The reactivity of niobium and tantalum pentahalides with imines

Article abstract of DOI:10.1016/j.poly.2016.04.034

The reactivity of NbCl₅, NbF₅ and TaCl₅ with a selection of commercial imines was investigated for the first time by using dichloromethane as reaction medium. NbCl₅ reacted with Ph₂C=NH, in 1:2 molar

Full text of DOI:10.1016/j.poly.2016.04.034

Accepted Manuscript
The Reactivity of Niobium and Tantalum Pentahalides with Imines
Fabio Marchetti, Guido Pampaloni, Stefano Zacchini
PII:
S0277-5387(16)30117-6
DOI:
http://dx.doi.org/10.1016/j.poly.2016.04.034
POLY 11961
Reference:
Polyhedron
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Accepted Date:
31 March 2016
21 April 2016
Please cite this article as: F. Marchetti, G. Pampaloni, S. Zacchini, The Reactivity of Niobium and Tantalum
Pentahalides with Imines, Polyhedron (2016), doi: http://dx.doi.org/10.1016/j.poly.2016.04.034
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The Reactivity of Niobium and Tantalum Pentahalides with
Imines
Fabio Marchetti a,*, Guido Pampaloni a, Stefano Zacchini b
a
Università di Pisa, Dipartimento di Chimica e Chimica Industriale, Via Moruzzi 13, I-56124
Pisa, Italy
b
Dipartimento di Chimica Industriale “Toso Montanari”, Università di Bologna, Viale
Risorgimento 4, I-40136 Bologna, Italy
Received.......; accepted ………………
_______________________
* Corresponding author. Tel.: +390502219245. E-mail address: fabio.marchetti1974@unipi.it
Webpage: http://www.dcci.unipi.it/fabio-marchetti.html
1

Abstract
The reactivity of NbCl5, NbF5 and TaCl5 with a selection of commercial imines was investigated
for the first time by using dichloromethane as reaction medium. NbCl5 reacted with Ph2C=NH, in
1:2 molar ratio, affording [Ph2C=NH2][NbCl5(N=CPh2)], 1, in 55% yield, as result of imine self-
protonation. The iminium salt [PhCH=NHtBu][NbCl6], 2, was isolated in 52% yield from NbCl5
and PhCH=NtBu (1:1 molar ratio), while a low yield of [tBu2C=NH2][NbCl6], 3, was identified
from NbCl5/tBu2C=NH. The 1:1 reactions of NbF5 with Ph2C=NH and PhCH=NtBu were
accompanied by electron interchange and led to the isolation of the salts [Ph2C=NH2][NbF6], 4,
and [PhCH=NHtBu][NbF6], 5, respectively, in ca. 50% yields. Few crystals of
[Ph2C=NH2]2[Ta2Cl10O], 6, were recovered from TaCl5/Ph2C=NH, the anion being probably
generated by the action of adventitious water. Compounds 1-6 were characterized by elemental
analysis, IR and NMR spectroscopy. The structures of 1, 4 and 6 were ascertained by X-ray
diffraction studies.
Keywords: Niobium Pentahalides, Tantalum Pentahalides, Imines, Benzophenone Imine,
Azavinylidene
1. Introduction
The chemistry of niobium and tantalum pentahalides, MX5 [1], has experienced a significant
progress in the last decade, encouraged by cost effectiveness, low toxicity of the metal elements and
unusual reactivity patterns [2]. As a matter of fact, MX5 have found increasing application as
catalytic precursors in metal mediated organic synthesis [3], and also many aspects of the
coordination chemistry with oxygen [4], nitrogen [5] and soft donors [6] have been elucidated in the
recent years. In general, the polynuclear structure of MX5 undergoes cleavage by the addition of one
equivalent of a neutral Lewis base (L) in a non coordinating solvent; this cleavage usually takes
2

place selectively in either symmetric or asymmetric mode, depending on the nature of both X and L
[4c, 5c]. In the case of symmetric cleavage, neutral complexes of formula MX5L can be generated,
whereas the ionic complexes [MX4L2][MX6] are the result of the asymmetric rupture [7]. The
[MX6]− anions are quite stable, the stability decreasing on moving down the halogen group [4c], and
are suitable to the stabilization of organic cations, including otherwise reactive cations [2b, 4b-c, 8].
In those cases in which the reactions of MX5 with organic compounds are non selective, the
formation of [MX6]− salts of protonated species is sometimes observed [9]. Usually, both
adventitious water and the activation of the organic substrate are possible sources of protonation.
To the best of our knowledge, the direct interaction of MX5 with imines has not been investigated
heretofore. Herein, we describe the results of our study on the reactions of MX5 with limited
amounts (1-2 molar equivalents) of three commercial imines, i.e. benzophenone imine, N-
benzylidene-tert-butylamine and 2,2,4,4-tetramethyl-3-pentanone imine. The reactions were
carried out in dichloromethane, i.e. a non coordinating, privileged solvent for studying the
coordination chemistry of high valent transition metal halides [4c-d, 10]. The isolated metal
products were characterized by analytical and spectroscopic techniques, and by X-ray diffraction
in a number of cases.
2. Results and Discussion
The reaction of NbCl5 with a two-fold excess of benzophenone imine, in dichloromethane at
room temperature, led to the isolation in 55% yield of the salt [Ph2C=NH2][NbCl5(N=CPh2)], 1,
that was characterized by analytical and spectroscopic methods, and X-ray diffraction (Scheme 1).
Compound 1 was obtained, although with minor yield, also by using an imine/Nb 1:1 molar ratio.
Scheme 1. The reaction of NbCl5 with benzophenone imine.
3

Crystals suitable for X-ray analysis were collected from a CH2Cl2/hexane mixture maintained at
−30 °C. The molecular structure of 1 (Figure 1 and Table 1) consists of ionic packings of
[Ph2C=NH2]+ cations and [NbCl5(N=CPh2)]– anions, the iminium N-protons being involved in H-
bonds with the halide ligands of the anion (Table 4). The benzophenone iminium cation was
previously X-ray characterized [11], while the [NbCl5(N=CPh2)]– anion is unprecedented. It
consists of an octahedral Nb(V) centre bonded to five chlorides and one diphenylmethyleneamido
ligand, and represents the first structurally characterized Nb(V) complex containing an
azavinylidene (methyleneamido) ligand [12]. The C(1)−N(1) distance [1.290(4) Å] is typical for a
double bond [13] and, accordingly, C(1) displays a perfect sp2 hybridization [sum angles at C(1)
360.0(5)°]. The Nb(1)−N(1) distance [1.862(2) Å] is intermediate between a single and a double
bond [5a, 14]. The (N=CPh2)– ligand shows a linear coordination [C(1)-N(1)-Nb(1) 176.3(2)°].
Both linear and bent coordination modes have been observed for a terminal azavinylidene ligand
with miscellaneous transition metals [15].
Figure 1. View of the molecular structure of [Ph2C=NH2][NbCl5(N=CPh2)], 1,
with key atoms labeled. Displacement ellipsoids are at the 50% probability level.
Table 1. Selected bond lengths (Å) and angles (°) for 1
Nb(1)–Cl(1)
Nb(1)–Cl(3)
Nb(1)–Cl(5)
C(1)–N(1)
2.5258(8)
2.3893(9)
2.4005(9)
1.290(4)
Nb(1)–Cl(2)
Nb(1)–Cl(4)
Nb(1)–N(1)
C(20)–N(2)
2.3552(9)
2.4143(8)
1.862(2)
1.293(4)
4

C(1)–C(2)
C(1)–C(8)
1.474(4)
1.472(4)
C(20)–C(21)
C(20)–C(27)
1.581(4)
1.563(4)
N(1)–Nb(1)–Cl(1)
Cl(3)–Nb(1)–Cl(5)
N(1)–C(1)–C(2)
N(1)–C(1)–C(8)
C(2)–C(1)–C(8)
175.43(8)
170.12(3)
118.1(3)
119.5(3)
122.4(3)
Cl(2)–Nb(1)–Cl(4)
Nb(1)–N(1)–C(1)
N(2)–C(20)–C(21)
N(2)–C(20)–C(27)
C(21)–C(20)–C(27
176.83(3)
176.3(2)
118.9(3)
117.2(3)
123.9(3)
The IR spectrum of 1 (solid state) displays a broad band envelope around 1660 cm−1, probably
1
accounting for both [C=N] moieties. In the H NMR spectrum (CD2Cl2 solution), the N-bound
protons resonate at 11.4 ppm. Major 13C NMR features are represented by the resonances of the
iminium and azavinylidene carbons, occurring at 182.1 and 156.9 ppm, respectively. In the 93Nb
NMR spectrum, the presence of the [NbCl6]− anion is indicated by a sharp resonance at 10.4 ppm
[4c,5a,7,8], while the niobium nucleus belonging to the cation was not observed.
The formation of 1 appears the result of self-ionization of benzophenone imine, via intermolecular
H+ migration induced by the Lewis acidic Nb(V) centre. Azavinylidene ligands have been
obtained from a variety of building blocks, especially ketimino derivatives LiNCRR' [12a, 16]. On
the other hand, the in situ imine deprotonation has been more rarely observed. For instance, the
[M=C=CPh2] unit was previously in situ generated from benzophenone imine with a Ti(IV)
complex [17], a Ru(IV) porphyrin complex [18] and a Pd(II) acetato complex in the presence of
[NBu4]OH [19].
We moved to extend our study to the reactivity of NbCl5 with other imines. The iminium salt
[PhCH=NHtBu][NbCl6], 2, was isolated from NbCl5 and one equivalent of N-benzylidene-tert-
butylamine in 52% yield, and no other products could be identified (Scheme 2). The IR spectrum
of 2 clearly shows the band related to the iminium function at 1650 cm−1. Salient NMR features
(CD3CN solution) are the resonances due to the NH (10.85 ppm) and iminium groups (1H: 8.78
ppm; 13C: 168.1 ppm). The 93Nb NMR resonance of the [NbCl6]− anion has been found at −0.7
5

ppm. The observed variability of the chemical shift due to [NbCl6]− in a restricted ppm range may
be an effect of the solvent and/or the nature of the cation.
Also the 1:1 molar reaction of NbCl5 with 2,2,4,4-tetramethyl-3-pentanone imine led to the low
yield isolation of a solid containing the relevant iminium cation, in admixture with non identified
secondary products (Scheme 2). The iminium moiety, [tBu2C=NH2]+ [20], manifests itself by IR
absorptions at 3380 (N-H), 3277 (N-H) and 1639 (C=N) cm−1. The 1H NMR resonance due to the
two N-bound protons has been found at 9.95 ppm (CD3CN solution).
Scheme 2. Formation of iminium salts from the reactions of NbCl5
with N-benzylidene-tert-butylamine and 2,2,4,4-tetramethyl-3-
pentanone imine.
The formation of 2 and 3 from NbCl5 and imines is the consequence of a protonation reaction.
In principle, three possible sources may contribute to the protonation, i.e. trace water, the solvent
t
(CH2Cl2) and the imine itself. In particular, it is possible that the NH containing imine Bu2C=NH
undergoes self protonation in the presence of NbCl5, analogously to what demonstrated for
Ph2C=NH. We performed several experiments aimed to the identification of the possible co-
products of the proton transfer, but these did not provide conclusive results.
The 1:1 molar reactions of NbF5 with benzophenone imine and N-benzylidene-tert-butylamine
afforded the hexafluoroniobate iminium salts [Ph2C=NH2][NbF6], 4, and [PhCH=NHtBu][NbF6],
5, respectively, in ca. 50% yields (Scheme 3).
6

Scheme 3. Formation of iminium salts from the reactions of NbF5
with benzophenone imine and N-benzylidene-tert-butylamine.
The X-ray structure of 4 (Figure 2, Table 2) consists of ionic packings of [Ph2C=NH2]+ cations
and [NbF6]– anions, with H-bonds involving N-bound hydrogens and fluoride ligands (Table 4).
The [NbF6]– anion displays bonding parameters resembling those previously reported in the
literature [6b, 7b, 21].
Figure 2. View of the molecular structure of [Ph2C=NH2][NbF6], 4,
with key atoms labeled. Symmetry transformation used to generate
equivalent atoms: (1) –x+3/2, –y+1/2, –z; (2) –x+1, y, –z+3/2.
Displacement ellipsoids are at the 50% probability level.
Table 2. Selected bond lengths (Å) and angles (°) for 4
Nb(1)–F(1)
Nb(1)–F(3)
C(1)–C(2)
1.8838(10)
1.8786(10)
1.468(2)
Nb(1)–F(2)
C(1)–N(1)
1.9048(11)
1.293(3)
F(1)–Nb(1)–F(1_1) 180.00(6)
F(3)–Nb(1)–F(3_1) 180.00(5)
F(2)–Nb(1)–F(2_1) 180.00(7)
N(1)–C(1)–C(2) 119.08(10)
N(1)–C(1)–C(2_2) 119.08(10)
C(2)–C(1)–C(2_2)
121.8(2)
Symmetry transformation used to generate equivalent atoms: (1) –x+3/2,
–y+1/2, –z; (2) –x+1, y, –z+3/2.
The IR and NMR features related to the iminium cations within 4 and 5 resemble those already
7

discussed for the analogous [NbC6]− salts, 1 and 2. The [NbF6]− anion has been characterized by
means of NMR spectroscopy. Indeed this species gives raise to typical 19F and 93Nb resonances,
falling at ca. 104 ppm and −1550 ppm, respectively [4c, 5a,c, 6a-c, 7].
Consideration analogous to those discussed for 2 and 3 are still valid here in order to explain the
formation of the iminium cations contained in 4 and 5. In addition, two points should be considered.
First, the high stability of the [NbF6]− anion is an important factor which often forces the formation
of ionic species from the interaction of NbF5 with organic compounds [4c, 5c, 6a-c]. Second, 4 and 5
were isolated by precipitation from a CH2Cl2/hexane mixture, and magnetic analysis of the solid
residue recovered from the liquid phase evidenced the presence of paramagnetic species (the solid
residues recovered from NbCl5/imines, see above, revealed to be diamagnetic). Moreover,
hydrolysis [22] of the paramagnetic substrate obtained from NbF5/PhCH=NtBu gave a solution
whose NMR analysis indicated the presence of moderate amounts of benzonitrile. The formation of
PhC≡N from PhCH=NtBu indicates the occurrence of C−H bond activation to some extent, possibly
contributing to the main protonation reaction leading to 5.
Finally, we included a tantalum pentahalide, i.e. TaCl5, in the present study. The reaction of TaCl5
with benzophenone imine was less selective than the analogous one involving NbCl5, and afforded a
complicated mixture of products. After many attempts, few crystals of the salt
[Ph2C=NH2]2[Ta2Cl10O], 6, were collected. The presence of oxygen in the anion is unambiguous
evidence of the action of fortuitous water. The X-ray structure of 6 (Figure 3, Tables 3 and 4) is an
ionic packing of [Ph2C=NH2]+ and [Ta2Cl10O]2– ions. The [Ta2Cl10O]2– anion was
crystallographically characterized in the past, the anion displaying bonding parameters very close to
those found in the present case [23].
8

Figure 3. View of the molecular structure of [Ph2C=NH2]2[Ta2Cl10O], 6, with key
atoms labeled. Symmetry transformation used to generate equivalent atoms: (1) –
x+2, –y+a, –z+1. Displacement ellipsoids are at the 50% probability level.
Table 3. Selected bond lengths (Å) and angles (°) for 6
Ta(1)–Cl(1)
Ta(1)–Cl(3)
Ta(1)–Cl(5)
C(1)–N(1)
C(1)–C(8)
2.3669(11)
2.3665(11)
2.3554(10)
1.304(6)
Ta(1)–Cl(2)
Ta(1)–Cl(4)
Ta(1)–O(1)
C(1)–C(2)
2.3635(11)
2.3632(11)
1.8929(2)
1.461(6)
1.469(6)
Cl(1)–Ta(1)–O(1) 176.55(3)
Cl(3)–Ta(1)–Cl(5) 179.67(4)
Cl(2)–Ta(1)–Cl(4) 179.35(4)
Ta(1)–O(1)–Ta(1_1) 180.000(5)
N(1)–C(1)–C(2)
119.5(4)
N(1)–C(1)–C(8)
118.6(4)
C(2)–C(1)–C(8)
121.9(4)
Symmetry transformation used to generate equivalent atoms: (1) –
x+2, –y+a, –z+1.
Table 4. H-bond parameters for 1, 4 and 6.
d(D-H)
d(H···A)
d(D···A)
<(DHA)
1
N(2)–H(2A)···Cl(1)#1
N(2)–H(2B) ··Cl(1)#2
N(2)–H(2B) ··Cl(4)#1
0.941(10)
0.938(10)
0.938(10)
2.375(16)
2.70(2)
2.89(3)
3.268(3)
3.453(3)
3.405(3)
158(3)
138(3)
116(2)
3
N(1)–H(1)...F(2)#3
4
0.868(15)
2.004(15)
2.8606(16) 169(2)
N(1)–H(1A)...Cl(4)#4
N(1)–H(1B)...Cl(3)#5
0.878(10)
0.876(10)
2.55(3)
2.88(3)
3.311(4)
3.676(4)
145(4)
152(4)
Symmetry transformations used to generate equivalent atoms: #1 –x+1,–y+1,–z+1; #2
x,y–1,z; #3 x–1/2,–y+1/2,z+1/2; #4 x–1,y,z; #5 –x+1,–y+1,–z+1.
3. Conclusions
In the framework of our interest in the coordination chemistry of high valent metal halides, we
have reported on the reactivity of niobium (tantalum) pentahalides with imines, which has not
been explored up to now. In general, the reactions are scarcely selective and lead to the isolation
of iminium salts, favored by the basicity of the imine moiety and the stability of [MX6]− anions.
Possible sources of protonation are represented by the solvent (dichloromethane), trace water and
9

the activation of the imine reactant. In particular, the occurrence of benzophenone imine self
protonation has been unambiguously observed in the reaction with NbCl5, resulting in the first
crystallographic characterization of a Nb(V) azavinylidene species. Redox processes appear to be
operative in the reactions involving NbF5, presumably triggered by the strong oxidative power of
niobium pentafluoride [4b].
4. Experimental
4.1. General considerations. Air/moisture sensitive compounds were manipulated under
atmosphere of pre-purified argon using standard Schlenk techniques. The reaction vessels were
oven dried at 140°C prior to use, evacuated (10–2 mmHg) and then filled with argon. NbCl5
(99+%), NbF5 (99.5+%), and TaCl5 (99.9%) were purchased from Strem, then sublimed and
stored in sealed tubes under argon. Once isolated, the metal products were conserved in sealed
glass tubes under argon. Solvents (Sigma-Aldrich) were distilled from P4O10 before use. Imines
(TCI Europe), CD2Cl2 and CD3CN (Cortecnet) were commercial chemicals stored under argon
atmosphere as received. Infrared spectra were recorded at 298 K on a FT IR-Perkin Elmer
Spectrometer, equipped with UATR sampling accessory. Magnetic susceptibilities were measured
at 298 K on solid samples with a Magway MSB Mk1 magnetic susceptibility balance (Sherwood
1
Scientific Ltd.) [24]. H, 13C NMR and 93Nb NMR spectra were recorded on a Bruker Avance
DRX400 instrument equipped with a BBFO broadband probe. 1H and 13C NMR assignments were
assisted by DEPT, HSQC and HMBC experiments [25]. 19F NMR spectra were recorded on a
Varian Gemini 200BB instrument. All NMR experiments were performed at 298 K. The chemical
1
shifts for H and 13C were referenced to the non-deuterated aliquot of the solvent; the chemical
shifts for 19F were referenced to external CFCl3; the chemical shifts for 93Nb were referenced to
external [NEt4][NbCl6]. Carbon, hydrogen and nitrogen analyses were performed on Carlo Erba
mod. 1106 instrument. The chloride content was determined by the Mohr method [26] on
10

solutions prepared by dissolution of the solid in aqueous KOH at boiling temperature, followed by
cooling to room temperature and addition of HNO3 up to neutralization. The metal was analyzed
as M2O5 obtained by high temperature treatment of the solid sample with HNO3 solution, followed
by calcination in a platinum crucible.
4.2. Reactions of NbCl5 with imines.
4.2.1. Synthesis of [Ph2C=NH2][NbCl5(N=CPh2)], 1. A suspension of NbCl5 (0.250 g, 0.925
mmol) in CH2Cl2 (15 mL) was treated with benzophenone imine (0.320 mL, 1.91 mmol). The
mixture was stirred at room temperature for 18 h, and the resulting dark-brown mixture was
filtered in order to remove some insoluble material. Then the filtered solution was concentrated up
to ca. 5 mL, layered with pentane and stored at −30 °C. Compound 1 was isolated as a greenish-
brown solid after 72 h. Yield 0.322 g, 55%. Crystals suitable for X-ray analysis were collected
from a CH2Cl2 solution layered with hexane and stored at −30 °C for one week. Anal. Calcd. for
C26H22Cl5N2Nb: C, 49.36; H, 3.51; N, 4.43; Cl, 28.02; Nb, 14.69. Found: C, 49.45; H, 3.37; N,
4.36; Cl, 27.87; Nb, 14.39. IR (solid state): 3334w, 3267w, 3206w, 3066w, 1675m-sh (νC=N),
1652s (νC=N), 1591s-sh, 1583s, 1571m-sh, 1488m, 1462vs, 1453vs-sh, 1406vs, 1472vs, 1328s,
1292s, 1179m, 1164m-s, 1102w-m, 1026w, 995m, 938w, 822vs, 795s, 777m, 737s, 690s, 665s
1
cm−1. H NMR (CD2Cl2): δ = 11.4 (br, 2 H, NH), 8.15, 7.92, 7.74, 7.67 (m, 20 H, Ph) ppm. 13C
NMR (CD2Cl2): δ = 182.1 (CNH2), 156.9 (CNNb), 138.1, 136.4, 134.9, 134.4, 132.4, 131.6,
130.5, 129.6, 129.0, 127.1 (Ph) ppm. 93Nb (CD2Cl2): δ = 10.4 (s, ∆ν½ = 1.3×10 Hz) ppm.
4.2.2. Synthesis of [PhCH=NHtBu][NbCl6], 2 and [tBu2C=NH2][NbCl6], 3. Only the preparation
of 2 is described in detail, 3 being obtained in a similar way. N-benzylidene-tert-butylamine
(0.135 mL, 0.759 mmol) was added to a suspension of NbCl5 (0.200 g, 0.740 mmol) in CH2Cl2
(15 mL). The mixture was allowed to react at room temperature for 18 h. The final mixture was
11

treated with hexane (30 mL), and the abundant brown precipitate was separated and then dried in
vacuo. Yield 0.185 g, 52% (with reference to the organic reactant). Anal. Calcd. for
C11H16Cl6NNb: C, 28.24; H, 3.45; N, 2.99; Cl, 45.46; Nb, 19.86. Found: C, 28.07; H, 3.38; N,
3.06; Cl, 45.22; Nb, 19.97. IR (solid state): 3306w, 3256w, 3058w, 2978w, 1650s (νC=N), 1594s,
1455w, 1413w, 1382w, 1328w, 1310w, 1262w, 1237m, 1185s, 1023s, 896w, 821m, 744vs, 694m,
1
673vs cm−1. H NMR (CD3CN): δ = 10.85 (s, 1 H, NH), 8.78 (s, 1 H, CH), 8.10, 7.88, 7.73 (m, 5
13
H, Ph), 1.61 (s, 9 H, CMe3) ppm. C NMR (CD3CN): δ = 168.1 (C=N), 137.7, 132.5, 130.2 (Ph),
127.0 (ipso-Ph), 62.4 (CMe3), 27.5 (CMe3) ppm. 93Nb (CD3CN): δ = −0.7 (s, ∆ν½ = 1×102 Hz)
ppm.
[tBu2C=NH2][NbCl6], 3. light-yellow solid, 47% yield from NbCl5 (0.320 g, 1.18 mmol) and
tBu2C=NH (1.20 mmol). Anal. Calcd. for C11H16Cl6NNb: C, 24.13; H, 4.50; N, 3.13; Cl, 47.49;
Nb, 20.74. Found: C, 24.09; H, 4.38; N, 3.03; Cl, 47.72; Nb, 20.01. IR (solid state): 3380w (νN-H),
3277w (νN-H), 1639m (νC=N) cm−1. 1H NMR (CD3CN): δ = 9.95 (s, NH2); 1.65 (s, 18 H, tBu) ppm.
4.3. Reactions of NbF5 with imines.
4.3.1. Synthesis of [Ph2C=NH2][NbF6], 4. A mixture of NbF5 (0.150 g, 0.798 mmol), CH2Cl2 (20
mL) and benzophenone imine (0.135 mL, 0.806 mmol) was allowed to react for 18 h at room
temperature. The resulting solution was treated with hexane (30 mL), and the resulting red solid
was isolated and dried in vacuo. Yield 0.151 g, 48% (respect to the organic reactant). Crystals
suitable for X-ray analysis were collected from a CH2Cl2/hexane mixture stored at −30 C for one
week. Anal. Calcd. for C13H12F6NNb: C, 40.12; H, 3.11; N, 3.60; F, 29.29; Nb, 23.87. Found: C,
40.22; H, 3.02; N, 3.65; F, 29.58; Nb, 23.76. IR (solid state): 3260w, 3195w (νN-H), 3059w-m,
1970vw, 1669m (νC=N), 1596s (νC=C, Ph), 1567m-s, 1489w, 1447s, 1372s, 1299w, 1274w-m,
1192w-m, 1162m, 1074w-m, 1028w-m, 999w-m, 932m, 884s, 790s, 763m-s, 726m, 693vs cm−1.
12

1H NMR (CDCl3): δ = 10.54 (s, 2 H, NH2); 7.71-7.41 (m, 10 H, Ph) ppm. 13C NMR (CDCl3): δ =
180.3 (C=N); 136.4 (ipso-Ph), 132.2, 129.5, 128.7 (Ph) ppm. 19F (CDCl3): δ = 103.5 (m, [NbF6]−)
ppm. 93Nb NMR (CDCl3): δ = −1549 (m, [NbF6]−) ppm.
4.3.2. Synthesis of [PhCH=NHtBu][NbF6], 5. The reaction of N-benzylidene-tert-butylamine
(0.160 mL, 0.900 mmol) with NbF5 (0.160 g, 0.852 mmol) was carried out by a procedure similar
to that described for NbF5/Ph2C=NH. Yield 0.170 g, 54%. Anal. Calcd. for C11H16F6NNb: C,
35.79; H, 4.37; N, 3.79; F, 30.88; Nb, 25.17. Found: C, 35.22; H, 4.60; N, 3.65; F, 30.41; Nb,
25.00. IR (solid state): 3277vw (NH), 2988vw, 1657m (C=N), 1598m, 1458w, 1424vw, 1386w,
1
1330vw, 1310vw, 1237w, 1184m, 1036w, 906m, 749m cm−1. H NMR (CDCl3): δ = 10.90 (m, 1
t
H, NH); 8.80 (d, 1 H, CH=N); 8.08, 7.90, 7.72 (m, 5 H, Ph); 1.60 (s, 9 H, Bu) ppm. 13C NMR
(CDCl3): δ = 167.9 (C=N); 137.6, 131.0, 126.8 (arom CH); 126.8 (ipso-Ph); 62.4 (CMe3); 26.9
(CMe3) ppm. 19F (CDCl3): δ = 103.6 (m, [NbF6]−) ppm. 93Nb NMR (CDCl3): δ = −1550 (m,
[NbF6]−) ppm.
The CH2Cl2/hexane liquors obtained from the reactions of NbF5 with imines were separated and
then eliminated of the volatile materials in vacuo. Magnetic analyses on the resulting solid
corr
residues were as follows. From NbF5/Ph2C=NH: χM
=
1.02×10−3 cgsu. From
corr
NbF5/PhCH=NtBu: χM
= 5.63×10−4 cgsu. The solid obtained from NbF5/PhCH=NtBu was
treated with CDCl3 (1.5 mL) and H2O (0.2 mL). The mixture was stirred in contact with air for 48
h. Subsequent NMR analysis on the organic phase allowed to detect some PhC≡N.
4.4. Synthesis of [Ph2C=NH2]2[Ta2Cl10O], 6. The reaction of TaCl5 (0.340 g, 0.949 mmol) with
benzophenone imine (0.160 mL, 0.955 mmol) was carried out by using a procedure similar to that
described for NbCl5/Ph2C=NH. Crystals of 6·2CH2Cl2 were collected after several attempts by
settling aside an orange reaction mixture, layered with hexane, at −30 °C for one week. Yield
13

0.031 g, 5% (with reference to the organic reactant). Anal. Calcd. for C28H28Cl14N2OTa2: C, 26.55;
H, 2.23; N, 2.21; Cl, 39.18; Ta, 28.57. Found: C, 26.33; H, 2.15; N, 2.31; Cl, 39.02; Ta, 28.38. IR
(solid state): 3381w, 3333w, 3252w, 1659s (νC=N), 1592s, 1515m, 1488m, 1453s, 1375s, 1324w,
1
1299w, 1186w, 1164w, 1128w, 1087w, 998w, 938w, 777m, 739vs, 694vs, 665s cm−1. H NMR
(CD2Cl2): δ = 9.57 (br, 2 H, NH), 8.11, 8.01, 7.87, 7.78 (m, 10 H, Ph) ppm. 13C NMR (CD2Cl2): δ
= 184.6 (C=N), 137.9, 132.5, 130.3 (Ph), 129.4 (ipso-Ph) ppm.
4.6. X-ray crystallographic studies. Crystal data and collection details for 1·0.5CH2Cl2, 4 and
6·2CH2Cl2 are listed in Table 5. The diffraction experiments were carried out on a Bruker APEX
II diffractometer equipped with a CCD detector and using Mo-Kα radiation. Data were corrected
for Lorentz polarization and absorption effects (empirical absorption correction SADABS) [27].
Structures were solved by direct methods and refined by full-matrix least-squares based on all data
using F2 [27]. All non-hydrogen atoms were refined with anisotropic displacement parameters. C-
bonded hydrogen atoms were fixed at calculated positions and refined by a riding model, whereas
N bonded hydrogens have been located in the Fourier map and refined isotropically using the 1.2
fold Uiso value of the parent N-atom with restrained N−H distances. The CH2Cl2 molecule of
1·0.5CH2Cl2 is disordered over four positions, two by two correlated by an inversion centre. The
two independent images have been refined isotropically using similar U restraints [SIMU
command in SHELXL; s.u. 0.005] and restrained C−Cl distances [DFIX 1.75 command in
SHELXL; s.u. 0.01].
Table 5. Crystal data and details of the structure refinement for [Ph2C=NH2][NbCl5(N=CPh2)]·0.5CH2Cl2,
1·0.5CH2Cl2, [Ph2C=NH2][NbF6], 4, and [Ph2C=NH2][Ta2Cl10O]·2CH2Cl2, 6·2CH2Cl2.
Complex
Formula
Fw
1·0.5CH2Cl2
C26.5H23Cl6N2Nb
675.08
4
6·2CH2Cl2
C28H28Cl14N2OTa2
1266.72
C13H12F6NNb
389.15
T, K
100(2)
100(2)
100(2)
λ, Å
Crystal system
Space group
0.71073
Triclinic
P1
0.71073
Monoclinic
C2/c
0.71073
Triclinic
P1
14

a, Å
b, Å
c, Å
α, °
β, °
γ, °
9.9964(10)
11.1707(12)
14.6301(15)
71.4490(10)
87.0600(10)
65.0340(10)
1397.5(3)
2
10.050(2)
10.166(2)
13.885(3)
90
94.885(2)
90
1413.5(6)
4
1.829
7.9420(8)
10.8191(11)
13.3952(13)
67.6340(10)
76.1120(10)
84.4360(10)
1033.26(18)
1
Cell Volume, Å3
Z
Dc, g cm-3
1.604
1.024
2.036
6.223
µ, mm-1
0.910
F(000)
678
768
602
Crystal size, mm
θ limits, °
0.19×0.16×0.12
1.48–26.00
14538
0.24×0.21×0.15
2.86–27.99
6977
0.23×0.21×0.18
1.69–27.10
11623
Reflections collected
Independent reflections
Data / restraints /parameters
Goodness of fit on F2
R1 (I > 2σ(I))
wR2 (all data)
5470 [Rint = 0.0430] 1679 [Rint = 0.0306] 4525 [Rint = 0.0371]
5470 / 18 / 385
1.032
0.0375
1839 / 1 / 78
1.043
0.0212
0.0548
4525 / 2 / 220
1.042
0.0301
0.0701
0.0789
Largest diff. peak and hole, e Å-3 0.891 / –0.859
0.416 / –0.574
1.765 / –1.069
Supporting Information. CCDC contain the supplementary crystallographic data for the X-ray
studies reported in this paper. CCDC 1471160 (1·0.5CH2Cl2), 1471161 (4) and 1471162
(6·2CH2Cl2). For ESI and crystallographic data in CIF or other electronic format see DOI:
XXXXXXX.
Acknowledgements. The University of Pisa is gratefully acknowledged for financial support.
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18

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19

For the Table of Contents
Synopsis
Fabio Marchetti, Guido Pampaloni, Stefano Zacchini
The reactivity of niobium (tantalum) pentahalides with a selection of simple imines is
described, including the first crystallographic characterization of a Nb(V)-azavinylidene
complex.
21

For the Table of Contents
Pictogram
Fabio Marchetti, Guido Pampaloni, Stefano Zacchini
22