Halo, alkyl, aryl, and bis(imido) complexes of niobium supported by the β-diketiminato ligand

2926 Organometallics 2010, 29, 2926–2942

DOI: 10.1021/om1001827

Halo, Alkyl, Aryl, and Bis(imido) Complexes of Niobium Supported by

the β-Diketiminato Ligand

Neil C. Tomson, John Arnold,* and Robert G. Bergman*

Department of Chemistry, University of California, Berkeley, California 94720

Received March 8, 2010

Synthesis of the complex (BDI)Nb(N^tBu)Cl₂py (BDI=HC[C(Me)N(2,6-ⁱPr₂-C₆H₃)]₂) was achieved in

high yield following the treatment of Nb(N^tBu)Cl₃py₂with Li(BDI)(OEt₂). Substitution of the chlorides

for fluorides was effected by introducing 2.0 equiv of Me₃SnF in toluene, providing the pyridine-coordi-

nated difluoride complex (BDI)Nb(N^tBu)F₂py in modest yield. The pyridine ligands from both halide

compounds were removed by treatment with B(C₆F₅)₃, affording the Lewis base-free complexes (BDI)Nb-

(N^tBu)X₂(X=Cl, F). Additionally, the Lewis base-free dichlorides of the ^tBu-imido and Ar-imido (Ar=

2,6-ⁱPr₂-C₆H₃) complexes were obtained following treatment of Nb(NR)Cl₃(dme) (R = ^tBu, Ar) with

Li(BDI)(OEt₂). The pyridine-coordinated dichloride was alkylated and arylated to form the dimethyl

complex (BDI)Nb(N^tBu)Me₂(described previously, see text) and the mono(p-tolyl) complex (BDI)Nb-

(N^tBu)Cl(p-tol), the latter of which was methylated with MeMgBr to yield the mixed alkyl/aryl complex

(BDI)Nb(N^tBu)Me(p-tol) in good yield. A rare example of a group 5 bis(^tBu-imido) species was

synthesized in good yield via treatment of (BDI)Nb(N^tBu)Cl₂py with 2.0 equiv of LiNH^tBu to form

(BDI)Nb(N^tBu)₂py. Exchange of the coordinated pyridine ligand for either pyridine-d₅or dmap

(p-(dimethylamino)pyridine) was shown to occur through a dissociative mechanism, allowing for removal

of the coordinated Lewis base by treatment with B(C₆F₅)₃. The resulting average C_2v-symmetric tetra-

coordinate bis(imido) complex (BDI)Nb(N^tBu)₂was characterized in solution by NMR spectroscopy and

observed to undergo clean thermal decomposition to yield (BDI^#)Nb(N^tBu)(NH^tBu) (BDI^#=H₂Cd

C(NAr)CHdC(NAr)Me) over several hours at room temperature. Treatment of the four-coordinate

t

bis(imido) with BuNCO resulted in clean [2 þ 2] cycloaddition to yield an oxaazametallacyclobutane

t

complex, which was further observed to extrude BuNdCdN^tBu over 12 h at room temperature.

The molecular structures of (BDI)Nb(N^tBu)Cl₂py, (BDI)Nb(NAr)Cl₂, (BDI)Nb(N^tBu)Me₂, (BDI)Nb-

(N^tBu)Cl(p-tol), (BDI)Nb(N^tBu)₂py, and(BDI)Nb(N^tBu)₂(dmap) were determined crystallographically.

Finally, DFT (BP86) geometry optimization calculations on a model complex of the thermally unstable

four-coordinate bis(imido) species allowed for identification of the orbital interactions leading to

activation of the imido groups through mixing with the BDI frontier orbitals.

Introduction

monoimido systems capable of a wide range of π- and σ-bond

activation reactions^1-7to group 6 imidos that often act as

spectator ligands.^8,9Accordingly, group 5 monoimido com-

plexes have shown modest imido group reactivity, but such

species are known to lead to unique products,^10-13prompting

interest in developing methods for increasing the reactivity of

the MdNR (M=V, Nb, Ta) bond.

One interesting concept for activating imido (and other

multiply bonded) ligands is to electronically saturate the

metal’s d_πorbitals (commonly termed π-loading) by intro-

ducing multiple π-donating ligands onto the metal center.¹⁴

When implemented with the appropriate symmetry and

The reactivity of early metal imido functionalities depends in

part on the relative energies of the metal xd (x=3-5) and nitro-

gen 2p orbitals. As the energies of the xd orbitals decrease from

left to right on the periodic table, the covalency of MdNR

bonds should increase, leading to an overall stabilization of the

M-N π-interaction. Studies on the chemistry of these com-

plexes has led to an empirical trend to this effect, from group 4

*Corresponding authors. E-mail: arnold@berkeley.edu; rbergman@

berkeley.edu.

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pubs.acs.org/Organometallics

Published on Web 06/07/2010

2010 American Chemical Society

Article

Organometallics, Vol. 29, No. 13, 2010 2927

relative energy considerations, π-loading should have the

effect of either polarizing or entirely localizing some imido

π-electron density on the nitrogen atom,^15-22thereby decreas-

ing the stabilization from the metal center and activating the

π-electrons of the imido bond. This effect has been most

successfully implemented with respect to early metal imido

chemistry through combinations of the imido ligand with the

orbitally related cyclopentadienyl (Cp) ligands.^1,7,23-30

A related method for achieving the π-loading effect is to

form bis(imido) complexes.^6,31,32The bent bis(imido) motif

can be compared to bent-metallocene systems due to the simi-

larity between the donor orbitals of the cyclopentadienyl and

imido ligands (both 1σ,2π, Figure 1). This similarity indicates

that the remaining unoccupied frontier orbitals for bent bis-

(imido) complexes would mimic those of the bent-metallo-

cenes, with one b₂and two a₁orbitals located in the plane

bisecting the angle between the two imido groups.³³These

frontier orbitals (1a₁, 2b₂, and 2a₁) lack any out-of-plane

π-bonding component, suggesting that the incorporation of

a π-donor at these positions would create competition between

the imido π-donor orbitals and those of the non-imido ligands

for the occupied metal-based π-symmetry orbitals through

mixing with the 1b₂and 1b₁orbitals (Figure 1).

Nevertheless, in contrast to the group 6 metals, for which

bis(imido) dihalide starting materials are readily available,

the bis(imido) moiety is rare for group 5 metals, suggesting the

need for specialized auxiliary ligands. The diaryl-β-diketiminato

ligand class (Ar₂-BDI, HC[C(Me)NAr]₂) has many desirable

characteristics, being monoanionic, bidentate ligands with six-

electron π-systems capable of mixing with π-symmetric orbitals

on a metal center.³⁴Many examples are now known in which the

use of the Ar₂-BDI ligands have allowed for the stabilization of

rare and generally reactive first-row transition metal fragments,

Figure 1. (Left, center) Illustration of the isolobal relationship

between the frontier orbitals of Cp₂Mand (RN)₂M complexes. The

given orbital occupation corresponds to a d⁰electron configuration.

The charges p and q are related by p=qþ2 for isoelectronic com-

plexes of elements from the same group. (Center, right) Molecular

orbital interaction scheme depicting the symmetry and relative

energy considerations for forming π-loaded four-coordinate bis-

(imido) complexes by using a π-donating bidentate ligand.

including early metal alkylidene and alkylidyne complexes,^35-37

three-coordinate iron species,³⁸and late metal nitrene com-

plexes,^39-41suggesting that the Ar₂-BDI ligand may be suitable

for stabilizing the low-coordinate bis(imido)niobium fragment

as well.

In comparison to the wealth of 3d transition metal chem-

istry supported by Ar₂-BDI ligands, relatively few reports

exist describing the application of the Ar₂-BDI ligand class

toward the heavier congeners of the middle transition metals

(groups 5-8). Thus, while a number of group 4 d⁰complexes

are known,^34,42-46only the neutral and cationic d⁰Mo com-

plexes (Ar₂-BDI)(X)Mo(NR)(Y) (Y=CHCMe₂R, NR; see

Figure 2) have been reported for the 4/5d mid-transition

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2928 Organometallics, Vol. 29, No. 13, 2010

Tomson et al.

Scheme 2

the introduction of a Ar₂-BDI ligand onto the heavier middle

transition metals. Empirically, however, it appears that

Ar₂-BDI complexes with substituted aryl rings tend toward

four- and five-coordinate complexes,³⁴which indicates that

the steric imposition of the flanking aryl groups may lead to

unstable complexes on formation of (Ar₂-BDI)MX₄species

(M=Nb, Ta, Mo, W; X=halide) should the common pen-

tahalides be used as starting materials. As an example of the

circumvention of these steric requirements, the only known

niobium β-diketiminato complex is that of the niobium tetra-

halide (PhNdCPh-CMedCPh-NH)NbCl₄.⁶²While the pro-

duct was not structurally characterized to confirm the biden-

tate coordination mode of the ligand, it is notable that the

β-diketiminato nitrogens were minimally substituted (Ph, H).

Continuing our interest in the synthesis of early metal low-

coordinate complexes,^63-66we have sought group 5 species sup-

ported by the bulky diaryl-β-diketiminato ligand HC[C(Me)N-

(2,6-ⁱPr₂-C₆H₃)]₂(hereafter referred to as BDI). Our solution to

the electronic and steric requirements of ligand introduction

Figure 2. (a) Representation of the delocalized structure of

a Ar₂-BDI ligand. (b) Representation of the molybdenum imido

alkylidene complexes supported by the Ar₂-BDI ligands.^47,48

(c) Representation of the molybdenum bis(imido) complexes

supported by the Ar₂-BDI ligands.⁴⁹

Scheme 1

metals.^47-49Several late metal Ar₂-BDI systems are also

known (groups 9, 10), but these tend toward either higher

formal oxidation states or lower oxidation state metal cen-

ters bound by strong π-acids.^34,50-58

Two competing factors appear to have significantly limited

research into 4d and 5d middle transition metal Ar₂-BDI

complexes. The first is the propensity of the Ar₂-BDI ligand

to undergo reductive cleavage of the ligand N-C_iminebond to

yield terminal metal imido complexes bound by a five-mem-

bered monoazabutadiene ligand.^46,59,60This transformation is

the predominant reactivity pattern observed for (Ar₂-BDI)-

MX₃(M = Zr, Hf; X=monovalent, σ-donor ligand) systems

when the metal is reduced to the d²electronic state (Scheme 1).

This instability seemingly excludes common entries into

group 5 and related chemistry via the d²metal halides, which

would mimic the route taken by Budzelaar⁶¹in forming the

(Ar₂-BDI)MCl₂(M=Ti, V) complexes.

extends from the synthetic protocols taken by Schrock and

€

0

Mosch-Zanetti (see Figure 2), who both made use of d moly-

bdenum bis(divalent) ligand species as the direct synthetic pre-

cursors to their Ar₂-BDI complexes. Their success would indi-

cate that by concentrating two valencies at one coordination site,

the steric requirements for forming stable d⁰BDI metal com-

plexes can be satisfied while avoiding low-valent starting materi-

als. For our purposes, use of the imido group as the divalent

ligand has the advantages of (i) lending stability to the Nb(V)

metal center through its 1σ,2π-bonding character,⁶⁷(ii) allowing

study of the effects of a monoimido group on the BDI ligand,

and (iii) providing facile entry into four- and five-coordinate

bis(imido)niobium complexes. The following text describes our

efforts to develop a range of suitable starting materials for

exploratory BDI-supported imidoniobium chemistry. We will

conclude by discussing our preliminary studies of five- and four-

coordinate bis(imido) complexes, along with theoretical support

for the claim of π-loading resulting from mixing of the BDI

π-system with the NbdNR π-bonds.

This inaccessibility of common d^ng2starting materials for

4/5d early metal Ar₂-BDI complexes indicates that the use of

high oxidation state starting materials may be necessary for

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Results and Discussion

Synthesis of Dihalide Complexes. The reaction of Nb-

(N^tBu)Cl₃py₂(py = pyridine) with Li(BDI) OEt₂(BDI =

3

HC(C(Me)NAr)₂, Ar=2,6-ⁱPr-C₆H₃) in THF resulted in the

formation of a deep red solution, from which (BDI)Nb-

(N^tBu)Cl₂py (1, Scheme 2) was obtained in high yield as a

1

red crystalline material. The room-temperature H NMR

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Article

Organometallics, Vol. 29, No. 13, 2010 2929

Figure 4. Resonance structures used for describing the ground

state of β-diketiminato niobium imido complexes.

low-temperature ¹H NMR spectrum, the structure of 1is that of

a distorted octahedron, with the two chlorides mutually trans

and the imido group and the pyridine oriented trans to the

nitrogens of the BDI ligand. This geometry places one of the

^tBu methyl groups directly above the π-system of one of the BDI

˚

aryl rings (d(C_Me-C_Ar)=3.648-3.820 A), consistent with the

upfield chemical shift of the ^tBu group in the H NMR spec-

trum. The displacement of the chlorides toward the pyridine

1

(Cl(l)-Nb(1)-Cl(2)=160.68(2)°) suggests a weakened Nb-py

˚

binding interaction (Nb(1)-N(4) = 2.376(4) A) and is sugges-

tive of the trigonal-bipyramidal geometry of the related Lewis

base-free (BDI)Nb(N^tBu)Me₂pentacoordinate complex (see

below).

On closer inspection of the crystal structure of 1, somefeatures

of the π-bonding within the BDI NCCCN metallacycle impor-

tant to our later discussion of how the BDI π-system influences

NbdN^tBu bonding become apparent. The Nb(1)-N(2) bond

˚

length of 2.100(2) A is close to the range expected for a Nb

amide,⁷⁰indicating π-donation from N(2) to Nb(1). In contrast,

˚

the long Nb(1)-N(3) bond length of 2.420(2) A reflects the

strong trans influence of the imido ligand and indicates that any

N(3)(p_π) to Nb(d_π) donation is minimal. This asymmetric

binding of the BDI nitrogen atoms to the metal center is further

reflected in the bond lengths within the backbone of the BDI

ligand, as the nitrogen trans to the imido group has a shorter

N(3)-C(20)_iminebond length than that of the N(2)-C(18)_imine

bond of the nitrogen trans to the pyridine ligand (1.317(3) vs

Figure 3. Molecular structure of 1 as determined by a single-

crystal X-ray diffraction study. Hydrogen atoms are omitted for

clarity; thermal ellipsoids are set at the 50% probability level.

Selected bond lengths are given in A. Selected bond angles (deg):

Nb(1)-N(1)-C(1) 163.59(19), Cl(1)-Nb(1)-Cl(2) 160.68(2),

N(1)-Nb(1)-N(3) 171.92(8), N(2)-Nb(1)-N(4) 172.98(7).

˚

1.369(3) A, respectively). Still, the long N(2)-C(18) bond is

shorter than expected for an alkyl amine (1.469 A) and close to

˚

71

˚

that of N-C(O) bond lengths of N,N-dialkyl ureas (1.363 A).

A related trend is observed for the N(3) atom, as the N(3)-C(20)

distance reflects the bond’s greater imine character, with the

interaction found to be intermediate between that of an isolated

spectrum displays significant line broadening due to pyridine

dissociation, as confirmed by the rapid exchange of coordi-

nated pyridine for pyridine-d₅; NMR spectra obtained at 243

K reveal average C_ssymmetry (T_c= 293K, ΔG^‡= 14.1(1)

kcal/mol) with one singlet for each backbone methyl group,

consistent with a geometry that places the chlorides trans to

one another. The signals in the ¹H and ¹³C{¹H} NMR

spectra appear in regions similar to those observed for

related early transition metal BDI complexes.^42,68,69

˚

imine (1.279 A) and the N-C bond lengths within pyridine

(1.337 A). This bond length alternation propagates through

71

˚

the C-C bonds, with a shorter C(18)-C(19) bond length

˚

(1.372(4) A) than the C(19)-C(20) distance (1.421(4) A). Thus,

while the niobacycle formed by the BDI NCCCN ligand back-

bone has the electronic and orbital structure to form an aromatic

metallacycle,⁷²the bond lengths within the six-membered ring

more closely represent resonance structure A (Figure 4) than the

resonance-averaged structure Cthat is commonly found on early

metal systems.

t

The chemical shift of the Bu group in the ¹H NMR

spectrum (0.64 ppm) is notable, however, as the signal is

shifted ca. 1 ppm upfield from the starting material. This

effect is due to the proximity of the imido group to one of the

BDI aryl rings, as the ring current of aryl groups is well-

known to provide a shielding effect for nuclei situated

To improve the range of available starting materials for

further synthetic chemistry, we sought the synthesis of other

halide complexes, both with and without coordinated donor

ligands. The difluoride complex (BDI)Nb(N^tBu)F₂py (2)

was synthesized from 1 by treating the dichloride with 2.0

equiv of the mild fluorinating reagent Me₃SnF in toluene at

room temperature (Scheme 3).

1

centrally above the ring. The H NMR chemical shift of

the ^tBu-imido group has thus been a useful indicator of the

molecular geometry of these species in solution.

The molecular structure of 1 was determined crystallogra-

phically and is represented in Figure 3. As indicated by the

(70) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson,

D. G.; Taylor, R. J. Chem. Soc., Dalton Trans. 1989, S1–S83.

(71) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen,

A. G.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1987, S1–S19.

(72) Masui, H. Coord. Chem. Rev. 2001, 219, 957–992.

(68) Basuli, F.; Bailey, B. C.; Watson, L. A.; Tomaszewski, J.;

Huffman, J. C.; Mindiola, D. J. Organometallics 2005, 24, 1886–1906.

(69) Basuli, F.; Bailey, B. C.; Huffman, J. C.; Mindiola, D. J.

Organometallics 2005, 24, 3321–3334.

2930 Organometallics, Vol. 29, No. 13, 2010

Tomson et al.

Scheme 3

Scheme 5

The ¹H NMR spectrum of 2 indicates average C_ssymme-

try at room temperature. The sharp resonances observed at

298 K are consistent with tighter binding of pyridine (T_c=

348 K, ΔG^‡=16.3(1) kcal/mol) to a metal center that is both

less sterically encumbered and more electrophilic than 1 due

to the smaller size and higher electronegativity of fluorine.

The formation of a C_s-symmetric complex with a mirror

plane relating the two fluorides is further supported by a

single new ¹⁹F NMR resonance at 68.77 ppm;a region

common for group 5 d⁰fluoride complexes⁷³;and by an

that more than one fluxional process is responsible for the

averaged symmetry observed by NMR spectroscopy. The for-

mation of a fluoride-bridged dimer upon pyridine abstraction

could account for this behavior, as the low temperature would

slow any bridging fluoride exchange mechanisms, while the

high-temperature spectra would start to approximate that ex-

pected for the mononuclear complex (Scheme 5), which would

likely have average C_2vsymmetry in solution on the basis of

related compounds (see below).

t

upfield shifted Bu resonance (0.41 ppm) indicative of an

imido group situated cis to one BDI nitrogen and trans to the

other, placing the imido group centrally above one of the

flanking aryl rings.

Forming 14/16 e^-base-free dihalide species was accom-

plished by two routes: either by pyridine abstraction from

complex 1 or 2 or by ligand metalation on the readily

attainable Nb(NR)Cl₃(dme) (R=^tBu, Ar) starting materials.

Thus, addition of the known pyridine-abstracting reagent

B(C₆F₅)₃to benzene solutions of 1 or 2 yielded the base-free

dihalide complexes (BDI)Nb(N^tBu)X₂(3, X = Cl; 4, X=F;

Scheme 4, route a).⁷⁴In each case, the color of the solution

As mentioned above, the reaction of Nb(N^tBu)Cl₃(dme)

with Li(BDI) OEt₂in toluene resulted in a cherry red solu-

3

tion, from which (BDI)Nb(N^tBu)Cl₂(3; Scheme 4, route b)

was crystallized in modest yields. The room-temperature

¹H NMR spectrum of 3 is again broadened due to fluxional

1

motion in solution on the NMR time scale, but H and

¹³C{¹H} NMR spectra collected at 223 K indicate overall C_s

symmetry in solution. This solution-state symmetry along

with the downfield shift of the ^tBu resonance from 0.67 ppm

for 1 to 1.54 ppm for 3 suggests an overall square-pyramidal

Scheme 4

1

geometry with an apical imido group. The shift in the H

NMR resonance for the ^tBu group is in accord with a change

in position of the ^tBu group with respect to the nearest aryl

ring of the BDI ligand. The chemical shift of the imido group

t

for 3 lies in the range expected for early metal Bu imido

groups that are not flanked by aryl rings. Vapor diffusion

osmometry measurements on 3 in benzene at room tempera-

ture further support the formulation of 3 as a monomer,

ruling out the possibility of a dimer with bridging chlorides.

A related complex that is believed to have a geometric struc-

ture analogous to that of 3 was synthesized from the arylimido

precursor Nb(NAr)Cl₃(dme) (Ar = 2,6-ⁱPr₂-C₆H₃). Reaction

lightened slightly on addition of the borane, and the forma-

tion of py B(C₆F₅)₃was clearly observed by H and ¹⁹F

NMR. Complex 3 and py B(C₆F₅)₃were separated by

repeated fractional crystallization from pentane, yielding

a material that was found to be analytically identical to

that obtained from the reaction of Nb(N^tBu)Cl₃(dme) with

of Li(BDI) OEt₂with Nb(NAr)Cl₃(dme) in THF led to

3

1

isolation of the purple, base-free complex (BDI)Nb(NAr)Cl₂

(5; Scheme 4, route b) in 57% yield after crystallization from

Et₂O. The room-temperature ¹Hand¹³C{¹H} NMR spectra of 5

revealed that the geometry of the complex was static on the NMR

time scale, and the overall C_ssymmetry observed in solution was

supported by the crystallographic data collected for 5 (Figure 5),

wherein an approximate molecular mirror plane includes the

metal center, the R-carbon common to both BDI imine groups,

and the nitrogen and arene portions of the imido group.

As a likely mimic for the coordination geometry of 3,

complex 5 reflects the versatility of the binding orientation of

the imido group with respect to the BDI ligand. The ability of

the ligand to adapt to the electronic situation at the metal

center is indicated by the bond distances within the ligand

NCCCN backbone.

3

Li(BDI) OEt₂(see below).

3

Compound 4 was characterized in solution by NMR spec-

troscopy and found to have averaged C_2vsymmetry at room

1

temperature. The H NMR resonances were broad at this

temperature, indicating that the high symmetry may be attri-

butable to fluxional processes in solution. While low-tempera-

ture NMR experiments reveal an averaged C_s-symmetric

complex, NMR spectra collected at elevated temperatures begin

to rebroaden from the room-temperature spectra, indicating

In comparison to 1, which had alternating long and short

bond distances within the BDI metallacycle resulting from the

trans influence of the imido group, complex 5 has an apical

imido group, with chlorides occupying both coordination

(73) Mullins, S. M.; Hagadorn, J. R.; Bergman, R. G.; Arnold, J.

J. Organomet. Chem. 2000, 607, 227–232.

(74) Sanchez-Nieves, J.; Frutos, L. M.; Royo, P.; Castano, O.;

Herdtweck, E. Organometallics 2005, 24, 2004–2007.

Article

Organometallics, Vol. 29, No. 13, 2010 2931

Scheme 6

are known,^76-84only one of which has been structurally

characterized.^80aOwing to the stabilizing nature of both the

BDI ligand and imido groups toward high oxidation states, we

sought niobium complexes with common alkyl and aryl li-

gands, in an effort to probe the electronic factors controlling the

geometric parameters within the BDI ligand. A synthetic

procedure for the dimethyl complex (BDI)Nb(N^tBu)Me₂has

been previously communicated, but the compound was not

discussed in detail.⁸⁵A full analysis of this complex will be

presented for comparative purposes.

Synthesis and Characterization of (BDI)Nb(N^tBu)Me₂.

Dimethylation of 1 could be performed cleanly using the

Grignard reagent. Addition of a solution containing 2.0 equiv

of MeMgBr in Et₂O to a stirred slurry of 1 in Et₂O at -72 °C

caused the solution to turn from dark red to orange, then pale

yellow on warming to room temperature. The isolated product

(BDI)Nb(N^tBu)Me₂(6, Scheme 6) crystallized from pentane as

large yellow blocks in 70-80% yields. Complex 6 is thermally

unstable both in the solid state (days) and in solution (hours) at

room temperature, decomposing into a brown unidentified

material. Storage of 6 at -35 °C allowed the complex to persist

for several months without significant decomposition.

A

¹H NMR spectrum of 6 taken in THF-d₈at room

temperature displays broad peaks associated with a dynamic

molecular geometry on the NMR time scale; cooling the sample

to 233 K reveals a molecular geometry with overall C_ssymme-

try, in which the BDI ligand bridges axial and equatorial

positions and the Nb-Me groups occupy the remaining equa-

torial positions. This geometry is proposed from several key

features in the ¹H NMR spectrum of 6, including the appear-

ance of (i) two resonances for the backbone methyl groups (1.73

and 2.00 ppm), (ii) a single resonance for the two Nb-Me groups

at 0.41 ppm, and (iii) a downfield shifted ^tBu group (0.88 ppm)

due to its close proximity to an aryl group. These features rule

out the possibility of a square-pyramidal complex with basal

methyl groups, in contrast to the pentacoordinate dihalide

complexes 3 and 5, which place the imido group in the apical

position of a square-based pyramid. The ¹³C{¹H} NMR spec-

trum taken at 213 K exhibits a single broad resonance at 47.5

ppm, attributable to the two Nb-Me groups with unresolved

coupling to the ⁹³Nb (I = 9/2, 100%) nucleus.

Figure 5. Molecular structure of 5 as determined by a single-

crystal X-ray diffraction study. The hydrogen atoms were omit-

ted for clarity; the thermal ellipsoids were set at the 50% proba-

˚

bility level. Selected bond lengths are given in A. Selected bond

angles (deg): Nb(1)-N(1)-C(1) 173.27(16), N(1)-Nb(1)-N(2)

103.93(7), N(1)-Nb(1)-N(3) 98.99(7), N(1)-Nb(1)-Cl(2)

107.81(6), N(1)-Nb(1)-Cl(1) 104.00(6), N(2)-Nb(1)-Cl(2)

148.05(5), N(3)-Nb(1)-Cl(1) 156.86(5).

sites trans to the BDI nitrogens. This leads to roughly equi-

˚

valent Nb-N_BDIbond lengths (Nb(1)-N(2) 2.1699(17) A,

˚

Nb(1)-N(3) 2.1879(18) A) and statistically equivalent

˚

N-C_imine(N(2)-C(26) 1.332(3) A, N(3)-C(28) 1.336(3)

˚

A) and C_imine-C_R(C(26)-C(27) 1.392(3) A, C(27)-C(28)

1.395(3) A) bond lengths. The implication of these findings is

˚

(78) Sharp, P. R.; Astruc, D.; Schrock, R. R. J. Organomet. Chem.

1979, 182, 477–488.

that the asymmetric binding motif found for 1 is a conse-

quence of competing π-donation from ligands about the

basal plane of the metal and not a general feature of the

BDI ligand binding to a Nb(V) center.

Synthesis of Alkyl and Aryl Nb(V) Complexes. Due to the

relative ease of reduction to Nb(IV), substitution reactions at

Nb(V) centers involving hard nucleophiles are often ham-

pered by reduction reactions.⁷⁵This trend may be respon-

sible for the fact that only a handful of Nb(V) aryl complexes

(79) Antinolo, A.; Deilarduya, J. M.; Otero, A.; Royo, P.; Lanfredi,

A. M. M.; Tiripicchio, A. J. Chem. Soc., Dalton Trans. 1988, 2685–2693.

(80) (a) Chesnut, R. W.; Yu, J. S.; Fanwick, P. E.; Rothwell, I. P.;

Huffman, J. C. Polyhedron 1990, 9, 1051–1058. (b) Lockwood, M. A.;

Potyen, M. C.; Steffey, B. D.; Fanwick, P. E.; Rothwell, I. P. Polyhedron

1995, 14, 3293–3313.

(81) Cockcroft, J. K.; Gibson, V. C.; Howard, J. A. K.; Poole, A. D.;

Siemeling, U.; Wilson, C. J. Chem. Soc., Chem. Commun. 1992, 1668–1670.

(82) Etienne, M.; White, P. S.; Templeton, J. L. Organometallics

1993, 12, 4010–4015.

(83) Djakovitch, L.; Herrmann, W. A. J. Organomet. Chem. 1997,

546, 399–405.

(84) Chan, M. C. W.; Cole, J. M.; Gibson, V. C.; Howard, J. A. K.;

Lehmann, C.; Poole, A. D.; Siemeling, U. J. Chem. Soc., Dalton Trans.

1998, 103–111.

(85) Tomson, N. C.; Yan, A.; Arnold, J.; Bergman, R. J. Am. Chem.

Soc. 2008, 130, 11262–11263.

(75) Bott, S. G.; Hoffman, D. M.; Rangarajan, P. Inorg. Chem. 1995,

34, 4305–4310.

(76) Jamieson, G.; Lindsell, W. E. Inorg. Chim. Acta 1978, 28, 113–118.

(77) McLain, S. J.; Schrock, R. R.; Sharp, P. R.; Churchill, M. R.;

Youngs, W. J. J. Am. Chem. Soc. 1979, 101, 263–265.

2932 Organometallics, Vol. 29, No. 13, 2010

Tomson et al.

The fluxional motion observed in the room-temperature

NMR spectrum does not likely involve an ionic mechanism for

interchange of the methyl groups, as the energy barrier for the

fluxional process did not change appreciably when the com-

plex was dissolved in benzene or toluene as opposed to THF.

While a Berry pseudorotation mechanism involving inter-

change of the axial and equatorial substituents in the penta-

coordinate complex seems likely, a mechanism involving

dissociation of a neutral nitrogen donor of the BDI ligand⁸⁶

to give a four-coordinate complex capable of methyl group

exchange has not been ruled out. A lengthening of the Nb-

(1)-N(3) bond;as seen in 1;due to the trans influence of

the imido ligand could again be operative in 6, whereby the imido

group’s trans influence could lead to a trans effect, facilitating the

dissociation of the BDI nitrogen trans to the imido group.

The structure of 6 in the solid state was found to be distorted

trigonal bipyramidal⁸⁷(τ = 0.71), with an axial imido group,

equatorial methyl groups, and the BDI ligand spanning axial

and equatorial positions (Figure 6), matching the geometry

proposed from the NMR data. As seen for 1, the Nb-N_BDI

distance for the nitrogen trans to the imido group is significantly

longer than the Nb-N_BDIdistance for the equatorial nitrogen

˚

(2.3578(13) vs 2.1354(13) A), attributable to the trans influence

˚

of the imido group (Nb(1)-N(1) 1.7773(13) A). This bond

alternation proceeds around the metallacycle, although not to

the degree observed for 1. Also, the difference between the

˚

Nb(1)-C_Mebond lengths (Nb(1)-C(1) 2.1776(17) A; Nb(1)-

C(2) 2.1874(17) A) is consistent with the distorted nature of the

˚

trigonal bipyramid, placing the C(1) methyl group in a more

apical position on the continuum between an idealized trigonal

bipyramid and an idealized square-based pyramid.

Attempts at forming a monomethylniobium chloride pro-

duct by using <2.0 equiv of MeMgBr resulted only in

mixtures of 1 and the dimethyl complex 6, indicating either

that (i) disproportionation processes are occurring rapidly in

solution or (ii) the rate of addition of the second methyl

group to the methyl chloride complex is much faster than the

rate of addition of the first methyl group to the dichloride

complex. Notably, attempts at forming other alkyl com-

plexes resulted in intractable mixtures of products, presum-

ably due to sterically induced reactivity.

Figure 6. Molecular structure of 6 as determined by a single-

crystal X-ray diffraction study. The hydrogen atoms were omit-

ted for clarity; the thermal ellipsoids were set at the 50% proba-

˚

bility level. Selected bond lengths are given in A. Selected bond

angles (deg): N1-Nb1-N2 99.71(5), N1-Nb1-C1 94.83(6),

N2-Nb1-C1 113.11(6), N1-Nb1-C2 89.58(6), N2-Nb1-C2

129.55(6), C1-Nb1-C2 115.33(7), N1-Nb1-N3 172.01(5),

N2-Nb1-N3 83.33(5), C1-Nb1-N3 90.71(6), C2-Nb1-N3

82.83(6), C3-N1-Nb1 168.96(12).

Synthesis and Characterization of (BDI)Nb(N^tBu)X(p-tol)

(X = Cl, Me). Aryl ligands should be more stable toward

sterically induced decomposition reactions due to their lack of

R-hydrogens and their smaller volume compared to common

non-methyl alkyl ligands. The addition of 1.0 equiv of p-tol₂Mg

(p-tol = 4-MeC₆H₄) to 1 cleanly afforded (BDI)Nb(N^tBu)Cl-

(p-tol) (7) as an orange solid (Scheme 7). The complex exhibits

C₁symmetry by ¹H NMR spectroscopy, with a p-tolyl methyl

resonance at 2.28 ppm. The use of aryl Grignard reagents failed

to give clean conversion to monoaryl products, and over-

arylation was observed only when a greater excess of p-tol₂Mg

was allowed to react with 1 for longer periods of time. Although

complex 7is indefinitely stable in the solid state, it decomposes in

solution over a few days into unidentified products.

Scheme 7

Alkylation of 7 with MeMgBr cleanly formed the mixed

aryl/alkyl complex (BDI)Nb(N^tBu)Me(p-tol) (8, Scheme 7).

(86) (a) Wright, R. J.; Power, P. P.; Scott, B. L.; Kiplinger, J. L.

Organometallics 2004, 23, 4801–4803. (b) Clegg, W.; Coles, S. J.; Cope,

E. K.; Mair, F. S. Angew. Chem., Int. Ed. 1998, 37, 796–798.

(87) Determined using the continuous symmetry parameter τ =

(R - β)/60, where R and β are the largest and second largest angles about

the metal center, respectively: Addison, A. W.; Rao, T. N.; Reedijk, J.;

van Rijn, J.; Verschoor, G. C. J. Chem. Soc., Dalton Trans. 1984, 1349.

This compound is stable at room temperature and, judging

by the 298 K ¹H NMR spectrum, nonfluxional on the NMR

Article

Organometallics, Vol. 29, No. 13, 2010 2933

CpML₄. The geometric preference of 7 may be solely in-

dicative of a steric preference for placing the larger ^tBu group in a

basal position, away from the bulky ⁱPr substituents of the BDI

ligand. Still, this complex provides a close structural analogy to

5, allowing for comparison of the effect of the imido group on the

bonding within the NCCCN framework. The trans influence of

the imido group can again be seen in the lengthening of the

˚

Nb(1)-N(3) bond length to 2.332(4) A compared to the Nb(1)-

N(2) bond length of 2.114(4) A for the bond trans to the chloride.

˚

The BDI NCCCN bond lengths alternate as predicted by

resonance structure A in Scheme 3, although not to the degree

˚

seen for 1. The Nb(1)-C(5)_Arbond length of 2.145(5) A is

slightly longer than that observed previously for the other known

Nb(V) aryl complex (2.122 A).⁸⁰The aryl group is also canted

˚

slightly with respect to the niobium center (Nb(1)-C(5)_ipso

-

˚

C(8)_para=172°), andthemetallies0.223Afrom the mean plane

of the aryl group. This likely reflects crystal-packing forces, as

little π-interaction would be expected between the aryl group and

the metal center, and an intermolecular interaction between the

t

˚

p-tolyl methyl group and a nearby Bu group (2.516 A) can be

observed in the compound’s crystal-packing diagram.

The clean conversion of the halides into the aryl and alkyl

products indicates that the combination of the BDI ligand

with the imido group provides thermodynamic stability for

the complexes containing a formal Nb(V) center, allowing

for substitution reactions while resisting reduction to d^n>0

complexes. We were further interested in testing the ability of

this ligand framework to stabilize a low-coordinate 4d metal

center, similar to the stabilization imparted to low-coordi-

nate 3d metal systems.^1-7Such stabilization may then lead to

increased reactivity compared with that exhibited by more

coordinately saturated systems.

Synthesis and Characterization of Pentacoordinate Bis(tert-

butylimido) Complexes. (BDI)Nb(N^tBu)₂py. The BDI ligand

is well positioned to stabilize low-coordinate fragments, given

the size and orientation of the flanking aryl groups. Initial

attempts at substitution of the chlorides of 1 with LiNH^tBu in

ethereal solvents resulted in mixtures of products, all displaying

similar solubility properties. However, on stirring a slurry of

LiNH^tBu and 1 in pentane for 12 h at room temperature, the

solution turned bright yellow and was found to contain a single

complex that could be isolated by crystallization from Et₂O.

The resonances in the room-temperature ¹H NMR spectrum of

the product were broadened by fluxional processes, but a

spectrum taken in THF-d₈at 238 K revealed a C₁-symmetric

structure, with two ^tBu methyl resonances, one at 1.39 ppm and

the other at 0.27 ppm. The appearance of two ^tBu peaks could

reflect the presence of either the desired bis(tert-butylimido)

complex (BDI)Nb(N^tBu)₂py or the product from a single

amide substitution at 1, (BDI)Nb(N^tBu)(NH^tBu)Cl. Observa-

Figure 7. Molecular structure of 7 as determined by a single-

crystal X-ray diffraction study. The hydrogen atoms were omit-

ted for clarity; the thermal ellipsoids were set at the 50% proba-

˚

bility level. Selected bond lengths are given in A. Selected bond

angles (deg): N(1)-Nb(1)-N(2) 96.12(15), N(1)-Nb(1)-C(5)

101.31(19), N(2)-Nb(1)-C(5) 100.23(16), N(1)-Nb(1)-N(3)

151.03(17), N(2)-Nb(1)-N(3) 82.56(13), C(5)-Nb(1)-N(3)

107.42(16), N(1)-Nb(1)-Cl(1) 89.71(13), N(2)-Nb(1)-Cl(1)

159.03(11), C(5)-Nb(1)-Cl(1) 98.34(13), N(3)-Nb(1)-Cl(1)

82.64(9), Nb(1)-C(5)-C(8) 171.92.

time scale at room temperature. The spectrum indicates

molecular C₁symmetry, with eight HCMe₂doublets, four

(overlapping) HCMe₂septuplets, and two backbone methyl

singlets. We were intrigued by the low symmetry of these

complexes in solution, given the fluxional nature of related

pentacoordinate complex (BDI)Nb(N^tBu)Me₂. To deter-

mine the effect the aryl group had on the overall molecular

structure, we obtained crystallographic information on one

of the aryl complexes.

Single crystals of the p-tolyl chloride complex 7 were

grown from pentane; the molecular structure is represented

in Figure 7. The complex adopts a square-pyramidal geo-

metry (τ = 0.13), with an apical p-tolyl group. This geometry

is surprising given the preference of square-pyramidal com-

plexes with one multiply bonded ligand for placing the

multiply bonded ligand in the apical coordination site,²¹

akin to the common four-legged piano stool complexes

1

tion of primary amido N-H peaks in H NMR spectra can

be problematic due to their characteristically broad signals

in ill-defined chemical shift regions.⁸⁸However, a test for

halide was negative, and analytical combustion data were

consistent with the molecular formula of the bis(imido) com-

plex, allowing us to identify the material as (BDI)Nb(N^tBu)₂py

(9, Scheme 8). The fluxional process occurring in solution was

found to have ΔG^‡= 13.2(2) kcal/mol, based on the separation

of the two ^tBu resonances at 223 K and a T_c= 298 K. Addition

of anexcess of pyridine-d₅toa solution of 9in THF-d₈at 223 K

(88) Humphries, M.; Green, M.; Leech, M.; Gibson, V.; Jolly, M.;

Williams, D.; Elsegood, M.; Clegg, W. J. Chem. Soc., Dalton Trans.

2000, 4044–4051.

2934 Organometallics, Vol. 29, No. 13, 2010

Tomson et al.

Scheme 8

caused rapid ligand exchange, as evidenced by the appearance

of one equivalent of free C₅H₅N.

In contrast to group 6 chemistry, in which a number of

bis(^tBu-imido) complexes are known, complex 9 represents

just the second isolated example of a neutral, monomeric

bis(alkylimido) group 5 complex⁸⁹and the third example of

an isolated Nb bis(imido) complex with any imido group

substitution.^75,90The reaction leading to this product likely

proceeds through the aforementioned amido chloride complex

(BDI)Nb(N^tBu)(NH^tBu)Cl, which can be either directly de-

hydrohalogenated by the second equivalent of LiNH^tBu, for-

ming 9, or amidated to form the bis(amido) complex (BDI)Nb-

(N^tBu)(NH^tBu)₂; an R-hydrogen abstraction reaction would

then yield the bis(imido) complex and H₂N^tBu. Notably,

attempts at forming the monoamido complex (BDI)Nb-

(N^tBu)(NH^tBu)Cl by reaction of 1.0 equiv of LiNH^tBu with

1 in pentane yielded a 1:1 mixture of 1 and 9. Related reactions

designed to generate imido complexes have been shown to

proceed through an R-H abstraction mechanism;⁹⁰the main

factor limiting the potential for this mechanism in the present

t

study is the steric requirement of placing two Bu-amido

groups on the (BDI)Nb(N^tBu) fragment, a situation reminis-

cent of our unsuccessful attempts at preparing dialkyl com-

plexes with substituted methide groups.

The proposed molecular structure of 9 was confirmed by

X-ray crystallography (Figure 8). The compound crystal-

lized in the orthorhombic space group Pbca, and the mole-

cular geometry is distorted square pyramidal (τ = 0.23).

Figure 8. Molecular structure of 9 as determined by a single-

crystal X-ray diffraction study. The hydrogen atoms were omit-

ted for clarity; the thermal ellipsoids were set at the 50% proba-

t

The two Bu imido groups, while in different chemical en-

vironments, have remarkably similar metric parameters

˚

bility level. Selected bond lengths are given in A. Selected bond

˚

(Nb(1)-N(1)_apical= 1.800(2) A; Nb(1)-N(2)_basal= 1.809(2)

angles (deg): N(1)-Nb(1)-N(2) 113.10(11), N(1)-Nb(1)-

N(3) 104.52(10), N(2)-Nb(1)-N(3) 96.08(10), N(1)-Nb(1)-

N(5) 98.71(10), N(2)-Nb(1)-N(5) 83.60(9), N(3)-Nb(1)-

N(5) 154.75(9), N(1)-Nb(1)-N(4) 104.67(9), N(2)-Nb(1)-N(4)

140.91(9), N(3)-Nb(1)-N(4) 83.55(8), N(5)-Nb(1)-N(4)

81.09(8), C(1)-N(1)-Nb(1) 165.5(2), C(5)-N(2)-Nb(1) 166.7(2).

˚

A; Nb(1)-N(1)_apical-C(1) = 165.5(2)°; Nb(1)-N(2)_basal

-

C(5) = 166.6(2)°). The NbdN^tBu bond distances in 9 are

shorter than those found for the only other neutral, structu-

rally characterized Nb bis(imido) complex, Nb(NAr)₂Cl-

(PMe₃)₂(Ar = 2,6-ⁱPr₂C₆H₃) (Nb-N = 1.819(5), 1.827(5)

75

˚

A), consistent with the effect of localizing the π-bonding

between the nitrogen atoms and the niobium center, but longer

than the NbdN^tBu distances reported for the monoimido

compounds 1, 6, and 7, due to the anticipated π-loading effect.

The NbdN^tBu bond distances in 9 are close to those found for

Scheme 9

Ta(NAr)₂Clpy₂(1.809(6) and 1.812(6) A)⁹⁰and Ta(NSi^tBu₃)₂-

˚

Mepy₂(1.810(13) and 1.819(13) A),³¹but longer than the ave-

˚

rage ModN^tBu bond distance of the 25 crystallographically

characterized compounds containing the Mo(N^tBu)₂fragment

BDI ligand, indicating that the averaged resonance form C

(Scheme 3) provides the best description of the electronic

structure within the BDI backbone of 9. This symmetry implies

that the BDI ligand is not participating in π-bonding with the

metal center and that the asymmetry in Nb-N_BDIbond lengths

is the result of a σ-based trans influence.

˚

found in the Cambridge Structural Database by 0.075 A (Mod

t

N Bu bond distances range from 1.671 to 1.772 A; d_av(Mod

˚

t

0

˚

N Bu)=1.732 A), in accord with the larger atomic radius of d

Nb versus d⁰Mo. The angles about both BDI nitrogens in 9sum

to >359°, indicating that they are participating in significant

π-bonding. In comparison with complex 1, however, the bond

lengths within the NCCCN ligand framework are identical

for the local mirror plane, reflecting the two halves of the

(BDI)Nb(N^tBu)₂(dmap). In an attempt to perturb the

π-bonding within the complex and ascertain any effect that

further π-bonding may have on the structure, we synthesized

the related dmap (4-(dimethylamino)pyridine) adduct (BDI)-

Nb(N^tBu)₂(dmap) (10, Scheme 9) by addition of dmap to a

solution of 9 in toluene, followed by pyridine removal via

distillation as the toluene azeotrope.

(89) Kilgore, U.; Karty, J.; Pink, M.; Gao, X.; Mindiola, D. Angew.

Chem., Int. Ed. 2009, 48, 2394–2397.

(90) Chao, Y. W.; Wexler, P. A.; Wigley, D. E.; Eh Inorg. Chem. 1990,

29, 4592–4594.

Article

Organometallics, Vol. 29, No. 13, 2010 2935

Table 1. Comparison of the Δδ_rβValues for Compounds 1-4 and

6-11

compound

Δδ_Rβ

(BDI)Nb(N^tBu)Cl₂py

1

2

3

4

39.9

36.5

44.0

40.1

(BDI)Nb(N^tBu)F₂py

(BDI)Nb(N^tBu)Cl₂

(BDI)Nb(N^tBu)F₂

(BDI)Nb(N^tBu)Me₂

(BDI)Nb(N^tBu)Cl(p-tol)

(BDI)Nb(N^tBu)Me(p-tol)

(BDI)Nb(N^tBu)₂py

(BDI)Nb(N^tBu)₂(dmap)

(BDI)Nb(N^tBu)₂

6

7

8

9

10

11

35.2^a, 35.0^b

39.0

35.8

30.2^c, 31.7^d

29.7^c, 30.9^d

31.3

^aΔδ_Rβin THF-d₈at 213 K. ^bΔδ_Rβin toluene-d₈at 213 K. ^cFor the

1

2

apical imido group, as determined by J_CHand J_CHNMR ¹H-¹³C

correlation experiments (see Experimental Section). ^dFor the basal

imido, determined as for c.

(δ = 0.32, 1.36 ppm), with one shifted upfield due to its proxi-

mity to the flanking aryl group. The methyl signals for the

p-NMe₂group were still unresolved at this temperature, as

indicated by a broad singlet at 2.88 ppm integrating to 6H.

Cooling the sample further revealed isolated singlets for the

p-NMe₂group at 193 K, indicating an activation energy for

methyl group interchange of less than 10.8 kcal/mol.

The ¹³C{¹H} NMR spectrum of 10 at 223 K reveals the ex-

pected set of four signals for the C_Rand C_βcarbons of the imido

groups; with these the Δδ_Rβvalues (Δδ_Rβ= δ(C_R) - δ(C_β))

associated with each imido nitrogen can be determined. The

Δδ_Rβvalue has been shown to qualitatively represent the degree

of bonding between tert-butylimido nitrogens and the metal

centers to which they are bound.^92aThe difference is representa-

tive of the degree of shielding of the imido nitrogen, where larger

Δδ_Rβvalues are indicative of electron-poor or electronegative

metal centers and smaller Δδ_Rβvalues are representative of

localization of significant electron density on the imido nitrogen.

The Δδ_Rβvalues for complexes 9 and 10 are generally lower in

magnitude than the related monoimido complexes (Table 1).

These low values for 9and 10 indicate that π-loading of the metal

center has decreased the degree of imido bonding to the metal,

creating more electron-rich imido nitrogens.^92b

Figure 9. Molecular structure of 10 as determined by a single-

crystal X-ray diffraction study. The hydrogen atoms were omit-

ted for clarity; the thermal ellipsoids were set at the 50% proba-

˚

bility level. Selected bond lengths are given in A. Selected bond

angles (deg): N(1)-Nb(1)-N(2) 112.80(13), N(1)-Nb(1)-N(3)

103.75(12), N(2)-Nb(1)-N(3) 95.85(12), N(1)-Nb(1)-N(5)

96.98(12), N(2)-Nb(1)-N(5) 83.94(11), N(3)-Nb(1)-N(5)

157.48(10), N(1)-Nb(1)-N(4) 105.38(11), N(2)-Nb(1)-N(4)

140.94(12), N(3)-Nb(1)-N(4) 82.52(10), N(5)-Nb(1)-N(4)

83.60(10), C(1)-N(1)-Nb(1) 165.3(3), C(5)-N(2)-Nb(1)

166.7(3).

Analysis of the Solution-State Behavior of the Bis(imido)

Complexes 9 and 10. Rate law data on the reaction to form 10

and on the mechanism of pyridine group exchange on 9 could

not be obtained by NMR measurements due to the rapid rate of

the reactions over the temperature ranges investigated (-80/þ

20 °C), but circumstantial evidence points to a dissociative

mechanism (Scheme 10) for these reactions, whereby pyridyl

ligand dissociation forms a four-coordinate intermediate that can

then associate a second pyridyl ligand. This hypothesis is sup-

ported by the following observations: (1) In a qualitative sense,

complexes 9 and 10 are electronically saturated such that ligand

dissociation would generate a 16 e^-complex. To maintain the

18-electron configuration of the starting material, addition of a

second pyridyl ligand would force either dissociation of one of the

Complex 10 crystallizes from Et₂O in the space group P2₁/n;

it was found to have a square-pyramidal geometry (τ = 0.28),

with one apical and one basal imido group (Figure 9), similar to

that observed for 9. The imido group bond lengths within 10

are equivalent within experimental error (Nb(1)-N(1)_apical

=

˚

1.797(3) A; Nb(1)-N(2)_basal=1.795(3) A), and the difference

between the two Nb-N-C bond angles for the imido groups

is insignificant considering the flat potential energy surface

for moderately bent alkyl imido groups⁹¹(Nb(1)-N(1)_apical

-

C(1) =165.3(3)°; Nb(1)-N(2)_basal-C(5) =166.7(3)°). Of the

bonds to the metal and within the NCCCN framework, the only

differences between 9 and 10 that are statistically significant

are the Nb(1)-N(2)_basal, Nb(1)-N(3), and C(22)-C(23) dis-

˚

tances, with the Nb(1)-N(3) of 10 being longer by 0.012 A

and the Nb(1)-N(2)_basaland C(22)-C(23) of 10 being shorter

by 0.014 and 0.016 A, respectively.

As seen for 9, the product was fluxional on the NMR time

scale at room temperature, but a spectrum of 10 taken at 223 K

revealed a C₁-symmetric complex analogous to that observed

for 9. Again, two imido group signals can be clearly distinguished

(92) (a) Nugent, W. A.; Mayer, J. M. Metal-Ligand Multiple Bonds;

John Wiley & Sons: New York, 1988. (b) While the the values of Δδ_Rβcan

provide useful qualitative data for comparing species with closely related

ligand environments, care should be taken when comparing systems with

significantly different electronic properties. For example, the Δδ_Rβfor the

fluoride complexes 2 and 4 are lower than those for the chloride complexes 1

and 3, even though fluorine is more electronegative than chlorine. This

discrepency possibly indicates better Nb d orbital overlap with the fluoride 2p

orbitals than with the 3p orbitals of the chlorides. Due to the complexity of

these interactions, we have refrained from providing further analysis of the

Δδ_Rβvalues beyond what is presented in the text.

˚

(91) Ciszewski, J.; Harrison, J.; Odom, A. Inorg. Chem. 2004, 43,

3605–3617.

2936 Organometallics, Vol. 29, No. 13, 2010

Tomson et al.

Scheme 10

Scheme 11

BDI nitrogens or localization of a pair of electrons on one or both

of the imido groups. (2) Addition of an excess of either dmap or

pyridine to solutions of 9 and 10 does not cause a change in the

NMR spectral resonances attributable to the metal complexes

when compared with those of the isolated complexes, even

though the added ligand rapidly exchanges with the coordinated

ligand. This implies either that the fluxional behavior is intramo-

lecular or that pyridyl group exchange is occurring via a dis-

sociative mechanism. (3) NMR spectra collected at elevated

temperatures cause the peaks to coalesce into a pattern indicative

of a C_2v-symmetric metal complex with free pyridine, strongly

suggesting that the fluxional behavior in solution is due to pyri-

dine dissociation, not intramolecular rearrangement. (4) Addi-

tion of a suitable Lewis acid results in pyridine abstraction

from 9 (see below).

It is interesting to compare complexes 9 and 10 with the rela-

ted group 6 N,N-(dipyrrolyl-R-methyl)-N-methylamine (dpma)

bis(imido) complexes studied by Odom and co-workers.⁹¹

The authors made the observation that the combination of a tri-

dentate, dianionic ligand with two monodentate substituents on

a transition metal often leads to complexes with static molecular

geometries in solution on the NMR time scale. The M(N^tBu)₂-

(dpma) (M = Cr, Mo, W) compounds they synthesized dis-

played this behavior, retaining nonequivalent imido groups

from -80 to 80 °C. This contrasts with the majority of transition

metal bis(imido) complexes that exhibit equivalent imido

groups in solution regardless of dissimilarities in the solid

state.^75,93-95The Odom compounds all have SP geometries

(τ=0.19 (W), 0.22 (Mo), and 0.28 (Cr)), with the apical posi-

tion occupied by an imido group, very nearly approximating the

geometry of (BDI)Nb(N^tBu)₂py, which has one apical imido

group, one basal imido group, and a set of basal nitrogen ligands

that form a combination of anionic and neutral donors. By

analogy with the M(N^tBu)₂(dpma) compounds and by con-

sidering that the low-temperature spectra of 9 and 10 clearly

resolve the two imido groups, it seems likely that the fluxional

processes observed in the NMR spectra are a result of pyridyl

ligand dissociation.

to a solution of 9 in benzene quickly and quantitatively

formed py B(C₆F₅)₃and (BDI)Nb(N^tBu)₂(11, Scheme 11)

as judged by NMR spectroscopy. The NMR spectra of the

transition metal product indicate that it has averaged C_2v

symmetry in solution, with the ¹H spectrum revealing a

3

i

single, sharp Pr methine septuplet at 3.42 ppm, two Pr

t

methyl doublets integrating to 12 H each, and a single Bu

resonance at 1.33 ppm integrating to 18 H. The ¹³C{¹H}

NMR spectrum supports the proposed C_2v-symmetric for-

mulation and reveals Δδ_Rβ= 31.3 ppm, a value that lies

between the Δδ_Rβvalues for the two nonequivalent imido

groups in 9 and at slightly lower magnitude than those for 10.

The self-consistency in these values between the structurally

characterized bis(imido) complexes 9 and 10 and solution-

state characterized bis(imido) 11 supports the formulation of

11 as a four-coordinate monomer instead of an imido-

bridged dimer, the latter of which would have lower Δδ_Rβ

values for amido-like bridging imido groups.⁸⁸

While 11 was stable enough to be spectroscopically charac-

terized in solution, the complex undergoes clean thermal de-

composition over several hours at room temperature to form

(BDI^#)Nb(N^tBu)(NH^tBu) (12, BDI^#=H₂CdC(NAr)CHdC-

(NAr)Me) viaaC-H activation process (Scheme 11). Complex

1

12 exhibits H NMR signals characteristic of the new ligand

motif, with two pseudosinglets at 3.29 and 3.74 ppm shifted

upfield due to their proximity to an aryl ring and attributable to

the newly formed terminal methylene group. The overall C₁

symmetry is evidenced by the spectroscopic observation of four

Synthesis, Characterization, and Reactions of a Four-Co-

ordinate Bis(imido) Complex. The lability of the pyridine

ligand was important because it indicated that a four-co-

ordinate bis(imido) complex may be accessible via utilization

of a Lewis acid capable of preferentially binding pyridine in

solutions of 9 or 10. Indeed, addition of 1.0 equiv of B(C₆F₅)₃

i

nonequivalent Pr groups, each having two diastereotopic

methyl groups. A broad singlet at 5.98 ppm integrates to 1H

and lies in the range expected for primary alkylamido ligands

on Nb.⁸⁸Similar transformations have been observed before

on related BDI systems;^34,42,96however, the nuclearity of this

reaction has not been determined for the present system due

to inconsistent reaction rates.

(93) Bradley, D. C.; Hodge, S. R.; Runnacles, J. D.; Hughes, M.;

Mason, J.; Richards, R. L. J. Chem. Soc., Dalton Trans. 1992, 1663–

1668.

(94) Barrie, P.; Coffey, T. A.; Forster, G. D.; Hogarth, G. J. Chem.

Soc., Dalton Trans. 1999, 4519–4528.

(96) (a) Ding, Y.; Hao, H.; Roesky, H. W.; Noltemeyer, M.; Schmidt,

H.-G. Organometallics 2001, 20, 4806. (b) Meltzer, A.; Inoue, S.; Praesang,

C.; Driess, M. J. Am. Chem. Soc. 2010, 132, 3038–3046, and references

therein..

(95) Strong, J. B.; Yap, G. P. A.; Ostrander, R.; Liable-Sands, L. M.;

Rheingold, A. L.; Thouvenot, R.; Gouzerh, P.; Maatta, E. A. J. Am.

Chem. Soc. 2000, 122, 639–649.

Article

Organometallics, Vol. 29, No. 13, 2010 2937

Scheme 12

The decomposition reaction observed for 11 limits inves-

tigation into its reactivity with organic substrates; still, one

reaction that proceeded cleanly before the C-H activation

event could occur was observed with ^tBuNCO. On addition of

2.0 equiv of this substrate to a solution of 11 in C₆D₆, a single

new product grew in cleanly over a period of 3 h, and 1.0 equiv

of ^tBuNCO was left unreacted. The product was identified by

NMR spectroscopy as the oxaazaniobacyclobutane (BDI)-

Nb(N^tBu)(κ²-O,N-OC(N^tBu)N^tBu) (13, Scheme 12), formed

by a [2 þ 2] reaction of the isocyanate with one of the imido

groups on the metal.

1

The H NMR spectrum displays averaged C_ssymmetry

t

with three distinct Bu resonances (1.28, 1.34, and 1.47 ppm),

and the ¹³C{¹H} NMR spectrum indicates three nonequivalent

^tBu-C_Rresonances at 52.0, 57.3, and 69.8 ppm. The upfield

shifted C_Rresonances are in regions similar to those found

for organic imines and metal amides. After 14 h at room tem-

t

perature, 13 decomposes into BuNCN^tBu (1.18 ppm, C₆D₆;

Figure 10. Geometry-optimized (BP86) coordinates of 11a. The

hydrogen atoms were omitted for clarity. Selected bond lengths

are given in A. Selected bond angles (deg): N(2)-Nb(1)-N(7)

Scheme 12) and unidentified metal species.

˚

Computational Studies on the Electronic Structure of 11. In

contrast to earlier investigations, which have found little varia-

tion in the bonding within the NCCCN metallacycle of the BDI

ligand, the complexes discussed above reveal that the π-system

of the BDI ligand can desymmetrize as needed to accommodate

the electronic requirements of the metal center. For C₁- and

C_s- (with the mirror plane containing the NCCCN ring) sym-

metric complexes, this variability of the bond lengths within the

ligand metallacycle is indicative of the degree of mixing occur-

ring between the high-lying BDI orbitals and the frontier

orbitals on the metal center. The asymmetry of the metallacycle

bond lengths within complex 1 illustrates how the difference in

π-bonding ability of the imido ligand compared to the chloride

and pyridine ligands leads to nonequivalent C-N and C-C

metallacycle bond lengths. The asymmetry further indicates that

the BDI π-system is close enough in energy to the NbdNR

π-bonds to affect and be affected by the metal-imido bonding.

Thus, to evaluate the possibility that the BDI ligand is causing

π-loading in the bis(imido) complex 11, we performed DFT

calculations (BP86, see Experimental Section for details) on a

truncated form of the metal complex. The geometry-optimized

structure of the model complex BDI⁰Nb(NMe)₂(11a, Figure10,

BDI⁰=HC[C(Me)N(2,6-Me₂-C₆H₃)]₂) reveals overall C_2vsym-

metry in accord with the solution-state symmetry observed by

NMR for complex 11. The geometry of 11a is best approxi-

mated as a tetrahedron, reminiscent of the bent-metallocene

complexes Cp₂MX₂, withanN_imido-Nb-N_imidobonding angle

of 110.8° and an N_BDI-Nb-N_BDIbonding angle of 93.8°. The

110.8, N(12)-Nb(1)-N(13) 93.8, Nb(1)-N(2)-C(3) 157.3,

Nb(1)-Nb(7)-C(8) 153.9, N(imido)-Nb(1)-N(BDI⁰) varies

between 112.7° and 112.9°.

and 10. The imido groups are bent about the nitrogens to ca.

155°, a common feature of alkyl imido groups on early transi-

tion metals, and bonding within the NCCCN ring of the BDI

ligand is symmetric with respect to the molecular mirror plane

containing the two imido groups and the metal center.

We next examined the frontier orbitals of 11a as com-

pared to those for the molecular fragments BDI⁰^-and Nb-

(NMe)₂^þ. The fragment analysis was performed using the

coordinates generated by the geometry optimization on 11a,

resulting in the expected sets of orbitals based on previously

published computational studies. Most important among

these for the present study are the b₁-symmetric frontier

orbitals of both the BDI⁰^-and Nb(NMe)₂^þfragments. We

were particularly interested in determining whether the high-

est occupied molecular orbital (HOMO) of the BDI⁰anion

would lie at an appropriate energy to effectively mix with the

b₁-symmetric HOMO of the Nb(NMe)₂^þfragment, and if it

does, whether the resulting antibonding combination would

have more BDI⁰or Nb(NMe)₂character.

The anticipated π-loading effect can be clearly discerned

from the respective contribution of the BDI⁰and imido

ligands to the HOMO-2 (116) and HOMO (118) orbitals

(Scheme 13). These two orbitals lie 15.3 kcal/mol apart in

energy and account for the respective bonding and antibond-

˚

NbdNMe bond lengths are both calculated to be 1.829 A,

-

ing combinations of the b₁-symmetric HOMOs of the BDI⁰

indicating weakening of the Nb-N_imidointeraction compared

to those in the monoimido complexes described above and in

accord with the trend observed for the bis(imido) complexes 9

and Nb(NMe₂)^þfragments. Orbital 116 is comprised of

9.5% metal character, of which 5.5% comes from the 4d

2938 Organometallics, Vol. 29, No. 13, 2010

Tomson et al.

Scheme 13

orbitals (5.4% d_xz) and 4.1% from the 5p set (3.9% p_x). The

imido nitrogens account for 19.6% of this molecular orbital, and

the p orbitals of the NCCCN atoms of the BDI⁰ligand metalla-

cycle contribute 52.2% to the total. By comparison, orbital 118,

the molecular HOMO, is composed of 5.4% metal character,

46.3% imido nitrogen p-character, and 32.1% NCCCN metal-

lacycle p-character, indicating that mixing of the BDI π-system

with the imido π-bonding orbitals leads to an imido-based

HOMO at higher energy than would be found in the absence

of the π-bonding component from the BDI⁰ligand.

Interestingly, the calculations also revealed that the

HOMO-1 orbital results from a similar π-loading effect.

The 5b₂σ-donor orbital of the BDI⁰lig_þand forms an antibond-

ing combination with the Nb(NMe)₂HOMO-1 fragment

orbital, itself derived from the bonding combination of the Nb-

d_yzorbital with the imido-nitrogen p_yorbitals. The correspond-

ing bonding component to the molecular HOMO-1 orbital is

found at HOMO-8. The relative contributions of the BDI⁰and

imido ligands to these two orbitals follow the trend described

above for the HOMO and HOMO-2 orbitals, in that the

bonding combination between the BDI⁰ligand and the metal

center is largely BDI⁰-based and the antibonding combination

has a greater imido component, indicating that the NbdNR

π-bond has been effectively raised in energy compared to a

theoretical imido-based molecular orbital that lacks significant

non-imido ligand character. A more complete breakdown of the

frontier orbitals of 11a, including the relative contribution of

each fragment to the molecular orbitals, can be found in the

Supporting Information.

precursor. We believe that the imido group is particularly

useful for forming this product, as it both reduces the

required coordination number about the metal and supports

the d⁰oxidation state through its strong donor capability. A

number of halide, alkyl, and aryl complexes were character-

ized, which serve both as starting points for future investiga-

tions and as bases of comparison for the electronic properties

of the metal center and the nature of BDI-metal bonding

interactions.

A rare example of a group 5 metal bis(imido) complex was

prepared by introducing a second imido group via salt

metathesis. The pentacoordinate complex 9 was found to

lose the coordinated pyridine ligand, likely through a dis-

sociative mechanism, allowing for pyridyl ligand abstraction

by Lewis acids to generate the four-coordinate bis(imido)

complex (BDI)Nb(N^tBu)₂in solution. If left unsaturated,

this species underwent a rearrangement to form a cyclo-

bis(anilide) niobium complex, in which a hydrogen had been

transferred to one of the imido nitrogens. This reactivity

supports the conjecture that forming heavily π-loaded sys-

tems creates metal centers with more highly polarized M-L_π

bonds. The imido group in this case activates a C-H bond,

albeit presumably via an intramolecular reaction. Cycload-

t

dition of BuNCO was found to be competitive with the

rearrangement process; the metallacycle could be observed

t

spectroscopically and was found to yield BuNCN^tBu on

thermal decomposition.

Finally, a computational investigation into the electronic

structure of the four-coordinate bis(imido) complex 11a

revealed two distinct interactions between the BDI⁰ligand

and the NbdNMe π-bonds near the molecular frontier

orbitals. The HOMO and HOMO-1 orbitals were found

to be largely imido-based and to result from antibonding

combinations with the BDI⁰ligand, thus illustrating the

Summary and Conclusions

The introduction of the BDI ligand onto niobium was

accomplished via salt metathesis with an imidoniobium

Article

Organometallics, Vol. 29, No. 13, 2010 2939

intended π-loading effect, which results from the proper

balance of orbital occupation, symmetry, and relative energy

between the imido- and supporting ligand-based orbitals.

and polarization effects; no correction for crystal decay was

applied. Data were analyzed for agreement and possible absorp-

tion using XPREP.¹⁰⁵An empirical absorption correction based

on comparison of redundant and equivalent reflections was

applied using SADABS.¹⁰⁶Structures were solved by direct

methods with the aid of successive difference Fourier maps

and were refined on F²using the SHELXTL 5.0 software

package. Thermal parameters for all non-hydrogen atoms were

refined anisotropically. ORTEP diagrams were created using

the ORTEP-3 software package.¹⁰⁷

Experimental Section

General Considerations. Unless otherwise noted, all reactions

were performed using standard Schlenk line techniques or in an

MBraun inert atmosphere box under an atmosphere of nitrogen

(<1 ppm O₂/H₂O). Glassware, cannulae, and Celite were stored

in an oven at ca. 425 K. Pentane, hexane, Et₂O, THF, toluene,

benzene, and DME were purified by passage through a column

of activated alumina and degassed prior to use.⁹⁷Pyridine and

^tBuNH₂were distilled from CaH₂. Deuterated solvents were

vacuum-transferred from sodium/benzophenone and degassed

with three freeze-pump-thaw cycles. NMR spectra were re-

corded on Bruker AV-300, AVQ-400, and DRX-500 spectro-

meters. ¹H and ¹³C{¹H} chemical shifts are given relative to

residual solvent peaks. Proton and carbon NMR assignments

were routinely confirmed by ¹H-¹H (COSY) or ¹H-¹³C (HSQC

and HMBC) experiments. Vapor diffusion osmometry was

performed according to published procedures, using sub-

limed ferrocene as a standard.⁹⁸Infrared (IR) samples were

prepared as Nujol mulls and were taken between KBr disks.

(BDI)Nb(N^tBu)Cl₂py (1). THF (50 mL) was added to a solid

mixture of Nb(N^tBu)Cl₃py₂(5.0 g, 11.7 mmol) and Li(BDI)

3

OEt₂(5.81 g, 11.7 mmol) in a 250 mL Schlenk flask at room

temperature. The mixture immediately took on an orange-red

color. The solution was stirred at room temperature for 12 h,

resulting in a deep red solution with a fine precipitate. The volatile

materials were removed under vacuum, and the residue was

extracted with 200 mL of hot hexane. The hexane extracts were

filtered through a small pad of Celite. The filtrate was reheated to

reflux, then filtered again and left to cool to room temperature.

The red crystalline product that formed was collected and dried

under vacuum. Yield: 5.8 g, 68%. Higher yields of material of

comparable purity may be obtained by performing a Soxhlet

extraction with pentane or by extracting the crude product with

benzene, filtering, evacuating to dryness, and washing thoroughly

with room-temperature hexane (50 mL). Yield: 6.8-7.7 g,

NbCl₅was purified by sublimation prior to use. Li(BDI)

3

OEt₂,⁶¹Nb(N^tBu)Cl₃py₂,²³Me₃SnF,⁹⁹B(C₆F₅)₃,¹⁰⁰[Nb(N^tBu)-

(H₂N^tBu)Cl₃]₂,¹⁰¹and (p-tol)₂Mg,¹⁰²were prepared using the

literature procedures. LiNH^tBu was prepared by adding 1.0

1

80-90%. H NMR (300 MHz, C₆D₆, 298 K): δ 8.57 (d, 2H,

py), 7.15 (m, 6H, Ar), 6.61 (t,1H, py), 6.28 (t, 2H, py), 5.34 (s, 1H,

HC(C(Me)NAr)₂), 3.77 (br s, 4H, CHMe₂), 1.72 (br s, 12 H,

HC(C(Me)NAr)₂, CHMe₂), 1.17 (br s, 18H, CHMe₂), 0.68 ppm

(s,9H, ^tBu). ¹HNMR (500MHz, C₇D₈, 243 K):δ8.45 (d, 2H, py),

7.19-7.10 (m, 6H, Ar), 6.56 (t, 1H, py), 6.19 (t, 2H, py), 5.27 (s,

1H, HC(C(Me)NAr)₂), 3.75 (sept, 2H, CHMe₂), 3.63 (sept, 2H,

CHMe₂), 1.79 (d, 6H, CHMe₂), 1.75 (s, 3H, HC(C(Me)NAr)₂),

1.62 (s, 3H, HC(C(Me)NAr)₂), 1.26 (d, 6H, CHMe₂), 1.05 (d, 6H,

CHMe₂), 0.96 (d, 6H, CHMe₂), 0.64 ppm (s, 9H, ^tBu). ¹³C{¹H}

NMR (125 MHz, C₇D₈, 243 K): δ 170.5 (HC(C(Me)NAr)₂),

158.6 (HC(C(Me)NAr)₂), 153.7 (Ar), 153.5 (Ar), 145.7 (Ar), 143.4

(Ar), 142.5 (Ar), 128.4 (Ar), 126.7 (Ar), 125.8 (Ar), 124.6 (Ar),

123.6 (Ar), 122.8 (Ar), 105.7 (HC(C(Me)NAr)₂), 68.8 (C_R, ^tBu),

28.9 (C_β, ^tBu), 28.5 (CHMe₂), 28.2 (CHMe₂), 28.0 (HC(C(Me)-

NAr)₂), 25.9 (CHMe₂), 25.8 (HC(C(Me)NAr)₂), 25.7 (CHMe₂),

25.1 (CHMe₂), 23.7 ppm (CHMe₂). Anal. Calcd for C₃₈H₅₅Cl₂-

N₄Nb: C, 62.38; H, 7.58; N, 7.66. Found: C, 62.57; H, 7.67;

N, 7.68.

n

equiv of BuLi to a solution of H₂N^tBu in pentane; the pre-

cipitate was collected by filtration, washed thoroughly with

pentane, and dried under vacuum. All other reagents were

acquired from commercial sources and used as received. Ele-

mental analyses were determined at the College of Chemistry,

University of California, Berkeley. The X-ray structural deter-

minations were performed either at CHEXRAY, University of

California, Berkeley (1, 5, 7, 9, and 10) using either Bruker

SMART 1000 or SMART APEX diffractometers or at the

Advanced Light Source, Beamline 11.3.1, Lawrence Berkeley

National Lab, Berkeley, CA (6). Full crystallographic details for

6 can be found elsewhere.⁸⁵

Representative Procedure for X-ray Crystallography for Com-

pounds 1, 5, 7, 9, and 10. A crystal of appropriate size was coated

in Paratone-N oil and mounted on a Kaptan loop. The loop was

transferred to a diffractometer equipped with a CCD area

detector,¹⁰³centered in the beam, and cooled by a nitrogen flow

low-temperature apparatus that had been previously calibrated

by a thermocouple placed at the same position as the crystal.

Preliminary orientation matrices and cell constants were deter-

mined by collection of 60 30-s frames, followed by spot integra-

tion and least-squares refinement. An arbitrary hemisphere of

data was collected, and the raw data were integrated using

SAINT.¹⁰⁴Cell dimensions reported were calculated from all

reflections with I > 10σ. The data were corrected for Lorentz

(BDI)Nb(N^tBu)F₂py (2). Toluene (15 mL) was added to a

solid mixture of 1 (300 mg, 0.41 mmol) and Me₃SnF (150 mg,

0.82 mmol) at room temperature. The solution was stirred

vigorously for 24 h, during which time the color of the solution

gradually changed from dark red to yellow-orange as the

insoluble Me₃SnF reacted. The solution was then filtered and

concentrated to ca. 2 mL, at which point a microcrystalline

material began to precipitate. The flask was stored at -40 °C for

48 h, causing the precipitation of a yellow-orange crystalline

material. The solid was collected by filtration and dried under

vacuum. Yield: 113 mg, 40%. ¹H NMR (500 MHz, C₇D₈, 298

K): δ 8.40 (d, 2H, py), 7.13 (s, 3H, Ar), 6.92 (m, 1H, Ar), 6.85

(d, 2H, Ar), 6.73 (m, 1H, py), 6.40 (m, 2H, py), 5.11 (s, 1H,

HC(C(Me)NAr)₂), 3.80 (sept, 2H, CHMe₂), 3.43 (sept, 2H,

CHMe₂), 1.74 (d, 6H, CHMe₂), 1.67 (s, 3H, HC(C(Me)NAr)₂),

1.66 (s, 3H, HC(C(Me)NAr)₂), 1.29 (d, 6H, CHMe₂), 1.08 (d,

6H, CHMe₂), 1.03 (d, 6H, CHMe₂), 0.41 (s, 9H, ^tBu). ¹³C{¹H}

NMR (125 MHz, C₇D₈, 298 K): δ 168.4 (HC(C(Me)NAr)₂),

159.0 (HC(C(Me)NAr)₂), 154.4 (Ar), 151.0, 146.6 (Ar), 143.3

(97) Alaimo, P. J.; Peters, D. W.; Arnold, J.; Bergman, R. G. J. Chem.

Educ. 2001, 78, 64–64.

(98) Burger, B. J.; Bercaw, J. E. In Experimental Organometallic

Chemistry; Wayda, A. L., Darensbourg, M. Y., Eds.; American Chemical

Society: Washington, DC, 1987; pp 79-98.

(99) Vela, J.; Smith, J. M.; Yu, Y.; Ketterer, N. A.; Flaschenriem,

C. J.; Lachicotte, R. J.; Holland, P. L. J. Am. Chem. Soc. 2005, 127,

7857–7870.

(100) Koppe, K.; Bilir, V.; Frohn, H.; Mercier, H. P. A.; Schrobilgen,

G. J. Inorg. Chem. 2007, 46, 9425–9437.

(101) Bates, P. A.; Nielson, A. J.; Waters, J. M. Polyhedron 1985, 4,

1391–1401.

(102) Pajerski, A. D.; Squiller, E. P.; Parvez, M.; Whittle, R. R.;

Richey, H. G. Organometallics 2005, 24, 809–814.

(103) SMART: Area-Detector Software Package; Bruker Analytical

X-ray Systems, Inc.: Madison, WI, 2001-2003.

(104) SAINT: SAX Area-Detector Integration Program, V6.40;

Bruker Analytical X-ray Systems Inc.: Madison, WI, 2003.

(105) XPREP; Bruker Analytical X-ray Systems Inc.: Madison, WI,

2003.

(106) SADABS: Bruker-Nonius Area Detector Scaling and Absorp-

tion v. 2.05; Bruker Analytical X-ray Systems, Inc.: Madison, WI, 2003.

(107) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565.

2940 Organometallics, Vol. 29, No. 13, 2010

Table 2. Crystallographic Data for Complexes 1, 5, 7, 9, and 10

Tomson et al.

1

5

7

9

10

formula

fw

space group

C

731.67

P2₁/c

8.865(1)

17.986(2)

24.392(2)

90

99.792(1)

90

3832.8(6)

4

1.268

1544

0.48

0.917772

16 890

6495

0.0291

5096

406

0.0367, 0.0872

0.0531

1.206

1.014, -0.399

675084

P

₃₈H₅₅Cl₂N₄Nb

C₄₁H₅₈Cl₂N₃Nb

756.71

Pbca

17.840(5)

21.065(6)

21.778(6)

90

8184(4)

8

1.228

3200

0.46

0.90033

37 957

8076

0.0398

6046

426

0.0356, 0.0860

0.0560

1.117

C₄₀H₅₇ClN₃Nb

708.25

P2₁2₁2₁

13.757(2)

16.524(3)

17.039(3)

90

C₄₂H₆₄N₅Nb

731.89

Pbca

19.649(1)

18.887(1)

22.364(1)

90

8299.5(9)

8

1.171

3136

0.32

0.887106

33 520

5593

0.0517

4337

433

0.0349, 0.0862

0.0521

1.382

C₄₄H₆₉N₆Nb

774.96

P2₁/n

15.530(3)

12.284(2)

23.724(4)

90

107.887(3)

90

4307.3(14)

4

1.195

1664

0.31

0.921823

17 130

6328

0.0457

4105

460

0.0406, 0.0910

0.0792

1.071

˚

a (A)

˚

b (A)

˚

c (A)

R (deg)

β (deg)

γ (deg)

90

3

˚

V (A )

Z

3873.3(11)

4

1.215

1504

0.41

0.870265

17 262

6488

0.0626

5497

407

0.0473, 0.1137

0.0617

0.969

1.261, -0.298

758023

F

F₀₀₀

_calcd(g/cm³)

μ (cm^-1

)

T_min/T_max

no. rflns measd

no. indep rflns

R_int

no. obsd (I>2.00σ(I))

no. variables

b

R₁,^awR₂

R₁(all data)

goodness-of-fit on F²

resid peak, hole (e /A )

-

3

˚

0.882, -0.287

758024

P

0.417, -0.308

675086

0.717, -0.393

758025

CCDC ref #

P

^aR₁= (|F_o| - |F_c|)/ (|F_o|). ^bwR₂= [ {w(F_o²- F_c²)²}/ {w(F_o²)²}]^1/2

.

(Ar), 143.1 (Ar), 126.4 (Ar), 124.8 (Ar), 123.8 (Ar), 123.6 (Ar),

(Ar), 141.2 (Ar), 127.3 (Ar), 125.5 (Ar), 123.7 (Ar), 105.1 (HC-

t

101.6 (HC(C(Me)NAr)₂), 66.1 (C_R, Bu), 29.6 (C_β, Bu), 28.7

(CHMe₂), 28.2 (CHMe₂), 26.6 (HC(C(Me)NAr)₂), 26.2 (HC-

(C(Me)NAr)₂), 26.1 (CHMe₂), 25.6 (CHMe₂), 25.2 (CHMe₂),

24.5 (CHMe₂). ¹⁹F NMR (376.5 MHz, C₇D₈, 298 K): δ 68.77 (s,

2F, NbF₂). Anal. Calcd for C₃₈H₅₅F₂N₄Nb: C, 65.32; H, 7.93;

N, 8.02. Found: C, 63.14; H, 7.88; N, 7.88.

(C(Me)NAr)₂), 75.3 (C_R, Bu), 31.3 (C_β, Bu), 30.3, 29.2, 28.1,

26.1, 24.7, 24.5, 24.5. Anal. Calcd for C₃₃H₅₀Cl₂N₃Nb: C, 60.74;

H, 7.72; N, 6.44. Found: C, 61.03; H, 7.86; N, 6.22.

(BDI)Nb(N^tBu)Cl₂(3) from 1. To a solution of 1 (200 mg, 0.27

mmol) in 10 mL of C₆H₆was added a solution of B(C₆F₅)₃

(140 mg, 0.27 mmol) in 5 mL of C₆H₆. The color quickly changed

from dark red to light red. The solution was stirred, and after

30 min, the volatile materials were removed under vacuum, leaving

a bright red, oily solid. Pentane (5 mL) was added, and the

suspension was agitated to dissolve any remaining benzene. The

volatile materials were again removed under vacuum, the resi-

due was extracted with pentane (2 ꢀ 10 mL), and the combined

extracts were filtered. Storing the filtrate at -35 °C produced a

mixture of bright red and white crystalline material. This

material was redissolved in a minimal amount of pentane,

filtered, and stored at -35 °C. This process was repeated several

times until the crystalline material was homogeneous in appear-

ance and analyzed as expected (see above). Yield: 29 mg, 16%.

(BDI)Nb(N^tBu)F₂(4) from 1. To a solution of 2 (30 mg, 0.43

mmol) in 0.3 mL of C₆D₆was added a solution of B(C₆F₅)₃

(22 mg, 0.043 mmol) in 0.3 mL of C₆D₆. The solution color

changed slightly from dark orange to a lighter orange. The product

Nb(N^tBu)Cl₃(dme). A solution of [Nb(N^tBu)(H₂N^tBu)Cl₃]₂

(1.33 g, 1.94 mmol) in 40 mL of DME was cooled to -72 °C, and

a solution of HCl in Et₂O (1.94 mL, 3.87 mmol) was added by

syringe. The flask was allowed to warm to room temperature. A

white precipitate formed while the solution was stirred for 12 h.

The resulting yellow solution was filtered, and the volatile

material was removed under vacuum. The resulting oily residue

was extracted with toluene, and the extracts were combined and

evacuated to dryness. The residue was triturated with Et₂O (3 ꢀ

10 mL) to give a yellow powder. Yield: 1.16 g, 83%. The spectral

data agree with those reported in the literature.¹⁰⁸

(BDI)Nb(N^tBu)Cl₂(3) from Nb(N^tBu)Cl₃(dme). A 100 mL

flask containing solid Nb(N^tBu)Cl₃(dme) (723 mg, 2.01 mmol)

and Li(BDI) Et₂O (1.0 g, 2.01 mmol) was cooled to -72 °C, and

3

20 mL of toluene was added slowly. The solution was allowed to

warm to room temperature, by which point the color turned

dark red. Stirring was continued overnight; then the volatile

materials were removed under vacuum. The residue was ex-

tracted with hot hexane (2 ꢀ 25 mL), and the extracts were

combined and concentrated. Crystallization of the residue from

hot hexane (þ65/-35 °C) yielded the product as red blocks. The

solid was collected by filtration and dried under vacuum. Yield:

740 mg, 57%. ¹H NMR (500 MHz, C₆D₆, 298 K): δ 7.1 (br s, 6H,

Ar), 5.25 (s, 1H, HC(C(Me)NAr)₂), 3.79 (br s, 2H, CHMe₂),

2.92 (br s, 2H, CHMe₂), 1.6 (br s, 6H, HC(C(Me)NAr)₂), 1.54 (s,

9H, ^tBu), 1.31 (br s, 16H, CHMe₂), 0.98 ppm (br s, 6H, CHMe₂).

¹H NMR (500 MHz, C₇D₈, 223 K): δ 7.2-6.9 (m, 6H, Ar), 5.14

(s, 1H, HC(C(Me)NAr)₂), 3.76 (sept, 2H, CHMe₂), 2.79 (sept,

2H, CHMe₂), 1.63 (d, 6H, CHMe₂), 1.42 (s, 6H, HC(C(Me)-

was analyzed by NMR spectroscopy without purification. The ¹⁹

F

NMR data match those for py B(C₆F₅)₃. H NMR (500 MHz,

1

3

C₇D₈, 303 K): δ 7.10 (m, 6H, Ar), 5.35 (s, 1H, HC(C(Me)NAr)₂),

3.35 (br s, 4H, CHMe₂), 1.61 (s, 6H, HC(C(Me)NAr)₂), 1.33 (d,

12H, CHMe₂), 1.17 (s, 9H, ^tBu), 1.13 (d, 12H, CHMe₂). ¹³C{¹H}

NMR (125 MHz, C₇D₇): δ 170.1 (HC(C(Me)NAr)₂), 146.7 (Ar),

142.7 (Ar), 127.8 (Ar), 125.0 (Ar), 105.6 (HC(C(Me)NAr), 72.2

(N^tBu, C_R), 32.1 (^tBu, C_β), 28.9 (CHMe₂), 25.8 (CHMe₂), 25.3

(HC(C(Me)NAr)₂), 25.1 (CHMe₂). ¹⁹F NMR (376.5 MHz, C₆D₆,

298 K): δ 83.96 (br s, Δν_1/2=460 Hz).

(BDI)Nb(NAr)Cl₂(5). A solution of Li(BDI) OEt₂(644 mg,

3

1.29 mmol) in THF (10 mL) was added to a solution of Nb-

(NAr)Cl₃(dme) (600 mg, 1.29 mmol) in THF (10 mL) at room

temperature. The flask was sealed, and the solution was stirred

at room temperature for 12 h, after which time the color had

turned deep purple. The volatile materials were removed under

vacuum, and the product was extracted with Et₂O (2 ꢀ15 mL).

The filtered solution was concentrated until a microcrystalline

precipitate began to form, at which point the solution was

t

NAr)₂), 1.31 (s, 9H, Bu), 1.27 (d, 6H, CHMe₂), 1.23 (d, 6H,

CHMe₂), 0.98 (d, 6H, CHMe₂). ¹³C{¹H} NMR (125 MHz,

C₇D₈, 223 K): δ 168.5 (HC(C(Me)NAr)₂), 151.9 (Ar), 145.8

(108) Korolev, A. V.; Rheingold, A. L.; Williams, D. S. Inorg. Chem.

1997, 36, 2647–2655.

Article

Organometallics, Vol. 29, No. 13, 2010 2941

filtered again and stored at -40 °C. A dark purple crystalline

material was collected and dried under vacuum. Yield: 554 mg,

57%. X-ray quality crystals were grown by storing a concen-

trated Et₂O solution at 0 °C for several days. H NMR (500

were filtered and concentrated to ca. 10 mL. The flask was

stored at -35 °C until a yellow-orange crystalline material

formed. The material was collected and dried under vacuum.

1

Yield: 345 mg, 59%. H NMR (500 MHz, THF-d₈, 298 K): δ

MHz, C₆D₆, 298 K): δ 7.13 (br m, 2H, BDI Ar), 7.06 (br m, 4H,

BDI Ar), 7.01 (br d, 2H, imido Ar), 6.91 (br t, 1H, imido Ar),

5.42 (s, 1H, HC(C(Me)NAr)₂), 4.55 (sept, 1H, imido CHMe₂),

4.01 (sept, 1H, imido CHMe₂) 3.34 (sept, 2H, BDI CHMe₂),

3.09 (sept, 2H, BDI CHMe₂), 1.65 (s, 6H, HC(C(Me)NAr)₂),

1.45 (d, 6H, BDI CHMe₂), 1.31 (d, 6H, imido CHMe₂), 1.21 (d,

12H, overlapping imido and BDI CHMe₂), 1.11 (d, 6H, BDI

CHMe₂), 1.07 ppm (d, 6H, imido CHMe₂). ¹³C{¹H} NMR (125

MHz, C₆D₆, 298 K): δ 168.28, 153.45, 147.93, 142.05, 141.63,

141.60, 128.68, 127.81, 127.73, 125.49, 124.49, 124.08, 122.64,

104.51, 30.45, 29.13, 28.81, 28.44, 26.94, 25.62, 25.26, 25.21,

25.17, 24.65. Anal. Calcd for C₄₁H₅₈Cl₂N₃Nb: C, 65.07; H, 7.73;

N, 5.55. Found: C, 65.15; H, 7.66; N, 5.38.

7.81 (br s, 2H), 7.14 (s, 2H), 7.10 (d, 1H), 7.06 (m, 2H), 6.97 (m,

3H), 6.03 (s, 1H, HC(C(Me)NAr)₂), 3.00 (sept, 1H, CHMe₂),

2.69 (m, 3H, CHMe₂), 2.28 (s, 3H, 4-Me-C₆H₄), 1.92 (s, 3H,

HC(C(Me)NAr)₂), 1.76 (s, 3H, HC(C(Me)NAr)₂), 1.38 (d, 3H,

CHMe₂), 1.21 (d, 3H, CHMe₂), 1.09 (dd, 6H, CHMe₂), 0.98 (dd,

t

6H, CHMe₂), 0.94 (s, 9H, Bu), 0.90 (d, 3H, CHMe₂), 0.41 (d,

3H, CHMe₂). ¹H NMR (500 MHz, C₆D₆, 278 K): δ 8.05 (br s,

2H), 7.15 (m, 2H), 7.07 (m, 2H), 7.00 (m, 4H), 5.43 (s, 1H,

HC(C(Me)NAr)₂), 3.05 (sept, 1H, CHMe₂), 2.89 (sept, 3H,

CHMe₂), 2.81 (m, 6H, CHMe₂), 2.05 (s, 3H, 4-Me-C₆H₄), 1.63

(s, 3H, HC(C(Me)NAr)₂), 1.45 (s, 3H, HC(C(Me)NAr)₂), 1.38

(d, 3H, CHMe₂), 1.37 (d, 3H, CHMe₂), 1.09 (m, 18H, ^tBu and 3

ꢀ CHMe₂), 1.02 (d, 3H, CHMe₂), 1.01 (d, 3H, CHMe₂), 0.77 (d,

3H, CHMe₂). ¹³C{¹H} NMR (125 MHz, C₆D₆, 298 K): δ 171.0,

162.8, 154.9, 149.4, 141.7, 141.5, 141.2, 140.5, 140.42, 127.3,

126.9, 126.5, 125.8, 124.4, 123.9, 123.7, 105.4, 69.4, 30.4, 30.2,

29.8, 28.8, 28.4, 27.1, 26.1, 26.0, 25.1, 25.1, 24.6, 24.6, 24.5, 24.5,

24.4, 22.3. Anal. Calcd for C₄₀H₅₇ClN₃Nb: C, 67.83; H, 8.11; N,

5.93. Found: C, 67.80; H, 8.27; N, 5.94.

(BDI)Nb(N^tBu)Me₂(6). An alternative procedure for synthe-

sizing 6 has been reported previously.⁸⁵The following presents a

preferred synthetic method, along with more extensive NMR

data than had previously been reported. To a slurry of 1 (2.13 g,

2.91 mmol) in cold Et₂O (50 mL, -72 °C) was added MeMgBr

(1.94 mL, 5.82 mmol) by syringe. The solution immediately took

on an orange color. Upon allowing the solution to warm to

room temperature, the solution rapidly became bright yellow.

After 30 min at room temperature, the volatile materials were

removed in vacuo, and the residue was thoroughly extracted with

pentane. The filtrate was concentrated and stored at -40 °C

until crystalline material formed. The product was collected

from two crops as bright yellow blocks. Yield: 1.41 g, 79%. The

product is thermally unstable at room temperature both in

solution and in the solid state, decomposing into a brown,

unidentified product over several hours in solution and several

days in the solid state, but it may be stored at -40 °C for months

without significant decomposition. ¹H NMR (300 MHz, THF-d₈,

293 K): δ 7.16 (s, 6H, Ar), 5.57 (s, 1H, HC(C(Me)NAr)₂),

2.99 (br s, 4H, CHMe₂), 1.86 (br s, 6H, HC(C(Me)NAr)₂),

1.25 (br s, 18H, CHMe₂), 1.16 (d, 6H, CHMe₂), 0.91 (s, 9H,

^tBu), 0.44 (s, 6H, NbMe₂). ¹H NMR (500 MHz, C₆D₆, 293 K): δ

7.14 (br s, 6H, Ar), 5.26 (s, 1H, HC(C(Me)NAr)₂), 3.19 (br s, 4H,

CHMe₂), 1.67 (br s, 6H), 1.36 (br s, 12H), 1.16 (br s, 12H), 1.10

(s, 9H, ^tBu), 0.78 (s, 6H, NbMe₂). ¹H NMR (500 MHz, THF-d₈,

233 K): δ 7.21 (m, 6H, Ar), 5.63 (s, 1H, HC(C(Me)NAr)₂), 3.15

(sept, 2H, CHMe₂), 2.73 (sept, 2H, CHMe₂), 2.00 (s, 3H, HC-

(C(Me)NAr)₂), 1.73 (s, 3H, HC(C(Me)NAr)₂), 1.41 (d, 6H,

CHMe₂), 1.19 (d, 6H, CHMe₂), 1.13 (d, 6H, CHMe₂), 1.08 (d,

(BDI)Nb(N^tBu)Me(p-tol) (8). A solution of 7 (319 mg, 0.45

mmol) in 20 mL of Et₂O was cooled to -72 °C, and a solution of

MeMgBr in Et₂O (0.15 mL, 0.45 mmol) was added dropwise by

syringe. The solution became bright yellow as the flask was

allowed to warm to room temperature. After stirring at room

temperature for 30 min, the volatile materials were removed under

vacuum. The residue was extracted with pentane (3 ꢀ 15 mL) and

filtered from a fine white precipitate until the solution was entirely

clear. The filtrate was concentrated under vacuum until a yellow

crystalline material began to precipitate. The solution was then

warmedgently toredissolvetheprecipitatedmaterialandstoredat

-40 °C, leading to the formation of yellow blocks, which were

collected by filtration and dried under vacuum. Yield: 133 mg,

43%. ¹H NMR (500 MHz, C₆D₆, 298 K): δ 7.28 (br s), 7.17 (s),

7.09 (m), 7.03 (m), 5.56 (s, 1H, HC(C(Me)NAr)₂), 2.91 (m, 4H,

CHMe₂), 2.16 (s, 3H, 4-MeC₆H₄), 1.82 (s, 3H, HC(C(Me)NAr)₂),

1.59 (s, 3H, HC(C(Me)NAr)₂), 1.35 (d, 3H, CHMe₂), 1.28 (d, 3H,

CHMe₂), 1.16 (d, 3H, CHMe₂), 1.14 (d, 3H, CHMe₂), 1.13 (s, 3H,

t

NbMe), 1.09 (s, 9H, Bu), 1.09 (d, 3H, CHMe₂), 1.04 (d, 3H,

CHMe₂), 0.95 (d, 3H, CHMe₂), 0.67 (d, 3H, CHMe₂). ¹³C{¹H}

NMR (125 MHz, C₆D₆, 298 K): δ 169.8 (HC(C(Me)NAr)₂),

164.4 (HC(C(Me)NAr)₂), 154.0, 149.6, 142.9, 142.2, 141.2, 140.7,

137.6, 128.7, 127.7, 126.5, 125.6, 124.9, 124.7, 123.7, 102.5

(HC(C(Me)NAr)₂), 66.8 (C_R, ^tBu), 54.8 (NbMe), 31.0 (C_β, ^tBu),

30.1, 29.0, 28.7, 28.1, 27.5, 26.1, 25.7, 25.5, 25.5, 24.9, 24.8, 24.6,

24.6, 23.6, 23.1, 22.1. Anal. Calcd for C₄₁H₆₀N₃Nb: C, 71.59; H,

8.79; N, 6.11. Found: C, 71.78; H, 8.59; N, 5.83.

t

6H, CHMe₂), 0.88 (s, 9H, Bu), 0.41 (s, 6H, NbMe₂). ¹³C{¹H}

NMR (125 MHz, THF-d₈, 213 K): δ 170.0 (HC(C(Me)NAr)₂),

164.8 (HC(C(Me)NAr)₂), 153.6, 149.5, 126.8, 126.5, 125.1,

124.9, 101.9 (HC(C(Me)NAr)₂), 65.8 (C_R, ^tBu), 47.5 (NbMe₂),

t

1

(BDI)Nb(N^tBu)₂py (9). To a solid mixture of 1 (700 mg, 0.96

mmol) and LiNH^tBu (151 mg, 1.91 mmol) was added pentane

(50 mL) at room temperature. The slurry was stirred vigorously

for 12 h, after which time the solution had turned bright yellow

and a white precipitate was present. The volatile materials were

removed in vacuo, the product was extracted with Et₂O(3 ꢀ 20 mL),

and the combined extracts were filtered repeatedly until a

clear solution was obtained. The yellow solution was then

concentrated until crystalline material began to form. The flask

was stored at -35 °C for 48 h. The yellow crystalline product

was collected and dried in vacuo. Yield from two crops: 371 mg,

53%. ¹H NMR (500 MHz, C₆D₆, 298 K): δ 8.33 (br s, 2H, py),

7.15-6.90 (br m, 6H, Ar), 6.66 (br s, 1H, py), 6.35 (br s, 2H, py),

5.18 (br s, 1H, HC(C(Me)NAr)₂), 3.68 (br s, 4H, CHMe₂), 1.65

30.6 (C_β, Bu), 29.7, 29.0, 27.5, 26.2, 25.4, 25.2, 24.9, 24.6. H

NMR (500 MHz, C₇D₈, 213 K): δ 7.18-6.99 (m, 6H, Ar), 5.15

(s, 1H, HC(C(Me)NAr)₂), 3.29 (sept, 2H, CHMe₂), 2.82 (sept,

2H, CHMe₂), 1.77 (s, 3H, HC(C(Me)NAr)₂), 1.51 (d, 6H,

CHMe₂), 1.47 (s, 3H, HC(C(Me)NAr)₂), 1.21 (t, 12H, CHMe₂),

1.09 (m, 15H, ^tBu and CHMe₂), 0.88 (s, 6H, NbMe₂). ¹³C{¹H}

NMR (125 MHz, C₇D₈, 213 K): δ 168.6 (HC(C(Me)NAr)₂),

163.5 (HC(C(Me)NAr)₂), 152.6, 148.6, 141.4, 139.8, 125.9,

t

124.3, 124.0, 100.9 (HC(C(Me)NAr)₂), 65.2 (C_R, Bu), 47.6

(NbMe₂), 30.2 (C_β, ^tBu), 28.8, 28.1, 26.9, 25.4, 24.9, 24.8, 24.1,

24.0 (one aryl resonance could not be found). Anal. Calcd for

C₃₅H₅₆N₃Nb: C, 68.72; H, 9.23; N, 6.87. Found: C, 68.47; H,

9.52; N, 6.98.

(BDI)Nb(N^tBu)Cl(p-tol) (7).To a stirred suspension of 1(600mg,

0.82 mmol) in 20 mL of Et₂O, cooled to -72 °C, was added a

slurry of (p-tol)₂Mg (169 mg, 0.82 mmol) in 10 mL of Et₂O. The

color quickly turned yellow as the flask warmed to room

temperature. After stirring the mixture for 30 min at room

temperature, the volatile materials were removed in vacuo. The

residue was extracted with pentane (2 ꢀ 20 mL), and the extracts

t

(s, 6H, HC(C(Me)NAr)₂), 1.35 (br s, 30 H, Bu and CHMe₂),

1.25 (d, 12H, CHMe₂). ¹H NMR (500 MHz, THF-d₈, 238 K): δ

8.86 (br s, 1H), 7.54 (t, 2H), 7.31 (br s, 1H), 7.15-7.00 (m, 4H,

Ar), 6.91 (t, 1H), 6.73 (br s, 1H), 6.65 (d, 1H), 5.19 (s, 1H,

HC(C(Me)NAr)₂), 4.04 (sept, 1H, CHMe₂), 3.84 (sept, 1H,

CHMe₂), 3.50 (sept, 1H, CHMe₂), 3.35 (sept, 1H, CHMe₂),

2942 Organometallics, Vol. 29, No. 13, 2010

Tomson et al.

1.68 (s, 3H, HC(C(Me)NAr)₂), 1.65 (s, 3H, HC(C(Me)NAr)₂),

1.50 (d, 3H, CHMe₂), 1.44 (d, 3H, CHMe₂), 1.42 (d, 3H,

CHMe₂), 1.39 (s, 9H, Bu), 1.23 (d, 3H, CHMe₂), 1.19 (d, 6H,

(CHMe₂), 25.66 (CHMe₂), 24.74 (HC(C(Me)NAr)₂), 24.28

(CHMe₂).

(BDI^#)Nb(N^tBu)(NH^tBu) (12). To a stirred solution of 9 (232

mg, 0.32 mmol) in C₆H₆(25 mL) was added a solution of

B(C₆F₅)₃(162 mg, 0.32 mmol) in C₆H₆(5 mL) at room tem-

perature. The solution was stirred for 12 h at room temperature;

then the solvent was removed in vacuo. The residue was tritu-

rated with pentane (2 ꢀ15 mL) and then extracted into pentane

(10 mL), leaving a white solid. Removal of the volatile materials

from the yellow filtrate gave the product as a yellow pow-

t

2 ꢀ CHMe₂), 1.08 (d, 3H, CHMe₂), 0.67 (d, 3H, CHMe₂), 0.28

t

(s, 9H, Bu). ¹³C NMR (125 MHz, THF-d₈, 223 K): δ 167.51,

166.38, 154.57, 154.46, 151.00, 150.29, 143.71, 142.09, 142.05,

141.64, 138.04, 126.06, 125.48, 124.85, 124.58, 124.47, 124.44,

124.42, 123.05, 64.50, 63.93, 34.35, 32.23, 29.49, 29.25, 28.09,

28.03, 27.97, 26.73, 25.98, 25.86, 25.01, 24.92, 24.55. Anal. Calcd

for C₄₂H₆₄N₅Nb: C, 68.92; H, 8.81; N, 9.57. Found: C, 68.82; H,

8.81; N, 9.75.

1

der. Yield: 182 mg, 87%. H NMR (400 MHz, C₆D₆): δ 7.27

(BDI)Nb(N^tBu)₂(dmap) (10). Solid dmap (17 mg, 0.14 mmol)

was added to a solution of 9 (100 mg, 0.14 mmol) in toluene

(5 mL) at room temperature. The solution was stirred at room

temperature until all the solids had dissolved (ca. 5 min); then

the volatile materials were removed under vacuum. The solid

was redissolved in toluene and evacuated to dryness two more

times to remove the pyridine. The resulting yellow solid was

dissolved in Et₂O, filtered, and concentrated until a microcrys-

talline material began to form. The solution was then stored at

-35 °C to yield the product as yellow blocks. Yield: 43 mg, 40%.

¹H NMR (500 MHz, THF-d₈, 223 K): δ 8.35 (br d, 1H, Ar-dmap),

7.09 (t, 2H, Ar), 7.05-6.98 (m, 3H, Ar and Ar-dmap),

6.93 (t, 1H, Ar), 6.40 (d, 1H, Ar), 6.48 (br d, 1H, Ar-dmap), 5.89

(br d, 1H, Ar-dmap), 5.13 (s, 1H, HC(C(Me)NAr)₂), 4.14 (sept,

1H, CHMe₂), 3.87 (sept, 1H, CHMe₂), 3.49 (sept, 1H, CHMe₂),

3.32 (sept, 1H, CHMe₂), 2.88 (br d, 6H, p-NMe₂), 1.64 (s, 3H,

HC(C(Me)NAr)₂), 1.62 (s, 3H, HC(C(Me)NAr)₂), 1.51 (d, 3H,

CHMe₂), 1.41 (d, 6H, 2 ꢀ CHMe₂), 1.36 (s, 9H, ^tBu), 1.22 (d,

3H, CHMe₂), 1.19 (d, 3H, CHMe₂), 1.18 (d, 3H, CHMe₂), 1.08

(m, 1H, Ar), 7.22 (m, 2H, Ar), 7.10 (m, 3H, Ar), 5.98 (br s, 1H,

Nb(NH^tBu), 5.28 (s, 1H, HC(C(Me)NAr)₂), 3.85 (sept, 1H,

CHMe₂), 3.74 (m, 2H, CH₂and CHMe₂), 5.54 (sept, 1H,

CHMe₂), 3.35 (sept, 1H, CHMe₂), 3.29 (s, 1H, CH₂), 1.54 (d,

3H, CHMe₂), 1.50 (s, 3H, HC(C(Me)NAr)₂), 1.49 (d, 3H,

CHMe₂), 1.43 (d, 3H, CHMe₂), 1.43 (d, 3H, CHMe₂), 1.41 (d,

3H, CHMe₂), 1.38 (d, 3H, CHMe₂), 1.38 (s, 9H, Bu), 1.24 (d,

3H, CHMe₂), 1.19 (d, 3H, CHMe₂), 0.89 (s, 9H, ^tBu). Satisfac-

tory elemental analysis data could not be obtain due to con-

t

tamination of the product with residual py B(C₆F₅)₃.

3

(BDI)Nb(N^tBu)(K²-O,N-OC(N^tBu)N^tBu) (13). To a solution

of 9 (20 mg, 0.029 mmol) in 0.5 mL of C₆D₆was added a solution

of B(C₆F₅)₃(14 mg, 0.029 mmol) in 0.5 mL of C₆D₆. After 5 min,

^tBuNCO (6.2 μL, 0.055 mmol) was added by syringe. The

solution was transferred to an NMR tube, and the tube was

sealed under vacuum. The solution was allowed to stand at

room temperature for 3 h, then analyzed by NMR spectroscopy

without purification due to the thermal instability of the pro-

duct. ¹H NMR (500 MHz, C₆D₆): δ 7.20-7.05 (m, 6H, Ar), 5.36

(s, 1H, HC(C(Me)NAr)₂), 3.57 (sept, 2H, CHMe₂), 2.81 (sept,

2H, CHMe₂), 1.70 (s, 6H, HC(C(Me)NAr)₂), 1.47 (s, 9H, ^tBu),

t

(d, 3H, CHMe₂), 0.77 (d, 3H, CHMe₂), 0.32 (s, 9H, Bu). ¹³C

NMR (125 MHz, THF-d₈, 223K): δ 167.1, 166.0, 155.1, 154.8,

154.2, 150.8, 144.1, 142.0, 141.8, 141.7, 138.5, 129.9, 129.2, 126.3,

125.6, 125.2, 124.8, 124.6, 124.3, 124.1, 104.4, 99.4, 64.1, 63.6,

39.1, 34.4, 32.7, 29.4, 29.2, 28.2, 28.0, 27.9, 26.8, 26.0, 25.0, 24.9.

Anal. Calcd for C₄₄H₆₉N₆Nb: C, 68.19; H, 8.97; N, 10.84. Found:

C, 68.14; H, 9.17; N, 10.66.

t

1.38 (d, 6H, CHMe₂), 1.34 (s, 9H, Bu), 1.33 (d, 6H, CHMe₂),

t

1.28 (s, 9H, Bu), 1.20 (d, 6H, CHMe₂), 1.05 (d, 6H, CHMe₂).

¹³C NMR (125 MHz, C₆D₆): δ 170.92 (HC(C(Me)NAr)₂),

156.83 (OCN), 149.50 (C₆F₅), 147.60 (C₆F₅), 146.69 (Ar), 144.41

(Ar), 143.80 (Ar), 143.02 (Ar), 142.04 (Ar), 139.00 (C₆F₅),

137.05 (C₆F₅), 127.57 (Ar), 125.30 (Ar), 124.65 (Ar), 104.71

(HC(C(Me)NAr), 69.82 (NbdN^tBu, C_R), 57.34 (cycle, ^tBu,

(BDI)Nb(N^tBu)₂(11). To a stirred solution of 9 (232 mg, 0.32

mmol) in C₆H₆(25 mL) was added a solution of B(C₆F₅)₃(162 mg,

0.32 mmol) in C₆H₆(5 mL) at room temperature. The resul-

ting solution was stirred for 10 min; then the solvent was

removed in vacuo, leaving a yellow residue. The residue was

triturated with pentane (2 ꢀ 10 mL) until a solid was obtained;

this was extracted with pentane, and the combined extracts were

filtered from a white solid to give a clear yellow filtrate, which

was evaporated in vacuo, leaving a yellow solid. Yield: 178 mg,

85%. NMR analysis of the isolated material revealed residual

t

C_R), 52.03 (cycle, Bu, C_R), 33.14 (^tBu, C_β), 32.59 (^tBu, C_β),

31.88 (^tBu, C_β), 29.66 (CHMe₂), 28.06 (CHMe₂), 26.06 (CH-

Me₂), 25.85 (CHMe₂), 25.71 (CHMe₂), 25.56 (CHMe₂),

24.75 (HC(C(Me)NAr)₂).

Acknowledgment. We thank the American Chemical

Society Petroleum Research Fund (ACS-47249AC3), the

National Science Foundation (CHE-0416309), and the

National Institutes of Health (R01-GM025459-29) for

funding.

py B(C₆F₅)₃along with varying proportions of 12. This con-

3

tamination along with the thermal instability of the product

prevented full characterization of 11. ¹H NMR (400 MHz,

C₇D₈, 298 K): δ 7.06 (m, 6H, Ar), 5.35 (s, 1H, HC(C(Me)-

NAr)₂), 3.83 (sept, 4H, CHMe₂), 1.79 (s, 6H, HC(C(Me)NAr)₂),

1.38 (d, 12H, CHMe₂), 1.32 (s, 18H, ^tBu), 1.17 (d, 12H, CHMe₂).

¹³C{¹H} NMR (100 MHz, C₇D₈): δ 170.29 (HC(C(Me)NAr)₂),

144.94 (Ar), 141.77 (Ar), 126.78 (Ar), 124.16 (Ar), 100.13

(HC(C(Me)NAr), 64.65 (NbdN^tBu, C_R), 34.34 (^tBu, C_β), 28.04

Supporting Information Available: Complete crystallographic

data for compounds 1, 5, 6, 7, 9, and 10 (CIF) and complete

information on the DFT calculations performed on compound

11a. This information is available free of charge via the Internet

at http://pubs.acs.org.

Article Doi

Halo, alkyl, aryl, and bis(imido) complexes of niobium supported by the β-diketiminato ligand

DOI: 10.1021/om1001827

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Authors:

Article abstract of DOI:10.1021/om1001827

Full text of DOI:10.1021/om1001827

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