The electron spin resonance spectra of niobium(IV) species in hydrochloric acid and ethanol-hydrochloric acid mixtures

J. inorg,nucLChem.,1972,Vol. 34,pp. 679-686. PergamonPress. PrintedinGreatBritain

THE ELECTRON SPIN R E S O N A N C E SPECTRA OF NIOBIUM(IV) SPECIES IN HYDROCHLORIC ACID A N D E T H A N O L - H Y D R O C H L O R I C ACID MIXTURES D E N N I S P. J O H N S O N and ROBERT D. B E R E M A N t Depai'tment of Chemistry, State University of New York at Buffalo, Buffalo, New York 14214, U.S.A. (First received 26 April 1971 ; in revised form 28 June 1971)

Al~tract-Electron spin resonance studies of NbCh in concentrated hydrochloric acid show two species to be present. These are identified as the oxopentachloroniobate(IV) ion and the oxotetachloroaquoniobate(IV) ion from their electron spin resonance parameters. A third species, the hexachloroniobate(IV) ion, is also present and can be isolated as the cesium salt. Solutions of niobium(IV) produced by zinc reduction of Nb(V) in hydrochloric acid or HCl-saturated ethanol contain the oxotetrachloroaquoniobate(IV) ion and the oxotetrachloroethanolatoniobate(IV) ion respectively. Calculation of the normalization constant for the ground state IB)* molecular orbital for the oxo species indicate considerable delocalization of unpaired spin density into the equatorial chlorine p-Tr orbitals. INTRODUCTION

INTEREST in the bonding of oxohalide species has prompted our investigation of Nb(IV) in hydrochloric acid media. Cozzi and Vivarelli[1] first investigated Nb(IV) spectroscopically in various concentrations of hydrochloric acid and postulated the existence of both a hexachloro species and an oxo species. However, no complexes were isolated. N. S. Garif'yanov et al. [12] have examined the electron paramagnetic resonance (EPR) spectrum of Nb(IV) in ethanol prepared by reduction with zinc and hydrochloric acid. They found a frozen glass spectrum but no solution spectrum. More recently [3], the same authors have repeated their experiment in aqueous HCI and obtained significantly different parameters which they assign to NbOCI4 ~-. Lardon and Giinthard [4] examined the EPR spectrum of Nb(IV) in HCl-saturated alcohols obtained by electrolytic reduction. Low et al. [5]; have examined Nb(IV) in Cs2ZrCI6 and find it to be tetragonally distorted. Brubaker et al. [6], have examined the spectra of the NbCIs(OCHz) -~ ion in methanol. It is obvious that despite the work done on Nb(IV) chloro complexes, discrepancies exist regarding the identity of the various species present. To remove tTo whom inquiries should be addressed. 1. D. Cozzi and S. Vivarelli, Z. anorg, allg. Chem. 279, 165 (1955). 2. V. N. Fedotov, N. S. Garif'yanov and B. M. Kozyrev, Dokl. Akad. Nauk. SSSR. 145, 1318 (1962). 3. I. F. Gainullin, N. S. Garif'yanov and B. M. Kozyrev, Dokl. Akad. Nauk SSSR. 180, 858 (1968). 4. M. Lardon and H. H. Giinthard, J. chem. Phys. 44, 2010 (1966). 5. S. Maniv, W. Low and A. Gabay,Phys. Lett. 29A, 536 (1969). 6. P. G. Rasmussen, H. A. Kuska and C. H. Brubaker, Jr., lnorg. Chem. 4,343 (1965). 679

680

D . P . J O H N S O N and R. D. B E R E M A N

these discrepancies and to satisfy our own interest, the present work was undertaken. EXPERIMENTAL Materials Niobium pentachloride was obtained from Research Organic/Inorganic Chemical Corporation and sublimed repeatedly before use. Niobium tetrachloride was prepared by reducing niobium pentachloride with niobium metal by the thermal gradient method (400-200°C)[7]. Cesium chloride was obtained from Research Organic/Inorganic Chemical Corporation. Pure nitrogen was obtained by passing high purity nitrogen through a three foot BTS column[8] and subsequently through calcium chloride and molecular sieve drying towers. Commercial anhydrous ethanol and Zn metal were used.

A nalyses Microanalyses were performed by Galbraith Laboratories, Inc., Knoxville, Tennessee. Experimental methods All reactions, transfers, and sample preparation were carried out in an inert (dry N~) atmosphere or under vacuum. All drying was in vacuo and compounds were stored under nitrogen. Cs~NbCl6 Method 1 [9]. Cesium chloride and niobium tetrachloride were intimately ground together in a 2 : 1 molar ratio under strictly anhydrous conditions and heated together in a Pyrex tube for 48 hr at 480°C. The resulting mixture was then reground and the process repeated for 12 hr. A dark violet compound was obtained. Anal. Calc. for Cs2NbClr: Cs, 46"52; Nb, 16"26; Cl, 37.22. Found: Cs, 46.65; Nb, 16.10; C1, 37"01. Method 2 [10]. Niobium tetrachloride was dissolved in a minimum amount of 12N hydrochloric acid and two equivalents of cesium chloride in 12N hydrochloric acid were added immediately. The violet precipitate obtained was washed with hydrochloric acid and acetone and dried under vacuum. Anal. Found: Cs, 46.89; Nb, 16.19; Ci, 36.92.

Spectroscopic methods The visible and ultraviolet spectra were obtained by use of Nujol mulls or 2 cm solution cells on a Cary 14 spectrophotometer. X-Band electron spin resonance spectra were determined at 110°K and room temperature (300°K). A Varian V4502-19 spectrometer was used in conjunction with a Magnion proton oscillator gaussmeter and a Hewlett-Packard frequency meter to obtain accurate measurements of the magnetic field and microwave frequency. Spectra were scanned slowly to determine the hyperfine parameters accurately. RESULTS

No electron spin resonance signal could be obtained for the solid cesium hexachloroniobate from either preparative method at 297° or l l0°K. Electron spin resonance spectra were obtained from solutions of NbCh in 12N hydrochloric acid and I0N hydrochloric acid. Spectra were also obtained from solutions prepared by reducing NbCI5 with Zn metal either in 12N hydrochloric acid, or in HCl-saturated ethanol. The solution spectra consisted of 10 lines (I = 9/2, 100% 93Nb). Spectra of the glasses could be resolved into parallel and perpendicular components. 7. 8. 9. 10.

H. Schafer and C. Pietruck, Z. anorg, allg. Chem. 266, 151 (1951). A. D. Broadbent, J. chem. Educ. 44, 145 (1967). B.G. Korshunov and V. V. Safonov, Russ. J. inorg. Chem. 6, 383 (1964). I.S. Morozov and N. P. Lipatova, Russ. J. inorg. Chem. 11,550 (1966).

ESR spectraof niobium(IV)

681

Since the hyperfine splittings are on the order of 200 Gauss, the high field approximation cannot be applied rigorously and second order corrections should be employed. The perturbation of the Zeeman transition resulting from the hyperfine interactions was corrected by means of the following equations:

hv = g[3Ho

( 1)

for isotropic g

Ho = Hm+ ( a ) m , +

a~- [1(1+ 1) --mi 2] 2Ho

(2)

for gH A± 2 Ho = Hm+Aiim~+~'~o [ I ( I + 1) --mr 2]

(3)

for g± _

/All 2 + A ± 2 \

H ° = Hm + A ± m ' + ~ - ~ o

) [I(I + l ) -rn'z]

(4)

where Hr, is the magnetic field position of the ESR line due to the component mt of the nuclear spin 1, v is the klystron frequency, and (a), AH, and A± are the hyperfine splitting constants. The corrections are necessarily reiterative and were carried out by desk calculator. Normally three iterations were sufficient. The hyperfine splitting constants were determined from the positions of the fifth and sixth, fourth and seventh, third and eighth, second and ninth, and first and tenth lines where resolution permitted. The values for Al~, A±, (a), glf, gl and (g) are given in Table 1. The observed g and A values were used to calculate the coefficients in a simple molecular orbital scheme. The procedure was similar to that of Brubaker et al. [6], except contributions from charge-transfer bands were not included. The NbOCI4 (solvent) 2- and NbOC15~- species have one unpaired electron and the ground state is 2B~. The orbitals which are necessary for discussion are those which are bases of the bl, b2, and e representations. The L C A O - M O wave functions of these orbitals are

(5) IB1) * = N~,(dz,-u~ - h~6b,) [E) * = N,,1 (d~u or duz - h~raq~ee

-

-

h~r,C~ea)

(6) (7)

where the metal orbitals are d functions and h,51 and h~r 1 a r e the xz (yz) molecular orbital coefficients for the equatorial and axial ligands, respectively. The ligand orbitals or oxygen and chlorine involved in ~r bonding are taken as pure p orbitals. The ligand orbitals involved in tr bonding were taken as sp hybrids. The experimental g and A values can be related to the molecular orbitals by glt-- 2.0023 =

m

2

2

8~N,~,N,~, A(b~--bl)

(8)

277.0 (2)

294.4 (10)

269.3 (6)

NbOCI, (H,O)“-

NbOC1,3-

NbOCl, (HOEt)P-

Species

131.8 (9)

130-6 (12)

177-6

179.4

184.7 (12)

1.9194 (1)

1.941 (12) 1.9215 (lO)$ 1.926 (5)

1.9165 (l)$

1.921 (6)

*Splittingsgiven are in gauss; estimateddeviation of last significantIigurein parentheses. $Corrected for second order effects; A, for NbOCl,+ was assumed to be 130.6gauss.

aq. HCI NbCI,+ Zn+ aq. HCI NbCl, + aq. HCl NbCI,+ Zn+ HCI/EtOH

NbCl,+

Method of preparation

1.915 19029

1.8946 (2)

1.8943%

1907

1.910 (5)

1.8833 (2)$

1900 (1)

Table 1. Experimentalparametersfor Nb(IV) chloro species*

1.8983 (2)$.

1901 (1)

ESR spectra of niobium(IV) _

g± -- 2.0023 =

2

683 2

2~N,~,N,,

(9)

A (b2 - e) All-- a =

4N~,P 7

(10)

where A (b2 - bl) and A (b2 -- e) are the d-d transitions expected for a tetragonally distorted complex. ~ is the spin-orbit coupling constant and P = 2-0023 gsBeBn(r-3) avg. where Be and Bn are the Bohr and nuclear magnetons respectively and g~v is the nuclear g factor. The values o f f and P depend on the formal charge assigned to the niobium atom. In our considerations here, we assumed the formal charge was 4.0 and therefore ~ = 750 Gauss[11] and P = 205 Gauss[12]. The values for A ( b 2 - bl) and A ( b z - e) were taken from Cozzi and Vivarelli[1] for niobium(IV) in 10N hydrochloric acid as 20,200cm -1 and 14,200cm -1 respectively. The results for NbOC14(H20) 2- are; N ~ = 0.83, N~I = 0.97 and N ~ = 0.48; for NbOCI~ 3-, N ~ = 0.95. The same optical transitions were used in calculating the parameters for the above two species andA± was assumed to be the same for both species since frozen glass spectra of mixtures of NbOCI4(HzO) 2- and NbOCI52show only one set of lines in this area. The optical spectra obtained for the species investigated here are given in Table 2 with probable assignments and agree well with those reported by Cozzi and Vivarelli for the species of niobium(IV) in 1ON hydrochloric acid. Table 2. Spectroscopic features of compounds investigated*

Cs~NbCI6 (Nujol mull) NbOC14(H20)2-(I0N HCI) Nb(IV) in 10N HCI[1]

18,600 A(tz~- e~) 15,400 A(b2--e) 14,200 A(b2--e)

(broad and symmetrical) 19,600 A(b2-bl) 20,000 A(b2-bl)

*Values in cm-k

DISCUSSION

When NbC14 is dissolved in 12N hydrochloric acid, the hexachloroniobate (NbCle 2-) species is formed initially. This is certain since the cesium salt can be isolated in a pure form. However, this salt does not give an electron spin resonance spectra in the solid state which indicates a pure octahedral configuration and a degenerate ground state. Similar results have been found for VCIe2- in solution and in the solid state[13]. It is hard to imagine a solvent effect large enough to remove this degeneracy for NbCle 2- in solution. Therefore, the electron spin resonance spectra observed must be due to other species. Cs2NbOC4 has been 11. T. M. Dunn, Trans. FaradaySoc. 57, 1441 (1961). 12. B. R. McGarvey,J.phys. Chem. 71, 51 (1967). 13. R.D. Bereman and C. H. Brubaker, lnorg. Chem. 8, 2480 (1969).

684

D . P . J O H N S O N and R. D. B E R E M A N

isolated by Kharitonov and Lipatova[14] and the behavior of the frozen glass spectrum of NbCI4 in hydrochloric acid strongly suggests that the NbOCI4(H20) 2ion is the predominant species in this investigation. It is evident that the species investigated in this work differ significantly only in the value of their parallel hyperfind tensor. Since the electron spin resonance spectra exhibit axial symmetry, the species must differ only in axial coordination. If NbCI4 is dissolved in 12N hydrochloric acid, two paramagnetic species differing'only in A, values are obtained as shown in Fig. 1. If this solution is alternately frozen and thawed, the species with the larger A, value disappears.

\~

5,~.~

f"

~,"

"J 2"

k:" A

~q, L/

"."

C .......

Fig. i. Air Lines for NbOCh.H202- and NbOCI52- for M I - - - 9 / 2 , -7/2, and - 5 / 2 respectively. A = Nb(IV) species in 12N HCI. B = Nb(IV) species after thawing and freezing sample A. C = Nb(IV) species after thawing and freezing sample B.

This would indicate that the species with the larger A, value is more stable in solutions with higher C1- concentrations, since HC1 would be given off in this freezing and thawing process. Also, if NbCI4 is dissolved in 10N hydrochloric acid, only the species with the smaller A, value is found. One could postulate the following equilibria existing in hydrochloric acid. NbCla + 2C1- ~ NbCI62-

(11)

NbCI62- + HzO ~ NbOCh z- + 2HCI

(12)

NbOCI4 z- + CL- ~ NbOCI53-.

(13)

If the C1- concentration is much below 12N, the NbOCls 3- species is not stable. The NbCI62- ion, as found for most d 1 transition metal halides, is unstable to hydrolysis although in this case, somewhat kinetically stable. When solutions of niobium(IV) in ethanol or in hydrochloric acid are obtained by reduction of niobium(V) chloride with Zn, it was impossible under all conditions employed to isolate the hexachloroniobate(IV) species. This is probably due 14. Y. Y. Kharitonov and N. P. Lipatova, Izv. Akad. Nauk. SSSR, Neorg. Mater. 3, 405 (1967); Chem. Abstr. 67, 588873m.

NbOCl4 (H~O) ~NbCI62-*

NbCIs + Zn + conc. HCI NbCI4+ Cs2ZrCI6 melt NbCls + Zn + HCI-EtOH NbCI 5 + HCIEtOH Electrolytically 146 (10) 130-2 (2)

269.9 (4)

148.7 (3)

291-2 (2)

270 (10)

122 (5)

.4±

260 (5)

A,~

*Second order corrections made by investigators.

NbOCI4 (HOEt) 2NbOCI* (HOEt) z-

Probable species

Method of preparation

(a)ealc

176-8 (4)

187 (10)

196"2

168 (5)

Table 3

1-922 (1)

1.82

1-9184 (6)

1"943 (6)

gll

1.891 (1)

1-80

1-9515 (6)

1.932 (6)

g±

1-901 (1)

1-81

1-9404 (6)

1-936

(g)eale

[4]

[2]

[5]

[3]

Ref.

e-

686

D . P . J O H N S O N and R. D. BEREMAN

to the fact that the niobium(IV) chloride was hydrolyzed to the oxo species, i.e. NbOC14- before reduction so that there is never any NbCI6 z- present in the product. It is somewhat unusual that even after resaturating the hydrochloric acid solution of Nb(IV) obtained in this manner, no electron spin resonance spectra due to the NbOCI5 a- species could be obtained. This could be due to the strenth of the Nb-OH2 bond if water is in the emply coordination position. The larger ,41~ lines are assigned to the NbOCI5 z- species since it is felt the tetragonal distortion would be larger with the chloride in the sixth coordination position than if water were present and from the knowledge that Cs2NbOCI4 had been isolated. The niobium(IV) species obtained by reducing niobium(V) chloride in HCIsaturated 95% ethanol differs only slightly in its electron spin resonance parameters from that obtained under similar circumstances in hydrochloric acid. It is felt this species differs only in that an ethanol molecule is in the sixth coordination position. Various investigators have studied niobium(IV) in solution and the results of their electron spin resonance studies are given in Table 3 along with the probable species being studied. Our studies agree well with these results and it is felt that the assignments of the species are correct. Repeated attempts to isolate a pure oxo niobium(IV) species were unsuccessful and work with these systems to more fully understand their chemistry and bonding properties is underway. Acknowledgement- D. P. J. is indebted to the Department of Chemistry, State University of New York at Buffalo for financial support during this investigation.