CHM 331S – Assignment – Organometallic Developments of
Niobium
John Brownie
961687800
April 12/99
Prof. M. Denk
CHM 331S – Assignment – Organometallic Developments of Niobium
Introduction:
Niobium is a member of the Group 5 elements. It was first isolated in 1866 by C.W.
Blomstrand.1 Niobium is obtained primarily from a high-grade oxide ore in Brazil.1 The
comparative rarity of niobium ensures that starting materials will be of considerable expense.
NbCl5, the primary starting material for niobium chemistry, sells for approximately $5/gram.11
As stated in Chemistry of the Elements, “The organometallic chemistry of this group
developed rather slowly…”.1 However, a surge in interest in niobium and tantalum has occurred
recently because of their ability to catalyze several processes including polymerization and
hydrogenation.5,6 The primary areas of interest have been cyclopentadiene (Cp) derivatives.
This report will focus upon new Cp derivatives of niobium and other model compounds. Several
new compounds will be presented with supporting spectroscopy data. In addition, a brief review
of journal articles not covered in this report will be presented.
The developments in the organometallic chemistry of niobium of the last decade are too
great to cover in this report. Therefore, an emphasis will be placed upon the development of new
compounds, presented within the last 4 years, that have demonstrated catalytic ability. Several
computational studies will also be presented and examined. Other articles will be reviewed to
provide a quick reference guide for other compounds and articles of interest related to this
chemistry.
New Structures:
Reference [2]:
The first four compounds are from the same reference. The first compound serves as a
synthetic precursor for two compounds. This paper presents a large number of compounds of Ta
and Zr in addition to those presented in this report. All compounds presented in this paper use
carborane ligands. These ligands are used because they are isoelectronic to cyclopentadiene.
The article states, “Motivated for the potential for development for new, stable catalytic systems
involving early transition elements, we have initiated a study and characterization of a family of
Nb, Ta and Zr complexes of C2B3 and C2B4…”.2 These compounds can also be used as new
synthetic tools. Each compound presented in this article is well documented with spectroscopic
data. All compounds presented from this article [2] are air stable. All data for the next four
compounds is taken from reference [2].
(Et2C2B4H4)NbCl2Cp:
R
R
R = H, Me
R` = Me, Et, SiMe3
R
R
Cl2Nb
R`
R
H
B
R`
BH
BH
B
Synthesis:
Reaction carried out under an inert atmosphere. To a 250 mL flask NaH (1.00g, 42.0
mmol) and 125 mL of THF were added. At room temperature, Et2C2B4H6 (2.75g, 20.6 mmol)
was added dropwise over a period of 45 minutes to avoid excessive H2 evolution. The mixture
was stirred for one hour after addition was completed. The solution of NaEt2C2B4H5 was
allowed to settle and the supernatant was transferred by candula into a 250 mL flask containing a
suspension of CpNbCl4 (1.00 g, 3.36 mmol) in 50 mL of THF (a red-brown solution results).
This solution was then stirred for four hours, followed by evaporation in vacuo to another flask
to collect the neutral Et2C2B4H6. The residue (dark red) left in the reaction flask was filtered
twice through Celite with CH2Cl2 to yield a bright crystalline solid (0.878g, 2.45 mmol, 73%).
Note: Cp=C5H5 and Cp=C5Me5
Spectroscopic Data Provided:
1H
NMR (, CDCl3): 6.51 (C5H5, s, 5H), 3.12 (CH2, dq, 2H), 2.59 (CH2, dq, 2H), 1.25 (CH3, t,
6H).
13C NMR (, CDCl ): 130.9 (C B , br), 113.6 (Cp), 23.6 (CH ), 14.2 (CH ).
3
2 4
2
3
11B NMR (, CH Cl ): 31.7 (1B, d, J = 146 Hz), 18.0 (2B, d, J=158 Hz), -1.2 (1B, d, J=165 Hz).
2 2
FTIR (cm-1): 3215 m, 3124 m, 3115 m, 3088 w, 2968 m, 2935 w, 2877 w, 2575 s, 2561 s, 2538
m, 2361 w, 2343 w.
UV-vis (nm, in CH2CL2): 370 (20%), 250 (100%).
MS: m/z 358 (parent ion envelope), 322 (-Cl).
Elemental Analysis: C 37.06, H 5.61 (Expected, C 36.87, H 5.34).
*No crystal structure.
Hazards:
In this synthesis, several compounds that are used are toxic. These include THF, which is
highly carcinogenic and CH2Cl2, which is also carcinogenic. Data on the other compounds used
could not be obtained (they are unavailable commercially).
[Cp*Co(Et2C2B3H3)]NbCl2Cp:
R
R
R
R
Cl2Nb
Et
R=H
R
H
B
Et
B
H
Co
BH
Synthesis:
(Et2C2B3H5)CoCp* (0.500 g, 1.57 mmol) was dissolved in 50 mL THF. To this solution,
an equimolar amount of tBuLi (0.87 mL, 1.8M in hexane, 1.57 mmol) was added at room
temperature resulting in a red-orange solution. After 30 min the solution was added to CpNbCl4
(0.238g, 0.785 mmol) in 25 mL THF. The resulting solution was stirred for 4 h and the solvent
was removed by rotovap and the residue was purified by column chromatographed (silica gel) in
4:1 petroleum ether – CH2Cl2 to afford two bands, the first was (Et2C2B3H5)CoCp*. The second
band was dark green (desired product) (104 mg, 0.30 mmol, 38% based upon recovered starting
material).
Spectroscopic Data:
1H NMR (, CDCl3): 6.12 (Cp, s, 5H), 2.98 (CH2, dq, 2H), 2.61 (CH2, dq, 2H), 1.71 (Cp*, s,
15H, 1.32 (CH3, t, 6H).
13C NMR (, CDCl3): 112.2 (Cp), 110.5 (C2B3, br), 92.1 (C*5Me5), 23.4 (CH2), 14.2 (CH3), 9.4
(C*5Me5).
11B NMR (, CH2Cl2): 63.0 (1B, d, J=128 Hz), 31.7 (2B, d, J=122 Hz).
FTIR (cm-1): 3217 m, 3115 m, 3012 m, 2978 m, 2923 w, 2504 s, 2351 m, 2333m.
MS: m/z 539 (parent ion envelope), 504 (- Cl).
Elemental Analysis: C 47.00, H 6.23. Expected C 46.65, H 6.15.
*No crystal structure.
Hazards:
THF and CH2Cl2 are carcinogenic. tBuLi is pyrophoric and cannot be exposed to air. In
addition, tBuLi often reacts violently. Therefore care must be taken when using this substance.
(Et2C2B4H4)NbMe2Cp:
R
R
R
R
L
L = Me
L
H
B
Et
Et
R
Nb
BH
BH
B
H
Synthesis:
A 1.4 M solution of methylmagnesium bromide (4.9 mL, 6.9 mmol, 10 eq.) in ether was
added dropwise to a solution of (Et2C2B4H4)NbCl2Cp (250 mg, 0.69 mmol) in 25 mL of THF at
0C. The mixture was stirred for 30 min at this temperature and then for 1 h at room
temperature. Solvent was removed to yield yellow crystals (206 mg, 0.65 mmol, 95%).
Spectroscopy Data:
1H
NMR (, CDCl3): 6.01 (Cp, s, 5H), 2.75 (CH2, m, 4H), 1.33 (CH3, t, 6H), 0.23 (Nb-CH3, s,
6H).
13C NMR (, CDCl ): 108.8 (Cp), 44.4 (Nb-CH ), 23.5 (CH ), 15.2 (CH ).
3
3
2
3
11B NMR (, CH Cl ): 34.1 (1B, d, J=130 Hz), 23.6 (1B, d, J=138 Hz), -3.1 (1B, d, J=161 Hz).
2 2
FTIR (cm-1): 2937 s, 2874 m, 2361 s, 1458 m, 1379 m.
UV-vis (nm, in CH2Cl2): 372 (25%), 236 (100%).
MS: m/z 315 (parent ion envelope), 302 (- Me).
Elemental Analysis: C 48.44, H 7.58. Expected: C 49.18, 7.94.
Crystal Structure:
Hazards:
The Grignard reagent used, methylmagnesium bromide is flammable and moisture
sensitive.
(Et2C2B4H4)Nb(CH2Ph)2Cp:
R
R
R
R
L
L = CH2Ph
L
H
B
Et
Et
R
Nb
BH
BH
B
H
Synthesis:
A 2.0 M solution of benzylmagnesium chloride (0.84 mL, 1.35 mmol) in ether was added
dropwise to (Et2C2B4H4)NbCl2Cp (200 mg, 0.55 mmol) in 15 mL of THF. The colour slowly
changed from red-orange to brown-orange as Grignard reagent was added. The solution was
stirred for 30 min and the solvent was remove in vacuo. The brown residue was extracted with
CH2Cl2, filtered through Celite, and chromatographed (silica gel) with 4:1 petroleum etherCH2Cl2. One major orange band was eluted, yielding compound as a yellow solid (241 mg, 0.43
mmol, 96%).
Spectroscopic Data:
1H
NMR (, CDCl3): 7.19 (Ph, t, 2H), 6.95 (Ph, t, 1H), 6.72 (Ph, d, 2H), 5.46 (Cp, s, 5H), 3.53
(CH*2Ph, d, 2H), 2.97 (CH2, dq, 2H), 2.79 (CH2, dq, 2H), 1.43(CH3, t, 6H), 0.62 (CH*2Ph, d,
1H).
13C NMR (, CDCl ): 150.5 (ipso C ), 128.4 (Ph), 128.2 (Ph), 124.3 (Ph), 110.3 (Cp), 121
3
6
(C2B4, br), 75.6 (C*H2Ph), 22.9 (CH2), 15.4 (CH3).
11B NMR (. CH Cl ): 32.5 (1B, d, J=129 Hz), 23.8 (2B, d, J=124 Hz), 2.0 (1B, d, J=139 Hz).
2 2
MS: m/z 469(parent ion envelope), 378 (-CH2PH).
*No Crystal Structure.
Hazards:
Again, THF and CH2Cl2 are used as solvents (carcinogenic). A Grignard reagent is
used, which is flammable and air sensitive.
Reference [4]:
The next article presented provides synthetic and spectroscopy details on two
compounds. These compounds are niobium derivatives of “1,3-dimetallabenzene.” The crystal
structure of several tantalum compounds is given. As well, crystal structure data is provided for
one niobium compound, but the actual structure is not presented because the tantalum compound
is isostructural. The article states that these types of compounds represent intermediate forms in
catalytic cycles. All data for the next two compounds is taken from reference [4]. All methods
use standard Schlenk techniques.
[(cb)2Nb(-CSiMe3){-C(Et)C(Et)C(SiMe3)}Nb(cb)2:
Et
SiMe3
Et
cb
cb
Nb
Nb
cb
cb
SiMe3
Synthesis:
To 20 mL of toluene was added [(cb)2Nb(-CSiMe3)2Nb(cb)2] (0.40g, 0.40 mmol). To
this solution 1 eq. of 3-hexyne (0.03g, 0.40 mmol) was added. The resulting solution was heated
at 110C for 72 h. The resulting solution was filtered and the filtrate dried under vacuum to
leave a dark red solid. This solid was washed with hexane and dried under vacuum. The residue
was recrystallized from a concentrated solution of toluene layered with hexane to yield (0.16g,
0.15 mmol, 38%).
*Note cb = carbazole
Spectroscopic Data:
1H
NMR (, C6D6, 30C): -0.77 (s, SiMe3), 0.33 (s, Nb2CSiMe3), 0.13 and 0.81 (t, CH2CH3),
1.56 and 2.32 (br, CH2CH3).
13C NMR (,C D , 30C): 1.3 (SiMe ), 5.6 (Nb CSiMe )
6 6
3
2
3
*No crystal structure given.
Hazards:
The hazards present in this synthesis arise from the toxicity of the solvents. All solvents
are flammable and carcinogenic.
[(cb)2Nb(-CSiMe3){-C(SiMe3)CHC(SiMe3)}Nb(cb)2:
H
SiMe3
Me3Si
cb
cb
Nb
Nb
cb
cb
SiMe3
Synthesis:
To a solution of [(cb)2Nb(-CSiMe3)2Nb(cb)2] (0.25g, 0.24 mmol) in 15 mL of toluene,
an excess of Me3SiCCH (0.12g, 1.22 mmol). The solution was heated at 110C for 20 h. The
solution was then filtered and the filtrate was dried under vacuum to leave a dark red solid. This
solid was washed with hexane and dried under vacuum. It was then recrystallized from a
concentrated solution of toluene layered with hexane to yield 0.04g (0.04 mmol, 15%).
Spectroscopic Data:
1H
NMR (, C6D6, 30C): -0.70 (s, SiMe3), 0.07 (s, Nb2CSiMe3), 8.43 (s, C(5)-H), 8.60 (d, cb
oath-H).
13C NMR (, C D , 30C): 0.10 (SiMe ), 5.8 (Nb --CSiMe ), 267.9 (NbCSiMe ).
6 6
3
2
3
3
*A crystal structure of an isostructural compound is provided with data for the niobium
containing compound provided.
Crystal Structure:
Hazards:
Solvent toxicity.
Reference [3]:
The next three compounds presented come from a paper on half-sandwich compounds of
Nb and Ta with o-xylylene, anthracene and cyclooctatetraene. Three crystal structures are
presented in this article – two of which are niobium compounds. The synthesis and study of
these compounds was undertaken by this group because of the “unique catalytic ability similar to
group 4 metallocene complexes for the polymerisation of ethylene and norborane.”3 All
information from the next three compounds is taken from reference [3]. All methods employ
standard Schlenk techniques.
Cp*Nb[o-(CH2)2C6H4]Cl2:
Nb
Cl
Cl
Synthesis:
To a solution of Cp*NbCl4 (0.508g, 1.37 mmol) in THF cooled to -78C was added a
suspension of o-C6H4(CH2MgCl)2 (0.95 eq., 1.30 mmol) in THF (0.21M, 6.20 mL) using a
syringe. The mixture was allowed to warm to room temperature, stirred overnight and
evaporated to dryness. The product was extracted with hot hexane (240 mL) at 60C. It was
then recrystallized from toluene (3.0 mL) at -20C to yield green crystals in 16% (mp. 234.5237.0 (dec.)).
Spectroscopic Data:
NMR (, C6D6, 30C): 0.75 and 1.95 (4H, AB quartet, 2JHH=6.1 Hz, -CH2), 1.80 (15H, s,
C5Me5), 7.55 (4H, m, C6H4).
13C NMR (, C D , 30C): 12.2 (q, 1J =128 Hz, C Me ), 71.3 (br, t, 1J =147 Hz, -CH ), 125.8
6 6
CH
5
5
CH
2
(s, C5Me5), 131.2 and 137.9 (d, 1JCH=161 and 165 Hz respectively, C6H4), 127.3 (s, ipso-C6H4).
Elemental Analysis: C 53.76, H 5.73. Expected: C 53.62, H 5.75.
*Crystal Structure Provided
1H
Crystal Structure:
Cp*Nb(4-buta-1,3-diene)(cot):
Synthesis:
Nb
To a solution of NbCl2Cp*(4-buta-1,3-diene) (0.271 g, 0.768 mmol) in THF (50 mL)
cooled at -78C was added cot (cyclooctatetraene) (1.2 eq, 0.88 mmol) in THF (0.40M, 2.20 mL)
and CH3MgI (2.3 eq, 1.74 mmol) in ether (0.62 M, 2.80 mL) with a syringe. The reaction
mixture was allowed to warm to 20C, the colour of the solution changed from light green to
dark green. While the mixture was stirred for 10 h at 50C, the colour of the solution changed
from dark green to brown purple. The solution was evaporated under vacuum to give a residue.
This was extracted with 170 mL of hexane. The residue was recrystallized from toluene (4.0
mL) at -20C to yield purple black crystals (42% yield, mp 162-163C (dec.)).
Spectroscopic Data:
1H NMR (, C D , 30C): -0.36 (2H, m, =CH anti), 1.48 (15H, s, C Me ), 3.05 (2H, m, =CH
6 6
2
5
5
2
syn), 4.51 (2H, br, =CH-), 5.16 (8H, s, COT).
13C NMR (, C D , 30C): 10.8 (q, 1J =127 Hz, C Me ), 53.6 (t, 1J =149 Hz, =CH ), 103.6 (d,
6 6
CH
5
5
CH
2
1
JCH=154 Hz, COT), 110.2 (s, C5Me5), 126.4 (d, 1JCH=163 Hz, =CH-).
Elemental Analysis: C 67.71, 7.54. Expected: C 68.39, H 7.57.
*Crystal structure provided.
Crystal Structure:
Hazards:
The solvents are all toxic and carcinogenic. The Grignard reagent is flammable and air
sensitive.
Nb(PhCCPh)Cp*(cot):
No structure provided.
Synthesis:
Cp*Nb(4-buta-1,3-diene)(cot) (2.9 mg, 0.008 mmol) was dissolved in 0.3 mL of C6D6 in
a 5 mm NMR tube. To the solution was added PhCCPh (1.4 mg, 0.008 mmol) in 0.3 mL of
C6D6 at 25C. The NMR tube was sealed and heated in an oil bath at 50C for 5 h. The 1H
NMR was measured.
Spectroscopic Data:
1H NMR (, C D , 30C): 1.43 (15H, s, C Me ), 4.75 (8H, s, COT), 7.11 (2H, overlap of phenyl
6 6
5
5
signals of free PhCCPh peaks), 7.41 (4H, t, 3JHH=7.6 Hz, m-Ph), 7.97 (4H, d, 3JHH=6.9 Hz, oPh). Free butadiene peaks were also observed.
*No crystal structure.
Hazards:
Benzene is toxic. In this NMR scale reaction the primary risk comes when sealing the
NMR tube, which can explode if not done properly.
*Please note that the procedures and data in this section are taken almost entirely from the text of
the articles in which they are presented.
Computational Studies:
Two articles with computational studies were uncovered in the course of research. These
two articles (references [7] and [8]), present studies on delocalization of unpaired spin density in
some niobocene complexes [8], while the second article provides a study of transition structures
in methane elimination in NbMe5 and TaMe5 [7].
In the first article [8], extended Huckel calculations have been run on a series of prepared
compounds. In combination with qualitative MO theory, the calculations performed have been
used to analyze EPR data collected by the researchers. The ligands used are -donors, acceptors. Good correlation with experimental data was achieved in the calculations.
The second article focuses upon the mechanism of methane elimination in
pentamethylniobium and pentamethyltantalum. Ab initio quantum mechanic calculations were
used. “Geometry optimizations were preformed with the 3-21G and HW3 (equivalent to 631G*) basis sets.”7 These were then evaluated with MP2/HW3 calculations. It was found that
the complexes favour a square-planar geometry and the most favourable transition state for
unimolecular methane elimination is close to a trigonal-bipyramidal geometry.7 The abstraction
of hydrogen by CH3 is concerted, but a significant amount of interaction between M-H exists in
the transition state. Calculated free energies of 35.3 and 37.3 kcal/mol (Nb and Ta respectively)
were found at the best level of calculation.7 A dimeric mechanism of hydrogen abstraction is
found have a lower activation energy than the monomeric mechanism.7
Reviews:
Reference [6]:
This article focuses upon the catalytic ability of niobium and tantalum hydride
complexes. These complexes are homogeneous hydrogenation catalysts used in the
hydrogenation of arenes. A new method for synthesizing cyclohexylphosphine ligands is
reported using a niobium complex. The hydrogenations are highly stereo- and regio-specific in
the hydrogenation of benzenes and polynuclear aromatic hydrocarbons.
Reference [5]:
This article provides a new synthetic pathway for (5-CpR)M(CO)4 (M = Nb and Ta).
The new synthesis involves a reduction in the pressure needed to a point were the pressure of the
canister of CO is enough.5 New compounds synthesized are fully characterized by IR, NMR and
elemental analysis. Molecular structures of five compounds are presented with X-ray structures.
Conclusion:
New work on organometallic complexes of niobium seems to be moving in one direction:
substituted cyclopentadiene complexes. Through the information presented in this report, it
appears that this is the only major direction that niobium organometallic chemistry is headed.
The reason that niobium chemistry is moving in this direction is easy to comprehend:
polymerization catalysts. These substances are very important to industry and industry is willing
to pay for research in this area. However, this allows new areas of study to be explored without
competition and it also ensures that many areas have remained largely unexplored.
Library Resources:
To complete this report several key sources were used. To obtain articles on this subject
the CD- ROM’s of chemical abstracts were first consulted. A search using the key words,
niobium and organometallic, for each year from 1991-1998 was preformed. This resulted in
about ten articles per year. These were then sorted through to obtain relevant articles. From this
list, articles were then obtained, primarily from the Chemistry library downtown. In addition to
the CD ROM’s, Greenwood and Earnshaw’s Chemistry of the Elements, 2nd Edition, was
consulted to gain a basic understanding of the chemistry of niobium. Erindale library had the
initial resources needed to begin researching this report, but lacked the journal articles needed to
complete the project (especially the Journal of Organometallics).
*All articles used in this paper are provided.
References:
1.
Greenwood and Earnshaw, Chemistry of the Elements, 2nd Edition; ButterwothHeinemann Ltd. (Oxford: 1984), pg. 976-1001.
2.
Stockman, K.E.; Houseknecht, K.L.; Boring, E.A.; Sabat, M.; Finn, M.G.; and Grimes,
R.N.; Organometallics, 14 (1995), pg. 3014-3029.
3.
Mashima, K.*; Nakayama, Y.; Kaidzu, M.; Ikushima, N.; and Nakamura, A.; Journal of
Organometallics, 557 (1998), pg. 3-12.
4.
Riley, P.N.; Profilet, R.D.; Salberg, M.M.; Fanwick, P.E.; and Rothwell, I.P.*;
Polyhedron, 17, No. 5-6 (1998), pg. 773-779.
5.
Bitterwolf, T.E.*; Gallagher, S.; Bays, J.T.; Scallorn, B.; Rheingold, A.L.; Guzei, I.A.;
Liable-Sands, L.; Linehan, John C.; Journal of Organometallic Chemistry, 557 (1998),
pg. 77-92.
6.
Rothwell, Ian P.; Chem. Comm., 15 (1997), pg. 1331-1338.
7.
Wu, Yun-Dong*; Chan, K.W.K.; Xue, Z.; J. Am. Chem. Soc., 117 (1995), pg. 9259-9264.
8.
Antinolo, A.; Fajardo, M.; de Jesus, E.; Mugnier, Y.; Otero, A.*; Journal of
Organometallic Chemistry, 470 (1994), pg. 127-130.
9.
Nakamura, A.; and Mashima, K.; Journal of Organometallic Chemistry, 500 (1995), pg.
261-267.
10.
H. Sloan; Annual Reports on the Progress of Chemistry: Inorganic Chemistry; Vol. 92,
Section A., Royal Society of Chemistry (Cambridge, 1996), pg. 157-168.
11.
Aldrich Catalogue, 1996-97 and 1998.