Macrocyclic complexes of niobium(III): Synthesis, structure, and magnetic behavior of mononuclear and dinuclear species that incorporate the [P2N2] system

Article abstract of DOI:10.1021/om010356z

The coordination chemistry of the two mixed donor macrocyclic ligands ^R[P₂N₂] (where ^R[P₂N₂] = RP(CH₂SiMe₂NSiMe₂CH₂)₂PR, R = cyclohexy

Full text of DOI:10.1021/om010356z

3752
Organometallics 2001, 20, 3752-3761
Ma cr ocyclic Com p lexes of Niobiu m (III): Syn th esis,
Str u ctu r e, a n d Ma gn etic Beh a vior of Mon on u clea r a n d
Din u clea r Sp ecies Th a t In cor p or a te th e [P 2N2] System
Michael D. Fryzuk,* Christopher M. Kozak, Michael R. Bowdridge,
Weichang J in, Dean Tung, Brian O. Patrick,† and Steven J . Rettig†,‡
Department of Chemistry, University of British Columbia, 2036 Main Mall,
Vancouver, British Columbia, V6T 1Z1 Canada
Received May 1, 2001
The coordination chemistry of the two mixed donor macrocyclic ligands R[P2N2] (where
R[P2N2] ) RP(CH2SiMe2NSiMe2CH2)2PR, R ) cyclohexyl (Cy) or phenyl (Ph)) with niobium-
R
(III) is presented. The reaction of the dilithio precursors [P2N2]Li2(S) (R ) Cy, S ) THF; R
) Ph, S ) 1,4-dioxane) with NbCl3(DME) (DME ) 1,2-dimethoxyethane) generates the
R
complexes [P2N2]NbCl (R ) Cy, 1; R ) Ph, 2). For R ) Cy, single-crystal X-ray diffraction
studies and variable-temperature magnetic susceptibility measurements indicate that 1 is
mononuclear in the solid state; however, analogous variable-temperature magnetic data
suggest that 2 is dinuclear in the solid state due to the observation of antiferromagnetic
exchange. In solution, 2 is apparently monomeric similar to 1. Adduct formation between
these mononuclear complexes is also evident; reaction of 2 with neutral donors and
coordinating solvents produces the mononuclear derivatives Ph[P2N2]NbCl(L) (L ) py, CO,
PMe3, THF, MeCN), of which the pyridine adduct, 3e, has been characterized crystallo-
graphically. Subsequent replacement of the chlorides can be achieved to generate the
R
paramagnetic alkyl complexes [P2N2]NbR′ (R ) Cy, Ph; R′ ) CH2SiMe3, CH(SiMe3)2). The
representative compounds Cy[P2N2]NbCH2SiMe3 (4a ) and Ph[P2N2]NbCH(SiMe3)2 (5b) have
been characterized by X-ray crystallography.
In tr od u ction
is growing. Such low-valent derivatives of niobium have
found applications in N2 activation20-22 and C-N23-25
bond cleavage processes.
Our approach to developing the chemistry of the early
transition elements has been to design multidentate
When complexed to different ancillary ligand sets,
niobium can span a wide range of formal oxidation
states, from -3 (e.g., [Nb(CO)53-]) to +5 (e.g., Cp2-
NbH3).1 Depending on the oxidation state, niobium
complexes can be used in a variety of purposes; for
example, Nb(III) derivatives have been used in stoichio-
metric organic reactions2,3 and Nb(V) complexes as
arene hydrogenation catalysts.4-6 While the most com-
mon oxidation state of niobium by far is Nb(V),7 the
study of lower oxidation states containing cyclopenta-
dienyl8-13 and non-cyclopentadienyl14-19 ancillary ligands
(10) Alcalde, M. I.; de la Mata, J .; Gomez, M.; Royo, P.; Pellinghelli,
M. A.; Tiripicchio, A. Organometallics 1994, 13, 462.
(11) de la Mata, J .; Gomez, M.; Gomezsal, P.; Royo, P. Inorg. Chem.
1993, 32, 5454.
(12) de la Mata, J .; Galakhov, M. V.; Gomez, M.; Royo, P. Organo-
metallics 1993, 12, 1189.
(13) Gomez, M.; de Ilarduya, J . M. M.; Royo, P. J . Organomet. Chem.
1989, 369, 197.
(14) Etienne, M.; Carfagna, C.; Lorente, P.; Mathieu, R.; de Mon-
tauzon, D. Organometallics 1999, 18, 3075.
(15) Etienne, M.; Mathieu, R.; Donnadieu, B. J . Am. Chem. Soc.
1997, 119, 3218.
(16) Lorente, P.; Carfagna, C.; Etienne, M.; Donnadieu, B. Organo-
metallics 1996, 15, 1090.
* E-mail: Fryzuk@chem.ubc.ca.
† Professional Officer: UBC X-ray Structural Chemistry Laboratory.
‡ S.J .R. deceased October 27, 1998.
(1) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M.
Advanced Inorganic Chemistry, 6th ed.; J ohn Wiley and Sons: Toronto,
1999.
(2) Hartung, J . B. J .; Pedersen, S. F. J . Am. Chem. Soc. 1989, 111,
5468.
(3) Roskamp, E. J .; Pedersen, S. F. J . Am. Chem. Soc. 1987, 109,
6551.
(4) Rothwell, I. P. J . Chem. Soc., Chem. Commun. 1997, 1331.
(5) Visciglio, V. M.; Clark, J . R.; Nguyen, M. T.; Mulford, D. R.;
Fanwick, P. E.; Rothwell, I. P. J . Am. Chem. Soc. 1997, 119, 3490.
(6) Yu, J . S.; Felter, L.; Potyen, M. C.; Clark, J . R.; Visciglio, V. M.;
Fanwick, P. E.; Rothwell, I. P. Organometallics 1996, 15, 4443.
(7) Wigley, D. E. G., S. D. Comprehensive Organometallic Chemistry
II; Pergamon: New York, 1995; Vol. 5.
(17) Ruppa, K. B. P.; Gambarotta, S.; Yap, G. P. A. Inorg. Chim.
Acta 1998, 280, 143.
(18) Tayebani, M.; Kasani, A.; Feghali, K.; Gambarotta, S.; Bensi-
mon, C. J . Chem. Soc., Chem. Commun. 1997, 2001.
(19) Tayebani, M.; Feghali, K.; Gambarotta, S.; Yap, G. P. A.
Organometallics 1998, 17, 4282.
(20) Berno, P.; Gambarotta, S. Organometallics 1995, 14, 2159.
(21) 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.
(22) Caselli, A.; Solari, E.; Scopelliti, R.; Floriani, C.; Re, N.; Rizzoli,
C.; Chiesi-Villa, A. J . Am. Chem. Soc. 2000, 122, 3652.
(23) Tayebani, M.; Feghali, H.; Gambarotta, S.; Bensimon, C.
Organometallics 1997, 16, 5084.
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A. D.; Stephan, D. W. Organometallics 1997, 16, 3504.
(9) Bailey, N. J .; Green, M. L. H.; Leech, M. A.; Saunders: J . F.;
Tidswell, H. M. J . Organomet. Chem. 1997, 538, 111.
(24) Tayebani, M.; Gambarotta, S.; Yap, G. P. A. Organometallics
1998, 17, 3639.
(25) Kleckley, T. S.; Bennet, J . L.; Wolczanski, P. T.; Lobkovsky, E.
B. J . Am. Chem. Soc. 1997, 119, 247.
10.1021/om010356z CCC: $20.00 © 2001 American Chemical Society
Publication on Web 07/25/2001

Macrocyclic Complexes of Niobium(III)
Organometallics, Vol. 20, No. 17, 2001 3753
Sch em e 1
Sch em e 2
produces the macrocyclic dilithio derivative Cy[P2N2]Li2
in acceptable yield as large colorless crystals by low-
temperature crystallization from a saturated hexanes
solution. The yield can, however, be substantially
improved by isolating the less soluble THF adduct
Cy[P2N2]Li2(THF) generated by adding a small excess
of THF to the hexanes solution and removal of solvents
in vacuo. No advantage was found to using the adduct-
free ligand; therefore, subsequent metalation reactions
employed the use of the THF adduct.
ligands that can support a variety of oxidation states.
For example, the combination of amido and phosphine
donors in varying ratios has allowed access to a var-
iety of metal complexes in unusual oxidation states,
for example, Zr(III) and Hf(III).26 In this work we
examine the characteristic chemistry of Nb(III) com-
plexes that incorporate the R[P2N2] macrocyclic ligand,
where R[P2N2] ) RP(CH2SiMe2NSiMe2CH2)2PR (R ) Cy
or Ph).
The H NMR spectrum of Cy[P2N2]Li2(THF) is quite
1
similar to the previously reported Ph[P2N2]Li2(THF)27
ligand precursor albeit for the presence of cyclohexyl
methylene resonances in lieu of phenyl resonances. The
solution structure is consistent with C2v symmetry; only
two types of silyl methyl protons are observed in the
1H NMR spectrum at 0.30 and 0.53 ppm for 12H each.
The methylene protons of the macrocycle appear as AB
multiplets coupled to phosphorus centered at 0.69 ppm.
The resonances of the phosphine cyclohexyl protons
appear as broad multiplets at 1.13, 1.60, 1.67, and 1.84
ppm. The resolution of these resonances could not be
improved even upon cooling to -80 °C. Two sets of
triplet resonances appear for the coordinated THF
ligand at 1.27 and 3.57 ppm. In the 31P{1H} NMR
spectrum, the two equivalent phosphorus nuclei give
rise to an equal intensity quartet at -32.78 ppm due to
coupling to Li. In the 7Li{1H} NMR spectrum, two
resonances are observedsa broadened singlet at 1.22
ppm and a triplet at 2.99 ppmsas a result of coupling
to the two aforementioned equivalent 31P nuclei. This
lithium-7 spectrum confirms that this material has the
indicated syn stereochemistry similar to that found for
the phenyl analogue.27 The syn stereochemistry of the
THF-free dilithium salt Cy[P2N2]Li2 was confirmed un-
equivocally by single-crystal X-ray structure determi-
nation (see Supporting Information).
Resu lts a n d Discu ssion
Ma cr ocyclic Liga n d Syn th esis. We have previously
reported the preparation of the syn-isomer Ph[P2N2]Li2-
(THF) by the reaction of PhPH2 with HN(SiMe2CH2Cl)2
n
in the presence of BuLi; this preparation is generally
carried out stepwise to first generate HN(SiMe2CH2-
PPhH)2 in virtually quantitative yield followed by a
n
lithium-templated ring closure using 4 equiv of BuLi
and an additional 1 equiv of HN(SiMe2CH2Cl)2.27 In
diethyl ether, the isolation of Ph[P2N2]Li2 proceeds in
high yield without the need for high dilution techniques
as often found for the synthesis of macrocycles.28 When
we attempted to extend this two-step procedure to
cyclohexylphosphine (CyPH2), the initial diphosphine
intermediate HN(SiMe2CH2PCyH)2 could not be isolated
cleanly. Therefore, we examined an alternate procedure
based on earlier investigations on the preparation of
HN(SiMe2CH2NMe2)2.29 As shown in Scheme 1, protec-
tion of the silicon center by the diethylamido moiety
allows formation of CyPHCH2SiMe2NEt2, which can be
transaminated with NH3 to generate the desired diphos-
phine HN(SiMe2CH2PCyH)2 in excellent yield. Subse-
quent ring closure in a manner analogous to Ph[P2N2]
Syn th eses a n d Ch a r a cter iza tion of R[P 2N2]NbCl.
A convenient and versatile starting material for nio-
bium(III) chemistry is NbCl3(DME) (DME ) 1,2-
dimethoxyethane).2,3 Addition of 1 equiv of the ligand
precursor R[P2N2]Li2(S) (R ) Cy, S ) THF; R ) Ph, S )
1,4-dioxane) to a suspension of NbCl3(DME) in toluene
results in the slow formation of a dark green solution
over 48 h. Upon workup, Cy[P2N2]NbCl (1) is obtained
as green solid, while Ph[P2N2]NbCl (2) is isolated as a
brick red precipitate, both in high yield (Scheme 2). As
(26) Fryzuk, M. D.; Mylvaganum, M.; Zaworotko, M. J.; MacGillivray,
L. R. J . Am. Chem. Soc. 1993, 115, 10360.
(27) Fryzuk, M. D.; Love, J . B.; Rettig, S. J . J . Chem. Soc., Chem.
Commun. 1996, 2783.
(28) Lindoy, L. F. The Chemistry of Macrocyclic Ligand Complexes;
Cambridge University Press: Cambridge, U.K., 1989.
(29) Fryzuk, M. D.; Hoffman, V.; Kickham, J . E.; Rettig, S. J .;
Gambarotta, S. Inorg. Chem. 1997, 36, 3480.

3754 Organometallics, Vol. 20, No. 17, 2001
Ta ble 1. Cr ysta llogr a p h ic Da ta a
Fryzuk et al.
1
3e
4a
5b
formula
fw
C
24H54N2P2Nb2Si4Cl‚C6H14
C29H47N3NbP2Si4Cl
740.36
C28H65N2NbP2Si5
725.12
C31H60N2NbP2Si6
784.20
759.53
color, habit
cryst size, mm
cryst syst
space group
a, Å
green, prism
0.5 × 0.5 × 0.3
orthorhombic
Pbcn (#60)
15.5439(4)
24.327(1)
10.8181(3)
90
red, irregular
0.30 × 0.10 × 0.10
triclinic
P1h (#2)
10.281(3)
11.435(2)
17.230(2)
84.181(3)
88.241(3)
67.291(3)
1859.0(5)
2
blue, platelet
0.15 × 0.15 × 0.05
monoclinic
P21/n (#14)
23.139(2)
16.411(1)
22.802(1)
90
112.770(5)
90
7989.0(9)
8
green, irregular
0.50 × 0.20 × 0.20
monoclinic
C2/c (#15)
20.414(1)
11.964(1)
19.783(2)
90
119.149(7)
90
4219.7(7)
4
b, Å
c, Å
R, deg
â, deg
90
90
γ, deg
V, Å3
4090.7(2)
4
Z
T, °C
-100
-100
-100
-100
F
calc, g/cm3
1.233
1.323
1.206
1.234
F(000)
1624.00
Mo KR
5.76
0.7058-1.0000
ω
0.5
772.00
Mo KR
6.33
0.7477-1.0000
ω
0.5
full sphere
52.9
3104.00
Mo KR
5.51
0.7290-1.0000
ω
0.5
1660.00
Mo KR
5.54
0.7742-1.0000
ω
0.5
radiation
µ, cm-1
transmn factors
scan type
scan range, deg in ω
data collected
2θmax, deg
full sphere
55.7
full sphere
50.2
full sphere
55.8
cryst decay, %
total no. of reflns
no. of unique reflns
Rmerge
negligible
30 918
4656
negligible
15 166
6715
negligible
53 102
14 340
negligible
18 053
7944
0.051
0.119
0.042
no. reflns with I g 3σ(I)
no. of variables
R
Rw
gof
3871
182
0.050
0.090
1.32
4803
361
0.071
0.118
6973
679
0.087
0.155
6381
213
0.06
0.092
1.35
1.10
1.41
max ∆/σ
1.41
-0.79
1.16
-0.92
1.76
-1.96
1.39
-0.80
residual density, e/Å3
a
2
2
2
2
2
2 2
Rigaku/ADSC CCD diffractometer,R ) ∑||Fo | - |Fc ||/∑|Fo |; Rw ) (∑w(|Fo | - |Fc |)2/∑w|Fo | )1/2
.
will be discussed below, while the solution structures
of 1 and 2 are likely mononuclear, the solid state
formulations appear to be different.
1H NMR studies for each of 1 and 2 show only
broadened resonances from which no structural infor-
mation could be determined. However, despite the
significant line broadening and paramagnetic shift of
the resonances, assignments based upon integration and
contact shifts could be made that were useful in sample
identification and estimation of purity.30,31 These species
are also EPR silent, as would be expected for triplet
spin-states.32 For each complex, mass spectrometry
indicates mass/charge fragments corresponding to their
respective mononuclear parent peaks, namely, 672 for
Cy[P2N2]NbCl and 660 for Ph[P2N2]NbCl. Elemental
analyses were also consistent for these formulations.
Room-temperature magnetic susceptibility measure-
ments were conducted in solution by Evans’ method33,34
and indicate a magnetic moment of 2.2 µB for 1 and 2.7
µB for 2, corresponding to two unpaired electrons per
niobium and lowered by large spin-orbit couplings in
the Nb(III) metal centers.35
F igu r e 1. Molecular structure (ORTEP) of Cy[P2N2]NbCl
(1). Ellipsoids drawn at the 50% probability level, silyl
methyls omitted for clarity, and only ipso carbons of
cyclohexyl rings shown.
Attempts were made to obtain crystals suitable for
X-ray diffraction studies on 1 and 2. Slow evaporation
of a toluene solution of 2 yielded a large crop of dark
red prisms, but structure determination was consis-
tently thwarted by crystal twinning. However, green
crystals of 1 were obtained from evaporation of a
hexanes solution which were suitable for molecular
structure determination. The solid state molecular
structure of 1 is shown in Figure 1; crystallographic data
are shown Table 1, and selected bond lengths and angles
in Table 2.
(30) Lamar, G. N.; Horrocks, W. D.; Holm, R. H. NMR of Paramag-
netic Molecules; Academic Press: New York, 1973.
(31) Drago, R. S. Physical Methods for Chemists; 3rd ed.; W. B.
Saunders: Orlando, 1996.
(32) Ebsworth, E. A. V.; Rankin, D. W. H.; Craddock, S. Structural
Methods in Inorganic Chemistry, 2nd ed.; Blackwell Scientific Publica-
tions: Oxford, 1991.
(33) Sur, S. K. J . Magn. Reson. 1989, 82, 169.
(34) Evans, D. F. J . Chem. Soc. 1959, 2003.
(35) Carlin, R. L. Magnetochemistry; Springer-Verlag: Heidelberg,
1986.
Crystals of 1 contain 0.5 equiv of cocrystallized hexane
per Cy[P2N2]NbCl molecule. The molecular structure
indicates unequivocally that 3 is monomeric in the solid
state; it resides on a crystallographic 2-fold axis of

Macrocyclic Complexes of Niobium(III)
Organometallics, Vol. 20, No. 17, 2001 3755
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles (d eg) in Cy[P 2N2]NbCl, 1
Nb(1)-N(1)
Nb(1)-Cl(1)
2.089(1)
2.4047(7)
Nb(1)-P(1)
2.5973(4)
N(1)-Nb(1)-N(1*)
N(1)-Nb(1)-Cl(1)
P(1)-Nb(1)-N(1)
Nb(1)-P(1)-C(7)
112.33(9)
123.84(4)
86.92(4)
P(1)-Nb(1)-P(1*)
P(1)-Nb(1)-Cl(1)
P(1)-Nb(1)-N(1*)
167.60(2)
96.20(1)
86.19(4)
130.43(6)
P(1)-Nb(1)-P(1*)-C(7*)
166.58(8)
Cl(1)-Nb(1)-P(1)-C(7)
-13.42(8)
rotation coincident with the molecular C2 symmetry axis
defined along the Nb-Cl bond. The niobium center
displays a distorted trigonal bipyramidal geometry with
the phosphine donors occupying slightly pinched back
apical positions giving a P(1)-Nb-P(1*) angle of 167.60-
(2)°. This angle is larger than the P-M-P angles
previously observed in other monomeric early transition
metal complexes of the [P2N2] ligand.36,37 The chloride
and two amide ligands lie in the trigonal plane, display-
ing a combined equatorial angle of 360.0°. The Nb-N
bond distance of 2.089(1) Å is longer than that reported
in Nb[N(SiMe3)2]2Br2,38 where a Nb-N bond distance
of 1.957(4) Å has been found. The Nb-N bonds in 1 are,
however, shorter than those found in zirconium and
36
tantalum complexes of Ph[P2N2]. Both Ph[P2N2]ZrCl2
F igu r e 2. Magnetic susceptibility (O) and moment (4)
versus temperature plot for Cy[P2N2]NbCl (1). The zero-field
splitting model was employed to generate the lines with D
) 2.03 cm-1, g ) 1.74, and P ) 0.0068 (F ) 0.0151).
and Ph[P2N2]TaMe3 exhibit M-N lengths averaging
37
2.1755 Å. It is likely the somewhat longer M-N bonds
in the Zr and Ta complexes are because of steric
crowding. The Nb-P bond distance of 2.5973(4) Å is
similar to that observed in these related monomeric Zr
and Ta species, although these complexes contain six-
and seven-coordinate d0 metal centers.37 It is, however,
marginally shorter than the Nb-P length in most other
reported Nb(III) and Nb(IV) phosphine complexes,
where bond lengths on the order of 2.65 Å prevail.39-44
Ma gn etism Stu d ies of 1 a n d 2. Although the
structure determination of Cy[P2N2]NbCl (1) has shown
it to be monomeric in the solid state, the solid state
structure of Ph[P2N2]NbCl (2) is still in question. Whereas
the color of Cy[P2N2]NbCl (1) is green in both the solid
and solution states, Ph[P2N2]NbCl (2) is dark red in the
solid state but gives dark green solutions in toluene or
benzene. One possible explanation is that in the solid
state 2 may exist as a chloride-bridged dimer. Similar
observations were made for NbCl4(PR3)2 complexes.43,45,46
These dark red or brown solids were found to commonly
give green solutions and were strongly solvent depend-
ent. When relatively small phosphines were used (PMe3,
PMe2Ph), these species were shown to be dimeric in the
solid state and exhibited surprisingly low magnetic
moments (0.50 µeff for NbCl4(PMe3)2), suggesting con-
siderable metal-metal interaction. Variable-tempera-
ture solid state magnetic susceptibility measurements
on complexes 1 and 2 were conducted to possibly
elucidate the nature of the coordination environment
around niobium in 2 and to compare the two systems.
The magnetic susceptibility, øm, of 1 versus temper-
ature indicates a steady rise in susceptibility at low
temperature consistent with a paramagnetic system
undergoing no antiferromagnetic exchange. Attempts to
model the system using the Curie or Curie-Weiss laws
did not provide good fits. Complexes with S ) 1 spin-
states exhibit zero-field splittings that cause deviations
from Curie law behavior, particularly those complexes
containing second- or third-row transition metals. The
magnetic susceptibility data for 1 were therefore ana-
lyzed using the equation for the zero-field splitting of a
S ) 1 state:35
where x ) D/kT and C ) NAg2µB2/kT. To account for the
presence of a small amount of paramagnetic impurity,
the expression was combined with the Curie law term
(36) Fryzuk, M. D.; Love, J . B.; Rettig, S. J . Organometallics 1998,
17, 846.
(37) Fryzuk, M. D.; J ohnson, S. A.; Rettig, S. J . Organometallics
1999, 18, 4059.
according to
(38) Bott, S. G.; Hoffman, D. M.; Rangarajan, S. P. Inorg. Chem.
1995, 34, 4305.
(39) Cotton, F. A.; Lu, J . Inorg. Chem. 1995, 34, 2639.
(40) Al-Soudani, A.-R. H.; Edwards, P. G.; Hursthouse, M. B.; Malik,
K. M. A. J . Chem Soc., Dalton Trans. 1995, 355.
(41) Cotton, F. A.; Roth, W. J . Inorg. Chim. Acta 1983, 71, 175.
(42) Cotton, F. A.; Roth, W. J . Inorg. Chem. 1984, 22, 3654.
(43) Cotton, F. A.; Duraj, S. A.; Roth, W. J . Inorg. Chem. 1984, 23,
3592.
(44) Cotton, F. A.; Roth, W. J . Polyhedron 1986, 4, 1103.
(45) Manzer, L. E. Inorg. Chem. 1977, 16, 525.
(46) Labauze, G.; Samuel, E.; Livage, J . Inorg. Chem. 1980, 19, 1384.
where P represents the fraction of paramagnetic S ) 1
impurity. Experimental susceptibility versus tempera-
ture data for 1 are compared with the best fits from
theory (Figure 2) with D ) 2.03, g ) 1.74, and P )
0.0068 (F ) 0.0151). The small g value can be rational-
ized by the large degree of spin-orbit coupling exhibited
by second- and third-row metals. This effect is made

3756 Organometallics, Vol. 20, No. 17, 2001
Fryzuk et al.
further evident by the plot of magnetic moment versus
temperature. At 300 K a magnetic moment of 1.84 µB
is observed, significantly lower than the expected spin-
only moment of 2.83 µB for two unpaired electrons. As
the temperature is lowered, a maximum magnetic
moment of 2.34 µB is observed at 40 K. Below 40 K the
moment begins to drop more rapidly, since zero-field
splitting factors begin to take prominence. In the
geometrically distorted trigonal bipyramid the normally
degenerate dxz and dyz orbitals may experience varia-
tions in energy, resulting in the removal of this degen-
eracy. In this case, a d2 complex will generate a
diamagnetic spin-paired configuration. If the energy
difference between the resulting nondegenerate dxz and
dyz orbitals is smaller than the spin-pairing energy, a
triplet spin-state will result. The low magnetic moment
of 1.83 µB at room temperature implies that there exists
a thermal equilibrium between the two possible spin
states, S ) 1 and S ) 0. As the temperature is lowered,
the system approaches a pure S ) 1 spin state with a
moment of 2.34 µB. This implies a spin-equilibrium
where the ground state is the high spin-state. The high-
low spin equilibrium case is well documented in the
literature,35,47 but to our knowledge there are no ex-
amples of spin equilibrium between S ) 1 and S ) 0
Nb(III). A related system displaying a high-spin ground
state to low-spin thermally populated state is FeCl3-
(PCy3)2.48 Variable-temperature magnetic data on this
Fe(III) species show a steady increase in magnetic
moment with decreasing temperature, indicating a S
) 5/2 ground state to S ) 3/2 spin-state transition.
Changing the phosphine from PCy3 to PMe3 causes the
reverse equilibrium to be observed with a low-spin
ground state.
F igu r e 3. Magnetic susceptibility (O) and moment (4)
versus temperature plot for Ph[P2N2]NbCl (2). The S ) 1
Heisenberg dimer model was employed to generate the
lines with J ) -75.74 cm-1, g ) 2.11, P ) 0.0861, θ ) -1.20
K, TIP ) 1800 × 10-6 cm3 mol-1 (F ) 0.00918).
been examined.50 Although many reports exist on the
magnetic behavior of vanadium-containing species,51-54
there is little data on low-valent niobium-containing
dimers other than those in which Nb-Nb bonds are
suggested.55 Our primary interest lies in the comparison
of the magnetic nature of 1 and 2 and the structural
differences these data imply. In the absence of a direct
crystallographic comparison of the two chloride com-
plexes, the magnetic behavior clearly indicates the
structural differences in the solid state.
Ad d u ct F or m a tion Rea ction s of P h [P 2N2]NbCl.
Whereas Cy[P2N2]NbCl (1) is a green solid which dis-
solves in most hydrocarbon solvents (including hexanes
and sparingly in pentane) to give green solutions, red
Ph[P2N2]NbCl (2) is not particularly soluble in hydro-
carbon solvents other than aromatics, such as toluene
and benzene, in which it gives deep green solutions.
Variable-temperature magnetic susceptibility data sug-
gest that complex 2 may be dimeric in the solid state
(see above). This color change in solution may imply
dissociation of a weakly bound chloride-bridged dimer.
In an effort to examine simple adduct formation of 2,
we examined its reactivity with donor ligands; similar
reactivity was observed for 1, but these adducts were
characterized only by their UV-visible spectra.
The plot of magnetic susceptibility of the phenyl
phosphine-substituted complex 2 versus temperature
reveals a broad maximum in susceptibility around 180
K, consistent with the presence of antiferromagnetic
coupling. A possible explanation is that species 2 is a
chloride-bridged dimer, as previously speculated.49 The
data can be modeled using the Heisenberg model for a
S ) 1 dimer where C ) NAg2µB2/kT:35
Upon dissolution in THF, 2 generates a yellow solu-
tion that corresponds to the formation of the THF
adduct Ph[P2N2]NbCl(THF) (3a ). Upon removal of the
THF, a brown solid could be obtained, but elemental
analysis consistent with the formula of 3a could not be
obtained; furthermore, dissolution of this isolated mate-
rial in aromatic solvents gave green solutions typical
of the starting, adduct-free chloride 2. A similar set of
observations are obtained for the acetonitrile adduct
Ph[P2N2]NbCl(NCMe) (3b). Addition of CO gas to a green
benzene-d6 solution of Ph[P2N2]NbCl (2) quickly yields
Paramagnetic impurities in 2 were accounted for as in
the analysis of 1 above, albeit using the Curie-Weiss
law term, and the data were corrected for temperature-
independent paramagnetism (TIP). The best fit between
experiment and theory, employing a TIP of 1800 × 10-6
cm3 mol-1 (per dimer unit), shown in Figure 3, yielded
J ) -75.74 cm-1, g ) 2.11, P ) 0.0861, and θ ) -1.20
(F ) 0.00918). Using smaller values of TIP yielded
smaller fractions of P, but unrealistic g values. It should
also be noted that contributions of zero-field splitting
and spin-orbit coupling have been omitted to simplify
analysis; complicated models employing these factors
for magnetically related Ni(II) systems, however, have
(50) Ginsberg, A. P.; Martin, R. L.; Brookes, R. W.; Sherwood, R. C.
Inorg. Chem. 1972, 11, 2884.
(51) Choukroun, R.; Lorber, C.; Donnadieu, B.; Henner, B.; Frantz,
R.; Guerin, C. J . Chem. Soc., Chem. Commun. 1999, 1099.
(52) Choukroun, R.; Donnadieu, B.; Malfant, I.; Haubrich, S.; Frantz,
R.; Guerin, C.; Henner, B. J . Chem. Soc., Chem. Commun. 1997, 2315.
(53) Dean, N. S.; Bond, M. R.; O’Conner, C. J .; Carrano, C. J . Inorg.
Chem. 1996, 35, 7643.
(47) Kahn, O. Molecular Magnetism; VCH: New York, 1993.
(48) Walker, D.; Poli, R. Inorg. Chem. 1989, 28, 1793.
(49) Fryzuk, M. D.; Leznoff, D. B.; Rettig, S. J .; Thompson, R. C.
Inorg. Chem. 1994, 33, 5528.
(54) Ferguson, R.; Solari, E.; Floriani, C.; Osella, D.; Ravera, M.;
Re, N.; Chiesi-Villa, A.; Rizzoli, C. J . Am. Chem. Soc. 1997, 119, 10104.

Macrocyclic Complexes of Niobium(III)
Organometallics, Vol. 20, No. 17, 2001 3757
Ta ble 3. UV-Vis Sp ectr a l Da ta a n d Solu tion
Ma gn etic Mom en ts for R[P 2N2]NbCl(L)
ligand
color
UV-vis (nm)
ꢀ
µeff
R ) Ph
none
green
yellow
616
439
452
672 2.70
997 2.69
1210 2.72
THF (3a )
MeCN (3b) amber
CO (3c)
PMe3 (3d ) orange
bright yellow 440
912 diamagnetic
801 2.02
1179 2.34
425 (shoulder)
540
py (3e)
R ) Cy
none
THF
MeCN
CO
red
green
630
614
458
599
601
lime green
pale yellow
dark yellow
406
427 (shoulder)
427 (shoulder)
F igu r e 4. Molecular structure (ORTEP) of Ph[P2N2]NbCl-
(py), 3e. Ellipsoids drawn at the 50% probability level, silyl
methyls omitted for clarity, and only ipso carbons of phenyl
rings shown.
PMe3
py
bright yellow 380 (shoulder) 1036
purple 549 1489
a yellow solution, which reverts to its original green
color upon removal of CO under vacuum. The H and
1
Sch em e 3
31P{1H} NMR spectra show the adduct is diamagnetic
upon coordination of the strong field ligand CO, the
degenerate dxz and dyz orbitals of the trigonal bipyra-
midal (or square pyramidal) starting material become
inequivalent, and the electrons pair. A solution IR
measurement shows only one assignable CO stretch at
νCO ) 1920 cm-1, consistent with the formulation
Ph[P2N2]NbCl(CO) (3c). This vibration lies within the
range observed for other Nb(III) carbonyl deriva-
tives.12,56-58 Any attempts to isolate a solid product were
hindered by the lability of the CO. The addition of PMe3
to a solution of Ph[P2N2]NbCl in toluene or cyclohexane
also induces a color change from green to orange. The
resulting adduct Ph[P2N2]NbCl(PMe3) (3d ) is paramag-
netic, showing only broadened and shifted resonances
1
in the H NMR spectrum.
N(3) and P(1)-Nb(1)-Cl(1) angles (92.47° and 92.74°)
are smaller than the corresponding P(2)-Nb(1)-N(3)
and P(2)-Nb(1)-Cl(1) angles (98.22° and 99.47°). The
pyridine ring also adopts a twisted coordination angle
with respect to the axis defined by P(1)-Nb(1)-P(2);
that is, the P(1)-Nb(1)-N(3)-C(29) torsion angle is
44.6°. This evidently minimizes steric interaction be-
tween the pyridine ligand and the chloride while also
avoiding close contact with the phenyl rings on [P2N2].
The phenyl rings themselves are each twisted to differ-
ent degrees in order to avoid steric congestion caused
by interaction with the pyridine. Whereas the phenyl
ring bound to P(2) adopts a nearly parallel plane with
the P(1)-Nb(1)-P(2) axis, the other phenyl ring ap-
proaches a perpendicular orientation. This is demon-
strated by the small Nb(1)-P(2)-C(19)-C(20) torsion
angle of 7.2° as compared to 70.9° for Nb(1)-P(1)-
C(13)-C(18). The Nb-N, -P, and Cl bond lengths are
not unusual and compare to other Nb-amido and phos-
phine complexes including Cy[P2N2]NbCl, above.39-44
In contrast to the above labile adducts, the addition
of pyridine results in the formation of a stable adduct
of formula Ph[P2N2]NbCl(py) (3e). This complex does not
dissociate pyridine even under vacuum. NMR spectro-
scopic measurements indicate that 3e is paramagnetic,
as evidenced by the broadened and shifted resonances
1
observed in the H NMR spectrum. Room-temperature
solution magnetic susceptibility measurements indicate
a magnetic moment of 2.34 µB, consistent with two
unpaired electrons. Formation of all of the adducts 3a -e
can be conveniently followed using UV-visisble spec-
troscopy (Table 3); all show single isosbestic points upon
titration of the starting chloride with the donor ligand.
Interestingly, only 1 equiv of pyridine is necessary to
complete the formation of 3e, whereas >10 equiv of
donor is necessary for all of the other adducts to form
completely. Red single crystals of pyridine adduct 3e
were obtained from a 1:1 toluene-hexanes solution. The
solid state X-ray molecular structure of Ph[P2N2]NbCl-
(py) (3e) is shown in Figure 4; crystal data are given in
Table 1, and selected bond lengths and angles are
detailed in Table 4. Structural analysis shows that 3e
is monomeric, with the niobium occupying a distorted
octahedral geometry. The equatorial plane is defined by
N(1), N(2), N(3), and Cl(1); P(1) and P(2) lie in elongated
axial positions. The P(1)-Nb(1)-P(1) bond angle of
164.98(4)° is comparable to that observed in other group
5 [P2N2] complexes,37,59 but slightly smaller than that
of 2. The chloride and pyridine ligands are asymmetri-
cally coordinated to niobium in that the P(1)-Nb(1)-
Rea ction of R[P 2N2]NbCl w ith LiCHR′(SiMe3) (R′
) H, SiMe3). The niobium chloride complexes 1 and 2
(55) Tayebani, M.; Feghali, K.; Gambarotta, S.; Yap, G. P. A.;
Thompson, L. K. Angew. Chem., Int. Ed. 1999, 38, 3659.
(56) Bunker, M. J .; Green, M. L. H. J . Organomet. Chem. 1980, 192,
C6.
(57) Antinolo, A.; de Ilarduya, J . M.; Otero, A.; Royo, P.; Manotti-
Lanfredi, A. M.; Tiripicchio, A. J . Chem. Soc., Dalton Trans. 1988, 2685.
(58) J alon, F. A.; Otero, A.; Royo, P.; Balcazar, J . L.; Florencio, F.
J . Chem. Soc., Dalton Trans. 1989, 79-84.
(59) Fryzuk, M. D.; J ohnson, S. A.; Rettig, S. J . Organometallics
2000, 19, 3931-3941.

3758 Organometallics, Vol. 20, No. 17, 2001
Ta ble 4. Selected Bon d Len gth s (Å) a n d An gles (d eg) in P h [P 2N2]NbCl(p y), 3e
Fryzuk et al.
Nb(1)-N(1)
Nb(1)-N(2)
Nb(1)-N(3)
2.125(3)
2.152(3)
2.398(3)
Nb(1)-P(1)
Nb(1)-P(2)
Nb(1)-Cl(1)
2.568(1)
2.596(1)
2.511(1)
N(2)-Nb(1)-N(1)
N(2)-Nb(1)-Cl(1)
N(1)-Nb(1)-Cl(1)
N(2)-Nb(1)-P(1)
N(1)-Nb(1)-P(1)
Cl(1)-Nb(1)-P(1)
Cl(1)-Nb(1)-P(2)
103.0(1)
164.5(1)
92.2(1)
85.6(1)
85.1(1)
P(1)-Nb(1)-P(2)
Cl(1)-Nb(1)-N(3)
P(1)-Nb(1)-N(3)
P(2)-Nb(1)-N(3)
N(1)-Nb(1)-N(3)
N(2)-Nb(1)-N(3)
164.98(4)
79.91(9)
92.47(9)
98.22(9)
171.7(1)
84.7(1)
92.74(4)
99.47(4)
N(1)-Nb(1)-N(3)-C(29)
P(1)-Nb(1)-P(2)-C(19)
-29(1)
-166.6(2)
Cl(1)-Nb(1)-P(1)-C(13)
P(2)-Nb(1)-P(1)-C(13)
49.9(2)
-165.6(2)
Sch em e 4
F igu r e 5. Molecular structure (ORTEP) of Cy[P2N2]NbCH2-
SiMe3, 4a . Ellipsoids drawn at the 50% probability level,
silyl methyls omitted for clarity, and only ipso carbons of
cyclohexyl rings shown.
Ta ble 5. Selected Bon d Len gth s (Å) a n d An gles
(d eg) in Cy[P 2N2]NbCH2SiMe3, 4a
Nb(1)-N(1)
Nb(1)-N(2)
Nb(1)-C(25)
2.112(7)
2.119(8)
2.21(1)
Nb(1)-P(1)
Nb(1)-P(2)
2.571(3)
2.583(2)
react with bulky alkyllithium reagents to generate
stable monomeric niobium alkyl complexes (Scheme 4).
Complexes of smaller hydrocarbyl moieties such as
methyl can also be synthesized, and this work will be
the subject of a future report.60 The addition of LiCH2-
SiMe3 to toluene solutions of 1 and 2 gives dark blue
paramagnetic products in each case, whereas the more
sterically demanding LiCH(SiMe3)2 gives green prod-
ucts.
N(1)-Nb(1)-N(2)
N(1)-Nb(1)-C(25)
P(1)-Nb(1)-N(1)
P(2)-Nb(1)-N(1)
Nb(1)-C(25)-Si(5)
N(1)-Nb(1)-C(25)
115.7(3)
123.84(4)
85.7(3)
130.43(6)
123.3(5)
123.0(3)
P(1)-Nb(1)-P(2)
P(1)-Nb(1)-C(25)
P(1)-Nb(1)-N(2)
P(2)-Nb(1)-N(2)
P(1)-Nb(1)-C(25)
166.9(1)
96.20(1)
87.0(2)
85.4(2)
95.6(3)
N(1)-Nb(1)-C(25)-Si(5) 1.8(8) P(1)-Nb(1)-C(25)-Si(5) -86.7(6)
As with the chloride precursors, these complexes show
only broadened and shifted resonances in the H NMR
Cy[P2N2]NbCH2SiMe3 crystallizes with two molecules
in the asymmetric unit cell. As with the chloride
precursor, 1, the niobium atoms in each asymmetric
molecule occupy a distorted trigonal bipyramidal geom-
etry. The phosphine atoms are pinched back from the
axial position giving P(1)-Nb(1)-P(2) and P(3)-Nb(2)-
P(4) angles of 166.9(1)° and 163.94(9)°, respectively. The
alkyl and two amide ligands lie in the trigonal plane,
displaying a combined equatorial angle of 360.0° about
niobium. The N-Nb-N angles in each asymmetric
molecule are quite different, giving a N(1)-Nb(1)-N(2)
angle of 115.7(3)° and a N(3)-Nb(2)-N(4) angle of
121.2(3)°. The Nb-N and Nb-P bond lengths are only
slightly longer than those exhibited by 1, indicating that
the sterically larger alkyl group does not impose a
significant effect upon the macrocycle binding in this
case. The Nb-C bond length of 2.21(1) Å and Nb-C-
Si bond angle of 123.3(5)° are typical of other niobium
(trimethylsilyl)methyl complexes.15,61-64 The position of
1
and no 31P{1H} resonances. They are also EPR silent.
X-ray crystallography and magnetic studies were, there-
fore, employed to characterize these R[P2N2]NbR′ com-
plexes. Room-temperature magnetic susceptibility
measurements were conducted in solution by Evans’
method33,34 and indicate magnetic moments of 2.6 and
2.5 µB for 4a and 5a , respectively, corresponding to two
unpaired electrons per niobium.
Single crystals of Cy[P2N2]NbCH2SiMe3, 4a , suitable
for X-ray diffraction studies were grown by slow cool-
ing of a saturated hexanes solution. The analogous
Ph[P2N2]NbCH2SiMe3, 5a , exhibits significantly higher
solubility in even the most nonpolar solvents such as
pentane and hexamethyldisiloxane and could be isolated
only as an amorphous solid by drying under high
vacuum for several hours. The solid state X-ray molec-
ular structure of 4a is shown in Figure 5; crystal data
are given in Table 1, and selected bond lengths and
angles are detailed in Table 5.
(61) Fu, P.; Khan, M. A.; Nicholas, K. M. Organometallics 1992, 11,
2607.
(60) Fryzuk, M. D.; Kozak, C. M.; Patrick, B. O. Manuscript in
preparation.
(62) Fu, P.; Khan, M. A.; Nicholas, K. M. Organometallics 1991, 10,
382.

Macrocyclic Complexes of Niobium(III)
Organometallics, Vol. 20, No. 17, 2001 3759
Ta ble 6. Selected Bon d Len gth s (Å) a n d An gles (d eg) in P h [P 2N2]NbCH(SiMe3)2, 5b
Nb(1)-N(1)
Nb(1)-C(13)
2.108(2)
2.270(3)
Nb(1)-P(1)
2.5832(5)
N(1)-Nb(1)-N(1*)
N(1)-Nb(1)-C(13)
P(1)-Nb(1)-N(1)
Nb(1)-C(13)-Si(3)
N(1)-Nb(1)-C(13)
N(1*)-Nb(1)-C(13)
111.36(9)
123.0(1)
86.06(4)
114.1(2)
123.0(1)
124.5(1)
P(1)-Nb(1)-P(1*)
P(1)-Nb(1)-C(13)
P(1)-Nb(1)-N(1*)
P(1)-Nb(1)-C(13)
P(1*)-Nb(1)-C(13)
167.63(2)
106.13(8)
86.98(4)
106.13(8)
86.24(8)
N(1)-Nb(1)-C(13)-Si(3)
-32.7(2)
P(1)-Nb(1)-C(13)-Si(3)
-78.2(2)
bond length significantly. All Nb-P bonds in 4a and
5b are approximately 2.58 Å. Also, the relatively similar
Nb-P-Cipso angles shown in each complex imply that
the coordination geometry is not sensitive to the sub-
stituents at phosphorus.
Con clu sion s
In this report the synthesis of a cyclohexylphosphine-
containing mixed donor macrocycle, Cy[P2N2], and its
utility in stabilizing Nb(III) complexes are described.
The chloride complex Cy[P2N2]NbCl (1) is monomeric in
the solid state, as evidenced by X-ray diffraction and
variable-temperature magnetic susceptibility studies.
It is a useful precursor to the hydrocarbyl derivatives
Cy[P2N2]NbCH2SiMe3 and Cy[P2N2]NbCH(SiMe3)2 by
metathesis type procedures. The related complex having
phenyl-substituted phosphine donor atoms, Ph[P2N2]-
NbCl (2), has also been prepared and studied by
variable-temperature magnetic susceptibility measure-
ments. The plot of øm versus temperature indicates
the presence of antiferromagnetic exchange between
two d2 centers, which is consistent with a dinuclear
structure for 2 in the solid state, possibly (Ph[P2N2]Nb)2-
(µ-Cl)2. The Lewis acidic nature of Nb(III) in 2 is evident
in its ability to readily form adducts with coordinating
solvents and small molecules generating monomeric
Ph[P2N2]NbCl(L) complexes. As in the cyclohexylphos-
phine analogue, metathesis of the chloride with alkyl-
lithium reagents generates the monomeric paramag-
netic organometallic complexes Ph[P2N2]NbCH2SiMe3
and Ph[P2N2]NbCH(SiMe3)2. Current work is being
directed at studying the reactivity patterns exhibited
by these Nb(III) alkyl complexes, especially hydro-
genolysis and insertion processes.
F igu r e 6. Molecular structure (ORTEP) of Ph[P2N2]NbCH-
(SiMe3)2, 5b. Ellipsoids drawn at the 50% probability level,
silyl methyls omitted for clarity, and only ipso carbons of
phenyl rings shown.
the Me3Si fragment of the substituent lies nearly
orthogonal to the P-Nb-P axis, as indicated by the
P(2)-Nb(1)-C(25)-Si(5) torsion angle of 94.3(6)°. Steric
hindrance by the cyclohexyl substituents on the phos-
phine atoms may force this preferred geometry, result-
ing in chemically equivalent phosphines. However, in
the absence of variable-temperature NMR data, barriers
to rotation of the alkyl group about niobium cannot be
determined.
The use of the bulkier CH(SiMe3)2 substituent allows
both the cyclohexyl- and phenylphosphine-containing
macrocycle complexes to be obtained as crystalline
products in high yield. As found for compounds 4a and
5a above, R[P2N2]NbCH(SiMe3)2 (R ) Cy, 4b; R ) Ph,
5b) are paramagnetic. Bright green crystals of 4b and
5b were obtained by slow evaporation of hexanes
solutions. The solid state X-ray molecular structure of
5b is shown in Figure 6; crystal data are given in Table
1, and selected bond lengths and angles are detailed in
Table 6.
The molecule sits on a 2-fold axis of rotation, with
C(13) being disordered about this axis. As with 4a , the
niobium atom occupies a trigonal bipyramidal geometry.
The P(1)-Nb(1)-P(1*) angle of 167.63(2)° is only mar-
ginally larger than that of 4a , while the N(1)-Nb(1)-
N(1*) angle of 111.36(9)° is considerably smaller, pos-
sibly as a result of the increased steric bulk of the alkyl
group. This is also reflected in the longer Nb(1)-C(13)
bond length of 2.270(3) Å. Interestingly, changing the
substituent from cyclohexyl to phenyl on the phosphine
donor atoms does not alter the niobium-phosphorus
Exp er im en ta l 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.65 Anhydrous diethyl ether and THF were
stored over sieves and distilled from sodium benzophenone
ketyl under argon. Dichloromethane was dried by refluxing
over calcium hydride. Nitrogen and argon were dried and
deoxygenated by passing the gases through a column contain-
ing molecular sieves and MnO. Deuterated benzene and
toluene were dried by refluxing over sodium and potassium
alloy, and THF-d8 was dried over sodium, in a sealed vessel
(63) Arnold, J .; Tilley, T. D.; Rheingold, A. L.; Geib, S. J . Organo-
metallics 1987, 6, 473.
(64) Dorado, I.; Garces, A.; Lopez-Mardomingo, C.; Fajardo, M.;
Rodriguez, A.; Antinolo, A.; Otero, A. J . Chem. Soc., Dalton Trans.
2000, 2375.
(65) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J . Organometallics 1996, 15, 1518.

3760 Organometallics, Vol. 20, No. 17, 2001
Fryzuk et al.
under partial pressure, and then trap-to-trap distilled. They
were degassed under three freeze-pump-thaw cycles. Unless
otherwise stated, all NMR spectra were recorded on either a
The yield can be improved by adding 1 equiv of THF to the
toluene solution before removal of solvents to generate the less
soluble Cy[P2N2]Li2(THF) species. Yield: 24.5 g (84%). 1H NMR
(C6D6, 400 MHz, 400 K): δ 0.30 and 0.45 (s, 24H total, SiCH3),
0.57 and 0.85 (m, 8H total, ring CH2), 1.12, 1.60, 1.67, and
1.83 (br, 22H total, C6H11P), 1.27 (m, 4H, THF-OCH2CH2), 3.57
(m, 4H, THF-OCH2CH2). 31P{1H} NMR (C6D6, 162 MHz, 300
1
Bruker AC-200 or a Bruker Avance 400 instrument. H NMR
spectra were referenced to residual protons in the deuterated
solvent. UV-vis spectra were recorded using a Hewlett-
Packard 8454 UV-visible spectrophotometer and quartz cu-
vettes with Teflon Kontes valves. FTIR spectra were recorded
on a BOMEM MB 100 spectrophotometer. 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 the range 2-300 K and at a field of 10 000 G
using a Quantum Design (MPMS) SQUID magnetometer.
The compounds syn-Ph[P2N2]Li2‚C4H8O2,27 NbCl3(DME),3 and
1
7
K): δ -32.78 (q, J PLi ) 52.1 Hz). Li{1H} (C6D6, 155.5 MHz,
1
300 K): δ 1.22 (s, 1Li), 2.99 (t, J LiP ) 52.1 Hz, 1Li). Anal.
Calcd for C24H54N2Si4P2Li2: C, 52.58; H, 9.74; N, 5.01. Found:
C, 51.84; H, 9.79; N, 4.90.
Cy[P 2N2]NbCl (1). A mixture of Cy[P2N2]Li2(THF) (7.00 g,
10.4 mmol) and NbCl3(DME) (3.00 g, 10.4 mmol) was sus-
pended in 100 mL of toluene. After stirring the red suspension
for 2 days a dark green solution formed. The solution was
evaporated to dryness, extracted into toluene, and filtered
through Celite. The toluene was removed in vacuo to produce
a dark green powder, which was washed in minimal hexanes
and recrystallized from slow cooling of a saturated toluene
solution, giving 2 as large dark green prisms. Yield: 5.17 g
66
LiCH(SiMe3)2 were prepared according to literature proce-
dures. LiCH2SiMe3 was purchased from Aldrich as a 1 M
solution in pentane and used as received. All other reagents
were obtained from a commercial source and purified by the
appropriate methods.67 CO gas was purchased from Matheson
and used without further purification.
ClCH2SiMe2NEt2. HNEt2 (25.0 g, 342 mmol) was added to
a cooled solution of Cl(ClCH2)SiMe2 (20.0 g, 140 mmol) in ether
at 0 °C and stirred for 2 h before warming to room tempera-
ture. Filtration of the flocculent precipitate and removal of
solvent under vacuum allowed isolation of a colorless oil.
Yield: 21.5 g (85.8%). 1H NMR (C6D6, 200 MHz, 300 K): δ
0.03 (s, 6H, SiCH3), 0.75 (t, 6H, CH2CH3), 2.40 (s, 2H, PCH2-
Si), 2.50 (q, 4H, CH2CH3)
1
(75%). H NMR (C6D6, 400 MHz, 300 K): δ 4.78 (w1/2 84 Hz,
12H, SiCH3CH3′); 11.19 (208 Hz, 12H, SiCH3CH3′); 11.95 (140
Hz, 4H, o-Ph); 39.65 (856 Hz, 4H, PCHH′) 56.16 (1244 Hz, 4H,
PCHH′). UV-vis (C6H12): 620 nm, ꢀ ) 712. Anal. Calcd for
C
24H54ClN2NbP2Si4: C, 42.81; H, 8.08; N, 4.16. Found: C,
43.01; H, 8.23; N, 4.30. MS (EI) m/z (%): 672, (60) [M+]. µeff
(Evans’ method) ) 2.20 µB.
P h [P 2N2]NbCl (2). A mixture of Ph[P2N2]Li2(dioxane) (6.65
g, 10.4 mmol) and NbCl3(DME) (3.00 g, 10.4 mmol) was
suspended in 100 mL of toluene. After stirring the red
suspension for 2 days a dark green solution formed. The
solution was evaporated to dryness, extracted into toluene, and
filtered through Celite. The toluene was removed in vacuo to
produce a brick red powder, which was washed in minimal
hexanes and recrystallized from slow cooling of a saturated
toluene solution, giving 3 as dark red cubes. Yield: 5.50 g
CyP HCH2SiMe2NEt2. n-BuLi (75 mL, 1.6 M, 120 mmol)
was syringed into a solution of CyPH2 (13.9 g, 120 mol) in Et2O
at -78 °C and allowed to warm to room temperature while
stirring. Upon stirring for 1 h at room temperature the yellow
solution was transferred to a cooled solution of ClCH2SiMe2-
NEt2 (21.5 g, 120 mmol) in Et2O at -78 °C. The mixture was
allowed to warm to room temperature while stirring. Filtration
and removal of solvent allowed isolation of a pale yellow oil.
1
(80%). H NMR (C6D6, 400 MHz, 300 K): δ 4.78 (w1/2 84 Hz,
12H, SiCH3CH3), 8.70 (28 Hz, 2H, p-Ph), 10.22 (48 Hz, 4H,
m-Ph), 11.19 (208 Hz, 12H, SiCH3CH3), 11.95 (140 Hz, 4H,
o-Ph), 39.65 (856 Hz, 4H, PCHH), 56.16 (1244 Hz, 4H, PCHH′).
UV-vis (C6H12): 616 nm, ꢀ ) 672. Anal. Calcd for C24H42ClN2-
NbP2Si4: C, 43.59; H, 6.04; N, 4.24; Cl, 5.36. Found: C, 43.34;
H, 6.32; N, 4.19; Cl, 5.49. MS (EI) m/z (%): 660, (70) [M+]. µeff
(Evans’ method) ) 2.70; (magnetic susceptibility balance) )
2.80 µB.
Rea ction s w ith Coor d in a tin g Liga n d s. Identical proce-
dures were used for the reaction of Ph[P2N2]NbCl and Cy[P2N2]-
NbCl with coordinating ligands. Experimental details and
characterization of only the Ph[P2N2]NbCl(L) complexes are
described.
1
Yield: 28.8 g (92%). H NMR (C6D6, 200 MHz, 300 K): δ 0.03
(s, 6H, SiCH3) 0.30 (m, 2H, PCH2Si), 0.80 (t, 6H, CH2CH3),
1.1, 1.4, 1.6, 1.8 (br, 11H total, C6H11P), 2.60 (q, 4H, CH2CH3),
1
3
3
2.80 (ddt, J HP ) 180 Hz, J HH ) 20 Hz, J HH ) 5 Hz CH2P-
(H)CH). 31P{1H} NMR (C6D6, 81 MHz, 300 K): δ -60.9 (s).
(CyP HCH2SiMe2)NH. NH4Cl (6.0 g, 112 mmol), CyPHCH2-
SiMe2NEt2 (28.8 g, 110 mmol), and CH2Cl2 (100 mL) were
cooled to -78 °C. NEt3 (15 mL) was added via syringe, and
the mixture was allowed to warm to room temperature and
stirred for 18 h. All volatiles were removed in vacuo and the
solids extracted into hexanes. Filtration and removal of
hexanes allowed isolation of a pale yellow viscous oil. Yield:
1
17.9 g (83%). H NMR (C6D6, 200 MHz, 300 K): 0.10 (s 12H,
P h [P 2N2]NbCl(THF ) (3a ). Ph[P2N2]NbCl (200 mg, 0.303
mmol) was dissolved in THF (15 mL), giving a deep yellow-
colored solution. The solution was stirred for 30 min before
all volatiles were removed in vacuo, allowing isolation of a dark
yellow solid. The THF molecule is considerably labile, and
attempts to dissolve the product in toluene and benzene caused
its displacement and regeneration of Ph[P2N2]NbCl. Yield: 220
mg (99%). 1H NMR only partially assignable (THF-d8, 400
MHz, 300 K): δ 4.20 (w1/2 56 Hz, 12H, SiCH3CH3), 5.96 (72
Hz, 12H, SiCH3CH3), 8.85 (22 Hz, 2H, p-Ph), 10.80 (46 Hz,
4H, m-Ph). UV-vis (C6H12/THF): 439 nm, ꢀ ) 997. µeff (Evans’
method) ) 2.69 µB.
P h [P 2N2]NbCl(CH3CN) (3b). Ph[P2N2]NbCl (200 mg, 0.303
mmol) was dissolved in acetonitrile (15 mL), giving a deep
yellow solution. The solution was stirred for 30 min before all
volatiles were removed in vacuo, allowing isolation of a dark
yellow-brown waxy solid. Dissolution in hexanes or toluene
caused slow product decomposition to Ph[P2N2]NbCl. [P2N2]-
NbCl(NCMe) Yield: 210 mg (99%). UV-vis (C6H12/MeCN):
452 nm, ꢀ ) 1210. µeff (Evans’ method) ) 2.72 µB.
SiCH3) 0.35 (m, 4H, PCH2Si), 1.1, 1.4, 1.6, 1.8 (br, 22H total,
1
3
3
C6H11P), 2.80 (ddt, J HP ) 180 Hz, J HH ) 20 Hz, J HH ) 5 Hz
CH2P(H)CH). 31P{1H} NMR (C6D6, 81 MHz, 300 K): δ -60.9
(s).
syn -Cy[P 2N2]Li2(THF ). HN(SiMe2CH2Cl)2 (10.49 g, 45.78
mmol) and (CyPHCH2SiMe2)NH (17.90 g, 45.78 mmol) were
dissolved in Et2O (200 mL) and cooled to -78 °C. MeLi (130.8
mL, 1.4 M, 184 mmol) was added, and the solution was allowed
to warm to room temperature while stirring, causing a white
precipitate to form. After stirring for 12 h all solvents were
removed in vacuo and the residue was extracted into toluene
and filtered. Evaporation of toluene and washing the crude
solids with hexanes allows isolation of a white powder.
Crystals suitable for single-crystal X-ray diffraction were
grown by cooling a hexanes solution to -40 °C. Yield: 10.9 g
(40%).
(66) Cowley, A. H.; Kemp, R. A. Synth. React. Inorg. Met.-Org. Chem.
1981, 11, 591.
(67) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals, 3 ed.; Butterworth-Heinemann Ltd.: Oxford, 1994.

Macrocyclic Complexes of Niobium(III)
Organometallics, Vol. 20, No. 17, 2001 3761
P h [P 2N2]NbCl(CO) (3c). A solution of Ph[P2N2]NbCl (10 mg)
in C6D6 was loaded into an NMR tube fitted with a Teflon tap
and degassed by three freeze-pump-thaw cycles. The sample
was then subjected to 1 atm of CO and sealed. The CO ligand
is highly labile and is readily removed under vacuum. 1H NMR
(C6D6, 400 MHz, 300 K): δ 0.24 (br, 24H total SiCH3), 1.74
(br, 8H, PCH2), 7.23, 8.18 (br, PPh). 31P{1H} NMR (C6D6, 162
MHz, 300 K): δ 10.70 (w1/2 852.5 Hz). IR vCO 1920 cm-1. UV-
vis (C6H12): 440 nm, ꢀ ) 912.
which was washed with minimal pentane. Yield: 1.18 g (90%).
1H NMR only partially assignable (C6D6, 400 MHz, 300 K): δ
4.50 (w1/2 90 Hz, 6H, SiCH3CH3), 5.20 (90 Hz, 6H, SiCH3CH3),
7.70 (500 Hz, 18H, Si(CH3)3), 8.60 (90 Hz, 6H, SiCH′3CH′3),
10.20 (90 Hz, 6H, SiCH′3CH′3), 13.80 (500 Hz, 2H, PCHH)
21.00 (500 Hz, 2H PCHH). Anal. Calcd for C31H73N2NbP2Si6:
C, 46.70; H, 9.23; N, 3.51. Found: C, 46.74; H, 9.28; N, 3.32.
MS (EI) m/z (%): 796, (30) [M+]. µeff (Evans’ method) ) 2.6 µB.
P h [P 2N2]NbCH(SiMe3)2 (5b). (a) To a solution of Ph[P2N2]-
NbCl (1.00 g, 1.51 mmol) in toluene (30 mL) was added LiCH-
(SiMe3)2 (250 mg, 1.51 mmol) at room temperature to give a
dark green solution. After stirring for 6 h the solution was
filtered through Celite and the solvent removed in vacuo to
yield green solids, which were washed with minimal pentane.
P h [P 2N2]NbCl(P Me3) (3d ). A solution of Ph[P2N2]NbCl (10
mg) in C6D6 was loaded into an NMR tube fitted with a Teflon
tap and degassed by three freeze-pump-thaw cycles. The
sample was then subjected to an excess of PMe3 added by
vacuum transfer and sealed. Upon warming to room temper-
ature, the solution turned from an intense green color to dark
yellow-orange. The PMe3 ligand is labile and is easily removed
1
Yield: 1.18 g (90%). H NMR (C6D6, 400 MHz, 300 K): δ 4.35
(w1/2 90 Hz, 6H, SiCH3CH3), 5.20 (90 Hz, 6H, SiCH3CH3), 7.70
(500 Hz, 18H, Si(CH3)3), 8.60 (90 Hz, 6H, SiCH′3CH′3), 9.20
(28 Hz, 2H, p-Ph), 10.20 (90 Hz, 6H, SiCH′3CH′3), 10.80, 11.20
(24 Hz, 36 Hz, 8H total, o, m-Ph), 13.80 (500 Hz, 2H, PCHH)
21.00 (500 Hz, 2H PCHH). Anal. Calcd for C31H61N2NbP2Si6:
C, 47.42; H, 7.83; N, 3.57. Found: C, 47.29; H, 7.75; N, 3.43.
MS (EI) m/z (%): 784, (80) [M+]. µeff (Evans’ method) ) 2.6 µB.
X-r a y Cr ysta llogr a p h ic An a lyses of 1, 3a , 4a , a n d 5b.
In all cases, suitable crystals were selected and mounted on a
glass fiber using Paratone-N crystal mounting oil and freezing
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 processed68 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.69,70 All struc-
tures were solved by direct methods71 and expanded using
Fourier techniques.72 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.
1
under vacuum. H NMR (C6D6, 400 MHz, 300 K): δ 3.78 (w1/2
172 Hz, 12H, SiCH3CH3′); 8.82 (22 Hz, 2H, p-Ph); 9.61 (36 Hz,
4H, m-Ph); 10.38 (132 Hz, 12H, SiCH3CH3′). UV-vis (C7H8):
425 nm, ꢀ ) 801. µeff (Evans’ method) ) 2.02 µB.
P h [P 2N2]NbCl(p y) (3e). Ph[P2N2]NbCl (200 mg, 0.303 mmol)
was dissolved in pyridine (15 mL), giving a deep red solution.
The solution was stirred for 30 min before all volatiles were
removed in vacuo, allowing isolation of a blood red solid. Slow
cooling to -35 °C of a saturated 50:50 toluene/hexanes solution
produced a crop of bright red prisms. Yield: 180 mg (80%).
1H NMR, only partially assignable (C6D6, 400 MHz, 300 K): δ
-35.61 (w1/2 828 Hz) -20.56 (1008 Hz) 4.03 (63 Hz, 12H,
SiCH3CH3) 6.13 (63 Hz,12H, SiCH3CH3) 8.37 (18 Hz) 10.63
(48 Hz) 11.96 (288 Hz) 15.24 (57 Hz) 39.30 (1011 Hz). UV-vis
(C6H12): 540 nm, ꢀ ) 1179. µeff (Evans’ method) ) 2.34 µB. Anal.
Calcd for C29H47N3NbP2Si4: C, 47.05; H, 6.67; N, 4.97. Found:
C, 47.25; H, 6.76; N, 4.86. MS (EI) m/z (%): 739, (10) [M]+,
660, (80) [M+ - py].
Syn th esis of P a r a m a gn etic Niobiu m Alk yl Com p lexes.
Cy[P 2N2]NbCH2SiMe3 (4a ). Cy[P2N2]NbCl (3.00 g, 4.46 mmol)
and LiCH2SiMe3 (420 mg, 4.46 mmol) were dissolved in toluene
(50 mL) and stirred for 12 h to give a bright indigo solution.
Solvents were removed in vacuo, resulting in a dark blue
residue that was extracted into hexanes and filtered through
Celite. Removal of hexanes afforded an indigo-colored solid
that was washed with hexamethyldisiloxane to give an ana-
lytically pure product. X-ray quality crystals of the paramag-
netic product were obtained by extracting the solids into
minimal hexamethyldisiloxane and slowly evaporating the
Ack n ow led gm en t. Professor Robert C. Thompson
and Victor Sanchez (UBC) and Professor Natia L. Frank
(University of Washington) are gratefully thanked for
help in measurement of the magnetic susceptibilities
of 1 and 2, respectively, and for stimulating discussions
on the subject of magnetism. We thank both the NSERC
of Canada and the Petroleum Research Fund adminis-
tered by the American Chemical Society for generous
financial support.
1
solvent over a period of weeks. Yield: 3.00 g (95%). H NMR
only partially assignable (C6D6, 400 MHz, 300 K): δ 4.74 (w1/2
76 Hz, 12H, SiCH3CH3), 7.55 (120 Hz, 9H, Si(CH3)3), 10.49
(108 Hz, 12H, SiCH3CH3), 48.61 (748 Hz, 4H, PCHH), 85.02
Hz, 4H, PCHH). Anal. Calcd for C28H65N2NbP2Si5: C, 46.38;
H, 9.04; N, 3.86. Found: C, 46.18; H, 9.12; N, 3.79. MS (EI)
m/z (%): 724, (20) [M+]. µeff (Evans’ method) = 2.6 µB.
Su p p or tin g In for m a tion Ava ila ble: Complete crystal-
lographic data including ORTEP diagrams, bond lengths and
angles, final atomic coordinates and equivalent isotropic
thermal parameters, anisotropic thermal parameters, inter-
molecular contacts, and least squares planes are available for
complexes 1, 3e, 4a , 5b, and Cy[P2N2]Li2. This material is
available free of charge via the Internet at http://pubs.acs.org.
P h [P 2N2]NbCH2SiMe3 (5a ). To a solution of Ph[P2N2]NbCl
(2.48 g, 3.76 mmol) in toluene (30 mL) was added LiCH2SiMe3
(350 mg, 3.76 mmol) at room temperature. A color change from
dark green to dark blue was observed within moments. After
stirring for 12 h the solution was filtered through Celite and
the solvent removed in vacuo to yield a waxy blue-green solid
that was washed with minimal pentane. Yield: 2.50 g (95%).
1H NMR (C6D6, 400 MHz, 300 K): δ 4.74 (w1/2 76 Hz, 12H,
SiCH3CH3), 7.55 (120 Hz, 9H, Si(CH3)3), 8.91 (24 Hz, 2H, p-Ph),
10.49 (108 Hz, 12H, SiCH3CH3), 10.81 (52 Hz, 4H, m-Ph), 13.77
(288 Hz, 4H, o-Ph), 48.61 (748 Hz, 4H, PCHH), 85.02 Hz, 4H,
PCHH). Anal. Calcd for C28H53N2NbP2Si5: C, 47.17; H, 7.49;
N, 3.93. Found: C, 46.79; H, 7.29; N, 3.99. MS (EI) m/z, (%):
712, (60) [M+]. µeff (Evans’ method) ) 2.5 µB.
OM010356Z
(68) teXsan Crystal Structure Analysis Package; Molecular Structure
Corp.: The Woodlands, TX, 1995.
(69) International Tables for X-Ray Crystallography; Kluwer Aca-
demic: Boston, MA, 1992; Vol. C, pp 200-206.
(70) International Tables for X-Ray Crystallography; Kynoch
Press: Birmingham, U.K. (present distributer Kluwer Academic:
Boston, MA), 1974; Vol. IV, pp 99-102.
(71) Altomare, A.; Burla, M. C.; Cammali, G.; Cascarano, M.;
Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna,
A. SIR97: A new tool for crystal structure determination and refine-
ment; 1999.
(72) 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: The Netherlands, 1994.
Cy[P 2N2]NbCH(SiMe3)2 (4b). To a solution of Cy[P2N2]NbCl
(1.00 g, 1.49 mmol) in toluene (30 mL) was added LiCH(SiMe3)2
(250 mg, 1.50 mmol) at room temperature to give a dark green
solution. After stirring for 6 h the solution was filtered through
Celite and the solvent removed in vacuo to leave a green solid,