Heterobinuclear 0x0-Bridged Complexes. 111* CrlI1 OMoV ...

Aust. J. Chem., 1992, 45, 889-896

Heterobinuclear 0x0-Bridged Complexes. 111* CrlI1OMoV Compounds-S ynthesis and Magnetic Properties Robyn L. Elliott, Paul Kruger, Keith S. Murray and Bruce 0. West Department of Chemistry, Monash University, Clayton, Vic. 3168.

Abstract Redox reactions between crIVO(p) (P is a porphyrin dianion) and hfoNO(dtc)z (dtc is a dithiocarbamate anion) result in the formation of 0x0-bridged C ~ ' ~ ' O M O compounds. ~ The variation of magnetic susceptibility with temperature to 25 K shows evidence for strong antiferromagnetic coupling for the pyrrolidyldithiocarbamate complex with J = -300 cm-l.

Introduction A number of ~ t u d i e s l -have ~ established that 0x0-bridged hIoV complexes can be formed by redox reactions between cis-dioxo MoV1 and monooxo MoIV species. We have carried out redox reactions between CrIV 0x0 and M O I ~compounds to achieve heterobinuclear Crl"-0-MoV complexes. The same Cr"'/MoV products can be formed by reaction of the related CrI1 and MO"' reagents. A preliminary report4 on such reactions has appeared, and this paper provides full experimental details for the reactions together with a magnetic susceptibility study over a temperature range to determine the degree of magnetic coupling between the metals. Results and Discussion Synthesis

A dark wine-red coloured solution of an oxochromium(~v)porphyrin, CrO(P) [P is the 5,10,15,20-tetraphenylporphyrindianion (tpp) or the p-methoxyphenyl analogue (tmp)], in tetrahydrofuran changes slowly to red-green then darkens upon addition of a solution of the pink-violet M 0 ' ~ 0 ( d t c )or~ M O ' ~(NR) (dtc)2 complexes [dtc = diethyldithiocarbamate (edtc) or pyrrolidyldithiocarbamatet (pdtc) anions]. The reactions were carried out under strictly anaerobic and anhydrous conditions. * Part 11, Aust. J. Chem., 1990, 43, 707. t The term 'pyrrolidyldithiocarbamate' is used to represent the pyrrolidine-1-carbodithioate anion [C4HsNC(S)S-] for convenience in comparison with the diethyldithiocarbamate anion. Barral, R., Bocard, C.,'Serre de Roch, I., and Sajus, L., Tetrahedron Lett., 1972, 17, 1693. Matsuda, T., Tanaka, K., and Tan&, T., Inorg. Chem., 1979, 18, 454. Reynolds, M. S., Berg, J. M., and Holm, R. H., Inorg. Chem., 1984, 23, 3057. Elliott, R. L., Nichols, P. J., and West, B. O., J. Chem. Soc., Chem. Commun., 1986, 840. Manuscript received 22 October 1991

00049425/92/050889$05.00

R. L. Elliott et al.

The desired complexes could be precipitated by addition of hexane as brown-purple solid products in relatively high yields. The materials are soluble in a range of solvents giving dichroic (redlgreen) solutions. The complexes have the empirical formula (P)CrOMo(O)( d t c ) ~ . Reactions between Crl'(tpp) and Mov102( d t c ) ~ complexes gave the same products. The compounds have room-temperature magnetic moments (Table 1) which are reduced below the spin-only value expected for an uncoupled d3 :dl CrlI1. . -MoV system, p, = 4.24 BM. Table 1. Magnetic susceptibility and infrared spectral data for CrOMo compounds and related substances Complex

v (cm-l)

P ~ R

(calc. per dimer unit) 300K 4.2K

CrOMo 160

180

MOO 160

lSo

4

The spin-only value for an uncoupled S = # ~ r " ' , S = M O ~system is 4.24 (BM). An increase in intensity occurs in porphyrin ligand vibrations centred at these frequencies. Liston, D. J., Kennedy, B. J., Murray, K. S., and West, B. O., Inorg. Chem., 1985, 24, 1561. Diamagnetic.

A

Infrared Spectra The two characteristic absorptions for terminal Mo=O stretching frequencies observed in c i s - ~ o ~ I ( O ) ~ ( dcompounds tc)~ around 910 and 880 cm-I are reduced to a single peak in the region 980-920 cm-I arising from Mo=O vibrations in ~0 the homobinuclear [ ( d t ~ ) ~ ~ o complexe~.~ ~0] The heterobinuclear compounds prepared in this work each show a single frequency near 930 cm-l. Such a band is absent from the spectrum of the complex prepared from CrIvO(tpp) and the monoorganoirnido species [MoIV(~p-tol)(pdtc)z] (N-p-to1 represents the p-tolylimido ion), a result indicating that Cr-0-Mo bridging must occur in that compound. Devore and Maatta6 have reported that the reaction of MoIv(0) (edtc)~and MoV1(N-p-tol) (0)(edtc)2 results in an MoV-0-MoV bridged complex. The Cr-0-Mo(N-p-tol) complex of the present work does show an absorption at 830 cm-I similar in position and intensity to bands found in the spectra of the other heterobinuclear species (Table 1). The symmetrical homobinuclear 0x0-bridged complexes do not display bands in this region, and the absorptions in the 830-845 cm-I region were therefore assigned to stretching frequencies associated with the Cr-0-Mo bridge. Support for the assignment was sought by examining the Stiefel, E. I., Prog. Inorg. Chem., 1977, 22, 1. Devore, D. D., and Maatta, E. A., Inorg. Chem., 1985, 24, 2846.

Heterobinuclear 0x0-Bridged Complexes. I11

180-labelled complex (tpp)Cr180Mo(180)(pdt~)2, prepared by the reaction of CrI1(tpp) with MoV1(180)2(pdt~)2(the latter having been synthesized with a 30% '0 enrichment). The 929 cm-l band of the normal complex was much diminished in intensity. The MO-~~O frequency expected at 885 cm-' was not directly observable because of porphyrin ring vibration bands in that region. Some increase in general intensity of absorption in that region did occur however. ]~O from MorVO(pdtc)2 and The homobinuclear complex [Mo(O)( p d t ~ ) ~ prepared ls0-labelled ~ o ~ ' ( O ) ~ ( ~ shows d t c ) an ~ intense new band at 884 cm-l with a greatly diminished band at 929 cm-l which would be normally allocated to the terminal Mo=O stretching frequency, the Mo=O bond being normally cis to the Mo-0-Mo bridging oxygen in such dithiocarbamate complexe~.~ The only known trans O=MoOMo=O arrangement to have been confirmed by an X-ray 0 structural study has been found in the porphyrin complex [ ~ o ~ ( O ) ( t p p ) ] 2and the infrared frequency considered to arise from vibrations of the trans terminal Mo=O groups, although still in the 900 cm-I region, is reported to be of weak intensity.' The rather strong bands in the 900 cm-I region observed for the present complexes appear to confirm the cis arrangement of terminal and bridging oxygens. The ls0-labelled heteronucleax complex also shows diminished intensity for a band at 835 cm-l found in the unlabelled complex while a detectable increase in intensity occurred in a porphyrin ligand vibration centred at c. 800 cm-l. The other CrOMo bridged complexes each show an intense band in the region 830-845 cm-l, and it is concluded that these frequencies relate to vibrations in the CrOMo bridge. The labelled complex would be expected to show a new peak at 785 cm-I, the region hidden by porphyrin ligand vibrational bands. Stability of the Complexes Exposing a sample of (tpp)CrOM~(O)(pdtc)~ to laboratory air and monitoring its infrared spectrum from time to time over several weeks disclosed that the solid complex was undergoing change. In particular, the strong absorption at 929 cm-l, indicative of the terminal Mo=O group, diminished in intensity over several weeks while a pronounced shoulder appeared at 915 cm-I together with a new peak at c. 880 cm-l. The band at 835 cm-l indicative of the CrOMo bridge decreased in intensity. A broad absorption in the 3200 cm-I region developed, indicating OH was now present. These spectral changes occurred more rapidly for the ethyldithiocarbamate complex and were quite noticeable after several days. The evidence suggests that the MoV1(0)2unit was forming in the complexes together with CrlI1-OH species by a combination of hydrolytic attack and aerial oxidation. Attempts to obtain accurate visible spectra of the complexes at the low concentrations ( 1 0 - 6 ~ ) needed to recognize the intense Soret band of the porphyrin ligands in the complexes were also frustrated by the facile hydrolysis of the materials even in solvents dried by conventional means. Each complex recorded a 'porphyrin' spectrum similar to the Cr1"-OH porphyrin species reported previou~l~.~*~ Ledon, H. J., Bonnet, M. C., Brigandat, Y., and Veresion, F., Inorg. Chem., 1980, 19, 3488.

* Liston, D. J., and West, B. O., Inorg. Chem., 1985, 24, 1568.

Liston, D. J., Kennedy, B. J., Murray, K. S., and West, B. O., Inorg. Chem., 1985, 24, 1561.


The effects observed in the solid state suggest that Hz0 attack on the CrOMo linkage may provide the initial reaction in both cases since the requirements of a rigid lattice would seem to prevent a dissociation process. Magnetic Susceptibility-Variation

with Temperature

Although the room-temperature moments shown in Table 1 are notably less than expected for uncoupled ~ r " ' / M o ~d3 :dl systems, they are also clearly within the region of Cr"' magnetic moments uninfluenced by any coupling. The observation, from infrared spectral measurements, that the complexes can undergo decomposition in the solid state, if allowed to come in contact with moisture to give Cr"' and (presumably) M O ~ ' species, has been supported by a temperature variation-magnetic susceptibility study of a sample of (tpp)CrOMo(O)(edtc)z after it had been exposed to laboratory air for a week. The magnetic behaviour of the sample was exactly that of a ~ r " ' complex and no evidence for antiferromagnetic coupling was observed. The pyrrolidyldithiocarbamate complex (tpp)CrOMo(O)(pdtc)z was found to be considerably more resistant to decomposition over many days of observation and studies of the variation of magnetic susceptibility with temperature were confined to this complex for convenience. x,/T curves are shown in Fig. 1.

Temperature (K)

Fig. 1. The magnetic susceptibility/temperature curve for (tpp)CrOMo(O)(pdtc)z.

The variation of susceptibility with temperature above 25 K can be satisfactorily interpreted on the basis of a simplified Heisenberg-Van Vleck isotropic coupling model by assuming strong antiferromagnetic coupling between Crl" (S = %)and MoV (S = %)centres. No allowance was made for zero-field splitting on the Crl" centre; this gave a small, rapid decrease in the moment from 3 . 0 to 2.89 BM between 20 and 4.2 K. The derived expression for molar susceptibility varying with temperature


has been compared by least-squares methods with the observed data. The best fit was obtained with J = -300 cm-I and g = 2.15. Strictly, two g values should be employed in the X , expression appropriate to each metal centre,1° but the present approximation was made because of the usual similarity of g values for Cr"' and M O ~0x0 species. Such values are normally close to and just less than 2 - 0 . Since g is essentially a scaling factor in the above expression the value for best fit was that appropriate to the plateau value of the moment (per mole) in the 25-300 K region, viz. 3.05 BM. Strong antiferromagnetic coupling between S = and S = centres should lead to an S' = 2 excited state well separated from an S = 1 ground state with a value of the moment of 2 83 BM. While the slope of the X,-l/T plot is as expected for an S t = 1 ground state it is not clear to us the reason for the p e value ~ being increased above 2.83 BM. It is most likely due to small quantities of uncoupled CrlI1 being present, despite the great care used for synthesis and handling. Such decomposition, and its influence on peff values, is discussed further below. It should be noted that the value of J deduced for best fit should probably better be regarded as the minimum value of negative J, one which still yields a very small thermal population of the S' = 2 state at temperatures above 250 K with a corresponding slight gradual increase in pea from 3-04 BM at 250 K to 3.06 BM at 300 K. A plateau value of 3.04 BM is calculated in the region 250-20 K. The experimental peff values show this small increase in the high-temperature region although such changes are difficult to distinguish with certainty from the errors in pee. Rlrthermore, contributions from second-order field effects (temperature-independent paramagnetism), admittedly small for Cr'I1 and MoV, have been ignored in the susceptibility calculations. If the actual value of J is considerably more negative than -300 cm-I it would result in a constant value of pefi being predicted over the whole temperature range, by the model used. Previous magnetic studies of (P)CrOFe(L) c ~ m ~ l e x have e s ~ shown ~ ~ ~ strong antiferromagnetic coupling between crlI1 and ~ e " ' across the 0x0 bridge with J values in the range from -115 to -150 cm-l. In these S = :S = $ cases, however, the coupling was not sufficiently strong to yield a constant value of p e ~ for the S t = 1 ground state until below c. 100 K, in which region the expected plateau value of 2-83BM was usually observed. Interestingly some L = pdtc and edtc examples did show increased petf values." The homobinuclear [ M 0 ( 0 ) ( d t c ) ~ ] ~complexes O are diamagnetic,12 which indicates that very strong coupling occurs in these compounds through n-type overlap of oxygen p and molybdenum dZY (n) orbital^.^ Orbitals of this symmetry are available in the present complex and a coupling mechanism of this kind would appear to be feasible. However, structural determination of the metal to p-0x0 distances would be required to help elucidate the appropriateness of spin-spin coupling models against delocalized molecular orbital bonding models in such systems.

q

Journaux, Y., Kahn, O., Zarembowitch, J., Galy, J., and Jaud, J., J. Am. Chem. Soc., 1983, 105, 7585. l1 Bakshi, E. N., Elliott, R. L., Murray, K. S., Nichols, P. J., and West, B. O., Aust. J. Chem., 1990, 43, 707. Casey, A. T., Mackey, D. J., Martin, R. L., and White, A. H., Aust. J. Chem., 1972, 25, 477. lo


Wieghardt and coworkers13 have recently developed superexchange interpretations for spin-spin coupling in a wide range of mixed-metal p-0x0 di-pacetato metal triazacyclononane systems but without any example of a C~"'-O-MO~ complex, to date. They have reported a M O I ~ ~ - M system O I ~ (S = $, S = 1) the magnetism of which showed strong antiferromagnetic coupling (J = -150 cm-l) compatible with a spin-spin coupling model. Interestingly the structure of the complex also showed direct Mo-Mo bonding.14

0

F i g .2. The variation with time of the magnetic moment of (tpp)CrOMo(O)(pdtc)z exposed to the air.

I

0

1000

2000

3000

Time (min)

Measuring the magnetic moment at various times of a sample of (tpp)CrOMo(O)( p d t c ) ~exposed to air provided further proof of the slow decomposition of the compound (Fig. 2). The initial pefi value of 3-05BM, indicative of strong coupling, increases with time to a value c. 3.65 BM over several days; this indicates conversion into a system containing largely uncoupled Crl" species. Possible MO~'=O+C$~ILewis Acid-Base Behaviour In the light of the various pieces of evidence indicating that the 0x0-bridged complexes are converted into HO-crlI1. -O=MoV1 mixtures on exposure to air and moisture, an attempt has been made to detect whether coordination could occur between CrlI1 and an 0x0 group of M ~ ~ I ( o ) ~ ( dacting t c ) ~ as a Lewis base. Such behaviour could retain the general structure of the initial 0x0 bridge complex. Thus

air

Hz0

(tpp)Cr~0=Mov1(dtc)2

I

OH

II

0

Complexes of this type have been prepared by Goedken et who established that the 0x0 group of a particular T ~ I ~macrocyclic O complex could bond to a variety of transition metals including ~ r " ' in (tpp)CrCl. A noticeable shift in the T i 0 stretching frequency also accompanies such bond formation. However, reacting (tpp)CrCl with Mov1(0)z(pdtc)2 under various experimental conditions failed to show evidence for association, and each starting complex could be identified separately in the products recovered. ~

l3

l4

1

~

~

Knopp, P., and Wieghardt, K., Inorg. Chem., 1991, 30, 4061, and references therein. Wieghardt, K., Bossek, U., Neves, A., Nuber, B., and Weiss, J., Inorg. Chem., 1989, 28,

432. l6 l6

.

Goedken, V. L., Yang, C. H., and Ldd, J. A., J. Coord. Chem., 1988, 19, 235. Goedken, V. L., and Yang, C. H., Inorg. Chim. Acta, 1986, 117, L19.

3

~

~


Experimental General All syntheses were conducted under an atmosphere of dry nitrogen by using standard Schlenk apparatus and cannulation techniques with stainless steel tubing. The nitrogen was passed through a column of BASF R311 catalyst to remove oxygen. Magnetic susceptibility measurements were made as previously described.' Infrared spectra were measured on PE180 or Jasco IRA-1 instruments. Microanalyses were performed by the National Analytical Laboratories Pty Ltd. Water isotopically enriched with ''0 (60 atom %) was obtained from Yeda Research and Development Co. Ltd (Israel). Oxo(5,10,15,20-tetraphenylporphyrinato)chromium(~v) [CrO(tpp)] was prepared by the method of Groves et al.17 involving the reaction of Cr(tpp)Cl with iodosylbenzene, and purified by chromatography on basic alumina in CHzC12. (5,10,15,20-Tetraphenylp~rph~rinato)chromi(~~) [Cr(tpp)] was prepared by reduction of Cr(tpp)Cl with bis(acetylacetonato)chromium(~~)ls in toluene solution following Reed et al." It was isolated as a bistoluene solvate, an air-sensitive, dark blue, solid which was used immediately after preparation. (N,N-Diethyldithiocarbamato)dioxornolybdenum(v~) [MoOz(edtc)n] and the pyrrolidyldithiocarbamate [MoOz(pdtc)2] derivative were prepared by the method of Moore and arson^' by the reaction of acidified sodium molybdate with the appropriate sodium dithiocarbamate salt in water. The products were recrystallized from CHC13/X4 petrol before use. Oxobis(pyrrolidyldithiocarbamato)molybdenum(1v) [MOO(pdtc)~]and the ethyl derivative were prepared by reduction of a mixture of sodium molybdate and the sodium salt of the dithiocarbamate in aqueous solution according to the procedure of Jowitt and itche ell.^' labelled dioxobis(pyrrolidyldithiocarbamato)molybdenum(v~).-An intensively dried sample of recrystallized MoOz(pdtc)z (0.5 g) was dissolved in dried dichloromethane (100 ml). "0-enriched water (0-25ml) was added and the solution stirred for 16 h. The final product was isolated after reducing the solvent volume and finally adding ether. 1.r. spectrum: 1560vs, 13329, 1251s, 1219m, 1185sh, 1158s, 1038w, 1004m, 950s, 912s, 901s, 8809, 866s, 849s, 722m cm-'. Bis(pyrrolidy1dithiocarbamato)(p-tolyimido)molybdenum(~v)Mo(N-p-tol) ( p d t c ) ~was prepared according to the procedure of ~ a a t t a 'for the diethyl derivative involving reaction of PPh3 with Mo(O)(N-p-tol)(pdtc)z. Triphenylphosphine (0.05 g, 0.19 mmol) was added to a stirred solution of Mo(O)(N-p-tol)(pdtc)2 (0.1 g, 0.19 mmol) in tetrahydrofuran (50 ml) under nitrogen. The orange solution was heated for 20 min, during which time the colour first changed to the deep purple characteristic of a p-0x0 species then to orange/brown. Evaporation of the final solution afforded the desired complex as an air-sensitive brown powder. In most experiments the compound was prepared and reacted in situ. Synthesis of CrOMo Bridged Complexes The following description represents a typical reaction procedure.

p-0xo-[(5,10,15,20-tetmphenylporphyrinato)chromium(111)][oxobis(py~o1idyldithiocarbamato)molybdenum(v)] [(tpp) CrOMo(O)(pdtc)z] Method 1.-CrO(tpp) (0 - 167 g, 0.245 mmol) was dissolved in dry, deoxygenated tetrahydrofuran (70 ml). MoO(pdtc)~(0 1g, 0.245 mmol) in tetrahydrofuran (40 ml) was added and the mixture stirred. The initial red-purple solution became tinged with green as reaction proceeded. After 1-2 h the solution was filtered, the volume was reduced to c. 10 ml, dry l7 Groves, J. T., Kruper, W. J., Hausehalter, R. C., and Butler, W. M., Inorg. Chem., 1982, 21, 1363. Is Ocone, L. R., and Block, B. P., Inorg. Synth., 1966, 8, 125. Reed, C. A., Kouba, J. K., Grimes, C. J., and Cheung, S. K., Inorg. Chem., 1978, 17, 2666. 20 Moore, F. W., and Larson, M. L., Inorg. Chem., 1967, 6, 998. 21 Jowitt, R. N., and Mitchell, P. C. H., J. Chem. Soc. A, 1969, 2632.


deoxygenated hexane (50 ml) was added, and the mixture cooled to 0' to precipitate the product. The brown-purple product was washed with hexane, and dried under vacuum (Found: C, 59.5; H, 4.4; N, 8.0; S, 11.7. Cs4H4aCrMoNsOzS4 requires C, 59.7; H, 4.1; N, 7.7; S, 11.8%). 1.r. spectrum (Nujol) [medium (m) to very strong (vs) peaks only]: 1596m, 15098, 1343m, 1204m, 1070m, 1009vs, 951m, 929m, 835s, 798s, 750s, 719m, 702s cm-l. Method 2.-MoOz(pdtc)z (65.3 mg, 0- 156 mmol) was dissolved in dry, deoxygenated tetrahydrofuran (70 ml), and the solution added to a sample of ~ r " ( t ~ ~ ) ( ~ e(132 ~ hmg, )z 0.156 mmol) in a Schlenk tube. The resulting solution was stirred for I h, filtered, and reduced to 10 ml by passage of a nitrogen stream; then deoxygenated hexane (50 ml) was added and the mixture cooled to 0'. The product was filtered off, washed with hexane, and dried under vacuum Torr).

Method 1.-(Found: C, 59.0; H, 4.7; N, 7.9. C S O H ~ & ~ M O N ~requires O Z S ~ C, 59.5; H, 4.4; N, 7.7%). 1.r. spectrum (Nujol) (m to vs peaks only): 1595m, 1510s, 1345m, 1280m, 1205m, 1070m, 1010vs, 940m, 932m, 880m, 838m, 8009, 750s, 705s cm-l.

Method 1.-(The Cr/Mo ratio only was checked as 1:1 by electron microprobe analysis for this preparation.) 1.r. spectrum (Nujol) (m to vs peaks only): 1606m, 1510s, 1342m, 1287m, 1248s, 1205m, 1176s, 1021m, 10079, 925m, 920m, 845m, 813m, 798s, 721m cm-l. p- 0x0-[(5,10,15, 20-tetraphenylporph~rinato)~hromium(~~~)][bi~ (pyrro1idyldithiocarbamato)-

(p-tolylimido)molybdenum(v)][(tpp) CrOMo(N-p-tol) (pdtc)~] Method 1.-Mo(N-p-tol)(pdtc)z was synthesized in situ and reacted with CrO(tpp) in tetrahydrofuran as solvent (Found: C, 62.4; H, 4.4; N, 8.4. C61H51CrMoN70S4 requires C, 62.4; H, 4.3; N, 8.4%). 1.r. spectrum (Nujol) (m to vs peaks only): 1590m, 1493s, 1340m, 1178m, 1065m, 1005s, 995sh, 880m, 830m, 820m, 795s, 750s cm-l. An Attempted Reaction Between Cr(tpp)Cl and M O ~0' 2 (pdtc)~ Cr(tpp)Cl (100 mg, 0.245 mmol) was reacted with Mo(Oz)(pdtc)z (0.167 g, 0.245 mmol) under the following conditions. (i) Compounds were dissolved in tetrahydrofuran, and the solution was stirred for I h, then reduced in volume by pumping; hexane was added to precipitate product. 1.r. spectral examination indicated a mixture of starting materials only. (ii) Reactants were dissolved in acetonitrile/toluene 1: 1, and the mixture was heated for 12 h at 80'. The solution was cooled to room temperature and the precipitate recovered. 1.r. spectral examination again only showed the characteristics of a mixture of both reactants.

Acknowledgments The work was supported by grants from the Australian Research Council (B. 0. West and K. S. Murray) and the award of a Commonwealth Postgraduate Research Award (to Dr R. L. Elliott). The help of Dr A. Markiewicz with the magnetic work is gratefully acknowledged.