Homogeneous catalytic hydrogen formation using a ditungsten cluster and low valency metal ions in aqueous acidic solutions

Polyhedron Vol. 8, No. 4, pp. 469472, Printed in Great Britain

0277-5387189

1989

0

S3.00+.00

1989 Pergamonpress plc

HOMOGENEOUS CATALYTIC HYDROGEN FORMATION USING A DITUNGSTEN CLUSTER AND LOW VALENCY METAL IONS IN AQUEOUS ACIDIC SOLUTIONS CONSTANTINOS

MERTIS”

and NIKOS PSAROUDAIUS

University of Athens, Inorganic Chemistry Laboratory, Greece

Navarinou

13A 10680 Athens,

(Received 20 June 1988 ; accepted 15 August 1988)

Abstract-The formally triply-bonded nonachlorodimetallates [wz(j&l)&16]3(6) and [Re2(p-C1)3C16]- (8) undergo a two-electron homogeneous reduction by the chromous or vanadous ions to give the quadruply-bonded species [w2Cls]4- (4) and [Re2ClJ2- (7). In aq. HCl solutions complex 6 is an effective catalyst for the anaerobic oxidation of Cf’ and V” to Cf” and V”’ with simultaneous dihydrogen formation.

Direct commercial production of hydrogen from water by electrolysis is uneconomic and considerable research into the possibility of splitting water using light’ or thermal’ energy is being carried out. Industrially, hydrogen is produced heterogeneously from synthesis gas (CO-H,) by the water-gas shift reaction. Homogeneous catalysts for this reaction are known and effective on a laboratory scale but require drastic conditions and much effort has been devoted to obtain hydride complexes3a operating at lower temperatures. There exist neutral, basic, and also acidic [Rh(CO)J;, KI, CH3C02H, HCl, Hz0 or K,PtCl,, SnC14*5H20, CH3C02H, HCl, Hz0 at lOO’c] systems.3b Although not strictly related to the above processes, relevant homogeneous hydrogen-producing acidic systems (K3M02HC1,, CrCl,, H20, HCl at 25”) have been recently reported4 based on the ability of the dimolybdenum species to form hydrides which then react with the low-valency metal ion, equation (1) :

(M = MO, 2 ; M = W, 5) coupled with diprotonation and loss of hydrogen to produce the analogous nonachloro derivatives [M2(&1)3C16]3(M = W, 6) are well established reactions, eqs (2) and (3) : [M,Cl;]4-~[M2HCl,]3-++[M2Clg]3-. (2)

(3;

The structures both of the reactants 1,4 and of the products 2,3,6 are known.’ However, reaction (1) requires strenuous conditions in the case of molybdenum (60°C in 12 M HCl), whereas the rhenium derivative [RezCls]2- (7) resists protonation even after refluxing for several hours. Accordingly, ~02(p-c1)3c16]3(3) and [Re&Cl),Cl,](8) are best prepared by electrochemical oxidation6 of 2 or chlorine oxidation7 of 7, respectively. In contrast, the isoelectronic and isostructural tungsten compound 4, reacts’ at -78°C giving 6, presumably via the reactive’ unstable hydride 5. Reactions (2) and (3) are easily reversible for H+ I 4,1Oand also the reduction of 8 to 7 for molybdenum [Mo2Cls14[Mo~HC18]3-+2Cr2t+H+rhenium7 whereas for tungsten they are not.8’1’*12 +2Cr3++:H2. (1) This enhanced susceptibility of WLW bonds to Proton oxidative addition to the quadruplyprotons or other oxidants and the irreversibility of bonded octachlorodimetallate anions [M2Clg14- these reactions has been the reason for : (M = MO, 1; M = W, 4) yielding the formally (a) the slow development in the chemistry of the triply-bonded hydrides [M&H)@-Cl)&,]‘W:+ unit and the long and numerous efforts for the isolation of 4 achieved eventually by the Na/Hg reduction8*‘3 of (WCl3, or *Author to whom correspondence should be addressed.

470

C. MERTIS

W,Cl,(THF),

in tetrahydrofuran,

and N. PSAROUDAKIS

eq. (4) :

W,Cl,(THF),-tZNa/Hg$&+ jNa4(THF),W2C18+

“WC12”.

(4)

The product is thermally unstable at room temperature, in any solvent it dissolves,’ eq. (5) :

4NaCl+2“WCI,“;

(5)

and (b) the belief that these allegedly important catalytic species are in fact kinetic and thermodynamic sinks and therefore relatively uninteresting in terms of their catalytic properties. We now describe the facile homogeneous twoelectron reduction of the diamagnetic 6 and 8 to give 4 and 7, respectively by the paramagnetic Crcti or V$$ ions and the low-temperature catalytic anaerobic oxidation of Cr” and Vi’ to Cr”’ and Vii’ by 6 with simultaneous hydrogen evolution in aq. HCI solutions.

EXPERIMENTAL The potassium salts of [Mo$&]~-, ~2C19]3and the tetra-n-butyl ammonium salts of [Re2Cls]*and [Re2C19]- were prepared by literature methods.7~“~‘4*‘5 CrCl, was supplied by Ventron and VCll * 4H20 was prepared electrochemically. ’ 6 All reagents were of analytical grade and thoroughly purified. Demineralized by ion exchange and doubly-distilled water was used. Solvents were very carefully deoxygenated by freeze pumping and all reactions were carried out under strictly anaerobic conditions (atmosphere of argon). Microanalyses were performed in this Laboratory and at the Research Center “Democritos”. Tungsten was determined by neutron activation and chloride ions, photometrically. Electronic spectra were recorded with a Hitachi and a Carry 17 ; Dihydrogen was detected by gas chromatography (GC; Varian 90-P). Reaction of tripotassium-l,I,I-trichloro-2,2,24richloro-tris-p-chloro-ditungsten(II1) with CrCl, or

VClz To a solution of K3W2C19 (0.240 g, 0.298 mmol) and KC1 (0.023 g, 0.309 mmol) in aq. 6 M HCl(lO0

cm3), CrCl, (0.327 g, 3.02 mmol) or VC12- 4H20 (0.596 g, 3.07 mmol) was added at room temperature and under stirring. After ca 2.5 h the mixture was filtered, the solvent was removed in vacua and the residues were extracted with tetrahydrofuran. The solution was removed by filtering and the residue was washed with small portions of tetrahydrofuran, dried in oacuo, and identified by elemental analysis and spectroscopy to be K3W,Cl,; yield 80%. Reaction of tripotassium 1,l,l-trichloro-2,2,2-trichloro-tris-p-chloro-dimolybdenum(II1) with CrC12

or VCI, The same procedure as described for K3W2Clg or the corresponding [(n-C4H9)4Nj3M02C194 was employed for the reduction of 3 to 1 (6 M HCI, t = 2.5 h); yield 87%. The reaction was monitored by visible spectroscopy in 8-10 M HCI, and mixtures of 1 and 2 depending on the reaction time, were isolated following the same work-up procedure. Reaction of tetra-n-butylammonium-l,l,l-trichlorowith 2,2,2-trichloro-tris-y-chloro-dirhenium(IV) CrC12 or VC12 To a solution of [Bu’&wRe,Cl, (0.502 g, 0.538 mmol) and (Bu’+N)Cl (0.150 g, 0.541 mmol) in tetrahydrofuran (20 cm3) was added at room temperature a solution of CrCl, (0.399 g, 3.25 mmol) or VC12*2CH30H (0.61 g, 3.28 mmol) dissolved in 6 M HCl(5 cm3). The mixture was left to react under stirring ca 2 h and a blue precipitate was formed. This was filtered, washed with methanol dried in uacuo, and identified by elemental analysis and spectroscopy to be (Bu”,--N),Re,Cl, ; yield, 92%.

RESULTS

AND DISCUSSION

When carefully deoxygenated aqueous solutions of one equivalent of K3W2Ci9/KCl and an excess of CrCl,, 9, were mixed at room temperature or at - 5”C, the colour quickly changes and a precipitate is formed. The mixture was filtered, washed with water and tetrahydrofuran leaving behind a greyblack powder insoluble in most common organic solvents. The light green filtrate contains a mixture of CrF* (broad peak at 650-700 nm) and Cri$ (peaks at 412 and 590 nm).” Similar results are obtained when an aq. 0.125 M HCl solution of VCl, (10) was employed. The composition of the precipitate depends on the reaction conditions (acid con-

471

Homogeneous catalytic hydrogen formation

(b)

Fig. 1. (a) Spectrum of K3W2Cl, (6.5 x 10m4 M) in 6 M HCl at 25°C. (b) Spectral changes of a mixture of K3WzC19 (6.9 x 10m4 M) and CrCl, (1.6 x lo-* M) in 6 M HCl at 25°C recorded at 15 min intervals.

and approximately analyses as “WCl,_,(OH),“, x = 0. . . 1. The insolubility of various salts of 6 [K+, (Ph,As)+, (Bu”,N)+] in tetrahydrofuran precluded the reaction at low temperature in this medium where the intensely blue-coloured 4 is more stable’ (598 nm, OC) and attempts to detect or to isolate it as its TMEDA adduct. ’ 3 In mixture(s) of tetrahydrofuran-water, in which all the reactants are soluble, a transient formation of a bluish colour at -20°C is observed but it is followed by fast decomposition as described above. However, the same reaction when carried out in aq. HCl solutions (from 6 to 12 M) produced only small amounts of the insoluble precipitate and hydrogen evolution was observed (identified by GC). The reaction can be followed spectroscopically (Fig. 1). The intensity of the peak at 452 mn attributed to 6 increases whereas a peak at 6 15 nm due to Cr&, probably as a mixture of [Cr(H20)&1]*+ (peaks at 430, 605 nm) and [Cr(H20)4C12]2+ (peaks at 450, 635 run), ’ 7appears. Also, the peak due to V,$$ (860, 560 nm) at 860 mn disappears and a shoulder at 400 nm characteristic of the V&$ ion (peaks at 588, 400, 263)” appears next to the peak of 6 at 452 nm.19 After removing the solvent in uacuo, the residues were extracted with tetrahydrofuran leaving behind a deep green solid identified as 6 ; the filtrates contain Cr”’ in the form of the dichloro complex [Cr(THF),Cl,]+ (peaks at 445 and 640 nm) or V”’ centration)

(peaks at 502, 751 mn). I8 If a large excess of the reducing metal ion was used (from 4 to 200 mol) under identical conditions, the reaction proceeds exothermically and cooling is necessary. Again hydrogen is evolved, 6 was recovered unchanged and 9 or 10 were oxidized quantitatively to their trivalent states. In view of the instability of 4, expected to be even greater in water than in organic solvents, we propose that in aqueous media a two-electron reduction occurs to give 4 [eq. (6)] followed by its fast decomposition and/or hydrolysis :

6

4

+2Cr3++C1-.

(6)

In acid, decomposition is prevented because reactions (2) and (3) intercept reproducing 6, thus the W;+/W:+ couple participates in the catalytic cycle (Fig. 2). The hydrogen evolved comes from reactions (2) and (3). The electrons required are provided by the low-valency metal ions which are consumed (oxidized), respectively. Parallel formation of hydrogen from the reduction of the tungsten hydride (5), similarly as for molybdenum [eq. (l)] is possible, and may occur to a small extent at low acid concentrations in which reaction (3) is slower. With the potassium salt of 3 analogous results are obtained but because the rate of reaction (2) is slow, and (3) does not practically proceed under the reaction conditions, the hydrogen evolved derives from reaction (2). The Mo$+/Moz+ couple essen-

472

C. MERTIS Ha

and N. PSAROUDAKIS

the implications to chemical or biochemical catalysis are further examined.

2M2+

Acknowb&ement-We thank the Greek General Secretariat of Research and Technology for support.

REFERENCES

2M3++Cl-

[‘A$HClg13-

[W,Cl,F

HCI

cl-

Fig. 2. The binuclear tungsten complex 6 catalyses the evolution of Hz in HCl solutions in the presence of excess M& M = Cr, V.

tially catalyses reaction (7)” which is usually very slow even at HCl concentrations as high as 12 M, M2+ +H+ +

M3++jH

(7)

2

M = Cr, V. This cycle can be monitored by electronic spectroscopy (3 with lo), the interconversions +2e-

3-ls2

and V$j -V$$

are dis-

cernible, and the intermediate products could be isolated by controlling the acid concentration (3 with 9). Also, the rhenium complex 8 was easily converted to 7 from the room temperature interaction of one equivalent of Re,Cl,(Bu’+N)/(Bu;-N)Cl in tetrahydrofuran with two equivalent of 9 or 10 in aq. HCl(6 M). The solution instantly from green turns blue and after a while blue crystals precipitate which were characterized to be the tetra-n-butyl ammonium salt of 7. The Cr” and V“ were oxidized to Cr‘” and VI”, respectively, eq. (8) : [Re,Cl,]- + 2M2+ z 8

[Re2Cls127 +2M3++C1-

(8)

M = Cr, V. The inability of the (R&Re)6+ core to undergo reaction (2) renders the reaction stoichiometric instead of catalytic. The mechanistic aspects of these homogeneous electron transfer reactions and

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