Nitride formation by thermolysis of a kinetically stable niobium dinitrogen complex

Article abstract of DOI:10.1021/ja025997f

The reduction of [P₂N₂]NbCl (where [P₂N₂] = PhP(CH₂SiMe₂NSiMe₂ CH₂)₂PPh) with KC₈ under a dinitrogen atmosphere generates the paramagnetic dinuclear dinitrogen complex ([P₂N₂]Nb)₂(μ-N₂) (2). Complex 2 has been characterized crystallographically and by EPR spectroscopy. Variable-temperature magnetic susceptibility measurements indicate that 2 displays antiferromagnetic coupling between two Nb(IV) (d¹) centers. A density functional theory calculation on the model complex [(PH₃)₂(NH₂)₂Nb]₂ (μ-N₂) was performed. Thermolysis of ([P₂N₂]Nb)₂(μ-N₂) in toluene generates the paramagnetic bridging nitride species where one N atom of the dinitrogen ligand inserts into the macrocycle backbone to form [P₂N₂]- Nb(μ-N)Nb[PN₃] (3) (where [PN₃] = PhPMe(CHSiMe₂NSiMe₂CH₂P(Ph) CH₂SiMe₂NSiMe₂N)). Complex 3 has been characterized in the solid state as well as by variable-temperature magnetic susceptibility measurements. The reaction of ([P₂N₂]Nb)₂(μ-N₂) with phenylacetylene displaces the dinitrogen fragment to generate a paramagnetic η²-alkyne complex, [P₂N₂]Nb(η²-HCCPh) (4).

Full text of DOI:10.1021/ja025997f

Published on Web 06/20/2002
Nitride Formation by Thermolysis of a Kinetically Stable
Niobium Dinitrogen Complex
Michael D. Fryzuk,* Christopher M. Kozak, Michael R. Bowdridge,
Brian O. Patrick,† and Steven J. Rettig†,‡
Contribution from the Department of Chemistry, UniVersity of British Columbia,
2036 Main Mall, VancouVer, British Columbia, V6T 1Z1, Canada
Received February 21, 2002
Abstract: The reduction of [P2N2]NbCl (where [P2N2] ) PhP(CH2SiMe2NSiMe2CH2)2PPh) with KC8 under
a dinitrogen atmosphere generates the paramagnetic dinuclear dinitrogen complex ([P2N2]Nb)2(µ-N2) (2).
Complex 2 has been characterized crystallographically and by EPR spectroscopy. Variable-temperature
magnetic susceptibility measurements indicate that 2 displays antiferromagnetic coupling between two
Nb(IV) (d1) centers. A density functional theory calculation on the model complex [(PH3)2(NH2)2Nb]2(µ-N2)
was performed. Thermolysis of ([P2N2]Nb)2(µ-N2) in toluene generates the paramagnetic bridging nitride
species where one N atom of the dinitrogen ligand inserts into the macrocycle backbone to form [P2N2]-
Nb(µ-N)Nb[PN3] (3) (where [PN3] ) PhPMe(CHSiMe2NSiMe2CH2P(Ph)CH2SiMe2NSiMe2N)). Complex 3
has been characterized in the solid state as well as by variable-temperature magnetic susceptibility
measurements. The reaction of ([P2N2]Nb)2(µ-N2) with phenylacetylene displaces the dinitrogen fragment
to generate a paramagnetic η2-alkyne complex, [P2N2]Nb(η2-HCCPh) (4).
Introduction
plexes cleave N2 to form Mo(VI) nitrides was the first of a series
of important studies of nitride formation from coordinated
The literature concerning the synthesis and characterization
of dinitrogen complexes is experiencing a resurgence. This has
been fueled by a number of separate events that include the
X-ray characterization of the active site of a nitrogenase
enzyme,1 the observation of new bonding modes for the
dinitrogen ligand in coordination chemistry,2,3 the reports of new
transformations of coordinated dinitrogen that involve reaction
with H2 and silanes,4 and the formation of nitrides from
coordinated N2.5-9 Taken together these results suggest that this
area is undergoing a renaissance.10
dinitrogen.5-7,14,15 Many of these systems involve the formation
of a dinuclear dinitrogen complex as an observed intermediate.6,8
Irrespective of observable intermediates, a total of six electrons
must be formally added to the dinitrogen unit, and so far this
has been best accomplished by using two metal centers.
In this paper we present our attempts to design a system that
can activate dinitrogen and furthermore induce N-N bond
cleavage to generate a nitride complex. Our success with the
formation of the side-on bound dinuclear zirconium dinitrogen
complex,4 ([P2N2]Zr)2(µ-η2:η2-N2) (where [P2N2] ) PhP(CH2-
SiMe2NSiMe2CH2)2PPh), provided a starting point since incor-
poration of the same macrocyclic ancillary ligand system onto
a group 5 metal (i.e., V, Nb, or Ta) would formally provide
one extra electron per metal over that found for the aforemen-
tioned zirconium analogue. If one assumes that the formation
of ([P2N2]Zr)2(µ-η2:η2-N2) corresponds to a formal addition of
4 electrons to neutral N2, then the availability of an additional
two electrons per dinuclear group 5 complex would provide
the necessary six electrons required for cleavage of dinitrogen.
We recently reported the synthesis of niobium(III) complexes
of the [P2N2] ligand system.16,17 In this work, we examine the
preparation and characterization of a dinuclear dinitrogen
complex of niobium, its reactivity with protons and alkynes,
and its thermolysis to form a dinuclear nitride species.
The cleavage of coordinated dinitrogen to generate metal
nitride derivatives is considered to be a fundamental process in
nitrogen fixation both industrially and biologically.3,11-13 The
recent discovery that bulky molybdenum(III) tris(amide) com-
* Corresponding author. E-mail: fryzuk@chem.ubc.ca.
† UBC X-ray Structural Chemistry Laboratory.
‡ Deceased October 27, 1998.
(1) Kim, J.; Rees, D. C. Nature 1992, 360, 563.
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Koetzle, T. K. J. Am. Chem. Soc. 2001, 123, 3960.
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9
10.1021/ja025997f CCC: $22.00 © 2002 American Chemical Society
J. AM. CHEM. SOC. 2002, 124, 8389-8397
8389

A R T I C L E S
Fryzuk et al.
Results and Discussion
Synthesis and Structure of the Niobium Dinitrogen
Compound, ([P2N2]Nb)2(µ-N2) (2). Our continued interest in
the synthesis of dinitrogen compounds and the availability of
the Nb(III) starting complex [P2N2]NbCl (1)16 prompted us to
examine its ability to bind N2 under reducing conditions. Within
group 5, there are a number of dinitrogen complexes of
vanadium and tantalum that have been reported;2,18-30 however,
those of niobium continue to be rare.8,31-34 Reduction of [P2N2]-
NbCl (1) by KC8 in toluene under 1 atm of N2 for 48 h allows
isolation of a dark brown paramagnetic solid in 85% yield that
has been characterized as ([P2N2]Nb)2(µ-N2) (2). Varying the
reaction conditions by changing the solvent to THF, increasing
the pressure of N2 to 4 atm, or performing the reaction at
temperatures from -78 to +65 °C did not appear to affect the
yield. Reaction of 1 equiv of PbCl2 with a brown toluene
solution of ([P2N2]Nb)2(µ-N2) overnight caused the appearance
of a deep green solution and a gray precipitate of metallic lead.
Filtration followed by removal of solvent allowed isolation of
a red solid that was spectroscopically characterized as the
starting niobium(III) chloride, [P2N2]NbCl (1) (Scheme 1).
The 1H NMR spectrum of dinitrogen complex 2 shows only
broadened resonances from which no structural information
could be determined. However, mass spectrometry indicates a
m/z peak of 1278, which correlates to the dinuclear complex 2
in Scheme 1. The presence of a dinitrogen ligand was confirmed
by protonation of 2 by Me3N‚HCl and the successful detection
of hydrazine (62%). Dark brown, X-ray quality crystals were
obtained from slow evaporation of a toluene solution. The solid-
state molecular structure is shown in Figure 1, with crystal-
lographic data listed in Table 1. The X-ray data indicate the
dinitrogen ligand is bound in an end-on fashion bridging the
two niobium centers. The coordination geometry of each
niobium center is a distorted trigonal bipyramid, with the three
nitrogen ligands (two from the macrocycle and one from the
bridging N2) occupying the equatorial plane (sum of the
equatorial angles is 360.0°); the phosphorus donors lie in slightly
pinched back apical positions giving P(1)-Nb(1)-P(2) and
P(3)-Nb(2)-P(4) angles of 161.07(5)° and 162.35(5)°, respec-
tively, which are consistent with those observed in other early
Figure 1. Molecular structure (ORTEP) of ([P2N2]Nb)2(µ-N2) (2). El-
lipsoids are drawn at the 30% probability level, silyl methyl groups are
omitted for clarity, and only ipso carbons of phenyl rings are shown. Selected
bond lengths (Å), angles (deg), and dihedral angles (deg): N(5)-N(6),
1.272(5); Nb(1)-N(5), 1.853(4); Nb(2)-N(6), 1.850(4); Nb(1)-P(1),
2.5729(14); Nb(1)-P(2), 2.5799(14); Nb(1)-N(1), 2.171(4); Nb(1)-N(2),
2.157(4); Nb(2)-P(3), 2.5709(15); Nb(2)-P(4), 2.5720(15); Nb(2)-N(3),
2.156(4); Nb(2)-N(4), 2.149(4); Nb(1)-N(5)-N(6), 179.5(3); Nb(2)-
N(6)-N(5), 179.1(3); P(1)-Nb(1)-P(2), 161.07(5); N(1)-Nb(1)-N(2),
116.1(2); P(3)-Nb(2)-P(4), 162.35(5); N(3)-Nb(2)-N(4), 114.9(2);
N(1)-Nb(1)-N(5), 123.8(2); N(2)-Nb(1)-N(5), 120.1(2); N(3)-Nb(2)-
N(6), 121.7(2); N(4)-Nb(2)-N(6), 123.4(2); N(1)-Nb(1)-P(1),
85.77(11); N(1)-Nb(1)-P(2), 83.57(11); N(2)-Nb(1)-P(1), 84.35(11);
N(2)-Nb(1)-P(2), 86.36(12); N(3)-Nb(2)-P(3), 86.47(12); N(3)-Nb-
(2)-P(4), 84.26(12); N(4)-Nb(2)-P(3), 84.09(13); N(4)-Nb(2)-P(4),
86.24(13); P(1)-Nb(1)-Nb(2)-P(4), 59.6.
Scheme 1
(18) Song, J.-I.; Berno, P.; Gambarotta, S. J. Am. Chem. Soc. 1994, 116, 6927.
(19) Berno, P.; Hao, S. K.; Minhas, R.; Gambarotta, S. J. Am. Chem. Soc. 1994,
116, 7417.
(20) Buijink, J. K. F.; Meetsma, A.; Teuben, J. H. Organometallics 1993, 12,
2004.
(21) Ferguson, R.; Solari, E.; Floriani, C.; Osella, D.; Ravera, M.; Re, N.; Chiesi-
Villa, A.; Rizzoli, C. J. Am. Chem. Soc. 1997, 119, 10104.
(22) Ferguson, R.; Solari, E.; Floriani, C.; Chiesivilla, A.; Rizzoli, C. Angew.
Chem., Int. Ed. Engl. 1993, 32, 396.
(23) Rehder, D.; Woitha, C.; Priebsch, W.; Gailus, H. J. Chem. Soc., Chem.
Commun. 1992, 364.
(24) Woitha, C.; Rehder, D. Angew. Chem., Int. Ed. Engl. 1990, 29, 1438.
(25) Leigh, G. J.; Prietoalcon, R.; Sanders, J. R. J. Chem. Soc., Chem. Commun.
1991, 921.
(26) Turner, H. W.; Fellman, J. D.; Rocklage, S. M.; Schrock, R. R.; Churchill,
M. R.; Wasserman, H. J. J. Am. Chem. Soc. 1980, 102, 7809.
(27) Schrock, R. R.; Wesolek, M.; Liu, A. H.; Wallace, K. C.; Dewan, J. C.
Inorg. Chem. 1988, 27, 2050.
(28) Rocklage, S. M.; Turner, H. W.; Fellman, J. D.; Schrock, R. R. Organo-
metallics 1982, 1, 703.
(29) Churchill, M. R.; Wasserman, H. J. Inorg. Chem. 1981, 20, 2899.
(30) Churchill, M. R.; Wasserman, H. J. Inorg. Chem. 1982, 21, 218.
(31) Berno, P.; Gambarotta, S. Organometallics 1995, 14, 2159.
(32) Zanotti-Gerosa, A.; Solari, E.; Giannini, L.; Floriani, C.; Chiesi-Villa, A.;
Rizzoli, C. J. Am. Chem. Soc. 1998, 120, 437.
transition metal complexes of [P2N2].16,35-37 The two [P2N2]-
Nb ends of 2 are staggered with respect to each other about the
axis defined by the Nb(1)-N(5)-N(6)-Nb(2) unit; the P(1)-
Nb(1)-Nb(2)-P(4) dihedral angle is 59.6°. The P-Nb-P and
N-Nb-N angles displayed in the macrocycle are not unusual
with respect to those previously observed in pentacoordinate
(35) Fryzuk, M. D.; Johnson, S. A.; Rettig, S. J. Organometallics 1999, 18,
4059.
(33) Dilworth, J. R.; Henderson, R. A.; Hills, A.; Hughes, D. L.; MacDonald,
C.; Stephens, A. N.; Walton, D. R. M. J. Chem. Soc., Dalton Trans. 1990,
1077.
(36) Fryzuk, M. D.; Johnson, S. A.; Rettig, S. J. Organometallics 2000, 19,
3931.
(37) Fryzuk, M. D.; Love, J. B.; Rettig, S. J. Organometallics 1998, 17, 846-
(34) Rocklage, S. M.; Schrock, R. R. J. Am. Chem. Soc. 1982, 104, 3077.
853.
9
8390 J. AM. CHEM. SOC. VOL. 124, NO. 28, 2002

Nitride Formation
A R T I C L E S
Table 1. Crystallographic Dataa
2
3
4
formula
FW
C48H84N6Nb2P4Si8 C48H84N6Nb2P4Si8 C32H48N2NbP2Si4
1279.62
1279.62
727.94
color, habit
brown, prism
blue, block
red, irregular chip
crystal size, mm3 0.35 × 0.40 × 0.45 0.35 × 0.25 × 0.25 0.40 × 0.20 × 0.10
crystal system
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
monoclinic
P21/n (no. 14)
16.1151(11)
15.887(2)
26.0621(7)
90
92.8130(4)
90
6664.6(7)
4
monoclinic
P21/c (no. 14)
14.527(1)
13.4604(9)
33.633(3)
90
99.360(4)
90
6489.1(8)
4
orthorhombic
Pna21 (no. 33)
15.8295(6)
12.5210(4)
19.466(1)
90
90
90
3858.3(5)
4
Z
Figure 2. EPR spectrum of ([P2N2]Nb)2(µ-N2) (2) in toluene, g ) 1.975,
a(93Nb) ) 108.2 G, a(31P) ) 22.9 G at 300 K.
T, °C
F
-93 ( 1
-100 ( 1
-75 ( 1
calc, g/cm3
1.275
1.310
1.253
µ(Mo KR), cm-1 6.18
R(F, I g 3σ(I)) 0.073
Rw(F, I g 3σ(I)) 0.069
6.35
0.061
0.141
0.101
0.1537
1.01
5.42
0.027
0.031
0.048
0.070
0.83
R(F2, all data)
0.114
Rw(F2, all data) 0.124
gof 2.59
a Rigaku/ADSC CCD diffractometer, R ) ∑||Fo | - |Fc ||/∑|Fo |;
2
2
2
2
2
2 2
Rw ) (∑w(|Fo | - |Fc |)2/∑w|Fo | )1/2
.
Table 2. N-N Bond Lengths in Niobium Dinitrogen Complexes
compd
N−N bond length (Å)
ref
[(Et2NCS2)3Nb]2(µ-N2)
[(Cy2N)3Nb]2(µ-N2)
1.252(16)
1.34(1)
1.390(17)
1.237(4)
1.272(5)
33
31
8
[{p-But-calix[4]-(O)4Nb}2(µ-N2)]Na2
([NPN]NbCl)2(µ-N2)
2
([P2N2]Nb)2(µ-N2)
this work
Figure 3. Magnetic susceptibility (O) and moment (4) versus temperature
plot per mole of ([P2N2]Nb)2(µ-N2) (2). The Bleaney-Bowers model was
employed to generate the lines with J ) -67.5 cm-1, g ) 1.975, TIP )
0.0027 cm3 mol-1, and P ) 0.061 (F ) 0.0077).
niobium chloride and alkyl complexes.16 The niobium atoms
themselves are joined by a linear end-on bridging N2 unit with
Nb(1)-N(5)-N(6) and Nb(2)-N(6)-N(5) angles of 179.5(3)°
and 179.1(3)°, respectively. The N-N bond length of 1.272(5)
Å is slightly longer than that observed in diazobenzene (1.255
Å) but shorter than that observed in hydrazine (1.46 Å). A
comparison of N-N bond lengths in previously reported
niobium dinitrogen complexes appears in Table 2.
magnetism (TIP) likely account for the observed moment. The
data can be modeled (Figure 3) by using the Bleaney-Bowers
equation, assuming the interaction between the two spin centers
can be described by the Heisenberg spin Hamiltonian H )
-2JS1‚S2, considering a pair of exchange-coupled centers where
1
S1 ) S2 ) /2. To account for the presence of a small amount
If one assumes that the N-N bond length in 2 more closely
approximates a N-N double bond of a diazenido type linkage,
an N22- unit, then each niobium center should correspond to a
formal oxidation state of +3 (the [P2N2] ligand is assigned a
-2 charge). However, as will be discussed below, the EPR data
and variable-temperature magnetic studies are more consistent
with a formalism in which both metal centers are Nb(IV).
The EPR spectrum of 2 displays a decet of triplets centered
at g ) 1.975 (Figure 2). The 30 line pattern results from
coupling of a single unpaired electron to a I ) 9/2 93Nb nucleus
(a(93Nb) ) 108.2 G) and two I ) 1/2 31P nuclei (a(31P) ) 22.9
G). The solid-state magnetic susceptibility (øm) of the dinitrogen
compound, 2, was measured from 2 to 300 K with a SQUID
magnetometer. The plot of magnetic susceptibility per dimer
vs temperature shows a broad maximum at 120 K, indicating
the presence of antiferromagnetic exchange. The magnetic
moment decreases upon cooling the sample to 2 K, consistent
with the sample approaching a singlet ground state. The room-
temperature magnetic moment of 2.9 µB per mole of dimer
appears high if one assumes the presence of two d1 Nb(IV)
centers in dinitrogen complex 2. However, large spin-orbit
coupling characteristic of second- and third-row transition metals
in addition to the presence of temperature-independent para-
of paramagnetic impurity the expression was combined with
the Curie law term,
NAg2µB S(S + 1)
2
øpara
)
3kT
according to
øm ) [1 - P]øNb + Pøpara
where P represents the fraction of paramagnetic S ) 1 impurity.
The presence of temperature-independent paramagnetism
(TIP) was also included in the model. The resulting analysis of
the collected data yielded g ) 1.975, J ) -67.5 cm-1, P )
0.061, and TIP ) 0.0027 cm3 mol-1. These results indicate a
significant amount of antiferromagnetic coupling between the
1
two S ) /2 spin centers through the N-N bridge and despite
the comparatively short N-N bond the coordinated N2 ligand
can be formally considered a hydrazido, N24-, moiety. Anti-
ferromagnetic coupling via N-N bonds has been observed
previously for related dinuclear vanadium dinitrogen complexes,
but the magnitude of the coupling is very small or negligible.38
9
J. AM. CHEM. SOC. VOL. 124, NO. 28, 2002 8391

A R T I C L E S
Fryzuk et al.
Table 3. Selected Bond Lengths, Bond Angles ,and Dihedral
Angles for the ab Initio DFT Geometry Optimization of the Model
Complex [(H3P)2(H2N)2Nb]2(µ-N2) (2A); Total Energy ) -479.309
Ha
atom
atom
distance (Å)
atom
atom
distance (Å)
Ndinitrogen
Nb
Ndinitrogen
Ndinitrogen
1.255
1.925
Nb
Nb
P
2.636
2.065
Namide
atom
atom
atom
angle (deg)
atom
P(1)
atom
atom angle (deg)
Nb(1) N(5)
N(1)
N(5)
N(6)
179.99
113.00
100.00
Nb(1) P(1)
160.00
84.18
84.82
Nb(1) N(2)
Nb(1) P(1)
N(1) Nb(1) P(1)
N(1) Nb(1) P(2)
Figure 4. Depiction of ligand geometries for the crystallographically
observed 59.6° and the calculated 96.8° P-Nb-Nb-P torsion angles in 2.
atom
atom
atom
atom
dihedral angle (deg)
qualitatively for two ML4 fragments with a single N2 unit.42
The ML4 fragments possess three available d orbitals (dxy, dxz,
and dyz) that point between the M-L and N2 ligands while the
P(1)
P(1)
Nb(1)
Nb(1)
Nb(2)
N(5)
P(3)
N(6)
96.8
58.9
2
2
remaining dz2 and dx -y orbitals are utilized in σ-bonding with
the ligands. In the end-on bridging mode, the symmetrical
interaction with the π*-orbitals of the N2 molecule generates
two molecular orbitals in the M-(µ-N2)-M fragment with
π-symmetry, which, if occupied, would be bonding for the
metal-nitrogen interaction, but antibonding with respect to the
N2 unit itself. For the analogous side-on bonding mode, as
observed in ([P2N2]Zr)2(µ-η2:η2-N2), one generates two bonding
molecular orbitals: one of π-symmetry and the other of
δ-symmetry. These too are both bonding with respect to the
metal-nitrogen interaction but antibonding for the N2 fragment.
However, the δ-molecular orbital is less stabilized than the
respective π-molecular orbital and as such the end-on bonding
mode that favors π-interaction is preferred on energetic grounds.
The existence of an end-on bonded dinitrogen fragment in 2
rather than the side-on bonding mode exhibited by ([P2N2]Zr)2-
(µ-η2:η2-N2) can be explained by an examination of the occupied
molecular orbitals. As has been shown by previously reported
ab initio and semiempirical studies of bridging dinitrogen
compounds,38,43,44 the metal-nitrogen bonding molecular orbit-
als are present in the nearly degenerate HOMO-2 and HOMO-3
orbitals. The HOMO and HOMO-1 are the entirely metal
centered orbitals of dδ symmetry, as expected. For a Nb(IV)
(d1) center, the HOMO is singly occupied. In ([P2N2]Zr)2-
(µ-η2:η2-N2) this orbital is vacant and can therefore participate
in a δ-bonding interaction with the π* orbital of the N2 fragment.
Because the δ orbitals have no contribution from the bridging
N2 ligand, it is surprising that any significant antiferromagnetic
coupling is observed between the two formally d1 metal centers.
A π(N-N) bonding combination and a π(M-N) antibonding
combination of orbitals describe the LUMO. Normally this
precludes any orbital overlap, but by nature of the crystallo-
graphically imposed “twist” there appears to be a possibility
for M-N orbital overlap resulting in a potential pathway for
antiferromagnetic coupling to occur.
Density Functional Theory Analysis of the Bonding in
([P2N2]Nb)2(µ-N2). Geometry optimizations were performed by
using density functional theory. To simplify the calculations
the model complex [(H3P)2(H2N)2Nb]2(µ-N2) (2A) was studied
with restrictions placed on the Ndinitrogen-Nb-Namide-H dihedral
angles. These restrictions were intended to mimic the rigid
geometry imposed by the [P2N2] ligand, which prevents the
amido donors from freely rotating. The geometry of model
complex 2A was optimized (with the exception of the previously
stated constraints) with use of the Gaussian 98 program39 and
the hybrid functional B3LYP method.40 The basis functions and
effective core potentials (ECP) used were those in the LANL2DZ
basis set41 but with additional d-polarization functions added
to P atoms with the exponent of the d-functions set at 0.37.
This level of theory has been used previously in the study of
dinitrogen complexes.2,3
Selected optimized bond lengths, bond angles, and dihedral
angles for the model complex 2A are shown in Table 3. The
numbering scheme used is the same as for the structure of
complex 2 shown in Figure 1. The general features of the
optimized geometry of this model complex are in good
agreement with the structure of complex 2 except that the model
complex attains a geometry where the [P2N2] ligands exhibit a
P(1)-Nb(1)-Nb(2)-P(3) torsion angle of 96.8° about the Nb-
N2-Nb axis (Figure 4). Also, the model displays slightly
elongated Nb-P and Nb-Ndinitrogen bonds. This inconsistency
between model complex 2A and the structure of complex 2 may
be attributed to crystal packing effects that may be strongly
influenced by the presence of phenyl groups on the phosphine
donor atoms.
The experimental evidence for dinitrogen to bind end-on to
transition metals is overwhelming,3,11,12 although more examples
of the side-on bridging mode are becoming known. The
predominance of the end-on binding mode has been rationalized
Thermolysis of ([P2N2]Nb)2(µ-N2) (2). It has been shown
that a molybdenum(III) tris(amide) readily coordinates dinitro-
gen and subsequently undergoes thermolysis to generate a
molybdenum nitride.5-7 Bearing in mind that the two Nb(IV)
centers in our ([P2N2]Nb)2(µ-N2) complex can in theory each
act as a one-electron reductant, the further reduction of the
hydrazido N24- ligand to nitrido N3- seems plausible. Heating
(38) Ferguson, R.; Solari, E.; Floriani, C.; Osella, D.; Ravera, M.; Re, N.;
ChiesiVilla, N.; Rizzoli, C. J. Am. Chem. Soc. 1997, 119, 10104.
(39) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Petersson, V. A.; Ayala, P.
Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari,
K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.;
Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen,
W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Repogle, E. S.; Pople,
J. A. Gaussian 98, Revision A.7; Gaussian, Inc.: Pittsburgh PA, 1998.
(40) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(42) Fryzuk, M. D.; Haddad, T. S.; Mylvaganam, M.; McConville, D. H.; Rettig,
S. J. J. Am. Chem. Soc. 1993, 115, 2782.
(43) Re, N.; Rosi, M.; Sgamellotti, A.; Floriani, C.; Solari, E. Inorg. Chem.
1994, 33, 4390.
(41) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270.
(44) Powell, C. B.; Hall, M. B. Inorg. Chem. 1984, 23, 4619.
9
8392 J. AM. CHEM. SOC. VOL. 124, NO. 28, 2002

Nitride Formation
A R T I C L E S
Figure 6. Proposed orbital interactions in phosphorus-nitride coupling.
Nb(2)-N(3) distance of 2.271(5) Å is longer than the Nb(2)-
N(4) and Nb(2)-N(5) bond lengths of 2.135(4) and 2.130(4)
Å, respectively. This elongated Nb-N bond may be a product
of constraints within the macrocycle. The P(3)-C(28) bond
length of 1.672(7) Å is shorter than the distances observed
between P(4)-C(26), 1.812(6) Å, and P(4)-C(27), 1.830(5) Å,
and suggests a PdC double bond. The P(3)-N(3) distance of
1.554(6) Å is shorter than expected for a typical P-N single
bond and may possess some double bond character. Another
possibility is that N(3) and C(28) may be disordered over two
sites. However, the long interatomic distance between C(28)
and Nb(2) of 2.537 Å suggests the absence of a bonding
interaction. The paramagnetic nature of 3 prevents the observa-
tion of any interpretable NMR spectrum. Nitride 3 is also EPR
silent, suggesting the existence of zero-field splitting. This is
consistent with the variable-temperature magnetic susceptibility
data (see below).
Several groups have studied the mechanism of N-N bond
cleavage in early transition metal complexes.6,8,9 The cleavage
of the dinitrogen ligand in 2 may be compared to the N-N
bond cleavage exhibited by a niobium calix[4]arene complex
where it has been suggested that reduction of niobium by sodium
generates a “biradical system”, effectively creating two Nb-
(IV), d1 centers.8 Cleavage proceeds via a side-on bound
dinitrogen intermediate stabilized by a resulting Nb-Nb bond
that undergoes thermally induced intramolecular transfer from
the metal-metal bond to the N-N bond. In our system, the
initial reduction step is not required as both niobium centers
are already at the required Nb(IV) oxidation state. The insertion
of one of the resulting nitrido groups into the macrocycle
backbone is less straightforward, likely proceeding via nitride
formation followed by reaction of a phosphine donor with an
electrophilic nitride (Scheme 2).45 Nitride coupling reactions
have been proposed to occur via an unsymmetrical transition
state geometry in which the lone pair of one nitride interacts
with an empty MtN π* orbital of another.46,47 In such a
transition state, one nitride acts as a nucleophile at the nitrogen
whereas the other acts as an electrophile. In our system, the
phosphine lone pair can be envisioned to donate into the MtN
π* orbital of the nitride (Figure 6). Electron transfer from the
nitride into the C-Si bond generates a phosphamethylene and
subsequent proton transfer occurs resulting in the formation of
an ylide-type fragment.
Figure 5. Molecular structure of [P2N2]Nb(µ-N)Nb[PN3], 3. Ellipsoids are
drawn at the 50% probability level, silyl methyl groups are omitted for
clarity, and only ipso carbons of phenyl groups on P(1), P(2), and P(4) are
shown. Selected bond lengths (Å), angles (deg), and dihedral angles (deg):
Nb(1)-N(1), 2.119(5); Nb(1)-N(2), 2.146(4); Nb(1)-P(1), 2.5761(13);
Nb(1)-P(2), 2.564(2); Nb(1)-N(6), 1.913(4); Nb(2)-N(3), 2.271(5); Nb-
(2)-N(4), 2.135(4); Nb(2)-N(5), 2.130(4); Nb(2)-N(6), 1.866(4); Nb-
(2)-P(4), 2.6089(13); Nb(2)‚‚‚P(3), 2.9169(19); P(3)-N(3), 1.554(6); P(3)-
C(28), 1.672(7); P(3)-C(25), 1.844(8); P(1)-Nb(1)-P(2), 156.23(6); N(1)-
Nb(1)-N(2), 110.39(17); P(1)-Nb(1)-N(6), 99.14(12); P(2)-Nb(1)-N(6),
104.43(12);
N(1)-Nb(1)-N(6),
118.58(18);
N(2)-Nb(1)-N(6),
129.96(17); Nb(1)-N(6)-Nb(2), 168.8(2); N(4)-Nb(2)-N(5), 115.57(18);
N(3)-Nb(2)-N(4), 66.6(2); N(3)-Nb(2)-N(5), 104.04(19); N(3)-Nb(2)-
N(6), 116.5(2); N(4)-Nb(2)-N(6), 115.43(19); P(4)-Nb(2)-N(6),
95.76(12); N(5)-Nb(2)-N(6), 123.81(18).
a degassed brown toluene solution of ([P2N2]Nb)2(µ-N2) to 110
°C in a sealed ampule for 12 h causes the formation of a
paramagnetic blue-green product. Mass spectrometry shows the
absence of a m/z peak at 1278 but instead displays an intense
peak at 639 corresponding to a [P2N2]NbN fragment. This peak
is absent in the mass spectrum of 2. The structure of the
thermolysis product 3 was determined by X-ray crystallography
and is shown in Figure 5 along with selected bond lengths and
angles. Crystallographic data are given in Table 1. What
becomes apparent is that the N-N bond has been cleaved, but
rather than generating either a monomeric terminal nitride or
dimeric bridging species, a single nitride bridging two niobium
nuclei is observed. The second nitrogen atom of the N2 fragment
has instead inserted into the backbone of one of the [P2N2]
macrocycles to form the bimetallic bridging nitride complex
[P2N2]Nb(µ-N)Nb[PN3] (3) (where [PN3] ) PhPMe(CHSiMe2-
NSiMe2CH2P(Ph)CH2SiMe2NSiMe2N)) (eq 1).
The thermolysis can be followed by using UV-visible
spectroscopy. The dark brown color of the dinitrogen complex
in toluene solution is observed to change to blue-green as the
solution is heated at 110 °C over time. The UV-visible
spectrum shows the diminishment of a peak at 440 nm
(45) Nugent, W. A.; Mayer, J. M. Metal-Ligand Multiple Bonds; John Wiley
& Sons: Toronto, Canada, 1988.
(46) Ware, D. C.; Taube, H. Inorg. Chem. 1991, 30, 4605.
(47) Seymore, S. B.; Brown, S. N. Inorg. Chem. 2001, 41, 462.
The structure of 3 was found to be disordered but was refined
without invoking any restrictions on atom designation. The
9
J. AM. CHEM. SOC. VOL. 124, NO. 28, 2002 8393

A R T I C L E S
Fryzuk et al.
Scheme 2
Figure 7. Magnetic susceptibility (O) and moment (4) versus temperature
plot for [P2N2]Nb(µ-N)Nb[PN3], 3. The zero field splitting model was
employed to generate the lines with D ) 15.05 cm-1, g ) 1.49, TIP )
0.0025, and P ) 0.0088 (F ) 0.007).
presence of a small amount of paramagnetic impurity were
accounted for as in the analysis of 2, above. Experimental data
for 3 are compared with the best fits from theory yielding
D ) 15.05 cm-1, g ) 1.49, TIP ) 0.0025, and P ) 0.0088
(F ) 0.007) (Figure 7). The small g value can be rationalized
by the large degree of spin-orbit coupling exhibited by Nb.
Such observed behavior is consistent with pairs of d2-d0 ions
rather than pairs of d1-d1 ions, suggesting a change in oxidation
states of the niobium centers to Nb(III)-Nb(V). This is
supported by the crystallographic evidence that indicates a
shorter Nb(2)-N(6) bond than Nb(1)-N(6), suggesting Nb(2)
has been oxidized to Nb(V) whereas Nb(1) has undergone a
reduction to Nb(III).
Reaction of 2 with Phenylacetylene. We have previously
reported the unprecedented reactivity of the zirconium dinitrogen
complex ([P2N2]Zr)2(µ-η2:η2-N2) with H2 and primary silanes
where the dinitrogen ligand undergoes N-H and N-Si bond
formation, respectively.4 The reactivity of ([P2N2]Nb)2(µ-N2)
(2) with small molecules was also examined; however, 2 shows
no reactivity with H2 or H3SiR.
attributable to the starting dinitrogen compound and the growth
of an absorption at 652 nm that is ascribed to the nitride
complex, 3. The presence of a single isosbestic point indicates
that no stable intermediate product, namely the bridging bis-
(nitride) species, is formed and that any such intermediates are
short-lived.
Magnetic Studies of Nitride (3). The solid-state magnetic
susceptibility of complex 3 was studied between 2 and 300 K
with a SQUID magnetometer. Unlike complex 2, the plot of
magnetic susceptibility vs temperature does not indicate any
antiferromagnetic coupling. The magnetic moment shows a
steady decrease as the temperature is lowered from 300 to 20
K. The moment then drops abruptly as the temperature ap-
proaches 2 K. These results are consistent with an isolated S )
1 spin center possessing a singlet ground state and a thermally
populated triplet state.
The magnetic moment of 3.2 at 300 K is higher than expected
for two unpaired electrons, however, this may be due to a large
TIP. Attempts to model the data by using simple Curie or
Curie-Weiss behavior did not provide good fits. As we have
observed for previous noncoupled Nb(III) complexes,16 contri-
butions from zero-field splitting effects in second- and third-
row transition metals are significant. The magnetic susceptibility
data for 3 were therefore analyzed by using the equation for
the zero-field splitting of a S ) 1 state:
Terminal acetylenes have been observed to induce N-C bond
formation at the dinitrogen moiety in ([P2N2]Zr)2(µ-η2:η2-N2).48
Although the dinitrogen unit in 2 is bound in the end-on fashion,
its susceptibility to protonation suggested that it might be
reactive toward terminal acetylenes. Addition of 2 equiv of
phenylacetylene to a brown toluene solution of 2 results in an
immediate reaction to form a dark red solution. Although the
dark red paramagnetic product obtained gives a broadened and
shifted 1H NMR spectrum, mass spectrometry shows a m/z peak
corresponding to a [P2N2]Nb(HCCPh) fragment. This formula-
tion was confirmed by elemental analysis.
(2/x)(1 - e-x)
1 + 2e-x
2e-x
øNb ) 1/3C
+ 2/3C
-x
[
]
[
]
1 + 2e
where x ) D/kT and C ) NAg2µB /kT. Contributions from the
2
9
8394 J. AM. CHEM. SOC. VOL. 124, NO. 28, 2002

Nitride Formation
A R T I C L E S
Figure 9. (a) Overlap of alkyne π MO with metal dxy orbital shared by
σ-bond formation with [P2N2] amide donors. (b) Interaction of the filled
dxz orbital with the vacant π|* orbital.
expected for a Nb-C single bond and the C(25)-C(26) bond
length of 1.307(5) Å corresponds to a bond order of 2.2.49 Also,
the C(25)-C(26)-C(27) angle shows a considerable distortion
from the free ligand. These lengths and angles agree with those
previously reported for systems described as containing four-
electron-donating alkyne fragments and are consistent with the
complex being viewed as a niobacyclopropene fragment with
the metal in a formally +4 oxidation state.50,51
Figure 8. Molecular structure (ORTEP) of [P2N2]Nb(η2-HCCPh), 4.
Ellipsoids are drawn at the 30% probability level, silyl methyl groups are
omitted for clarity, and only ipso carbons of phenyl rings are shown. Selected
bond lengths (Å), angles (deg), and dihedral angles (deg): C(25)-C(26),
1.307(5); Nb(1)-C(25), 2.100(3); Nb(1)-C(26), 2.124(3); Nb(1)-P(1),
2.5756(8); Nb(1)-P(2), 2.5449(8); Nb(1)-N(1), 2.116(2); Nb(1)-N(2),
2.127(2); C(25)-C(26)-C(27), 137.4(3); P(1)-Nb(1)-P(2), 152.04(3);
N(1)-Nb(1)-N(2), 110.76(9); P(1)-Nb(1)-N(1), 85.52(6); P(1)-Nb(1)-
N(2), 79.20(7); P(2)-Nb(1)-N(1), 79.72(6); P(2)-Nb(1)-N(2), 84.00(7);
P(1)-Nb(1)-C(25), 123.67(9); P(1)-Nb(1)-C(26), 87.62(9); P(2)-Nb-
(1)-C(25), 84.29(9); P(2)-Nb(1)-C(26), 120.34(9); N(1)-Nb(1)-C(25),
118.4(1); N(1)-Nb(1)-C(26), 120.7(1); N(2)-Nb(1)-C(25), 126.2(1);
N(2)-Nb(1)-C(26), 125.5(1); P(1)-Nb(1)-C(25)-C(26), 0.6(3); P(2)-
Nb(1)-C(26)-C(27), -171.0(5); N(1)-Nb(1)-C(25)-C(26), -104.1(2);
N(2)-Nb(1)-C(25)-C(26), 102.6(2).
Solution magnetic susceptibility measurement by Evans
method gives a magnetic moment of 1.4 µB consistent with one
unpaired electron. Alkyne complex 4 is EPR active and, like
dinitrogen complex 2, displays a decet of triplets. The 30 line
spectrum is centered at g ) 1.985 and results from coupling of
9
the unpaired electron to the I ) /2 93Nb nucleus (a(93Nb) )
108.4 G) and two I ) /2 31P nuclei (a(31P) ) 21.6 G). No
1
coupling to the acetylene proton was observed.
The coordination of the alkyne ligand is coplanar to the
phosphine donors. Assuming the alkyne ligand occupies one
coordination site, the niobium center can be considered to
possess a mildly distorted trigonal-bipyramidal geometry, thus,
the frontier orbital description is similar to that presented for
dinitrogen complex 2, above. The phosphine donors are
considered to lie along the z-axis while the amide donors occupy
The displacement of the dinitrogen ligand by phenylacetylene
was confirmed by single-crystal X-ray molecular structure
determination. The structure of [P2N2]Nb(η2-HCCPh) is shown
in Figure 8 and crystallographic data are given in Table 1. The
reaction of phenylacetylene with 2 therefore appears to involve
the displacement of the bound dinitrogen moiety as dinitrogen
gas, and the binding of phenylacetylene to produce an η2-
acetylide [P2N2]Nb(η2-HCCPh), 4 (eq 2). The niobium atom
resides in a distorted trigonal bipyramidal geometry if the alkyne
ligand is considered to occupy one coordination site. The two
amides and the alkyne occupy the equatorial positions while
the phosphine groups occupy slightly pinched back apical
positions. Interestingly, the coordination of the alkyne group is
coplanar to the apical phosphine donors; a brief description of
the bonding interactions in 4 is discussed below.
The Nb-P and Nb-N bond lengths are fully comparable to
those observed in 2, above. The N-Nb-N and P-Nb-P angles
of 110.76(9)° and 152.04(3)°, respectively, in 4 are, however,
significantly smaller than the corresponding angles observed
in complex 2. Steric effects imposed upon the macrocycle by
the phenylacetylene unit likely cause this decrease in bond
angles. The alkyne ligand in 4 is best described as being a four-
electron donor where both π-systems are involved in the bonding
to the transition metal. The system can therefore be described
as containing a formally Nb(II) (d3) center that adopts a
resonance form where the metal is in an oxidized Nb(IV) (d1)
state. An examination of the relevant bond lengths confirms
this description, thus the system formally possesses a niob-
ium(IV), d1 center. The Nb(1)-C(25) and Nb(1)-C(26) bond
lengths of 2.100(3) and 2.124(3) Å, respectively, are in the range
2
the xy-plane. Thus, the vacant dxy and dz orbitals are stabilized
by σ-bonding with the amide and phosphine donors, respec-
tively. The bonding of the alkyne ligand is accomplished by
three molecular orbitals of π-symmetry, π , π|, and π|*. The
filled dxz orbital is of appropriate symmetry to interact with the
vacant π|* orbital. The filled π| orbital is therefore well suited
2
2
to interact with the empty dx -y orbital to form a σ-bond. Last,
the filled π orbital of the alkyne is of appropriate symmetry
to donate into the dxy orbital, which is also experiencing σ-bond
interaction with the amide donors (Figure 9). The dyz orbital
undergoes no interaction with the alkyne; as shown in the DFT
calculations for dinitrogen complex 2 above the dyz orbital is
the singly occupied HOMO.
Conclusion
The goal of this work was to examine the formation of nitrides
by investigation of the coordination chemistry of [P2N2] with a
group 5 metal. Reduction of [P2N2]NbCl under dinitrogen was
expected to lead to the formation of a niobium-nitride complex
on the basis of a total of six electrons added. Instead, the
isolation of the kinetically stable dinuclear dinitrogen complex
([P2N2]Nb)2(µ-N2), 2, was observed. The variable-temperature
(49) Curtis, M. D.; Real, J.; Kwon, D. Organometallics 1989, 8, 1644.
(50) Etienne, M.; Mathieu, R.; Donnadieu, B. J. Am. Chem. Soc. 1997, 119,
3218.
(51) Templeton, J. L. AdV. Organomet. Chem. 1999, 29, 1.
(48) Fryzuk, M. D.; Love, J. B.; Morello, L. Unpublished results.
9
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A R T I C L E S
Fryzuk et al.
(216 Hz, 24H, SiCH3CH3′); 6.60 (120 Hz, 8H, m-Ph); 7.67 (32 Hz,
4H, p-Ph); 32.39 (2720 Hz, 4H, PCHH′). EPR (C7H8) giso ) 1.975;
a(93Nb) ) 108.2 G, 1 Nb; a(31P) ) 22.9 G, 2 P. MS (EI) m/z, (%)
1278, (80) [M]+. µeff(Evans method) ) 2.8 µB (per dinuclear molecule).
Anal. Calcd for C48H84N6Nb2P4Si8: C, 45.05; H, 6.62; N, 6.57.
Found: C, 44.86; H, 6.47; N, 4.13. (Formation of stable niobium nitrides
during microanalysis cause consistently low observed values for N.)
magnetic properties of complex 2 were examined and indicate
the existence of antiferromagnetic exchange between two Nb-
(IV) (d1) centers. However, thermolysis of 2 does result in the
formation of nitrides, albeit complicated by ligand rearrange-
ment. The isolation of [P2N2]Nb(µ-N)Nb[PN3], 3, not only
illustrates that nitrides can be formed by using the combination
of group 5 elements and the [P2N2] ligand set, but that the
nitrides can be functionalized; in this case, N-P and N-Si bond
formation is observed although the overall process is stoichio-
metric. The overall transformation of 2 to 3 is also significant
in that attack of the phosphine donor on the putative niobium
nitride results in the oxidation of P(III) to P(V) in 3. The N-N
bond cleavage product 3 indicates a promising route to the
functionalization of coordinated N2 and work is currently
underway to examine the reactivity of niobium nitride species
stabilized by [P2N2] with nucleophilic and electrophilic sub-
strates.
[P2N2]Nb(µ-N)Nb[PN3] (3): ([P2N2]Nb)2(µ-N2) (1.00 g, 0.782 mmol)
was dissolved in toluene (50 mL) and placed in a thick-walled glass
reactor fitted with a Teflon Kontes valve. The solution was heated to
110 °C for 24 h. The resulting deep blue solution was cooled and filtered
through Celite, and the solvent was removed in vacuo to obtain a dark
blue solid. Recrystallization from a saturated hexanes solution cooled
to -40 °C produced a crop of dark blue crystals. Yield: 0.74 g (74%).
MS (EI) m/z, (%) 639, (80) [M]+. µeff(Evans method) ) 2.7 µB (per
dinuclear molecule). Anal. Calcd for C48H84N6Nb2P4Si8: C, 45.05; H,
6.62; N, 6.57. Found: C, 44.95; H, 6.57; N, 5.23.
[P2N2]Nb(η2-HCCPh) (4): ([P2N2]Nb)2(µ-N2) (1.00 g, 0.782 mmol)
was dissolved in toluene (50 mL) and phenylacetylene (160 mg, 1.564
mmol) was added slowly to the stirred brown solution. After 1 h the
solution turned dark orange. The reaction was stirred for a further 11
h and filtered and solvents were removed in vacuo to yield a dark
orange-brown solid. Yield: 1.00 g (88%). Large dark red crystals were
obtained by slow evaporation of a saturated toluene solution. EPR
(C7H8) giso ) 1.985; a(93Nb) ) 108.4 G, 1 Nb; a(31P) ) 21.6 G, 2 P.
MS (EI) m/z, (%) 727, (100) [M]+. µeff(Evans method) ) 1.4 µB. Anal.
Calcd for C32H48N2NbP2Si4: C, 52.80; H, 6.65; N, 3.85. Found: C,
52.85; H, 6.85; N, 3.81.
Experimental Section
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 system and a
-40 °C freezer). Hexanes and toluene 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.52 Anhydrous diethyl ether and THF were stored
over sieves and distilled from sodium benzophenone ketyl under argon.
Nitrogen and argon were dried and deoxygenated by passing the gases
through a column containing molecular sieves and MnO. Deuterated
benzene and toluene were dried by refluxing over sodium and potassium
alloy, d8-THF was dried over sodium, in a sealed vessel under partial
pressure, and then trap-to-trap distilled. They were degassed under 3
freeze-pump-thaw cycles. Unless otherwise stated, all NMR spectra
were recorded on a Bruker AVA 400 instrument operating at 400.132
Reaction of 2 with PbCl2. PbCl2 (70 mg, 0.25 mmol) was added to
a brown solution of 2 (300 mg, 0.23 mmol) in toluene (30 mL) and
the mixture was stirred for 12 h. The resulting green solution was
filtered through Celite and toluene was removed in vacuo. NMR and
mass spectral data were identical with those for 1.
Reaction of 2 with Me3N‚HCl. Me3N‚HCl (45 mg, 0.47 mmol)
was added to a brown solution of 2 (300 mg, 0.23 mmol) in toluene
(30 mL) and the mixture was stirred for 12 h. The resulting green
solution was filtered through Celite and toluene was removed in vacuo.
NMR and mass spectral data indicate the product to be 1.
1
MHz for H spectra. Evans method experiments were performed on a
1
Bruker AC 200 instrument operating at 200.132 MHz for H spectra.
1H NMR spectra were referenced to residual protons in the deuterated
solvent. EPR spectra were obtained on a Bruker ECS 106 spectrometer.
UV-vis spectra were recorded with a Hewlett-Packard 8454 UV-
visible spectrophotometer and quartz cuvettes with Teflon Kontes
valves. Elemental analyses were performed by Mr. P. Borda of this
department. Mass spectrometry was performed on a Kratos MS 50 by
Mr. M. Lapawa, also of this department. The variable-temperature
magnetic susceptibility of powdered samples of 2 and 3 was measured
over a temperature range of 2-300 K and at a field of 10 000 G with
a Quantum Design (MPMS) SQUID magnetometer. The orbitals
obtained by DFT calculations were visualized by using the Molden53
program.
Hydrazine Analysis. Analysis for hydrazine was performed ac-
cording to published procedure.54
X-ray Crystallographic Analyses of 2, 3, and 4. In all cases,
suitable crystals were selected and mounted on a glass fiber with use
of Paratone-N oil and frozen to -100 °C. All measurements were made
on a Rigaku/ADSC CCD area detector with graphite monochromated
Mo KR radiation. Crystallographic data appear in Table 1. In each case
the data were processed55 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
Crystallography.56,57 All structures were solved by direct methods58 and
expanded by using Fourier techniques.59 All non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were included but not refined.
Hydrogen atoms were fixed in calculated positions with C-H ) 0.98
Å.
([P2N2]Nb)2(µ-N2) (2): A mixture of [P2N2]NbCl (3.0 g, 4.54 mmol)
and KC8 (0.68 g, 5.00 mmol) was placed in a thick-walled glass reactor
fitted with a Teflon Kontes valve. The flask was evacuated and cooled
to -196 °C and toluene (100 mL) was added by vacuum transfer; the
flask was sealed under an atmosphere of dinitrogen and allowed to
warm to 25 °C. After being stirred for 48 h the brown solution was
filtered through Celite and solvent removed in vacuo, leaving a brown
residue. The solids were washed with hexanes. Yield: 2.5 g (85%).
Crystals suitable for X-ray diffraction were obtained by slow evapora-
tion of a saturated toluene solution. 1H NMR (δ, 400 MHz, C6D6, ppm):
-11.52 (w1/2 768 Hz, 8H, o-Ph); 0.82 (88 Hz, 24H, SiCH3CH3′); 5.44
(54) Watt, G. W.; Chrisp, J. D. Anal. Chem. 1952, 24, 2006.
(55) teXsan, Crystal Structure Analysis Package; Molecular Structure Corp.:
The Woodlands, TX, 1995.
(56) International Tables for X-ray Crystallography; Kluwer Academic: Boston,
MA, 1992; Vol. C, pp 200-206.
(57) International Tables for X-ray Crystallography; Kynoch Press: Birming-
ham, U.K. (present distributer Kluwer Academic: Boston, MA), 1974; Vol.
IV, pp 99-102.
(58) Altomare, A.; Burla, M. C.; Cammali, G.; Cascarano, M.; Giacovazzo, C.;
Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, A. SIR97:
new tool for crystal structure determination and refinement, 1999.
a
(52) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers,
F. J. Organometallics 1996, 15, 1518.
(53) Schaftenaar, G. Molden, 3.5 ed.; CAOS/CAMM Center, University of
Nijmegen: Nijmegen, The Netherlands, 1991.
(59) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.; de Gelder,
R.; Israel, R.; Smits, J. M. M. DIRDIF94; The DIRDIF-94 program system,
Technical Report of the Crystallography Laboratory; University of
Nijmegen: Nijmegen, The Netherlands, 1994.
9
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Nitride Formation
A R T I C L E S
Acknowledgment. We gratefully acknowledge Professor
Robert C. Thompson and Victor Sanchez (UBC) and Professor
Natia L. Frank (University of Washington) for help in measure-
ment of the variable-temperature magnetic susceptibilities of 2
and 3 and for stimulating discussions on the subject of
magnetism. Barry Liboiron is thanked for assistance with EPR
spectroscopy. We thank both the NSERC of Canada and the
Petroleum Research Fund, administered by the Ameri-
can Chemical Society, for funding in the form of research
grants.
Supporting Information Available: Complete crystallo-
graphic data (CIF) for complexes 2, 3, and 4; density functional
theory calculation results for model complex 2A (PDF). This
material is available free of charge via the Internet at
http://pubs.acs.org.
JA025997F
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