18
Reactions Halides
of
with
Some
Niobium(V) and Tantalum(V)
Pyridine
R. E. McCARLEY, B. G. HUGHES, J. C. BOATMAN, and B. A. TORP
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Institute for Atomic Research and Department of Chemistry, Iowa State University, Ames, Iowa
The reaction of pyridine with NbBr proceeded by oxidation-reduction to yield the niobium(IV) derivative NbBr (C H N) , and the oxidation products 1-(4-pyridyl) pyridinium bromide (I) and pyridinium bromide. I was identified by comparison of the ultraviolet spectra in acidic and basic solutions with those of its known salts. Reduction of NbC by pyridine was also observed but a different reaction stoichiometry was obtained. The reactions of TaC and TaBr with pyridine afforded only the adducts TaC (C H N) and TaBr (C H N). In contrast to the behavior of NbC and NbBr , both Nbl and Ta dissociated in pyridine to give the adducts M (C H N) , and the pyridine adduct of elemental iodine. 5
4
5
5
2
l5
l5
5
l5
5
5
5
l5
5
5
5
5
l5
l4
5
5
2
In previous studies (6, 7) of the reaction of niobium(V) chloride with ammonia and the three methylamines it was found that aminolysis was the primary mode of reaction. The extent of displacement of chloride in the reactions followed the order NH Me > N H = NHMe > NMe . While the products obtained from the reactions with N H , NH Me, and NHMe were separated and identified, the products of the reaction with NMe could not be separated. However, the over-all molar ratio of Nb:Cl:N of approximately 1:5:2 for the latter product led to the authors' suggestion that the product was a simple adduct or a mixture of tetramethylammonium chloride and tetrachloro (dimethylamido) niobium (V). The possibility of reduction was not considered. Other work (I) has shown that the reaction of titanium(IV) chloride and trimethylamine yielded a mixture which was 90% tetrachloro ( trimethylamine )titanium(IV) and 10% trichlorobis ( trimethylamine) titanium (III). The oxidation product of the reaction was not identified. However, the authors suggested that the oxidation products could be amine polymers or chloroamine derivatives. Many other workers (2, 4, 5, 15) have reported the reduction of various metal halides with ammonia and amines, but the oxidation products of such reactions have never been positively characterized. Generally the metal must be in the maximum valence state consistent with stability of the metal halide before reduction will take place. Where an acidic 2
3
3
2
3
2
2
3
243 In Reactions of Coordinated Ligands; Busch, D.; Advances in Chemistry; American Chemical Society: Washington, DC, 1962.
ADVANCES IN CHEMISTRY SERIES
244
proton is available, as w i t h primary and secondary amines, the metal-halogen bond may be broken b y solvolysis of the type: M X » N H R
2
+
NHR
2
MX„_i-NR
=
+
2
N H R 2
2
+
X ~
(1)
In the absence of an acidic proton, as w i t h tertiary amines, the cleavage of halogen may be accomplished b y a reduction of the type:
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MXn-NR
+
3
e~
=
MX„_rNR
3
+
X '
(2)
A l t h o u g h the source of the electron is k n o w n to be the amine itself, the oxidation products of such a reaction have not been characterized. Experimentally it has been f o u n d that primary and secondary amines react b y solvolysis, w h i l e only the tertiary amines generally produce reduction, if re duction is observed. It thus seemed appropriate to study the reaction of n i obium ( V ) halides w i t h pyridine, where proton dissociation need not be con sidered a n d any reaction w o u l d necessarily lead to a simple adduct of pyridine or reduction of the metal halide. I n this work, reduction of the niobium ( V ) halides was observed, a n d the reaction products were characterized. E l u c i d a t i o n of the pyridine oxidation products has permitted an interpretation of the reaction mecha nism i n terms of the two-electron reduction of niobium ( V ) b y the pyridine molecule. Experimental Materials. N i o b i u m metal of low oxygen content was prepared by carbon reduction of h i g h purity n i o b i u m ( V ) oxide obtained from Fansteel Metallurgical C o r p . T h e metal then was arc-melted, cut into fine turnings, and outgassed at a pressure of 8 X 1 0 ~ m m . of H g and a temperature of 2 0 5 0 ° for 2 hours. ANALYSIS. F o u n d : O , 35 p . p . m . ; N , 575 p . p . m . ; C , 3500 p . p . m . Tantalum metal was obtained from the National Research C o r p . as a granular powder of l o w oxygen content and a stated purity of 9 9 . 9 + % . Chlorine was distilled from cylinders directly into a side arm of the reaction tubes for halide preparations, using a dry ice-acetone bath. After distillation the l i q u i d was frozen i n l i q u i d nitrogen and outgassed under h i g h v a c u u m . Reagent-grade bromine was dried over phosphorus ( V ) oxide and distilled into dry, evacuated flasks. Samples then were vacuum-distilled into reaction tubes for the halide preparations. Reagent-grade iodine was transferred directly to the reaction tubes for halide preparations a n d was outgassed thoroughly at room temperature under h i g h vacuum. P y r i d i n i u m halides were prepared b y b u b b l i n g anhydrous hydrogen halide gas through a solution of pyridine i n anhydrous ethyl ether. T h e precipitates then were filtered, washed w i t h ethyl ether, and dried under h i g h vacuum at room temperature. l - ( 4 - P y r i d y l ) pyridinium dichloride was obtained from Eastman Organic Chemicals. It was purified by dissolving the solid i n concentrated hydrochloric acid, boiling the solution w i t h activated charcoal, evaporating to a small volume, and stirring into absolute ethanol. T h e pure white crystalline product was then dried under h i g h vacuum at room temperature. ANALYSIS. C a l c d . for C H N C l : C I , 30.94. F o u n d : CI, 30.43. l - ( 4 - P y r i d y l ) p y r i d i n i u m dibromide was prepared b y dissolving l - ( 4 - p y r i d y l ) p y r i d i n i u m dichloride i n concentrated hydrobromic acid and evaporating nearly to dryness three times w i t h concentrated hydrobromic acid. T h e solution then was stirred into cold absolute ethanol. The pale yellow precipitate w h i c h formed was filtered and d r i e d under h i g h vacuum at room temperature. 5
1 0
1 0
8
2
In Reactions of Coordinated Ligands; Busch, D.; Advances in Chemistry; American Chemical Society: Washington, DC, 1962.
McCARLEY ET AL.
Nb and Ta Halides with Pyridine
245
ANALYSIS. C a l c d . for C H N B r 2 : B r , 50.20. F o u n d : B r , 49.97. Spectro-grade pyridine was dried over calcium hydride, thoroughly outgassed on the vacuum line, distilled onto fresh barium oxide, a n d again outgassed. T h e l i q u i d was finally distilled into a clean, d r y , evacuated flask from w h i c h the samples for investigation were distilled. Reagent-grade chloroform was distilled through a 30-plate, fractional distilla tion column at a reflux ratio of 10 to 1 and collected over outgassed calcium hy dride. T h e purified solvent then was frozen i n l i q u i d nitrogen a n d outgassed under h i g h vacuum while it warmed slowly to room temperature. Spectro-grade acetonitrile was distilled through a 30-plate, fractional dis tillation column at a reflux ratio of 10 to 1 and collected over thoroughly outgassed Molecular Sieves. T h e purified solvent was frozen i n l i q u i d nitrogen a n d out gassed under h i g h vacuum as i t warmed slowly to room temperature.
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1 0
1 0
2
Spectra. T h e visible and ultraviolet spectra for various solutions were de termined using a C a r y 14 recording spectrophotometer a n d 1-cm. quartz cells. Infrared spectra were obtained using a Perkin-Elmer 13 infrared spectrophotometer w i t h sodium chloride optics and cells. X-Ray Powder Patterns. Samples for x-ray pattern determinations were sealed i n 0.2-mm. glass capillary tubes under an atmosphere of argon. T h e samples were then exposed to nickel-filtered, C u K a radiation i n an 11.459-cm. Debye-Scherrer camera for 18 to 20 hours. Preparation of Niobium(V) and Tantalum(V) H a l i d e s . T h e n i o b i u m ( V ) and tantalum ( V ) chloride a n d bromide were prepared i n a system of sealed, evacuated bulbs b y reacting the pure metal w i t h gaseous halogen. F o r the chlo rides the metal was maintained at 3 0 0 ° to 3 5 0 ° under a chlorine pressure of ap>roximately 7 0 m m . of H g ; the chlorine pressure was maintained 6 y keeping the i q u i d chlorine immersed i n a d r y ice-acetone bath. F o r the bromides the metal was maintained at a temperature of 4 0 0 ° to 4 5 0 ° under a bromine pressure of approximately 250 m m . of H g , maintained b y leaving the l i q u i d bromine at room temperature. W h e n the reactions were complete, excess halogen was isolated b y freezing i t in l i q u i d nitrogen. T h e pure product was then separated from the system i n a sealed b u l b , w h i c h was left unopened u n t i l the product was needed. T h e n i o b i u m ( V ) a n d tantalum ( V ) iodides were prepared b y the method reported b y Corbett and Seabaugh for n i o b i u m ( V ) iodide (3).
{
Analytical data for the pentahalides are given i n Table I. Table I·
Analytical Data for M X
5
Calcd. Compound NbCl NbBr Nbl TaCl TaBr Tal
5
6
5
6
6
5
%M 34.39 18.85 12.77 50.53 31.16 22.10
Found %X 65.61 81.14 87.23 49.47 68.84 77.90
%M 34.49 18.59 12.77 50.40 31.10 22.30
%X 65.37 79.81 87.20 49.35 68.00 77.60
Reactions w i t h P y r i d i n e . T h e reaction products of n i o b i u m ( V ) a n d tan talum ( V ) halides w i t h pyridine were prepared i n a sealed, evacuated flask w h i c h contained a stirring bar and was designed for easy weighing on the balance. T h e flask was first evacuated and weighed. A sample of metal halide was transferred to the flask i n the glove box, and the flask was again evacuated and weighed to obtain the weight of sample. Approximately 30 m l . of anhydrous pyridine were condensed o n the sample of metal halide and the mixture was stirred at room temperature until the reaction was complete. T h e reactions were considered complete w h e n n o unreacted
In Reactions of Coordinated Ligands; Busch, D.; Advances in Chemistry; American Chemical Society: Washington, DC, 1962.
246
ADVANCES IN CHEMISTRY SERIES
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metal halide could be observed visually. Reactions where the reagents were stirred over a m u c h longer period of time ( 2 to 3 days) gave results entirely i n agreement w i t h those obtained for the shorter reaction time. Excess pyridine was distilled from the flask, and the resulting product was dried under h i g h vacuum at room temperature to a constant weight. T h e amount of pyridine utilized i n the reaction could then be determined b y weight difference before and after reaction. Complete analytical results for the crude products obtained w i t h the chlorides and bromides are given i n Table II. Because the tantalum ( V ) and niobium ( V ) iodides both yielded free iodine i n the reaction w i t h pyridine, their products were washed w i t h chloroform, as described later, before analysis. Table II. Analytical Data for Products of Reaction of Niobium(V) and Tantalum(V) Chlorides and Bromides with Pyridine Reoctants
%M
Found
Calcd. for MX^py %X %py
%M
%X
%py
Py/M
Ratios X/M
NbCU-py
36.86 to 38.52
19.81 to 20.72
44.33» to 40.76
2.39 to 2.63
4.98 to 4.87
Nblhvpy
12.46 to 12.49
52.67 to 52.96
34.87» to 34.55
3.37 to 3.46
4.96 to 4.93
40.26 60.30
41.05 27.10
17.80 11.82
0.99 1.00
5.01 5.04
TaCUpy TaBr py 6
a
40.55 60.60
41.40 27.41
18.08 11.98
D e t e r m i n e d b y difference.
W a s h i n g Procedures. T h e metal halide-pyridine reaction mixtures were washed i n a sealed apparatus designed for continuous extraction i n vacuo. Samples were transferred to and from the apparatus i n a glove box under an atmosphere of argon. Washings were considered to be complete when the filtrate coming through the filter became colorless. Oxidation-Reduction Titrations. T h e extent of reduction resulting from reaction of niobium ( V ) chloride and bromide w i t h pyridine was determined b y indirect titration of crude reaction mixtures w i t h standard ammonium tetrasulfatocerate(IV) solution. Samples were stirred overnight i n a stoppered flask w i t h an excess of iron (III) ammonium sulfate. A n y iron (II) formed b y reaction w i t h the niobium complex mixture was then titrated w i t h the standard tetrasulfatocerate(IV) solution using ferroin as indicator. Results of these determinations are given i n Table I I I . Table III.
Sample
Results of Oxidation-Reduction Titrations
Titrant
Solvent
NbCU-pyridine reaction p r o d u c t
IN H 2 S O 4 excess Fe" "
NbBr -pyridine reaction p r o d u c t
IN H S 0 excess Fe" "
6
1
2
1
2.63 2.63 2.53 2.53
64.7 62.9 66.1 70.5
Ce" " i n UVH2SO4
3.44 3.44
88.1 89.5
1
4
3
%.
Reduction
Ce in IN H 2 S O 4 + 4
3
Pyridine/ Metal for Sample
4
Degradation of l-(4-Pyridyl) pyridinium Ion. Samples of l - ( 4 - p y r i d y l ) - p y r i d i n i u m dichloride a n d dibromide dissolved i n dilute hydrochloric acid were treated w i t h concentrated sodium hydroxide solution u n t i l strongly basic. The In Reactions of Coordinated Ligands; Busch, D.; Advances in Chemistry; American Chemical Society: Washington, DC, 1962.
McCARLEY ET AL.
247
Nb and Ta Halides with Pyridine
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solutions turned a bright yellow color a n d gave the absorption spectrum shown as A i n Figure 1. T h e broad peak was centered at approximately 432 ταμ, w i t h the sharper peak centered at 365 πΐμ. T h e yellow solutions were then heated near the boiling point. T h e spectrum obtained after heating the basic solution is shown as Β i n Figure 1. T h e broad peak at 432 πΐμ h a d disappeared, w h i l e the peak at 365 τημ h a d sharpened and intensified. 0.6
ι
0.5
\ / \ 1; ι 1
0.3
1 /
"
//
8 0.2|
ω
'
