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Preparation and characterisation of [M(CN)4O(pz)]2− complexes (M=Mo or W) and their reactivity towards molecular oxygen
www.elsevier.nl/locate/poly Polyhedron 19 (2000) 1503 – 1509
Preparation and characterisation of [M(CN)4O(pz)]2 − complexes (M= Mo or W) and their reactivity towards molecular oxygen Dariusz Matoga a, Janusz Szklarzewicz a, Alina Samotus a,*, John Burgess b, John Fawcett b, David R. Russell b a
Department of Inorganic Chemistry, Faculty of Chemistry, Jagiellonian Uni6ersity, R. Ingardena 3, 30 -060 Cracow, Poland b Department of Chemistry, Uni6ersity of Leicester, Leicester, LE1 7RH, UK Received 14 February 2000; accepted 29 March 2000
Abstract The synthesis and characterisation of (PPh4)2[M(CN)4O(pz)]·3H2O (M = Mo or W; pz =pyrazine) are presented. The salts are reactive towards molecular oxygen, both in solution and in the solid state, with formation of (PPh4)2[M(CN)4O(O2)]. The X-ray crystal structure of the molybdenum compound confirmed the presence of a peroxo ligand cis to the MO bond; the OO bond distance is 1.41 A, . The IR spectra exhibit two absorption bands in the 950 – 850 cm − 1 region assigned to the terminal MO group [917 (Mo) and 933 (W) cm − 1] and the peroxo group [893 (Mo) and 871 (W) cm − 1]. The possible mechanism of molecular oxygen uptake by pyrazine complexes is discussed. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Molybdenum complexes; Tungsten complexes; Pyrazine complexes; Peroxo ligands; Crystal structures
[M(CN)4O(H2O)]2 − + LLn −
1. Introduction It is well known that in aqueous solutions tetracyanodioxometallates(IV), [M(CN)4O2]4 − (M =Mo, W) undergo stepwise protonation reactions K1
[M(CN)4O2]4 − +H+ X [M(CN)4O(OH)]3 − K2
[M(CN)4O(OH)]3 − + H+ X [M(CN)4O(H2O)]2 −
(1) (2)
with the equilibrium constants given by: log K1 =13.00, log K2 = 9.70 and log K1 =11.60, log K2 =7.76 for M = Mo and W, respectively [1,2]. It is also known that the [M(CN)4O(H2O)]2 − ion easily undergoes a considerable number of substitution reactions. It reacts with a variety of monodentate ligands yielding complexes of general formula [M(CN)4O(L)](n + 2) − , where L =CN−, NCS−, F−, N3− , HCN and pyridine (py) [2 – 5]. [M(CN)4O(H2O)]2 − +Ln − [M(CN)4O(L)](n + 2) − +H2O
(3)
Bidentate ligands are coordinated according to the reaction * Corresponding author. Fax: +48-12-6340-515. E-mail address: [email protected] (A. Samotus).
[M(CN)3O(LL)](n + 1) − + H2O+ CN −
(4)
where LL= 2,2%-bipyridyl (bipy), 1,10-phenanthroline (phen), pyridine-2-carboxylate (pic) and several Schiff bases [6]. There is also one known example of a neutral seven-coordinate complex [M(CN)2O(LLLL)] formed in the reaction [M(CN)4O(H2O)]2 − + LLLL [M(CN)2O(LLLL)]+ H2O+ 2CN −
(5)
where the tetradentate ligand LLLL= N,N%-bis[1(pyridin-2-yl)ethylidene]ethane-1,2-diamine [7]. The considerable number of relatively fast reactions of [M(CN)4O(H2O)]2 − with different ligands prompted us to use a similar procedure to obtain tetracyanocomplexes with pyrazine (pz), which forms a variety of complexes with different transition metals [8,9]. The pyrazine ligand seemed interesting for us due to its small dimensions, which imply that the steric factor does not interfere with its coordination and, moreover, it can act as a monodentate ligand or as a bridging ligand. We have found, however, that the only species formed under our experimental conditions was
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D. Matoga et al. / Polyhedron 19 (2000) 1503–1509
[M(CN)4O(pz)]2 − , in which pyrazine is monodentate. The syntheses of new complexes, with their spectroscopic and physicochemical characterisation, and their reactivity towards molecular oxygen are the subject of this report. There is only one known example of molecular oxygen uptake by a similar type of complex. The anhydrous salt (PPh4)3[Mo(CN)5O] has been reported [10] to react with dioxygen according to the equation (PPh4)3[MoIV(CN)5O]+O2 (PPh4)2[MoVI(CN)4O(O2)] +(PPh4)CN
(6)
In this paper we present the spectroscopic and physicochemical characteristics of peroxo complexes of formula (PPh4)2[M(CN)4O(O2)] obtained by us in the reaction of hydrated pyrazine salts with molecular oxygen. We also discuss the mechanism of molecular oxygen incorporation, our proposal differing from that presented earlier [11].
2. Experimental
Tetraphenylphosphonium tetracyanooxoperoxomolybdate(VI), (PPh4)2[Mo(CN)4O(O2)] (3), was prepared by extraction of 1.1 g (1.07 mmol) of 1 three times with 30 ml of acetone and filtering the extracts. The resulting yellow filtrates were combined and the solvent was evaporated in air at room temperature within 20 min. The yellow crystals were washed three times with ethanol and dried in air. Yield: 0.46 g (46%). Anal. Found: C, 67.38; H, 4.25; N, 5.94. Calc. for C52H40MoN4O3P2: C, 67.39; H, 4.35; N, 6.05%. Tetraphenylphosphonium tetracyanooxoperoxotungstate(VI), (PPh4)2[W(CN)4O(O2)] (4), was prepared by dissolving 0.45 g (0.4 mmol) of 2 in 30 ml of acetonitrile. After 60 min, the initially yellow solution turned colourless; it was evaporated in air at room temperature. The resulting white product was washed three times with ethanol and dried in air. Yield: 0.35 g (85%). Anal. Found: C, 61.34; H, 3.92; N, 5.29. Calc. for C52H40N4O3P2W: C, 61.55; H, 3.97; N, 5.52%.
2.3. Reaction of (PPh4)2[M(CN)4O(O2)] with molecular oxygen
2.1. Materials K3Na[M(CN)4O2]·6H2O (M=Mo or W) were prepared as described in the literature [12], and their purity confirmed by their IR spectra. Tetraphenylphosphonium chloride [(PPh4)Cl], pyrazine (pz), triphenylphosphine (PPh3) and standard laboratory reagents were of analytical grade (Sigma, Aldrich) and used as supplied.
2.2. Syntheses Tetraphenylphosphonium tetracyanooxopyrazinemolybdate(IV) was prepared as its trihydrate, (PPh4)2[Mo(CN)4O(pz)]·3H2O (1), by dissolving 2.80 g (5.8 mmol) of K3Na[Mo(CN)4O2]·6H2O and 1.92 g (24 mmol) of pz in 15 ml of water. The pH of the solution was adjusted to ca. 8 with 1 M HCl and a solution of 4.5 g (12 mmol) of (PPh4)Cl in 15 ml of water added. The resulting green crystals were filtered off, washed four times with water and dried in air. Yield: 5.80 g (97%). Anal. Found: C, 65.80; H, 4.96; N, 8.09. Calc. for C56H50MoN6O4P2: C, 65.37; H, 4.90; N, 8.17%. Tetraphenylphosphonium tetracyanooxopyrazine tungstate(IV) was prepared as its trihydrate, (PPh4)2[W(CN)4O(pz)]·3H2O (2), by dissolving 3.3 g (5.8 mmol) of K3Na[W(CN)4O2]·6H2O and 1.92 g (24 mmol) of pz in 15 ml of water. The pH of the solution was adjusted to ca. 7 with 1 M HCl. It was heated to 50°C for ca. 20 min, followed by addition of 4.5 g (12 mmol) of (PPh4)Cl in 15 ml of water. The resulting dark green crystals were filtered off, washed four times with water and dried in air. Yield: 2.46 g (38%). Anal. Found: C, 60.40; H, 4.77; N, 7.53. Calc. for C56H50N6O4P2W: C, 60.22; H, 4.51; N, 7.52%.
The salts 3 or 4 were dissolved in acetonitrile and left in air for about 2 h. In the case of salt 4 no changes were observed and after evaporation of solvent 4 was quantitatively recovered. In the case of 3 its yellow colour disappeared, and after removal of solvent by evaporation a white product of formula (PPh4)2[Mo2O7] was isolated. IR: n(MoO) = 880 cm − 1, n(MoOMo) = 790 cm − 1. Anal. Found: C, 58.06; H, 3.92; N, 0.06. Calc. for C48H40Mo2O7P2: C, 58.66; H, 4.07; N, 0.00%.
2.4. Analytical methods and physical measurements Carbon, nitrogen and hydrogen were determined by organic microanalysis. UV–Vis absorption spectra (Shimadzu 2101 PC) were recorded in the normal way. Reflectance spectra were measured in BaSO4 pellets versus BaSO4 as a reference (Shimadzu 2101 PC equipped with ISR-260 attachment). IR spectra were measured in KBr pellets on a Bruker IFS 48 spectrometer. 1H NMR spectra in CDCl3 were recorded on a Tesla BS-567A spectrometer. ESR spectra were measured on a Bruker ELEXSYS spectrometer at room and liquid nitrogen temperatures using diphenylpicrylhydrazide (dpph) as an internal standard. To estimate the amount of paramagnetic centres, VOSO4 was used as standard.
2.5. Crystal structure determination Crystals of 3 suitable for X-ray crystallography were selected from the recrystallised material prepared as described above. Crystal data for 3, and a summary of
D. Matoga et al. / Polyhedron 19 (2000) 1503–1509
data collection and structure refinement parameters, are given in Table 1. The positions of Mo and P atoms were determined by Patterson methods; the remaining non-hydrogen atoms were located in successive difference Fourier syntheses. Hydrogen atoms were included in the structure factor calculations at idealised positions and were not refined. In the final refinement cycle all non-hydrogen atoms were refined anisotropically. The largest peaks in the final difference maps were located near the Mo atom. All calculations were performed using SHELXTL-PC [13]; scattering factors and anomalous dispersion factors were those given in SHELXTL.
3. Results
3.1. Characterisation of the complexes The reaction of [M(CN)4O(H2O)]2 − with pyrazine under appropriate conditions results in aqua ligand substitution and pyrazine coordination according to the equation [M(CN)4O(H2O)]2 − +pz = [M(CN)4O(pz)]2 − +H2O (7)
Table 1 Crystal data and structure refinement for 3
The equilibrium lies strongly towards the reactants — formation of the product complex can be observed only in concentrated solutions in the presence of a large excess of pyrazine. Complexes 1 and 2 were isolated as tetraphenylphosphonium salts. The (PPh4)+ cation forms insoluble salts only with the product complex thus shifting equilibrium (7) to the right. The results of analysis and physicochemical measurements indicate that pyrazine acts as a monodentate ligand; the possibility of its acting as a bridging ligand between two complex units can be excluded. On the basis of the crystal structure data of all known [M(CN)4O(L)]n − type complexes [3,5,11], the pyrazine ligand is expected to be in the position trans to the MO bond. Both light green 1 and dark green 2 are diamagnetic. In the solid state they are stable when kept under restricted anaerobic conditions. The salts are almost insoluble in water, but soluble in polar organic solvents such as MeCN, CH2Cl2 and DMSO. In dilute aqueous solutions, the only species which can be detected by visible spectroscopy is the [M(CN)4O(H2O)]2 − ion, even in the presence of a large excess of pyrazine. In organic solvents in the presence of air, a fast change of colour from green to yellow is observed. After evaporating the solvent, yellow 3 and white 4 were isolated, indicating that the colour change is connected with molecular oxygen uptake: [M(CN)4O(pz)]2 − + O2 [M(CN)4O(O2)]2 − + pz
Complex
3
Empirical formula Formula weight Temperature (K) Wavelength (A, ) Crystal system Space group Unit cell dimensions a (A, ) b (A, ) c (A, ) a (°) b (°) g (°) Volume (A, 3) Density (Mg m−3) Absorption coefficient (mm−1) Min./max. transmission F(000) Crystal size (mm) q range data collection (°) Reflection collected Independent reflections Observed data [I\2s(I)] Free parameters R1 wR2 Goodness-of-fit Drmax, Drmin (e A, −3)
C52H40N4O3P2Mo 926.76 200(2) 0.71073 triclinic P1( 10.235(4) 10.912(3) 22.557(8) 84.44(2) 82.21(3) 63.64(2) 2234.6(13) 1.377 0.413 0.863, 0.934 952 0.54×0.32×0.08 1.82–26.00 9174 8688 3828 464 0.1127 0.2437 1.058 +1.271, −3.017
1505
(8)
Similar colour changes were observed when solid salts 1 and 2 were kept in air. The final products of solid state reactions were found to be identical with those isolated from the solutions. They were found not to be contaminated with free pyrazine thanks to its high volatility. The salts 3 and 4 are insoluble in water and alcohols and soluble in common organic solvents, such as MeCN, CHCl3, CH2Cl2, DMSO and DMF. As solids they are stable in air at room temperature. In solution, complex 4 is thermally stable, whereas salt 3 undergoes slow decomposition to (PPh4)2[Mo2O7] as confirmed by the elemental analysis and IR spectra of the product formed. Molecular oxygen uptake by cyano-oxo complexes of Mo(IV) was found only for anhydrous (PPh4)3[Mo(CN)5O], and was postulated to be impossible for hydrated salts [11]. Here we have succeeded, for the first time, in demonstrating molecular oxygen uptake by the hydrated oxotetracyano complexes and in isolating the tungsten salt analogous to (PPh4)2[Mo(CN)4O(O2)] described in the literature [10].
3.2. Description of the structure of 3 The structure of 3 is presented in Fig. 1 and selected bond distances and angles are collected in Table 2. The
D. Matoga et al. / Polyhedron 19 (2000) 1503–1509
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in the literature. The Mo(1)O(1) bond distance is shorter (1.671(10) compared with 1.694(3) A, ) and the Mo(1)C(4) and Mo(1)C(3) bond distances are longer (2.296(16) compared with 2.197(4) A, and 2.196(11) compared with 2.167(5) A, , respectively). It is difficult to compare the bond angles with the literature data due to some mistakes in the assignments of angles (e.g. the O(1)Mo(1)C(4) angle (O3MoC4 in their numbering scheme) was reported to be 88.20°, which is impossible for atoms in trans positions). It seems, however, that there are probably some differences in bond angles in the structures of both salts, though the angle subtended by the peroxo ligand is the same in both determinations (43.1(4)° in 3, 43.2(2) in Ref. [10]). The OO bond distance in our crystal, 1.410(14) A, , is the same as that in the earlier reference, 1.416(5) A, ; these distances do not differ significantly from that in the analogous complex [Mo(CN)3O(O2)(hmpa)]−, 1.397(10) A, [10]. Fig. 1. Structure of the [Mo(CN)4O(O)2]2 − ion in 3.
anion has a distorted pentagonal bipyramidal geometry with the OO group cis to the MoO bond. The OO bond distance of 1.41 A, is typical for the peroxo group [14 – 17] and thus the formal oxidation state of the metal is + 6. A salt of the same formula was obtained earlier by Arzoumanian et al. [10] in the reaction of (PPh4)3[Mo(CN)5O] in CH2Cl2 with molecular oxygen and its structure was determined by X-ray crystal analysis. There are, however, some differences in the bond distances and angles between salt 3 and that described
Table 2 Selected bond distances (A, ) and angles (°) for (PPh4)2[Mo(CN)4O(O2)] (3), with estimated standard deviations in parentheses Mo(1)O(1) Mo(1)O(3) Mo(1)C(2) Mo(1)C(4) C(1)N(1) C(3)N(3)
1.671(10) 1.927(9) 2.188(16) 2.296(16) 1.145(18) 1.150(14)
Mo(1)O(2) Mo(1)C(1) Mo(1)C(3) O(2)O(3) C(2)N(2) C(4)N(4)
1.914(9) 2.156(16) 2.196(11) 1.410(14) 1.156(17) 1.145(17)
O(1)Mo(1)O(2) O(2)Mo(1)O(3) O(2)Mo(1)C(1) O(1)Mo(1)C(2) O(3)Mo(1)C(2) O(1)Mo(1)C(3) O(3)Mo(1)C(3) C(2)Mo(1)C(3) O(2)Mo(1)C(4) C(1)Mo(1)C(4) C(3)Mo(1)C(4) O(2)O(3)Mo(1) N(2)C(2)Mo(1) N(4)C(4)Mo(1)
106.5(4) 43.1(4) 81.1(5) 92.3(5) 80.9(5) 87.9(5) 153.5(5) 76.5(5) 86.6(4) 80.3(5) 78.3(5) 68.0(5) 178.1(13) 177.6(12)
O(1)Mo(1)O(3) O(1)Mo(1)C(1) O(3)Mo(1)C(1) O(2)Mo(1)C(2) C(1)Mo(1)C(2) O(2)Mo(1)C(3) C(1)Mo(1)C(3) O(1)Mo(1)C(4) O(3)Mo(1)C(4) C(2)Mo(1)C(4) O(3)O(2)Mo(1) N(1)C(1)Mo(1) N(3)C(3)Mo(1)
106.7(4) 97.0(5) 123.2(5) 123.7(5) 149.7(5) 153.5(5) 75.1(5) 166.2(4) 85.8(4) 83.8(5) 69.0(5) 175.4(13) 176.1(13)
3.3. IR spectra Selected IR data for salts 1 to 4 are presented in Table 3. For comparison data for pyrazine and (PPh4)Cl are also included. The IR spectra of salts 1 and 2 show bands at 974 cm − 1 (1) and at 969 cm − 1 (2) which can be assigned to the stretching vibrations of the MO group. The pyrazine complexes have higher values of n(MO) than analogous six-coordinate cyano-oxo complexes of Mo(IV) and W(IV) with monodentate ligands having the nitrogen atom in the trans position to the MO bond [3,5,11,18] (for L= N3− , CH3CN, HCN band positions are at 953, 940 and 940 cm − 1 for Mo and for L=N3− , NCS−, py at 952, 954 and 964 cm − 1 for W, respectively). This suggests weak bonding of pyrazine and locates the pyrazine ligand between pyridine (n(WO) = 964 cm − 1) and H2O (n(WO) = 980 cm − 1) in the series of ligands arranged according to the decreasing value of n(WO), viz. OH − \ CN − \ F − \ N3− \ NCS − \ pyridine \ pyrazine\ H2O given in the literature [5]. The analogous series for Mo complexes [3,10,11,19] is: OH − \ CN − \ HCN\CH3CN\N3− \ pyrazine \ H2O. In the CN stretching region four bands are observed (Table 3). The larger number of bands with respect to analogous complexes of the [M(CN)4O(L)]n − type indicates lower symmetry for the anions in 1 and 2. The 400–1800 cm − 1 region is dominated by overlapping bands of (PPh4)+ and pyrazine. There are four bands assigned to coordinated pyrazine, at 1017, 1037, 1147 and 1415 cm − 1 for salt 1 and at 1017, 1040, 1148 and 1415 cm − 1 for salt 2. The positions of the bands
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Table 3 Selected IR absorption bands (cm−1) a Compound
n(OO)
n(MO)
n(CN)
1
974 s
2085 m, 2088 s, 2094 s, 2103 vw
2
969 vs
2071 s, 2079 sh, 2082 s, 2092 w
3
893 s
917 s
2127 vvw, 2150 vvw, 2201 vvw
4
871 m
933 s
2132 vvw, 2154 vvw, 2199 vvw
a
Others 712 s, 816 s, 870 s, 1010 s, 1036 w, 1145 m, 1183 vs, 1351 vs, 1408 vs, 1613 vs, 3077 s 444 m, 527 vs, 620 vw, 690 vs, 723 vs, 757 m, 763 m, 814 vw, 853 vw, 998 w, 1017 vw, 1027 vw, 1037 w, 1083 vw, 1108 vs, 1147 vw, 1162 vw, 1187 vw, 1316 w, 1338 vw, 1415 m, 1436 vs, 1442 vs, 1484 m, 1586 w, 1620 vw, 3061 vw, 3407 m, b, 3490 w, sh, 3654 w 444 m, 527 vs, 623 vw, 690 vs, 722 vs, 757 m, 763 m, 814 w, 853 vw, 998 m, 1017 vw, 1027 vw, 1040 w, 1085 vw, 1108 vs, 1148 vw, 1162 vw, 1188 w, 1318 w, 1340 vw, 1415 m, 1436 vs, 1441 vs, 1484 s, 1584 w, 1620 vw, 3061 vw, 3400 m, b, 3470 w, sh, 3651 w 415 w, 527 vs, 620 vw, 690 vs, 722 vs, 754 m, 761 m, 852 vw, 996 m, 1027 vw, 1085 vw, sh, 1108 vs, 1164 vw, 1186 vw, 1315 w, 1340 vw, 1437 s, 1443 s, sh, 1484 m, 1586 w, 1615 vw, 3061 vw 422 w, 526 vs, 623 w, 688 vs, 722 vs, 753 m, 762 m, 853 vw, 995 m, 1029 vw, 1086 vw, sh, 1107 vs, 1163 vw, 1185 vw, 1314 w, 1341 vw, 1435 s, 1441 s, 1483 m, 1585 w, 1612 w, 3056 vw
s, strong; m, medium; w, weak; v, very; sh, shoulder; b, broad.
are only slightly shifted in comparison with free pyrazine (1010, 1036, 1145 and 1408 cm − 1 [20]) in agreement with weak bonding of pyrazine to the metal centre. The IR spectra of 3 and 4 exhibit bands at 917 and 933 cm − 1, assigned to the respective MO stretching vibrations. The bands at 893 cm − 1 (3) and 871 cm − 1 (4) correspond to the stretching vibrations of side on bonded peroxo group [17,21]. The positions of the n(MO) and n(OO) bands are close to those of peroxo complexes of molybdenum(VI) and tungsten(VI) without cyanide ligands [14,15,22]. In the CN stretching region the IR spectra show bands of very low intensity at 2127, 2150 and 2201 cm − 1 for 3 and 2132, 2154 and 2199 cm − 1 for 4. The unusually high position of these CN vibrations in comparison with the cyano-oxo complexes of Mo(IV) and W(IV) is consistent with an increase of the formal oxidation state of the central atom resulting in the decrease of p-back donation which shifts the CN stretching band towards higher energy.
3.4. 1H NMR spectra 1
H NMR spectra for solutions of salts 1 and 2 exhibit signals of protons of pyrazine, aromatic rings of (PPh4)+ cations and water molecules. The four pyrazine protons give a singlet at 8.48 ppm for 1 and at 8.60 ppm for 2, in comparison with 8.60 ppm for free pyrazine [23]. One could expect that pyrazine coordinated as a monodentate ligand should exhibit two doublets of two groups of protons, positioned farther and closer to the metal centre, respectively [9]. The singlet resonances for salts 1 and 2 indicate very weak bonding of pyrazine which causes the protons of pyra-
zine to be chemically equivalent. The upfield shift of the signal for salt 1 shows that in CDCl3 solution weakly bonded pyrazine is still coordinated whereas for salt 2 decomposition accompanied by the release of pyrazine occurs.
3.5. UV–Vis spectra In the solid state reflection spectra show bands at 232, 270, 303 and 650 nm for salt 1 and at 229, 270, 309 and 620 nm for salt 2. In aqueous solutions, the bands observed in the visible region at 610 and 562 nm (for salts 1 and 2, respectively) can be attributed to the [M(CN)4O(H2O)]2 − ions which are formed according to equilibrium (7). In organic solvents such as MeCN, CH2Cl2 and acetone fast formation of 3 and 4 is observed. Fig. 2 presents typical spectral changes observed for a solution of 1 in dichloromethane in the presence of air, together with the spectrum of 3 in the same solvent. The decrease of the band at ca. 621 nm, characteristic of a d–d transition in 1, and the growth of a new band at ca. 393 nm, connected with dioxygen uptake and formation of 3, are observed. The diffuse isosbestic point (Fig. 2) indicates that two species are dominant in the system. The reflection spectra of salts 3 and 4 consist of bands originating from the respective cations at 231 and 276 nm and at 237 and 281 nm, and bands at 420 and 336 nm which can be assigned to charge-transfer to metal (CTTM) transitions. In organic solvents the latter bands are shifted towards higher energy, e.g. in acetonitrile the band maximum is observed at 393 (3) and 323 nm (4) (Fig. 3) with molar absorptivity equal to 465 and 635 M − 1 cm − 1, respectively.
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Fig. 2. Spectral changes of 1 in dichloromethane solution. The arrows show the direction of changes with time (first spectrum measured after 30 s from dissolution, time interval = 90 s). For comparison the spectrum of 3 is given (dashed curve). Conc. = 5.3× 10 − 3 M, optical path = 1 cm.
Fig. 3. Electronic spectra of 3 (solid curve) and of 4 (dashed curve) in acetonitrile.
hydrated salts are not reactive towards molecular oxygen [11]. We have now found that the hydrated tetracyano complexes of Mo(IV) and W(IV) of formula [M(CN)4O(pz)]2 − can react with molecular oxygen, both in solution and in the solid state. The presence of water molecules does not interfere with the reaction with dioxygen. This is probably caused by the fact that the weakly bonded pyrazine ligand does not participate in the network of hydrogen-bonds. Thus, the best site for nucleophilic attack of dioxygen seems to be in the vicinity of pyrazine, whereas the rest of the complex ion is surrounded by water molecules, hydrogen-bonded to cyano and oxo ligands, which hinder the reaction with O2. It seems also that the dissociative mechanism proposed by Arzoumanian et al. [11] is not valid. In this case the incoming dioxygen should be in the trans position to the MO bond, as found for all substitution reactions leading to the [M(CN)4O(L)]n − (n=2, 3) type complexes. In all these cases the coordinatively unsaturated [M(CN)4O]2 − does not undergo rearrangement in the coordination sphere and does not react with dioxygen. The associative mechanism presented in Scheme 1 seems more probable. In the first step the coordination of dioxygen occurs with the increase of coordination number to seven. Such a structure (distorted pentagonal bipyramid) was found by us earlier in [Mo(CN)2O(LLLL)]·H2O [7]. Release of pyrazine followed by a rearrangement in which a cyano group is transferred to the vacant trans position and the second oxygen of dioxygen is bonded to the metal centre could lead to the observed cis-tetracyanooxoperoxomolybdate(VI) anion.
3.6. ESR spectra The ESR spectra for salts 3 and 4 consist of very weak signals, which are typical for paramagnetic centres M(V)– d 1 [24] with g factors (at room temperature) equal to 1.970 and 1.915, respectively. In the case of salt 3 in liquid nitrogen (77 K) an anisotropic signal is observed with g =1.915 and 1.943 indicating the axial symmetry of the complex anion. The amount of M(V) centres in salts 3 and 4 is equal to 0.32 and 0.82%, respectively. This indicates that to some extent thermal charge transfer from the O22 − ligand to the metal occurs, resulting in M(V) and O2− formation.
4. Discussion It was found earlier that anhydrous (PPh4)3[Mo(CN)5O] can react with molecular oxygen giving (PPh4)2[Mo(CN)4O(O2)] [10]. It was also found that this reaction is possible only for the anhydrous salt and that
Scheme 1. Proposed mechanism for molecular oxygen incorporation into the [M(CN)4O(pz)]2 − ion.
D. Matoga et al. / Polyhedron 19 (2000) 1503–1509
5. Supplementary material Full crystallographic data may be obtained from the Cambridge Crystallographic Data Centre, where CIF files have been deposited (deposition no. 139915) Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44-1223-336033; email: [email protected] or www: http:// www.ccdc.cam.ac.uk).
[7] [8] [9] [10] [11] [12]
Acknowledgements [13]
This work was supported in part by the Polish Research Committee, KBN, Grant no. 3T09A 057 17.
[14] [15]
References [1] (a) E. Hejmo, A. Kanas, A. Samotus, Bull. Acad. Polon. Sci. Ser. Sci. Chim. 21 (1973) 311. (b) A. Kanas, M. Dudek, A. Samotus, Bull. Acad. Polon. Sci. Ser. Sci. Chim. 24 (1976) 43. [2] M. Dudek, A. Samotus, Transition Met. Chem. 10 (1985) 271. [3] (a) S.S. Basson, J.G. Leipoldt, I.M. Potgieter, A. Roodt, Inorg. Chim. Acta 103 (1985) 121. (b) J.G. Leipoldt, R. van Eldik, S.S. Basson, A. Roodt, Inorg. Chem. 25 (1986) 4639. (c) J.P. Smit, W. Purcell, A. Roodt, J.G. Leipoldt, J. Chem. Soc., Chem. Commun. (1993) 1388. [4] (a) J.G. Leipoldt, S.S. Basson, A. Roodt, I.M. Potgieter, S. Afr. J. Chem. 39 (1986) 179. (b) I.M. Potgieter, S.S. Basson, A. Roodt, J.G. Leipoldt, Transition Met. Chem. 13 (1988) 209. [5] A. Roodt, J.G. Leipoldt, S.S. Basson, I.M. Potgieter, Transition Met. Chem. 13 (1988) 336. [6] (a) S.S. Basson, J.G. Leipoldt, I.M. Potgieter, Inorg. Chim. Acta 87 (1984) 71. (b) S.S. Basson, J.G. Leipoldt, I.M. Potgieter, Inorg. Chim. Acta 90 (1984) 57. (c) J.G. Leipoldt, S.S. Basson, A. Roodt, I.M. Potgieter, Transition Met. Chem. 11 (1986) 323. (d) J. Szklarzewicz, A. Samotus, N.W. Alcock, M. Moll, J. Chem. Soc., Dalton Trans. (1990) 2959. (e) B. Burda, J. Burgess, S.A. Parsons, A. Samotus, J. Szklarzewicz, Transition Met. Chem. 20 (1995) 291. (f) B. Nowicka, A. Samotus, J. Szklarzewicz, F. Heinemann, H. Kisch, J. Chem. Soc., Dalton
.
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[17] [18] [19]
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Trans. (1998) 4009. (g) J. Szklarzewicz, A. Samotus, D. Matoga, J. Burgess, J. Fawcett, D.R. Russell, Coordination Chemistry at the Turn of the Century, Slovak Technical University Press, Bratislava, vol. 7, 1999, p. 139. J. Szklarzewicz, A. Samotus, J. Burgess, J. Fawcett, D.R. Russell, J. Chem. Soc., Dalton Trans. (1995) 3057. (a) C. Creutz, H. Taube, J. Am. Chem. Soc. 91 (1969) 3988. (b) C. Creutz, H. Taube, J. Am. Chem. Soc. 95 (1973) 1086. C.G. Johnson, R.E. Shepherd, Inorg. Chem. 22 (1983) 1117. H. Arzoumanian, J.F. Petrignani, M. Pierrot, F. Ridouane, J. Sanchez, Inorg. Chem. 27 (1988) 3377. H. Arzoumanian, M. Pierrot, F. Ridouane, J. Sanchez, Transition Met. Chem. 16 (1991) 422. (a) J.P. Smit, W. Purcell, A. Roodt, J.G. Leipoldt, Polyhedron 12 (1993) 2271. (b) A. Roodt, S.S. Basson, J.G. Leipoldt, Polyhedron 13 (1994) 599. G.M. Sheldrick, SHELXTL-PC Release 4.2, Siemens Analytical X-ray Instruments, Madison, WI, 1991. A.C. Dengel, W.P. Griffith, R.D. Powell, A.C. Skapski, J. Chem. Soc., Dalton Trans. (1987) 991. (a) W.P. Griffith, B.C. Parkin, A.J.P. White, D.J. Williams, J. Chem. Soc., Dalton Trans. (1995) 3131. (b) N.M. Gresley, W.P. Griffith, B.C. Parkin, A.J.P. White, D.J. Williams, J. Chem. Soc., Dalton Trans. (1996) 2039. (a) M.H. Dickman, M.T. Pope, Chem. Rev. 94 (1994) 569. (b) M. Sivak, V. Sucha, L. Kuchta, J. Marek, Polyhedron 18 (1998) 93 and references therein. (c) M. Kosugi, S. Hikichi, M. Akita, Y. Moro-oka, J. Chem. Soc., Dalton Trans. (1999) 1369. A. Butler, M.J. Clague, G.E. Meister, Chem. Rev. 94 (1994) 625. A. Roodt, J.G. Leipoldt, S.S. Basson, I.M. Potgieter, Transition Met. Chem. 15 (1990) 439. (a) M. Dudek, A. Kanas, A. Samotus, J. Inorg. Nucl. Chem. 41 (1979) 1135. (b) M. Dudek, A. Kanas, A. Samotus, J. Inorg. Nucl. Chem. 42 (1980) 1701. B. Klein, J. Berkowitz, J. Am. Chem. Soc. 81 (1959) 5160. L.J. Bellamy, The Infra-red Spectra of Complex Molecules, Chapman and Hall, London, 1975, p. 135. R. Bandyopadhyay, S. Biswas, S. Guha, A.K. Mukherjee, R. Bhattacharyya, Chem. Commun. (1999) 1627. C.J. Pouchert, J. Behnke, The Aldrich Library of 13C and 1H FT NMR Spectra, vol. III, Aldrich Chemical Company Inc., 1993. (a) B.R. McGarvey, Inorg. Chem. 5 (1966) 476. (b) R.A. Pribush, R.D. Archer, Inorg. Chem. 13 (1974) 2556. (b) P.M. Kiernan, W.P. Griffith, J. Chem. Soc., Dalton Trans. (1975) 2489. (c) T.J. Kemp, M.A. Shand, D. Rehorek, J. Chem. Soc., Dalton Trans. (1988) 285. (d) J. Szklarzewicz, A. Samotus, O. Traverso, S. Sostero, Polyhedron 13 (1994) 1755.
Preparation and characterisation of [M(CN)4O(pz)]2 − complexes (M= Mo or W) and their reactivity towards molecular oxygen Dariusz Matoga a, Janusz Szklarzewicz a, Alina Samotus a,*, John Burgess b, John Fawcett b, David R. Russell b a
Department of Inorganic Chemistry, Faculty of Chemistry, Jagiellonian Uni6ersity, R. Ingardena 3, 30 -060 Cracow, Poland b Department of Chemistry, Uni6ersity of Leicester, Leicester, LE1 7RH, UK Received 14 February 2000; accepted 29 March 2000
Abstract The synthesis and characterisation of (PPh4)2[M(CN)4O(pz)]·3H2O (M = Mo or W; pz =pyrazine) are presented. The salts are reactive towards molecular oxygen, both in solution and in the solid state, with formation of (PPh4)2[M(CN)4O(O2)]. The X-ray crystal structure of the molybdenum compound confirmed the presence of a peroxo ligand cis to the MO bond; the OO bond distance is 1.41 A, . The IR spectra exhibit two absorption bands in the 950 – 850 cm − 1 region assigned to the terminal MO group [917 (Mo) and 933 (W) cm − 1] and the peroxo group [893 (Mo) and 871 (W) cm − 1]. The possible mechanism of molecular oxygen uptake by pyrazine complexes is discussed. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Molybdenum complexes; Tungsten complexes; Pyrazine complexes; Peroxo ligands; Crystal structures
[M(CN)4O(H2O)]2 − + LLn −
1. Introduction It is well known that in aqueous solutions tetracyanodioxometallates(IV), [M(CN)4O2]4 − (M =Mo, W) undergo stepwise protonation reactions K1
[M(CN)4O2]4 − +H+ X [M(CN)4O(OH)]3 − K2
[M(CN)4O(OH)]3 − + H+ X [M(CN)4O(H2O)]2 −
(1) (2)
with the equilibrium constants given by: log K1 =13.00, log K2 = 9.70 and log K1 =11.60, log K2 =7.76 for M = Mo and W, respectively [1,2]. It is also known that the [M(CN)4O(H2O)]2 − ion easily undergoes a considerable number of substitution reactions. It reacts with a variety of monodentate ligands yielding complexes of general formula [M(CN)4O(L)](n + 2) − , where L =CN−, NCS−, F−, N3− , HCN and pyridine (py) [2 – 5]. [M(CN)4O(H2O)]2 − +Ln − [M(CN)4O(L)](n + 2) − +H2O
(3)
Bidentate ligands are coordinated according to the reaction * Corresponding author. Fax: +48-12-6340-515. E-mail address: [email protected] (A. Samotus).
[M(CN)3O(LL)](n + 1) − + H2O+ CN −
(4)
where LL= 2,2%-bipyridyl (bipy), 1,10-phenanthroline (phen), pyridine-2-carboxylate (pic) and several Schiff bases [6]. There is also one known example of a neutral seven-coordinate complex [M(CN)2O(LLLL)] formed in the reaction [M(CN)4O(H2O)]2 − + LLLL [M(CN)2O(LLLL)]+ H2O+ 2CN −
(5)
where the tetradentate ligand LLLL= N,N%-bis[1(pyridin-2-yl)ethylidene]ethane-1,2-diamine [7]. The considerable number of relatively fast reactions of [M(CN)4O(H2O)]2 − with different ligands prompted us to use a similar procedure to obtain tetracyanocomplexes with pyrazine (pz), which forms a variety of complexes with different transition metals [8,9]. The pyrazine ligand seemed interesting for us due to its small dimensions, which imply that the steric factor does not interfere with its coordination and, moreover, it can act as a monodentate ligand or as a bridging ligand. We have found, however, that the only species formed under our experimental conditions was
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[M(CN)4O(pz)]2 − , in which pyrazine is monodentate. The syntheses of new complexes, with their spectroscopic and physicochemical characterisation, and their reactivity towards molecular oxygen are the subject of this report. There is only one known example of molecular oxygen uptake by a similar type of complex. The anhydrous salt (PPh4)3[Mo(CN)5O] has been reported [10] to react with dioxygen according to the equation (PPh4)3[MoIV(CN)5O]+O2 (PPh4)2[MoVI(CN)4O(O2)] +(PPh4)CN
(6)
In this paper we present the spectroscopic and physicochemical characteristics of peroxo complexes of formula (PPh4)2[M(CN)4O(O2)] obtained by us in the reaction of hydrated pyrazine salts with molecular oxygen. We also discuss the mechanism of molecular oxygen incorporation, our proposal differing from that presented earlier [11].
2. Experimental
Tetraphenylphosphonium tetracyanooxoperoxomolybdate(VI), (PPh4)2[Mo(CN)4O(O2)] (3), was prepared by extraction of 1.1 g (1.07 mmol) of 1 three times with 30 ml of acetone and filtering the extracts. The resulting yellow filtrates were combined and the solvent was evaporated in air at room temperature within 20 min. The yellow crystals were washed three times with ethanol and dried in air. Yield: 0.46 g (46%). Anal. Found: C, 67.38; H, 4.25; N, 5.94. Calc. for C52H40MoN4O3P2: C, 67.39; H, 4.35; N, 6.05%. Tetraphenylphosphonium tetracyanooxoperoxotungstate(VI), (PPh4)2[W(CN)4O(O2)] (4), was prepared by dissolving 0.45 g (0.4 mmol) of 2 in 30 ml of acetonitrile. After 60 min, the initially yellow solution turned colourless; it was evaporated in air at room temperature. The resulting white product was washed three times with ethanol and dried in air. Yield: 0.35 g (85%). Anal. Found: C, 61.34; H, 3.92; N, 5.29. Calc. for C52H40N4O3P2W: C, 61.55; H, 3.97; N, 5.52%.
2.3. Reaction of (PPh4)2[M(CN)4O(O2)] with molecular oxygen
2.1. Materials K3Na[M(CN)4O2]·6H2O (M=Mo or W) were prepared as described in the literature [12], and their purity confirmed by their IR spectra. Tetraphenylphosphonium chloride [(PPh4)Cl], pyrazine (pz), triphenylphosphine (PPh3) and standard laboratory reagents were of analytical grade (Sigma, Aldrich) and used as supplied.
2.2. Syntheses Tetraphenylphosphonium tetracyanooxopyrazinemolybdate(IV) was prepared as its trihydrate, (PPh4)2[Mo(CN)4O(pz)]·3H2O (1), by dissolving 2.80 g (5.8 mmol) of K3Na[Mo(CN)4O2]·6H2O and 1.92 g (24 mmol) of pz in 15 ml of water. The pH of the solution was adjusted to ca. 8 with 1 M HCl and a solution of 4.5 g (12 mmol) of (PPh4)Cl in 15 ml of water added. The resulting green crystals were filtered off, washed four times with water and dried in air. Yield: 5.80 g (97%). Anal. Found: C, 65.80; H, 4.96; N, 8.09. Calc. for C56H50MoN6O4P2: C, 65.37; H, 4.90; N, 8.17%. Tetraphenylphosphonium tetracyanooxopyrazine tungstate(IV) was prepared as its trihydrate, (PPh4)2[W(CN)4O(pz)]·3H2O (2), by dissolving 3.3 g (5.8 mmol) of K3Na[W(CN)4O2]·6H2O and 1.92 g (24 mmol) of pz in 15 ml of water. The pH of the solution was adjusted to ca. 7 with 1 M HCl. It was heated to 50°C for ca. 20 min, followed by addition of 4.5 g (12 mmol) of (PPh4)Cl in 15 ml of water. The resulting dark green crystals were filtered off, washed four times with water and dried in air. Yield: 2.46 g (38%). Anal. Found: C, 60.40; H, 4.77; N, 7.53. Calc. for C56H50N6O4P2W: C, 60.22; H, 4.51; N, 7.52%.
The salts 3 or 4 were dissolved in acetonitrile and left in air for about 2 h. In the case of salt 4 no changes were observed and after evaporation of solvent 4 was quantitatively recovered. In the case of 3 its yellow colour disappeared, and after removal of solvent by evaporation a white product of formula (PPh4)2[Mo2O7] was isolated. IR: n(MoO) = 880 cm − 1, n(MoOMo) = 790 cm − 1. Anal. Found: C, 58.06; H, 3.92; N, 0.06. Calc. for C48H40Mo2O7P2: C, 58.66; H, 4.07; N, 0.00%.
2.4. Analytical methods and physical measurements Carbon, nitrogen and hydrogen were determined by organic microanalysis. UV–Vis absorption spectra (Shimadzu 2101 PC) were recorded in the normal way. Reflectance spectra were measured in BaSO4 pellets versus BaSO4 as a reference (Shimadzu 2101 PC equipped with ISR-260 attachment). IR spectra were measured in KBr pellets on a Bruker IFS 48 spectrometer. 1H NMR spectra in CDCl3 were recorded on a Tesla BS-567A spectrometer. ESR spectra were measured on a Bruker ELEXSYS spectrometer at room and liquid nitrogen temperatures using diphenylpicrylhydrazide (dpph) as an internal standard. To estimate the amount of paramagnetic centres, VOSO4 was used as standard.
2.5. Crystal structure determination Crystals of 3 suitable for X-ray crystallography were selected from the recrystallised material prepared as described above. Crystal data for 3, and a summary of
D. Matoga et al. / Polyhedron 19 (2000) 1503–1509
data collection and structure refinement parameters, are given in Table 1. The positions of Mo and P atoms were determined by Patterson methods; the remaining non-hydrogen atoms were located in successive difference Fourier syntheses. Hydrogen atoms were included in the structure factor calculations at idealised positions and were not refined. In the final refinement cycle all non-hydrogen atoms were refined anisotropically. The largest peaks in the final difference maps were located near the Mo atom. All calculations were performed using SHELXTL-PC [13]; scattering factors and anomalous dispersion factors were those given in SHELXTL.
3. Results
3.1. Characterisation of the complexes The reaction of [M(CN)4O(H2O)]2 − with pyrazine under appropriate conditions results in aqua ligand substitution and pyrazine coordination according to the equation [M(CN)4O(H2O)]2 − +pz = [M(CN)4O(pz)]2 − +H2O (7)
Table 1 Crystal data and structure refinement for 3
The equilibrium lies strongly towards the reactants — formation of the product complex can be observed only in concentrated solutions in the presence of a large excess of pyrazine. Complexes 1 and 2 were isolated as tetraphenylphosphonium salts. The (PPh4)+ cation forms insoluble salts only with the product complex thus shifting equilibrium (7) to the right. The results of analysis and physicochemical measurements indicate that pyrazine acts as a monodentate ligand; the possibility of its acting as a bridging ligand between two complex units can be excluded. On the basis of the crystal structure data of all known [M(CN)4O(L)]n − type complexes [3,5,11], the pyrazine ligand is expected to be in the position trans to the MO bond. Both light green 1 and dark green 2 are diamagnetic. In the solid state they are stable when kept under restricted anaerobic conditions. The salts are almost insoluble in water, but soluble in polar organic solvents such as MeCN, CH2Cl2 and DMSO. In dilute aqueous solutions, the only species which can be detected by visible spectroscopy is the [M(CN)4O(H2O)]2 − ion, even in the presence of a large excess of pyrazine. In organic solvents in the presence of air, a fast change of colour from green to yellow is observed. After evaporating the solvent, yellow 3 and white 4 were isolated, indicating that the colour change is connected with molecular oxygen uptake: [M(CN)4O(pz)]2 − + O2 [M(CN)4O(O2)]2 − + pz
Complex
3
Empirical formula Formula weight Temperature (K) Wavelength (A, ) Crystal system Space group Unit cell dimensions a (A, ) b (A, ) c (A, ) a (°) b (°) g (°) Volume (A, 3) Density (Mg m−3) Absorption coefficient (mm−1) Min./max. transmission F(000) Crystal size (mm) q range data collection (°) Reflection collected Independent reflections Observed data [I\2s(I)] Free parameters R1 wR2 Goodness-of-fit Drmax, Drmin (e A, −3)
C52H40N4O3P2Mo 926.76 200(2) 0.71073 triclinic P1( 10.235(4) 10.912(3) 22.557(8) 84.44(2) 82.21(3) 63.64(2) 2234.6(13) 1.377 0.413 0.863, 0.934 952 0.54×0.32×0.08 1.82–26.00 9174 8688 3828 464 0.1127 0.2437 1.058 +1.271, −3.017
1505
(8)
Similar colour changes were observed when solid salts 1 and 2 were kept in air. The final products of solid state reactions were found to be identical with those isolated from the solutions. They were found not to be contaminated with free pyrazine thanks to its high volatility. The salts 3 and 4 are insoluble in water and alcohols and soluble in common organic solvents, such as MeCN, CHCl3, CH2Cl2, DMSO and DMF. As solids they are stable in air at room temperature. In solution, complex 4 is thermally stable, whereas salt 3 undergoes slow decomposition to (PPh4)2[Mo2O7] as confirmed by the elemental analysis and IR spectra of the product formed. Molecular oxygen uptake by cyano-oxo complexes of Mo(IV) was found only for anhydrous (PPh4)3[Mo(CN)5O], and was postulated to be impossible for hydrated salts [11]. Here we have succeeded, for the first time, in demonstrating molecular oxygen uptake by the hydrated oxotetracyano complexes and in isolating the tungsten salt analogous to (PPh4)2[Mo(CN)4O(O2)] described in the literature [10].
3.2. Description of the structure of 3 The structure of 3 is presented in Fig. 1 and selected bond distances and angles are collected in Table 2. The
D. Matoga et al. / Polyhedron 19 (2000) 1503–1509
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in the literature. The Mo(1)O(1) bond distance is shorter (1.671(10) compared with 1.694(3) A, ) and the Mo(1)C(4) and Mo(1)C(3) bond distances are longer (2.296(16) compared with 2.197(4) A, and 2.196(11) compared with 2.167(5) A, , respectively). It is difficult to compare the bond angles with the literature data due to some mistakes in the assignments of angles (e.g. the O(1)Mo(1)C(4) angle (O3MoC4 in their numbering scheme) was reported to be 88.20°, which is impossible for atoms in trans positions). It seems, however, that there are probably some differences in bond angles in the structures of both salts, though the angle subtended by the peroxo ligand is the same in both determinations (43.1(4)° in 3, 43.2(2) in Ref. [10]). The OO bond distance in our crystal, 1.410(14) A, , is the same as that in the earlier reference, 1.416(5) A, ; these distances do not differ significantly from that in the analogous complex [Mo(CN)3O(O2)(hmpa)]−, 1.397(10) A, [10]. Fig. 1. Structure of the [Mo(CN)4O(O)2]2 − ion in 3.
anion has a distorted pentagonal bipyramidal geometry with the OO group cis to the MoO bond. The OO bond distance of 1.41 A, is typical for the peroxo group [14 – 17] and thus the formal oxidation state of the metal is + 6. A salt of the same formula was obtained earlier by Arzoumanian et al. [10] in the reaction of (PPh4)3[Mo(CN)5O] in CH2Cl2 with molecular oxygen and its structure was determined by X-ray crystal analysis. There are, however, some differences in the bond distances and angles between salt 3 and that described
Table 2 Selected bond distances (A, ) and angles (°) for (PPh4)2[Mo(CN)4O(O2)] (3), with estimated standard deviations in parentheses Mo(1)O(1) Mo(1)O(3) Mo(1)C(2) Mo(1)C(4) C(1)N(1) C(3)N(3)
1.671(10) 1.927(9) 2.188(16) 2.296(16) 1.145(18) 1.150(14)
Mo(1)O(2) Mo(1)C(1) Mo(1)C(3) O(2)O(3) C(2)N(2) C(4)N(4)
1.914(9) 2.156(16) 2.196(11) 1.410(14) 1.156(17) 1.145(17)
O(1)Mo(1)O(2) O(2)Mo(1)O(3) O(2)Mo(1)C(1) O(1)Mo(1)C(2) O(3)Mo(1)C(2) O(1)Mo(1)C(3) O(3)Mo(1)C(3) C(2)Mo(1)C(3) O(2)Mo(1)C(4) C(1)Mo(1)C(4) C(3)Mo(1)C(4) O(2)O(3)Mo(1) N(2)C(2)Mo(1) N(4)C(4)Mo(1)
106.5(4) 43.1(4) 81.1(5) 92.3(5) 80.9(5) 87.9(5) 153.5(5) 76.5(5) 86.6(4) 80.3(5) 78.3(5) 68.0(5) 178.1(13) 177.6(12)
O(1)Mo(1)O(3) O(1)Mo(1)C(1) O(3)Mo(1)C(1) O(2)Mo(1)C(2) C(1)Mo(1)C(2) O(2)Mo(1)C(3) C(1)Mo(1)C(3) O(1)Mo(1)C(4) O(3)Mo(1)C(4) C(2)Mo(1)C(4) O(3)O(2)Mo(1) N(1)C(1)Mo(1) N(3)C(3)Mo(1)
106.7(4) 97.0(5) 123.2(5) 123.7(5) 149.7(5) 153.5(5) 75.1(5) 166.2(4) 85.8(4) 83.8(5) 69.0(5) 175.4(13) 176.1(13)
3.3. IR spectra Selected IR data for salts 1 to 4 are presented in Table 3. For comparison data for pyrazine and (PPh4)Cl are also included. The IR spectra of salts 1 and 2 show bands at 974 cm − 1 (1) and at 969 cm − 1 (2) which can be assigned to the stretching vibrations of the MO group. The pyrazine complexes have higher values of n(MO) than analogous six-coordinate cyano-oxo complexes of Mo(IV) and W(IV) with monodentate ligands having the nitrogen atom in the trans position to the MO bond [3,5,11,18] (for L= N3− , CH3CN, HCN band positions are at 953, 940 and 940 cm − 1 for Mo and for L=N3− , NCS−, py at 952, 954 and 964 cm − 1 for W, respectively). This suggests weak bonding of pyrazine and locates the pyrazine ligand between pyridine (n(WO) = 964 cm − 1) and H2O (n(WO) = 980 cm − 1) in the series of ligands arranged according to the decreasing value of n(WO), viz. OH − \ CN − \ F − \ N3− \ NCS − \ pyridine \ pyrazine\ H2O given in the literature [5]. The analogous series for Mo complexes [3,10,11,19] is: OH − \ CN − \ HCN\CH3CN\N3− \ pyrazine \ H2O. In the CN stretching region four bands are observed (Table 3). The larger number of bands with respect to analogous complexes of the [M(CN)4O(L)]n − type indicates lower symmetry for the anions in 1 and 2. The 400–1800 cm − 1 region is dominated by overlapping bands of (PPh4)+ and pyrazine. There are four bands assigned to coordinated pyrazine, at 1017, 1037, 1147 and 1415 cm − 1 for salt 1 and at 1017, 1040, 1148 and 1415 cm − 1 for salt 2. The positions of the bands
D. Matoga et al. / Polyhedron 19 (2000) 1503–1509
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Table 3 Selected IR absorption bands (cm−1) a Compound
n(OO)
n(MO)
n(CN)
1
974 s
2085 m, 2088 s, 2094 s, 2103 vw
2
969 vs
2071 s, 2079 sh, 2082 s, 2092 w
3
893 s
917 s
2127 vvw, 2150 vvw, 2201 vvw
4
871 m
933 s
2132 vvw, 2154 vvw, 2199 vvw
a
Others 712 s, 816 s, 870 s, 1010 s, 1036 w, 1145 m, 1183 vs, 1351 vs, 1408 vs, 1613 vs, 3077 s 444 m, 527 vs, 620 vw, 690 vs, 723 vs, 757 m, 763 m, 814 vw, 853 vw, 998 w, 1017 vw, 1027 vw, 1037 w, 1083 vw, 1108 vs, 1147 vw, 1162 vw, 1187 vw, 1316 w, 1338 vw, 1415 m, 1436 vs, 1442 vs, 1484 m, 1586 w, 1620 vw, 3061 vw, 3407 m, b, 3490 w, sh, 3654 w 444 m, 527 vs, 623 vw, 690 vs, 722 vs, 757 m, 763 m, 814 w, 853 vw, 998 m, 1017 vw, 1027 vw, 1040 w, 1085 vw, 1108 vs, 1148 vw, 1162 vw, 1188 w, 1318 w, 1340 vw, 1415 m, 1436 vs, 1441 vs, 1484 s, 1584 w, 1620 vw, 3061 vw, 3400 m, b, 3470 w, sh, 3651 w 415 w, 527 vs, 620 vw, 690 vs, 722 vs, 754 m, 761 m, 852 vw, 996 m, 1027 vw, 1085 vw, sh, 1108 vs, 1164 vw, 1186 vw, 1315 w, 1340 vw, 1437 s, 1443 s, sh, 1484 m, 1586 w, 1615 vw, 3061 vw 422 w, 526 vs, 623 w, 688 vs, 722 vs, 753 m, 762 m, 853 vw, 995 m, 1029 vw, 1086 vw, sh, 1107 vs, 1163 vw, 1185 vw, 1314 w, 1341 vw, 1435 s, 1441 s, 1483 m, 1585 w, 1612 w, 3056 vw
s, strong; m, medium; w, weak; v, very; sh, shoulder; b, broad.
are only slightly shifted in comparison with free pyrazine (1010, 1036, 1145 and 1408 cm − 1 [20]) in agreement with weak bonding of pyrazine to the metal centre. The IR spectra of 3 and 4 exhibit bands at 917 and 933 cm − 1, assigned to the respective MO stretching vibrations. The bands at 893 cm − 1 (3) and 871 cm − 1 (4) correspond to the stretching vibrations of side on bonded peroxo group [17,21]. The positions of the n(MO) and n(OO) bands are close to those of peroxo complexes of molybdenum(VI) and tungsten(VI) without cyanide ligands [14,15,22]. In the CN stretching region the IR spectra show bands of very low intensity at 2127, 2150 and 2201 cm − 1 for 3 and 2132, 2154 and 2199 cm − 1 for 4. The unusually high position of these CN vibrations in comparison with the cyano-oxo complexes of Mo(IV) and W(IV) is consistent with an increase of the formal oxidation state of the central atom resulting in the decrease of p-back donation which shifts the CN stretching band towards higher energy.
3.4. 1H NMR spectra 1
H NMR spectra for solutions of salts 1 and 2 exhibit signals of protons of pyrazine, aromatic rings of (PPh4)+ cations and water molecules. The four pyrazine protons give a singlet at 8.48 ppm for 1 and at 8.60 ppm for 2, in comparison with 8.60 ppm for free pyrazine [23]. One could expect that pyrazine coordinated as a monodentate ligand should exhibit two doublets of two groups of protons, positioned farther and closer to the metal centre, respectively [9]. The singlet resonances for salts 1 and 2 indicate very weak bonding of pyrazine which causes the protons of pyra-
zine to be chemically equivalent. The upfield shift of the signal for salt 1 shows that in CDCl3 solution weakly bonded pyrazine is still coordinated whereas for salt 2 decomposition accompanied by the release of pyrazine occurs.
3.5. UV–Vis spectra In the solid state reflection spectra show bands at 232, 270, 303 and 650 nm for salt 1 and at 229, 270, 309 and 620 nm for salt 2. In aqueous solutions, the bands observed in the visible region at 610 and 562 nm (for salts 1 and 2, respectively) can be attributed to the [M(CN)4O(H2O)]2 − ions which are formed according to equilibrium (7). In organic solvents such as MeCN, CH2Cl2 and acetone fast formation of 3 and 4 is observed. Fig. 2 presents typical spectral changes observed for a solution of 1 in dichloromethane in the presence of air, together with the spectrum of 3 in the same solvent. The decrease of the band at ca. 621 nm, characteristic of a d–d transition in 1, and the growth of a new band at ca. 393 nm, connected with dioxygen uptake and formation of 3, are observed. The diffuse isosbestic point (Fig. 2) indicates that two species are dominant in the system. The reflection spectra of salts 3 and 4 consist of bands originating from the respective cations at 231 and 276 nm and at 237 and 281 nm, and bands at 420 and 336 nm which can be assigned to charge-transfer to metal (CTTM) transitions. In organic solvents the latter bands are shifted towards higher energy, e.g. in acetonitrile the band maximum is observed at 393 (3) and 323 nm (4) (Fig. 3) with molar absorptivity equal to 465 and 635 M − 1 cm − 1, respectively.
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Fig. 2. Spectral changes of 1 in dichloromethane solution. The arrows show the direction of changes with time (first spectrum measured after 30 s from dissolution, time interval = 90 s). For comparison the spectrum of 3 is given (dashed curve). Conc. = 5.3× 10 − 3 M, optical path = 1 cm.
Fig. 3. Electronic spectra of 3 (solid curve) and of 4 (dashed curve) in acetonitrile.
hydrated salts are not reactive towards molecular oxygen [11]. We have now found that the hydrated tetracyano complexes of Mo(IV) and W(IV) of formula [M(CN)4O(pz)]2 − can react with molecular oxygen, both in solution and in the solid state. The presence of water molecules does not interfere with the reaction with dioxygen. This is probably caused by the fact that the weakly bonded pyrazine ligand does not participate in the network of hydrogen-bonds. Thus, the best site for nucleophilic attack of dioxygen seems to be in the vicinity of pyrazine, whereas the rest of the complex ion is surrounded by water molecules, hydrogen-bonded to cyano and oxo ligands, which hinder the reaction with O2. It seems also that the dissociative mechanism proposed by Arzoumanian et al. [11] is not valid. In this case the incoming dioxygen should be in the trans position to the MO bond, as found for all substitution reactions leading to the [M(CN)4O(L)]n − (n=2, 3) type complexes. In all these cases the coordinatively unsaturated [M(CN)4O]2 − does not undergo rearrangement in the coordination sphere and does not react with dioxygen. The associative mechanism presented in Scheme 1 seems more probable. In the first step the coordination of dioxygen occurs with the increase of coordination number to seven. Such a structure (distorted pentagonal bipyramid) was found by us earlier in [Mo(CN)2O(LLLL)]·H2O [7]. Release of pyrazine followed by a rearrangement in which a cyano group is transferred to the vacant trans position and the second oxygen of dioxygen is bonded to the metal centre could lead to the observed cis-tetracyanooxoperoxomolybdate(VI) anion.
3.6. ESR spectra The ESR spectra for salts 3 and 4 consist of very weak signals, which are typical for paramagnetic centres M(V)– d 1 [24] with g factors (at room temperature) equal to 1.970 and 1.915, respectively. In the case of salt 3 in liquid nitrogen (77 K) an anisotropic signal is observed with g =1.915 and 1.943 indicating the axial symmetry of the complex anion. The amount of M(V) centres in salts 3 and 4 is equal to 0.32 and 0.82%, respectively. This indicates that to some extent thermal charge transfer from the O22 − ligand to the metal occurs, resulting in M(V) and O2− formation.
4. Discussion It was found earlier that anhydrous (PPh4)3[Mo(CN)5O] can react with molecular oxygen giving (PPh4)2[Mo(CN)4O(O2)] [10]. It was also found that this reaction is possible only for the anhydrous salt and that
Scheme 1. Proposed mechanism for molecular oxygen incorporation into the [M(CN)4O(pz)]2 − ion.
D. Matoga et al. / Polyhedron 19 (2000) 1503–1509
5. Supplementary material Full crystallographic data may be obtained from the Cambridge Crystallographic Data Centre, where CIF files have been deposited (deposition no. 139915) Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44-1223-336033; email: [email protected] or www: http:// www.ccdc.cam.ac.uk).
[7] [8] [9] [10] [11] [12]
Acknowledgements [13]
This work was supported in part by the Polish Research Committee, KBN, Grant no. 3T09A 057 17.
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