Synthesis and characterization of niobium(IV) and tantalum(V) guanidinates and benzamidinates: Molecular structure of the first Nb(IV) guanidinate NbCl2{CyNC(N(SiMe3)2)NCy}2

Article abstract of DOI:10.1016/S0277-5387(99)00190-4

The reactions between niobium pentachloride or NbCl₄(THF)₂ and [(Et₂N)₂TaCl₃]₂ with various lithium guanidinates or benzamidinates were investigated. [NbCl₂{RNCR′NR}₂] [R=Cy (Cyclohexyl), R′=N(SiMe₃)₂ (1); R=SiMe₃, R′=p-tolyl (2)] [TaCl₃{Me₃SiNC(p-tolyl)NSiMe₃}₂] (3) and [(Et₂N)₂TaCl₂{RNCR′NR}] [R=SiMe₃, R′=p-tolyl (4) and R=Cy, R′=N(SiMe₃)₂ (5)] were isolated and characterized by elemental analyses, FT-IR and ¹H NMR or ESR spectroscopy. The molecular structure of 1 corresponds to a centrosymmetric, monomeric [NbCl₂{CyNC(N(SiMe₃)₂)NCy}₂] species as established by X-ray diffraction studies with the chloride ligands in trans positions. The guanidinate ligand displays a symmetrical σ,σ′-coordination behaviour (Nb-N 2.161(8) A av.) Elsevier Science Ltd.

Full text of DOI:10.1016/S0277-5387(99)00190-4

Polyhedron 18 (1999) 2885–2890
www.elsevier.nl/locate/poly
Synthesis and characterization of niobium(IV) and tantalum(V)
guanidinates and benzamidinates
Molecular structure of the first Nb(IV) guanidinate
NbCl2hCyNC(N(SiMe3)2)NCyj2
b
Jean Manuel Decamsa, Liliane G. Hubert-Pfalzgrafa , , Jacqueline Vaissermann
*
a
´
ESA CNRS 5079, Universite Claude Bernard-Lyon 1, 43 boulevard du 11 novembre 1918, 69622 Villeurbanne Cedex, France
b
´
Laboratoire de Chimie des Metaux de Transition, URA-CNRS, 4, place Jussieu, 75230 Paris Cedex, France
Received 22 April 1999; accepted 5 July 1999
Abstract
The reactions between niobium pentachloride or NbCl4(THF)2 and [(Et2N)2TaCl3]2 with various lithium guanidinates or benzamidi-
nates were investigated. [NbCl2hRNCR9NRj2] [R5Cy (Cyclohexyl), R95N(SiMe3)2 (1); R5SiMe3, R95p-tolyl (2)]
[TaCl3hMe3SiNC( p-tolyl)NSiMe3j2] (3) and [(Et2N)2TaCl2hRNCR9NRj] [R5SiMe3, R95p-tolyl (4) and R5Cy, R95N(SiMe3)2 (5)]
were isolated and characterized by elemental analyses, FT-IR and 1H NMR or ESR spectroscopy. The molecular structure of 1
corresponds to a centrosymmetric, monomeric [NbCl2hCyNC(N(SiMe3)2)NCyj2] species as established by X-ray diffraction studies with
˚
the chloride ligands in trans positions. The guanidinate ligand displays a symmetrical s,s9-coordination behaviour (Nb-N 2.161(8) A av.)
1999 Elsevier Science Ltd. All rights reserved.
Keywords: Niobium; Tantalum; Benzamidinates; Guanidinates
1. Introduction
shown by the bisbenzamidinate tantalum(V) methyl,
methylidene and benzyl derivatives [23] and/or to undergo
unusual reactivity as observed for multiple metal–metal
species. Most group 5 derivatives correspond to species in
oxidation state 5 and, if some dinuclear species in low
oxidation state have been described [24,25], no species in
oxidation state 4 has been reported. The steric and (to a
lesser extent) electronic properties of amidinate type
ligands can easily be tuned by the variation of C- and
N-substituents. Steric analysis of this ligand system places
it as intermediate between the Cp and Cp* ligands [27,28].
We wish to report here the results of our investigations
with niobium penta or tetrachloride and bis-
Amido ligands with various denticity have received
increasing attention as ancillary ligands for the develop-
ment of catalysts [1–3] especially for olefins polymeri-
zation or of precursors of nitrides for diffusion barriers
[4–9]. Amidines RNHC(R9)5NR and their deprotonated
anions display a great variety of coordination modes with
transition metals [10–12]. Moreover, amidinate and related
type ligands, are accessible via a large range of synthetic
routes such as metathesis reactions using metal halides,
insertion of carbodimides into metal–hydrogen or metal–
alkyl bonds, insertion of nitriles into M–N bonds. Such
ligands have been extensively used recently especially for
group 4 and 13 metals [13–19] and to, a lesser extent for
niobium and tantalum [20–22]. Increasing interest has
been promoted by their ability to stabilize labile bonds as
diethylamidotantalum(V)
trichloride.
N,N9-bis-
trimethylsilylbenzamidinato [p-tolylC(NSiMe3)2] and
N,N9-bis(cyclohexyl)guanidinato hR2NC(NC6H11)2j lig-
ands were chosen because of the ease of preparation of
their lithium salts and the stabilizing properties they impart
to their metallic derivatives. Various mono and bis amidi-
nates derivatives have been isolated and were characterized
by microanalyses, FT-IR, 1H NMR or ESR spectroscopies.
*Corresponding author. Tel.: 133-4-7243-1570; fax: 133-4-7243-
1568.
E-mail address: hubert@univ-lyon1.fr (L.G. Hubert-Pfalzgraf)
0277-5387/99/$ – see front matter
PII: S0277-5387(99)00190-4
1999 Elsevier Science Ltd. All rights reserved.

2886
J.M. Decams et al. / Polyhedron 18 (1999) 2885–2890
The first niobium (IV) guanidinate [NbCl2hRNCR9NRj2]
(R5Cy, R95N(SiMe3)21, and benzamidinate (R5SiMe3,
R95p-tolyl 2) were obtained. The molecular structure of 1
was established by single crystal X-ray diffraction.
118w, 1158w, 1143w, 1112w, 1025m, 1008m, 981s,
949m, 929s, 906m; 838s; 761s, 717s, 689m, 643s, 607w,
586w; 574w; 487m, 473s, 455m nM–N. ESR (hexane,
RT): ,g. 1.907, ,A. 107G.
Compounds 1 and 2 were slightly soluble in hexane,
more soluble in toluene, diethylether, THF and DME.
2. Experimental
2.2. Synthesis of [hMe3SiNC(p-tolyl)NSiMe3 j2TaCl3 ](3)
All manipulations were routinely performed under a
nitrogen atmosphere using Schlenk tubes and vacuum line
techniques with dried and distilled solvents. NbCl4(THF)2,
[29] [(NEt2)2TaCl3]2 [30] and the lithium benzamidinates
[10–19] were prepared as reported in the literature.
LiCyNCR9NCy (R95N(SiMe3)2) was generated by react-
ing 1 equivalent of LiN(SiMe3)2 and 1,3-dicyclohexylcar-
bodiimide. 1H NMR spectra were recorded on solutions on
a Bruker AC-200 spectrometer. Infrared spectra were
registered with a Perkin Elmer Paraggon spectrometer as
Nujol mulls between KBr plates. ESR spectra were
obtained on a Bruker spectrometer. Analytical data were
obtained from the Centre de Microanalyses du CNRS. The
melting points as well as the decomposition temperatures
were measured in capillaries and are given uncorrected.
A solution of 2.00 g (4.88 mmol) of [hMe3SiNC( p-
tolyl)NSiMe3jLi(TMEDA)] in 40 ml of Et2O was added to
TaCl5 (0.874 g, 2.14 mmol) in 40 ml of Et2O cooled at
2788C. After stirring for 12 h at room temperature,
filtration was achieved. The filtrate was concentrated, 3
was isolated as yellow crystals (0.85 g, 42%) at 2208C.
m.p. 1408C, dec 2208C. Anal. found: C, 40.68, H, 6.25; N,
6.94, Cl, 12.51; calc. for C28H50N4Si4Cl3Ta: C, 39.92, H,
5.98, N, 6.65, Cl, 12.6. IR (cm21, Nujol): 1611 m, 1526m.
n(C5C, C5N); 1491 m, 1481 s, 1444 s, 1311 m, 1244 s
n(Si–Me); 1211 m, 1184 m, 1162 m, 1111 s, 1027 m, 1010
m, 990 vs; 842 s, 835 s, 821 s n(Si–C), 798 m, 765 s, 730
m, 717 m, 690 m, 649 m, 624 m, 617 m; 485 m, 481 m,
474 m n(M–N). 1H NMR (C6D6, 208C, ppm): 7.25 (d, J57
Hz, 2H, H(Ph), 6.73 [d, J57Hz, 2H, H(Ph)], 1.95 [s, 3H,
Me(Ph)], 0.31 (s, 18H, MeSi).
2.1. Synthesis of [hRNC(R9)NRj2NbCl2 ], R5Cy, R95
N(SiMe3 )2 (1) and R5SiMe3 , R95p-tolyl (2)
Compound 3 has solubility properties comparable to
those of 1 and 2
A
solution
of
2.8
g
(7.54
mmol)
of
Li[CyNChN(SiMe3)2jNCy] in 40 ml of diethylether was
added to a suspension of 1.02 g (3.77 mmol) of NbCl5 in
20 ml of diethylether. Formation of a brown precipitate
was observed while the reaction medium turned red.
Filtration was achieved after decantation for 12 h, the
filtrate was concentrated. Several crops of orange crystals
of 1 were obtained at 2208C (1.08 g, 32%). m.p. of 1708C,
dec 2208C. Anal. calc. for C38H80N6Cl2Si4Nb C, 50.87; H,
8.99; N, 9.37; Cl, 7.5. Found: 50.75, H 8.75, N, 9.15, Cl,
7.9% IR (cm21): 1660w, 1636w, 1605w n C5N; 1496m,
1447s, 1404s, 1360s, 1346s, 1310m, 1268m; 1258s nSi–
Me; 1192s, 1186s, 1142m, 1082w, 1063m, 1047w, 1028w,
1010m, 970m, 935s, 887m, 859s, 839s, 804m, 759m, 728s,
694s, 660m, 642s, 620w; 525s, 487m, 465m, 453w, 435m
nM–N. ESR (toluene, RT): kgl 1.913, kAl 110 G.
The same procedure applied to 1.14 g (2.9 mmol) of
NbCl4(THF)2 in toluene gave 1 (1.4 g, 54%). the com-
pound was dried under vacuum for 12h and was solvent
free (whereas the monocrystals were only briefly dried).
Compound 2 was obtained accordingly by using 0.611 g
(1.61 mmol) of NbCl4(THF)2 in hexane cooled to 2788C
and lithium benzamidinate complexed with TMEDA.
Several crops of yellow crystals of 2 were obtained at
2108C (0.44 g, 38%). m.p.51808C, dec 2808C; Anal. calc.
for C28H50Cl2N4Si4Nb: C, 46.78, H, 7.01, N, 7.79, Cl,
9.86; Found: C, 46.25, H, 7.08, N, 7.80, Cl: 9.82% IR
(cm21): 1660w, 1612m, 1549w, 1521m nC5C, C5N;
1502m, 1406s, 1308s, 1298s; 1246s nSi–Me; 1210w,
2.3. Synthesis of [(Et2N)2TaCl2 hRNC(R9)NRj], R5SiMe3 ,
R95p-tolyl (4) and R5Cy, R95n(SiMe3 )2 (5)
Compound 4 was obtained as orange crystals (71%) by
a similar procedure in toluene at 2788C using lithium
benzamidinate complexed with TMEDA and (Et2N)2TaCl3
(1:1 stoichiometry). Anal. found: C, 39.42, H, 6.48, N,
8.07, Cl, 10.20; calc. for C22H45N4Si2Cl2Ta: C, 39.23; H,
6.73, N, 8.32, Cl 10.35%. IR (cm21): 1613m, 1567w,
1517w nC5C, C5N; 1317m, 1308m, 1283m; 1248s nSi–
Me; 1211s, 1186m, 1167m, 1131m, 1110m, 1094m,
1063m, 1043m, 1026m, 994s, 968m, 950m, 909m; 882m,
840s; 790m, 765m, 686m, 651m, 632m; 576m, 528m,
472s, 401w nM–N. 1H NMR (C6D6, ppm): 7.1 d (J57 Hz,
CH, 2H), 6.75 d (J57Hz, CH, 2H), 4.25 q (J53Hz, CH2,
8H), 1.96 s [Me(tolyl), 3H], 1.16 t (J53Hz, Me, 12H), 0.1
s [Me(Si), 18H]; m.p.51008C, dec 1608C, sublimes at
1108C under 4.1024 mm of Hg.
A suspension of CyNC(N(SiMe3)2)NCyLi (0.8 g, 2.22
mmol) in 40 ml of toluene was added to a solution of
(Et2N)2TaCl3 (0.96 g, 2.22 mmol) in 20 ml of toluene
cooled at 2788C similar workup as for 3. Orange crystals
of 5 (1.28 g, 76%) were obtained at 2108C. m.p.51208C,
dec 1408C. Anal. found: C, 43.11, H, 7.60, N, 9.01, Cl,
9.21; calc. for C27H60N5Si2Cl2Ta C, 42.51, H, 7.93, N,
9.18, Cl, 9.30% IR(cm21): 1658w, 1641w, 1575w nC5N;
1450s, 1343s, 1311m; 1270m; 1256s nSi–Me; 1207s,

J.M. Decams et al. / Polyhedron 18 (1999) 2885–2890
2887
1185s, 1178s, 1142m, 1130m, 1092m, 1070m, 1062m,
1042m, 1013s, 996s, 929m, 906m, 897m; 881m, 873s,
860s, 835s, 802m; 788s, 767m, 755m, 708w, 694m, 651m,
645s, 620w; 577m, 548w, 525s, 489m, 476w, 465w, 438m
nM–N. 1H NMR (C6D6 ppm): 4.38q (J56Hz, CH2, 8H),
3.91 (t, J57Hz, 2H (Cy), 1.7 (m, Cy, 20H), 1.2 (t, J56Hz,
Me, 12H), 0.33 s (MeSi, 18H).
dispersion were taken from [32]. The structure was solved
by standard Patterson-Fourrier techniques and refined by
full-matrix least-squares. The asymmetric unit contains a
molecule of solvent (toluene) together with half a molecu-
lar unit of the Nb compound. Refinements were performed
using anisotropic thermal parameters for all non hydrogen
atoms of the Nb compound. The toluene molecule was
refined isotropically using restraints on the C–C bond
Compounds 5 and 4 are soluble in the usual solvents
(hexane, toluene, ether, . . . ).
˚
lengths (1.40(2) A) and bond angles (120(2)8). The
solvent is disordered as shown by the high thermal factors
of C(34) and C(37). Hydrogen atoms were introduced in
calculated positions on the carbon atoms of the Nb
compound and introduced in the last refinement with an
overall refinable isotropic thermal parameter.
2.4. X-ray structure determination of
[NbCl2 hCyNCN(SiMe3 )2NCyj2 , 2cC6H5CH3 ]
Suitable crystals of 1 were obtained directly from the
reaction medium with NbCl4(THF)2 as the starting materi-
al. The selected crystal was mounted in a Lindeman
capillary. Accurate cell dimensions and orientation ma-
trices were obtained by least-squares refinements of 25
accurately centered reflections. A decay of 50% was
observed for the intensities of two checked reflections
during data collection, the usual linear correction was
applied. Complete crystallographic data and collection
parameters are listed in Table 1. The data were corrected
for Lorentz and polarization effects. Computations were
performed by using the PC version of CRYSTALS [31].
Scattering factors and corrections factors for anomalous
3. Results and discussion
Niobium(IV) N,N9,N0-trialkylguanidinates or bis-
trimethylsilylbenzamidinates [NbCl2hRNCR9NRj2] [R5
Cy, R95N(SiMe3)2 (1) and R5SiMe3, R95p-tolyl) (2)]
were obtained by reacting an etheral suspension of NbCl5
with 2 equivalents of Li(RNCR9NR) or of the TMEDA
complex for the N,N9-bis(trimethylsilyl-p-tolyl benzamidi-
nate (TMEDA5N,N,N9,N9-tetramethylethylenediamine).
Better yields (.50%) were obtained if the reactions were
achieved with the metal halide in the appropriate oxidation
state, namely with NbCl4(THF)2 in toluene. By contrast,
similar reactions applied to TaCl5 in diethylether at low
temperature proceeded without reduction affording a tan-
talum(V) disubstituted derivative 3. No tractable products
could be isolated by using hexane as a solvent. The use of
[(Et2N)2TaCl3]2 as starting material allowed the isolation
of [(Et2N)2TaCl2(RNCR9NR)]); [R5SiMe3, R95p-tolyl
(4) and R5Cy, R95N(SiMe3)2 (5] (Eq. (1)) in good
yields. The best yields (.70%) were obtained by using
low temperature procedures. The various compounds in-
cluding the niobium(IV) ones were soluble in organic
media whereas 4 was also volatile (sublimation 1108C/
4.1024 mm of Hg). The different synthetic routes are
summarized in Scheme 1.
The novel compounds were characterized by elemental
analyses and FT-IR spectroscopy. IR spectra display
mainly the absorption bands of the organic moieties. Sharp
bands are observed between 1660 and 1600 cm21 for the
nC5N and nC5C vibrations. The presence of metal–
nitrogen bonds is illustrated by bands in the 550–400 cm21
region. This region is more complex for compounds 4 and
5 whose spectra show also absorption bands corresponding
to the NEt2 ligand. The presence of the trimethylsilyl
substituents leads to a strong absorption band around 1250
cm21, characteristic of the nSi-Me3 vibration. The diamag-
netic tantalum(V) derivatives were also characterized by
1H NMR. The diethylamido ligands appear magnetically
equivalent for 4 and 5 indicating a symmetrical and/or
Table 1
Crystal data for NbCl2[CyNC(NSiMe3)2)NCy]2
Formula
Fw
C52H96N6Si4Cl2Nb
1081.5
˚
a (A)
9.3418(9)
13.591(1)
13.799(1)
63.243(7)
79.934(8)
80.085(8)
1531.6(3)
1
Triclinic
¯
P1
3.87
1.17
MACH3 Enraf-Nonius
Mo Ka (l50.71069 A)
v/2u
0.810.345 tgu
1–25
Room temperature
0,11; 215,16; 216,16
5731
˚
b (A)
˚
c (A)
a (8)
b (8)
g (8)
3
˚
V (A )
Z
Crystal system
Space group
Linear absorption coefficient m (cm21
)
Density r(g.cm23
Diffractometer
Radiation
)
˚
Scan type
Scan range (8)
u Limits (8)
Temperature of measurement
Octants collected
Number of data collected
Number of unique data collected
Number of unique data used for refinement
R (int)
R5Suu Fo u2u Fc uu/Su Fo u
Rw5[Sw(u Fo u2u Fc u)2 /SwFo2]1 / 2
Extinction parameter
5369
2240(Fo)2.3s(Fo)2
0.0474
0.0601
0.0689w51.0
No
261
20.42
0.79
Number of variables
23
˚
˚
Dr min (e.A
)
)
23
Dr max (e.A

2888
J.M. Decams et al. / Polyhedron 18 (1999) 2885–2890
Scheme 1.
fluxional structure The spectroscopic data thus support the
structure represented in Eq. (1) based on a six-coordinated
metal center. The most notable feature for the spectrum of
4 is the presence of a sharp resonance at 3.91 ppm
attributed to the proton in a position of the cyclohexyl
groups. Only one type of SiMe3 groups is observed for
compound 3 suggesting a fluxional behaviour for this
species in which the metal is probably heptacoordinated.
The tetravalent niobium species 1 and 2 display a charac-
teristic ten line ESR pattern at room temperature (INb59/
2) suggesting the formation of monomeric species. This
was confirmed by an X-ray diffraction study for 1.
The structure is based on a centrosymmetric, monomeric
species in which the metal is in a slightly distorted
octahedral environment, the two chloride ligands being in
trans position. The coordination sphere of the Nb(IV)
center includes the four nitrogen atoms of the two chelat-
ing guanidinate anions. These display a s,s9-symmetrical
coordination behaviour and the Nb–N bond lengths have
˚
identical values (2.161(8) A av.). The Nb–Cl bond dis-
˚
tances have values of 2.384(3) A and are consistent with
these reported in the literature for other Nb(IV) derivatives
[33–38]. The four-membered Nb–N–C–N–Nb metallocy-
cles are planar, the bite angles are small, namely 60.7(3)8
but consistent with the value reported for similar ligands in
cis-[SnCl2hMe3SiNC(But)NSiMe3j2]
or
[hCyNC(NMe2)CNCyjTa(NMe2)4] [39]. The amido nitro-
gen atom N(3) is planar as well with a C(1)–N(3) bond
˚
distance of 1.40(1) A. This C–N distance is longer than
˚
the C–N distances within the chelating ligand (1.335 A av)
but in agreement with the literature body [10–12,40,41].
Its value militates against conjugation of the N–C–N
p-system and the exocyclic N(3) atom. Such a conjugation
was observed for the dianionic guanidinato ligand in
[hCyNC(NCy)CNCyjTa(NMe2)3] giving an extra cyclic
˚
C–N distance of 1.28(2) A and thus an imine character
(1)
[26]. The cyclohexyl rings of 1 are orientated nearly
parallel to each other, a situation which obviously mini-
mizes the steric repulsion between these bulky sub-
stituents. No short intermolecular contacts were observed.
The
molecular
structure
of
[NbCl2hCyNC(N(SiMe3)2)NCyj2] is depicted in Fig. 1.
Selected bond lengths and angles are collected in Table 2.
˚
The Nb . . . .C(11)–H distance (.3.5 A) is too long for

J.M. Decams et al. / Polyhedron 18 (1999) 2885–2890
2889
Fig. 1. Molecular structure of 1 showing the atom numbering scheme. Thermal ellipsoids at 30% probability.
effective interaction. The overall symmetry of 1 is compar-
able to that of the divalent vanadium derivatives
[VhCyNC(R9)NCyj2(THF)2] [42] but different to that of
the zirconium analog, [ZrCl2hCyNC(Me)NCyj2], for
which the chlorine occupy cis positions [15]. Molecular
modelling (Hyperchem V5) of 1 indicate comparable
energies for the cis and trans isomers suggesting that solid
state effects will govern. By contrast, for the bis ben-
zamidinate derivative 2, the cis isomer was found to
display a much higher stability (DE.500 kJ) than the
trans isomer. X-ray data are in agreement with these
observations since only cis-isomers have been structurally
characterized so far for bis benzamidinato derivatives.
The reported N,N9,N0-trialkylguanidinate or bis-
(trimethylsilyl) benzamidinate ligands are able to function
as supporting ligands for paramagnetic Nb(IV) species
displaying high solubility as well as modest sensitivity to
ambient atmosphere. Compounds 1 and 2 are the first
examples of Nb(IV) species based on such ligands. The
thermal stability was higher for the benzamidinate deriva-
tives since the guanidinates undergo easy decomposition
via cleavage of the external C–N bond giving
NH(SiMe3)2. Volatility could be achieved for a tan-
talum(V) species with an appropriate set of ligands
including a trimethylsilylbenzamidinato one. The presence
of dialkylamido ligands lead to a better volatility but
reduced the stability toward ambient atmosphere.
Supplementary data
Supplementary data (Tables of coordinates, thermal
parameters, of non-essential bond lengths and angles) are
available from the Cambridge Crystallographic Data Cen-
tre, 12 Union Road, Cambridge CB2 1EZ, UK on request,
quoting the deposition number 101 725.
Table 2
Interatomic
˚
distances
(A)
and
angles
(8)
for
[NbCl2h(CyN)2CN(SiMe3)2j2],(C7H8)2
References
Nb(1)–Cl(1)
Nb(1)–N(1)
Nb(1)–N(2)
Cl(1)–Nb(1)–N(1)
Cl(1)–Nb(1)–N(2)
N(1)–Nb(1)–N(2)
Cl(1)–Nb(1)–N(2)
N(1)–Nb(1)–N(2)
2.384(3)
2.162(8)
2.159(7)
90.1(2)
89.4(2)
60.7(3)
90.6(2)
119.3(3)
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