A structurally-characterized NbCl5-NHC adduct

Article abstract of DOI:10.1039/c4cc01575d

The selective reactions of niobium pentachloride with two bulky NHC carbenes afforded NbCl₅(NHC) complexes, bearing the highest oxidation state ever found for a metal centre in a transition metal halide-NHC adduct. The X-ray structure of 2a is the first one reported for a monodentate NHC-niobium species, and exhibits an abnormally long Nb-C bond. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc01575d

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A structurally-characterized NbCl5–NHC adduct†
Marco Bortoluzzi,a Eleonora Ferretti,b Fabio Marchetti,*b Guido Pampalonib and
Cite this: Chem. Commun., 2014,
50, 4472
Stefano Zacchinic
Received 2nd March 2014,
Accepted 10th March 2014
DOI: 10.1039/c4cc01575d
www.rsc.org/chemcomm
The selective reactions of niobium pentachloride with two bulky NHC the full characterization of the more stable WVIO2Cl2–NHC
carbenes afforded NbCl5(NHC) complexes, bearing the highest oxida- adduct.6 As a matter of fact, NHC adducts of homoleptic metal
tion state ever found for a metal centre in a transition metal halide– halides with the metal in an oxidation state of above +4 have
NHC adduct. The X-ray structure of 2a is the first one reported not been reported hitherto.
for a monodentate NHC–niobium species, and exhibits an abnormally
long Nb–C bond.
In the framework of our interest in the coordination chemistry
of group 5 pentahalides,4,7 we present herein the synthesis and
the characterization of two NbCl5 coordination compounds with
Since the discovery of the first stable N-heterocyclic carbene (NHC),1 bulky monodentate NHC ligands. The reactions of NbCl5, 1,8 with
the preparation of a huge variety of NHC complexes has followed equimolar amounts of NHC compounds9 were conducted in toluene
with reference to almost all the metal elements of the periodic at room temperature and afforded, after work-up, the products
table,2 stimulated by the outstanding features that such ligands may NbCl5(NHC), 2a–b, in good yields (Scheme 1).
confer on the resulting species.3 In this context, niobium still
Compounds 2a,b are more tolerant to air than the precursor
remains an exception, despite its attractive properties (cheapness, NbCl5 (see ESI† for details), and have been characterized using
1
low toxicity, peculiar catalytic behaviours).4 Indeed very few Nb–NHC elemental analysis and IR and NMR spectroscopy. The H and
derivatives have been prepared up to now,5 and the only 13C NMR spectra of 2a,b in C6D6 solution (Fig. S1–S4, ESI†)
crystallographic characterization is regarding a Nb(III) complex show the presence of single NHC species; the carbenic carbon
with a pincer 2,6-bis(imidazolylidene)pyridine.5a
of 2a has been recognized at 187.0 ppm by means of the 1H-13C
It should be also remarked that the chemistry of NHCs has HMBC experiment.
been primarily focused on low and medium valent metals,6
Crystals of 2a suitable for X-ray analysis (see ESI† for details)
while the isolation of derivatives of high valent metals may be a could be collected from a toluene–pentane mixture: the mole-
hard task. This is particularly true for the homoleptic halides of high cular structure is shown in Fig. 1.
oxidation state metals, due to the extreme moisture sensitivity
generally exhibited by the reaction systems. Indeed the progressive
replacement of halide ligands with oxo groups reduces the ‘‘water
hunger’’: for instance, the addition of 1,3-bis(2,6-diisopropyl-
phenyl)imidazol-2-ylidene to WVIOCl4 afforded an intractable
product whose controlled hydrolysis allowed the isolation and
a Ca’ Foscari University of Venice, Dipartimento di Scienze Molecolari e
Nanosistemi, Dorsoduro 2137, I-30123 Venezia, Italy
b University of Pisa, Dipartimento di Chimica e Chimica Industriale, Via
Risorgimento 35, I-56126 Pisa, Italy. E-mail: fabmar@dcci.unipi.it;
Web: http://www.dcci.unipi.it/~fabmar; Tel: +39 0502219245
c University of Bologna, Dipartimento di Chimica Industriale ‘‘Toso Montanari’’,
Viale Risorgimento 4, I-40136 Bologna, Italy
† Electronic supplementary information (ESI) available: Experimental details,
NMR spectra, DFT structures and their relevant data. CCDC 980212. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
Scheme 1 Synthesis of NHC complexes of NbCl5.
c4cc01575d
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Fig. 2 Isosurface of C–Nb–Cl s-bonding orbital in 2a (isovalue 0.05 e1/2
Fig. 1 ORTEP drawing of 2a. Displacement ellipsoids are at the 30% prob-
ability level. Only the min images of the disordered groups are represented.
H-atoms have been omitted for clarity. Selected bond lengths (Å) and angles
(deg): Nb(1)–C(1) 2.396(12), Nb(1)–Cl(1) 2.362(4), Nb(1)–Cl(2) 2.236(6), Nb(1)–
Cl(3) 2.336(2), Nb(1)–Cl(4) 2.304(3), Nb(1)–Cl(5) 2.326(2), C(1)–N(1) 1.387(11),
C(1)–N(2) 1.343(14), C(2)–N(1) 1.362(16), C(3)–N(2) 1.371(14), C(2)–C(3)
1.328(16), N(1)–C(1)–N(2) 104.1(10), C(1)–N(1)–C(2) 110.3(10), N(1)–C(2)–C(3)
106.9(10), C(2)–C(3)–N(2) 108.2(13), C(3)–N(2)–C(1) 110.5(9), C(1)–N(1)–C(4)
139.6(15), C(1)–N(2)–C(16) 132.0(10).
bohrꢁ3/2). H-atoms have been omitted for clarity.
The Nb(1) centre displays approximately octahedral coordi-
nation, and is slightly displaced [0.145 Å towards the apical
Cl(1)] from the plane individuated by the four equatorial
chlorides. This configuration is common with other NbCl5L
complexes (L = monodentate organic ligand).10 The steric effect
of the chloride ligands on the NHC unit is evident in that
especially one C6-ring is forced away from the Cl-equatorial
plane [the C(1)–N(1)–C(4) angle is 139.6(15)1].
|
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Fig. 3 DFT-calculated structure of NbCl5[CN(Ph)CHCHN(Ph)], 3 (hydro-
gen atoms omitted for clarity).
The Nb(1)–C(1) [2.396(12) Å] distance is significantly longer
than both Nb(V)–alkylidene moieties11 and even classical Nb(V)– The computational analysis suggests that the C–Nb bond essentially
alkyl s-bonds;12 for instance, the longest Nb–Ceq distance in consists of a s-donation from the carbon to a niobium d-type
NbCl2Me3 measures 2.152(4) Å.12a Interestingly, the Nb(III)–C orbital; the relevant molecular orbital includes the axial Cl–Nb
lengths in the chelating NHC adduct [(C–N–C0)NbCl3(thf)], bond (see Fig. 2).
C–N–C0 = 2,6-bis(imidazolylidene)pyridine, were found to be
around 2.20 Å.5a
The computational analysis of the hypothetical NbCl5 adduct
of 1,3-diphenylimidazol-2-ylidene, 3 (Fig. 3), was helpful in the
On the other hand, the carbene atom in 2a displays CꢀꢀꢀCl understanding of the niobium–NHC p bonding system of 2a.
contacts with the four equatorial Cl-ligands [3.14–3.36 Å] which are The formal replacement of the isopropyl groups in 2a with
within the sum of the van der Waals radii of the respective atoms hydrogen atoms, giving 3, determines a twisting of the NbCl5
[sum = 3.45 Å].13 An analogous situation was previously observed in frame respect to the imidazole ring, by about 241 (see Fig. 3 and
a range of d0 transition metal–chloride NHC complexes,6 including compare Tables S1 and S2, ESI†). A molecular orbital possible
VOCl3(IMes)14 and TiCl4(IPr)2,15 and was attributed to some p candidate for the p interaction in 2a receives a small contribution
interaction between the Cl-lone pairs and the p-orbital of the from the NHC ligand and is extended over both the equatorial
carbenic carbon. Such an interaction has been considered to chlorines with the Cl–Nb axis perpendicular to the imidazole ring,
be a form of back-bonding donation, made possible by the and the chlorine in the trans position with respect to the carbene
electron density on the Cl-ligands, in spite of the formal (Fig. S7A, ESI†). The contribution of the NHC moiety appears to be
absence of electrons at the metal centre.
nearly absent in the corresponding orbital calculated for 3; this
In order to shed light onto the structural aspects, we carried seems reasonable because the rotation of the NbCl5 frame around
out a DFT study. Thus the structure of 2a was optimised without the Nb–C axis inhibits overlapping of the Cl and NHC orbitals,
symmetry constraints using restricted DFT calculations (EDF2 respectively (Fig. S7B, ESI†). It may be concluded that no p
and PBE functionals).16 The calculated structure of 2a is provided interaction exists in 3. Upon considering that the Nb–C distance
in the ESI† (Fig. S5), together with relevant bonding parameters does not vary substantially in 3 with respect to 2a (Tables S1
(Table S1, ESI†). The latter are in good agreement with the and S2, ESI†), the Nb–NHC bond in 2a should be envisaged as an
corresponding data obtained using the X-ray diffraction study. essentially s one. Accordingly, the Nb–C dissociation energy
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values calculated for 2a (31 kcal molꢁ1, EDF2 functional) and 3
(29 kcal molꢁ1, EDF2 functional) are very close (Table S4, ESI†).
With reference to the Nb–C bond, it is noteworthy that
computer simulation of the hypothetical complex NbCl5(1,3-
methylimidazol-2-ylidene), 4 (Fig. S8, ESI†), has pointed out
that the Nb–C distance is negligibly shortened upon replacing
the hindered aromatic N-substituents (compounds 2a, 3) with
methyl groups (Table S3, ESI†).
The computational analysis of the charge distribution of 2a
has revealed some deficiency of electron density on the carbene
carbon (the Mulliken charge resulted to be about 0.3 a.u. more
positive than on the other C atoms of the NHC ring). This
feature suggests that 2a,b are Fischer-type carbene complexes
and, thus, provides an example of extension of the latter to
high-oxidation-state metal centers.14
In conclusion, we have obtained monodentate NHC derivatives
of niobium pentachloride by straightforward synthesis. Three main
points of novelty may be traced: (1) we have proved that stable NHC
adducts of homoleptic metal halides can be isolated with the metal
centre in the +5 oxidation state; (2) the X-ray structure ascertained for
2a is the first one comprising the Nb–NHC(monodentate) moiety; (3)
the Nb–C bond in 2a is abnormally elongated and lacks p contribu-
tion, in contrast to what was previously reported for a series of
d0 transition metal–NHC complexes. The structural features
highlighted for 2a contribute to the knowledge of the yet poorly
investigated high-valent metal–NHC bonding systems.
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4474 | Chem. Commun., 2014, 50, 4472--4474
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