb(1)ꢀN(5)ꢀ
/
/
N(6)ꢀ
/
Nb(2)
27(4)
62(2)
N(6)ꢀN(5)ꢀ
/
/
Nb(1)ꢀ/C(25)
ꢀ91(2)
/
N(5)ꢀN(6)ꢀNb(2)ꢀ
/
/
/C(50)
ꢃ/

M.D. Fryzuk et al. / Inorganica Chimica Acta 345 (2003) 53ꢁ
/62
59
Fig. 3. (a) Racemization via axial rotation about the Nbꢀ
/
N2ꢀNb axis assuming two octahedral Nb centres in solution. (b) Isomerization via methyl
/
group migration between capped trigonal bipyramidal and octahedral geometries in (Ph[P2N2]NbMe)2(m-N2) (5). Methyl groups on silicon are
omitted for clarity.
axis (Fig. 3(a)). The other possibility is that in solution
the mixed geometries of the niobium centres are
identical to that observed in the solid-state structure,
but a more complicated process ensues. Exchange of the
phosphorus environments occurs by a process where the
methyl group shifts from one face to another, likely via
an octahedral intermediate as shown in Fig. 3(b). In this
case, once both centres are octahedral, rotation about
the Nbꢀ
/
N2ꢀ
/
Nb axis can occur followed by methyl
migration on the opposite niobium centre. Variable
temperature NMR studies were performed on a de-
gassed sample that was sealed under vacuum. As the
temperature is lowered, the 31P{1H} NMR signal
broadens and then decoalesces at 220 K. The low-
temperature spectrum at 190 K consists of three broad
resonances at 30.14, 19.60 and 12.61 ppm. The large
quadrupole moment of the 93Nb nucleus is also a likely
source of increased line broadening of the resonances at
low temperatures. The effect of temperature on the
Fig.
(
4. Variable-temperature
31P{1H}
NMR
spectra
of
Ph[P2N2]NbMe)2(m-N2) (5).
resonance at 13.1 ppm indicating four equivalent
phosphorus centres. This last result contrasts the solid
state structure shown in Fig. 1 wherein all four
phosphorus donors reside in chemically inequivalent
environments. It is evident, therefore, that in solution
some fluxional process is operative. The following are
two possible rationalizations of the different solution
and solid state structures. First, the different geometries
at each niobium centre seen in the molecular structure of
5 may be a solid state effect, which, in solution converts
into two staggered octahedral geometries as observed in
the solid state molecular structure for 6. The resulting
phosphorus environments become equivalent if one
Fig.
(
5. Variable-temperature
31P{1H}
NMR
spectra
of
assumes an unhindered rotation about the Nbꢀ
/
N2ꢀ
/
Nb
Cy[P2N2]NbMe)2(m-N2) (6).

60
M.D. Fryzuk et al. / Inorganica Chimica Acta 345 (2003) 53ꢁ62
/
31P{1H} spectrum of (Ph[P2N2]NbMe)2(m-N2) is shown
in Fig. 4.
(
Ph[P2N2]NbMe)2(m-N2) (5) is not observed in 6. The
low-temperature 31P{1H} NMR data suggest that rota-
tion around the NbꢀN2ꢀNb axis is hindered giving rise
to two inequivalent phosphorus centres; one phosphorus
environment is eclipsed by the NbꢀMe group while the
other is eclipsed by an amido nitrogen of the [P2N2]
ligand. If free rotation about the NbꢀN2ꢀNb axis was
1
A variable temperature H NMR spectroscopy study
/
/
of 5 was also conducted. As the temperature was
lowered to 190 K the resonances, most notably those
of the silyl methyl and niobium bound methyl groups,
began to broaden. The low symmetry of the solid state
molecular structure suggests that each of the silyl methyl
groups would effectively be inequivalent giving rise to 16
resonances. However, over the temperature range from
300 to 190 K only four silyl methyl resonances are ever
observed and no low-temperature limiting spectrum
indicating the lack of symmetry present in the solid
state could be obtained.
/
/
/
occurring, the phosphorus environments would remain
identical and a single resonance would be observed as
for 5. Again, quadrupolar effects of the 93Nb nucleus
may also contribute to line broadening at lower
temperatures.
The room temperature 1H NMR spectrum of 6 is
similar to that of 5, albeit for the presence of cyclohexyl
rather than phenyl resonances. As in complex 5, four
sets of silyl methyl groups are detected. However, the
niobium methyl resonance occurs at 3.78 ppm, slightly
further downfield from that observed in 5. Coupling to
two equivalent 31P nuclei is also observed giving rise to a
triplet. The 13C{1H} chemical shift of the methyl group
3.3. Dinitrogen exchange and crossover in 5 and 6
While the paramagnetic methyl complexes 1 and 2
readily react with N2 at ambient temperature to form
diamagnetic dinitrogen complexes, heating toluene solu-
tions of 5 and 6 above 360 K under partial vacuum
shows loss of signal in their respective 31P{1H} NMR
spectra. The 1H NMR spectra also show broadened and
shifted resonances indicative of the presence of para-
magnetic species. The samples are also observed to
revert to the purple colour characteristic of the para-
magnetic methyl complexes, 1 and 2. These observations
indicate that dissociation of the dinitrogen ligand can be
induced thermally (Eq. (1)). Cooling the samples to 300
K causes reappearance of brown solutions characteristic
of the dinitrogen complexes and confirmed by their
is observed as a triplet at 86.77 ppm coupled to two
2
equivalent 31P nuclei with JPC
ꢀ14.8 Hz.
/
The room temperature 31P{1H} spectrum of 6 differs
from that of 5. Rather than a single peak, two broad
resonances are observed at 12.1 and 21.7 ppm, consis-
tent with inequivalent environments. As the temperature
is lowered to 255 K, these resonances sharpen and
resolve into a pair of doublets at 12.1 and 21.7 ppm,
2
with JPP
ꢀ155.9 Hz, which is consistent with two
/
1
respective H and 31P{1H} NMR spectra.
transdisposed phosphorus environments on the same
metal centre. Lowering the temperature further causes
loss of coupling resolution. As the temperature is raised
to 360 K, the two broad signals appear to coalesce into a
single broad resonance then disappear. This line broad-
ening likely arises from the fast N2 dissociation and
reassociation and from the presence of the resulting
paramagnetic niobium(III) methyl species. The effect of
temperature on the 31P{1H} spectrum of 6 is shown in
Fig. 5.
Above 340 K both 5 and 6 show N2 dissociation and
association (where very broad peaks are observed in the
31P{1H} spectra). Raising the temperature further
causes total absence of resonances in the 31P{1H}
NMR spectra indicating dissociation of the N2 to
generate the paramagnetic R[P2N2]NbMe species (see
below). The presence of inequivalent phosphorus envir-
onments in 6 arises from a less energetically accessible
That the dinitrogen ligand in 5 and 6 is labile is
supported by exchange experiments utilizing 15N2 (Eq.
(2)). A toluene solution of 5 was degassed and then
blanketed with one atm of 15N2. The solution was then
stirred for 72 h before removal of all volatiles under
vacuum. This procedure was repeated for 6. Character-
ization by mass spectrometry shows the presence of
parent peaks corresponding to (R[P2N2]NbMe)2(15N2)
(Rꢀ
Ph, 5-15N2 and Cy, 6-15N2) in addition to those of
/
the non-labelled species. The intensities of the peaks
were identical for both the naturally abundant and
isotopically enriched species suggesting a 1:1 mixture.
15N NMR spectra for 5-15N2 and 6-15N2 were unobser-
vable, likely because of line broadening resulting from
the quadrupolar 93Nb nucleus. Also, coupling to 15N
could not be determined in the 31P{1H} NMR spectrum
due to already broad line widths.
rotation about the Nbꢀ
/
N2ꢀ
/
Nb axis due to the increased
(R[P2N2]NbMe)2(m-14N2)ꢂ15 N2
bulk of the cyclohexyl moieties, which locks 6 into one
of two enantiomers (Fig. 3a). It is possible that the
phenyl groups may be capable of rotating past each
RꢀPh;
RꢀCy;
5
6
X(R[P2N2]NbMe)2(m-15N2)ꢂ14 N2
RꢀPh; 5- 15N2
(2)
other because of the favourability of pꢁ
is not possible for the cyclohexyl substituents. Racemi-
zation by the mechanisms proposed for
/
p stacking which
RꢀCy; 6-15N2

M.D. Fryzuk et al. / Inorganica Chimica Acta 345 (2003) 53ꢁ
/62
61
A crossover experiment was conducted whereby a
toluene solution containing a one-to-one mixture of 5
and 6 was stirred for 72 h (Eq. (3)). Removal of all
volatiles and characterization by NMR spectroscopy
and mass spectrometry shows the presence of the mixed
ligand species, (Cy[P2N2]NbMe)(m-N2)(NbMePh[P2N2]),
in addition to the characteristic 1H and 31P{1H}
resonances and masses, respectively, of 5 and 6 in an
approximately 2:1:1 mixture. These results were com-
pared to a control experiment whereby a toluene
The observation of exchange in the N2 fragment with
15N2 as well as metal centre crossover complicates the
fluxional processes described for 5 and 6. Clearly, any
dinitrogen ligand exchange process that is fast on the
NMR timescale would effectively prevent the observa-
tion of different phosphorus environments; racemiza-
tion would occur as a result of the niobiumꢁN2 bond
/
breaking and forming.
3.4. Reactivity of 5 and 6 with CO and H2
solution containing
a one-to-one mole ratio of
Ph[P2N2]NbCl and Cy[P2N2]NbCl was reacted with
MeMgCl (Eq. (4)). The products resulting from this
experiment also indicate an approximate 2:1:1 mixture
of (Cy[P2N2]NbMe)(m-N2)(NbMePh[P2N2]), 5 and 6 in
the 31P{1H} spectrum. The 31P{1H} NMR spectrum of
the mixed ligand species shows two singlets at 12.9 and
16.5 ppm indicating that the two phosphorus environ-
ments on each ligand are equivalent. Clearly, whereas
the presence of all cyclohexyl substituents in 6 prevents
the exchange of the phosphorus centres, fluxionality in
the mixed ligand derivative is possible.
The addition of one atmosphere of CO to toluene
solutions of (R[P2N2]NbMe)2(m-N2) causes the forma-
tion of dark brown paramagnetic products that display
a single band in the nCO region at 1620 and 1636 cmꢃ1
for 7 and 8, respectively, consistent with h2-acyl Nb-
COCH3 groups (Eq. (5)) [30]. Mass spectrometry and
elemental analyses indicate loss of the N2 fragment and
are in agreement with the formulation of the product as
R[P2N2]NbCOCH3. Single crystals of 7 were obtained
and indicate a dimeric species bridged by acyl ligands.
The carbonyl fragments in (Ph[P2N2]Nb)2(m-h2-
COCH3)2 bond to the metal centres in an h2 fashion.
Acceptable structure refinement was hindered by dis-
order of the bridging acetyl groups, but the overall
nature of the species is discernible from the crystal data.
(
Ph[P2N2]NbMe)2(m-N2)ꢂ(Cy[P2N2]NbMe)2(m-N2)
5
6
X2(Cy[P2N2]NbMe)2(m-N2)(NbMePh[P2N2])
(3)
(4)
0:5Ph[P2N2]NbCl
ꢂ
0
MeMgCl 5ꢂ6
Toluene
N2
0:5Cy[P2N2]NbCl
ð5Þ
ꢂ(Cy[P2N2]NbMe)(m-N2)(NbMePh[P2N2])
The labile nature of the dinitrogen ligand in 5 and 6 is
consistent with a weakly activated N2 fragment. Dini-
trogen complexes that exhibit N2 displacement under
vacuum are more commonly associated with the later
Addition of one atmosphere of H2 to toluene solu-
tions of 5 and 6 also causes the formation of dark brown
paramagnetic products. The products were identified by
mass spectrometry and elemental analyses as the para-
magnetic dinuclear Nb(IV) containing complexes,
(R[P2N2]Nb)2(m-N2) [5]. The precise mechanism by
which the formation of (R[P2N2]Nb)2(m-N2) occurs is
presently unclear. It is likely that hydrogenolysis of the
transition metals and typically possess Nꢀ
/
N bond
lengths shorter than 1.2 A [1]. A side-on bound N2
˚
moiety in (Me2Si(C5H2-2-SiMe3-4-CMe3)2Zr)2(m-h2:h2-
N2) (abbreviated [rac-BpZr]2(m-h2:h2-N2)) possessing a
˚
N bond length of 1.241(3) A, however, does displace
Nꢀ
/
N2 upon exposure to PMe3 to form the Zr(II) species
[rac-BpZr](PMe3)2 [28]. It is unusual that methyl com-
plexes 1 and 2 show the ability to reversibly form
Nbꢀ
/
C bond occurs followed by reduction of the metal
centre by elimination of H2.
dinitrogen complexes 5 and 6 where Nꢀ
/
N bond lengths
of 1.280(7) and 1.250(3) A are observed. A vanadium(II)
˚
complex [PhC(NSiMe3)2]2V(THF)2 exhibits loss of THF
upon heating to yield {[PhC(NSiMe3)2]2V}2(m-h1:h1-
4. Conclusion
˚
N2) [29]. Although the Nꢀ
/
N bond length of 1.235(6) A
for the above N2 complex implies activation to a N/22ꢃ
ligand, this N2 is weakly coordinated and easily
displaced by THF. Ready displacement of N2 despite
Whereas the utilization of large alkyl groups
(ꢀ
/
CH2SiMe3, or ꢀ
/
CH(SiMe3)2) on niobium(III) com-
plexes of R[P2N2] generates stable, paramagnetic mono-
meric species, we have shown that the much smaller
methyl substituent forms highly reactive R[P2N2]NbMe
significant elongation of the Nꢀ
/
N bond is common for
the first row transition metals [27].

62
M.D. Fryzuk et al. / Inorganica Chimica Acta 345 (2003) 53ꢁ62
/
[2] S. Gambarotta, J. Organomet. Chem. 500 (1995) 117.
[3] M. Hidai, Y. Mizobe, Chem. Rev. 95 (1995) 1115.
[4] M.D. Fryzuk, C.M. Kozak, P. Mehrkhodavandi, L. Morello,
B.O. Patrick, S.J. Rettig, J. Am. Chem. Soc. 124 (2002) 516.
[5] M.D. Fryzuk, C.M. Kozak, M.R. Bowdridge, B.O. Patrick, S.J.
Rettig, J. Am. Chem. Soc. 124 (2002) 8389.
species. The pentacoordinate niobiumꢁmethyl com-
/
plexes show a propensity for dinitrogen activation and
generate diamagnetic dinuclear organoniobium dinitro-
gen complexes, which contain end-on bound N2 frag-
ments as elucidated by X-ray molecular structure
determination. Variable-temperature 1H and 31P{1H}
NMR spectroscopies indicate fluxional processes in
dinitrogen complex 5. In the solid state, 5 possesses
different coordination geometries at each niobium atom.
One niobium occupies an octahedral coordination
environment and the second niobium centre occupies a
capped trigonal bipyramidal geometry. Exchange in 5
can proceed via rearrangement in solution to generate
two octahedral niobium centres with rotation about the
[6] M.D. Fryzuk, C.M. Kozak, M.R. Bowdridge, W. Jin, D. Tung,
B.O. Patrick, S.J. Rettig, Organometallics 20 (2001) 3752.
[7] A.B. Pangborn, M.A. Giardello, R.H. Grubbs, R.K. Rosen, F.J.
Timmers, Organometallics 15 (1996) 1518.
[8] teXsan Crystal Structure Analysis Package, Molecular Structure
Corp., The Woodlands, TX, 1995.
[9] International Tables for X-ray Crystallography, vol. C, Kluwer
Academic, Boston, MA, 1992, pp. 200ꢁ206.
/
[10] International Tables for X-ray Crystallography, vol. IV, Kynoch
Press, Birmingham, UK (present distributor Kluwer Academic,
Boston, MA), 1974, pp. 99ꢁ102.
/
Nbꢀ
/
N2ꢀ/Nb axis. In the absence of this geometric
[11] A. Altomare, M.C. Burla, G. Cammali, M. Cascarano, C.
Giacovazzo, A. Guagliardi, A.G.G. Moliterni, G. Polidori, A.
Spagna, SIR97: A New Tool for Crystal Structure Determination
and Refinement, 1999.
rearrangement, a methyl group migration followed by
axial rotation would result in equivalent phosphorus
environments. Complex 6 appears to be locked with
respect to axial rotation resulting in inequivalent
phosphorus centres that appear as two doublets in its
31P{1H} NMR spectrum. Whereas the planar phenyl
substituents are capable of rotating past each other in a
‘‘gear-like’’ fashion, the increased bulk of the cyclohexyl
substituents is believed to hinder rotation about the
[12] P.T. Beurskens, G. Admiraal, G. Beurskens, W.P. Bosman, R. de
Gelder, R. Israel, J.M.M. Smits, DIRDIF94; The DIRDIF-94
program system, Technical Report of the Crystallography La-
boratory, University of Nijmegen, Nijmegen, The Netherlands,
1994.
[13] M.D. Fryzuk, J.B. Love, S.J. Rettig, Organometallics 17 (1998)
846.
[14] D.F. Evans, J. Chem. Soc. (1959) 2003.
Nbꢀ
/
N2ꢀ/Nb axis.
[15] S.K. Sur, J. Magn. Res. 82 (1989) 169.
[16] O. Kahn, Molecular Magnetism, VCH, NewYork, 1993.
[17] R.L. Carlin, Magnetochemistry, Springer-Verlag, Heidelberg,
1986.
5. Supplementary material
[18] M.D. Fryzuk, S.A. Johnson, B.O. Patrick, A. Albinati, S.A.
Mason, T.K. Koetzle, J. Am. Chem. Soc. 123 (2001) 3960.
[19] A. Caselli, E. Solari, R. Scopelliti, C. Floriani, N. Re, C. Rizzoli,
A. Chiesi-Villa, J. Am. Chem. Soc. 122 (2000) 3652.
[20] P. Berno, S. Gambarotta, Organometallics 14 (1995)
2159.
Complete crystallographic data including ORTEP dia-
grams, bond lengths and angles, final atomic coordi-
nates and equivalent isotropic thermal parameters,
anisotropic thermal parameters, intermolecular contacts
and least-squares planes for complexes 5 and 6 have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC Nos. 184666 and 184667. Copies of
this information may be obtained free of charge from
The Director, CCDC, 12 Union Road, Cambridge CB2
[21] J.R. Dilworth, R.A. Henderson, A. Hills, D.L. Hughes, C.
MacDonald, A.N. Stephens, D.R.M. Walton, J. Chem. Soc.,
Dalton Trans. (1990) 1077.
[22] C.J. Brown, Acta Crystallogr. 21 (1966) 146.
[23] J.A. Bouwstra, A. Schouten, J. Kroon, Acta Crystallogr., C 39
(1983) 1121.
[24] L.E. Sutton, Tables of Interatomic Distances and Configuration
in Molecules and Ions; Chemical Society Special Publication No.
11, Chemical Society, London, 1958.
1EZ, UK (fax: ꢂ44-1223-336-033; e-mail: deposit@
ccdc.cam. ac.uk or www: http://www.ccdc.cam.ac.uk).
/
[25] A. Mostad, C. Romming, Acta Chem. Scand. 25 (1971)
3561.
Acknowledgements
[26] For a heterodinuclear niobium/molybdenum dinitrogen complex
see: D.J. Mindiola, K. Meyer, J.P.F. Cherry, T.A. Baker, C.C.
Cummins, Organometallics 19 (2000) 1622.
We thank NSERC of Canada for funding.
[27] G.J. Leigh, Acc. Chem. Res. 25 (1992) 177.
[28] P.J. Chirik, L.M. Henling, J.E. Bercaw, J. Am. Chem. Soc. 20
(2001) 534.
References
[29] S.K. Hao, P. Berno, R.K. Minhas, S. Gambarotta, Inorg. Chim.
Acta 244 (1996) 37.
[30] M.E. Kerr, N. Sarker, A.S. Hneihen, G.K. Schulte, J.W. Bruno,
Organometallics 19 (2000) 901.
[1] M.D. Fryzuk, S.A. Johnson, Coord. Chem. Rev. 200ꢁ
/202 (2000)
379.