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Synthesis and structure of the first sulfur-donor adduct of a diruthenium tetracarboxylate
www.elsevier.com/locate/ica Inorganica Chimica Acta 331 (2002) 330– 335
Note
Synthesis and structure of the first sulfur-donor adduct of a diruthenium tetracarboxylate Heather J. Gilfoy a, Katherine N. Robertson b, T. Stanley Cameron b, Manuel A.S. Aquino a,* a
Department of Chemistry, St. Francis Xa6ier Uni6ersity, PO Box 5000, Antigonish, NS, Canada B2G 2W5 b Department of Chemistry, Dalhousie Uni6ersity, Halifax, NS, Canada B3H 4J3 Received 22 June 2001; accepted 29 August 2001 Dedicated to Professor A.G. Sykes
Abstract The first sulfur-donor adduct of a diruthenium tetracarboxylate, [Ru2(m-O2CCH3)4(THT)2]PF6 (1), where THT =tetrahydrothiophene, has been synthesized and characterized by X-ray diffraction, IR and UV– Vis spectroscopy, magnetic susceptibility and electrochemistry. Complex 1 displays a typical RuRu bond length of 2.285(4) A, but a very long RuS axial bond of 2.627(13) A, . The redox potential for the [Ru2(m-O2CCH3)4(THT)2] + /0 couple is − 322 mV (vs. Fc/Fc+) which is comparable to other moderately strong Lewis base adducts. Axial ligand binding is driven by s-donation with little p-backdonation from the metal centers. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Diruthenium carboxylate; Crystal structures; Sulfur-donor; Electrochemistry
1. Introduction Axial diadducts of diruthenium tetracarboxylates, either of the homovalent form, Ru2(m-O2CR)4L2, or valence-averaged form, [Ru2(m-O2CR)4L2]X, (where R= alkyl or aryl group, L=Lewis base and X= anion) have received significant attention over the past 25 years due to their interesting magnetic and spectroscopic properties [1,2]. Recent work in our lab has centered on the synthesis and structural characterization of various [Ru2(m-O2CCH3)4L2]PF6 adducts, with L= N, O and P-donor bases, in order to assess the effect of varying the s-donor strength of L on the RuRu and RuL bond lengths, the metal-centered redox potentials and the magnetic susceptibility [3–5]. We report here the synthesis of the first sulfur-donor diadduct of a diruthenium tetracarboxylate, its crystal structure and electrochemistry, and how its structural
* Corresponding author. Tel.: +1-902-867 5336; fax: + 1-902-867 2414. E-mail address: [email protected] (M.A.S. Aquino).
and physical properties compare with previously studied adducts.
2. Experimental
2.1. Starting reagents All reagents and solvents were purchased from commercial sources and used without further purification. Ru2(m-O2CCH3)4Cl and [Ru2(m-O2CCH3)4(H2O)2](PF6) were prepared using literature methods [3,6].
2.2. Physico-chemical measurements IR spectra were obtained as KBr pellets on a BioRad FTS175 FT-IR spectrophotometer. UV –Vis spectra were recorded on a Varian Cary 100 UV –Vis spectrophotometer. Cyclic voltammetry was carried out on a BAS CV-50 Voltammetric Analyzer using a cell setup as described previously [4]. The ferrocene/ferrocenium (Fc/Fc+) couple was consistently measured at 392 mV versus Ag/AgCl with an anodic to cathodic peak to
0020-1693/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 0 1 ) 0 0 7 5 2 - 6
H.J. Gilfoy et al. / Inorganica Chimica Acta 331 (2002) 330–335
peak separation of 75–80 mV at 100 mV s − 1. Room temperature (r.t.) magnetic susceptibility measurements were carried out using a Johnson Matthey MSB-1 balance using HgCo(SCN)4 as calibrant (g =16.17× 10 − 6 c.g.s. units at 25 °C) [7]. Elemental analysis was performed by Canadian Microanalytical Services Ltd., Delta, BC.
2.3. Synthesis of [Ru2(v-O2CCH3)4(THT)2](PF6) (1) A 0.100 g (0.16 mmol) quantity of [Ru2(mO2CCH3)4(H2O)2](PF6) was dissolved in 5 ml of 2propanol. To this solution was added a fourfold excess of tetrahydrothiophene (THT) (0.056 g, 0.64 mmol) and the resulting solution was allowed to react, with stirring, for 5 min at r.t. The brown– yellow precipitate, which formed almost immediately, was filtered off, washed with a small amount of cold 2-propanol and dried in vacuo overnight. Crystals of (1) could be grown by slow evaporation from 1,2-dichloroethane. Yield, 82 mg (67%), veff (295 K)=4.1 B.M. Anal. Found: C, 24.96; H, 3.70; S, 8.30. Calc. C, 25.30; H, 3.71; S, 8.44%.
2.4. X-ray crystallography All measurements were made on a Rigaku AFC5R diffractometer with graphite monochromated Cu Ka radiation and a rotating anode generator. Cell constants and an orientation matrix for data collection, Table 1 Crystal Data for [Ru2(m-O2CCH3)4(THT)2](PF6) (1) Empirical formula Formula weight Crystal color/habit
C8H14F3O4P0.5RuS 379.81 Reddish–brown, rectangular plate 0.15×0.30×0.35 monoclinic C2/c
Crystal size (mm) Crystal system Space group Lattice parameters a (A, ) 22.77(2) b (A, ) 7.56(2) c (A, ) 16.55(3) i (°) 107.18(8) V (A, 3) 2723(8) Z 8 Dcalc (g cm−3) 1.853 v (Cu Ka) (cm−1) 117.2 Temperature (°C) 23(1) Scan method –2q Total reflections 2470 Observed reflections [I\2.00|(I)] 2212 R1 a 0.0549 wR2 b 0.1439 Goodness-of-fit 1.010 a b
R1 =_ Fo − Fc /_ Fo . wR2 = [_(w (Fo 2−Fc 2)2)/_w(Fo 2)2]1/2.
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obtained from a least-squares refinement using setting angles of 24 carefully centered reflections in the range 43.17B 2q B 71.48°, corresponded to a C-centered monoclinic cell. Based on the systematic absences of hkl: h + k9 2n and h0l: l9 2n, packing considerations, a statistical analysis of intensity distribution, and the successful refinement of the structure, the space group was determined to be C2/c (no. 15). All pertinent crystallographic data are summarized in Table 1. A total of 2470 reflections were collected of which 2212 were unique. The intensities of three representative reflections were measured after every 150 reflections. The linear absorption coefficient, v, for Cu Ka radiation is 117.2 cm − 1. An empirical absorption correction based on azimuthal scans of several reflections was applied which resulted in transmission factors ranging from 0.34 to 1.00. The data were corrected for Lorentz and polarization effects. A correction for secondary extinction was applied. The structure was solved by direct methods [8] and expanded using Fourier techniques [9]. The non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included but not refined. The five-membered tetrahydrothiophene ring was disordered. Its atoms were split over two positions, each with occupancy of one-half. The bond lengths within the ring were restrained to reasonable values. The A/B distances in each pair of atoms were also restrained and each atom of the A/B pairs, C(5), C(6) and C(7), was assigned equal thermal parameters. A similar approach was adopted for the PF− 6 anion which was also found to be disordered. The final cycle of full-matrix least-squares refinement on F2 was based on the 2212 observed reflections and 197 variable parameters. All calculations were performed using the TEXSAN [10] crystallographic software package of Molecular Structure Corporation except for refinement, which was performed using SHELXL-97 [11].
3. Results and discussion
3.1. Synthesis To synthesize complex 1, we employed a ‘rapid precipitation’ reaction from 2-propanol which has been used successfully in our lab in the past for N and P-donor p-acid ligands [4,5]. One advantage of using 2-propanol is that both starting materials are soluble in it and the product is not, leading to immediate precipitation of product with negligible axial to equatorial migration of the substituting ligand. This migration is often a problem for moderate to strong p-acid ligands such as 1-methylimidazole and triphenylphosphine [12– 14]. A second advantage, particularly in the case of relatively weak binding ligands such as we have here, is
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Fig. 1.
ORTEP
drawing of [Ru2(m-O2CCH3)4(THT)2](PF6) (1). The PF− 6 counterion has been omitted for clarity.
that 2-propanol does not readily compete with the incoming ligand due to its modest steric constraints.
3.2. X-ray structure of [Ru2(v-O2CCH3)4(THT)2](PF6) (1) The ORTEP diagram of complex 1 is shown in Fig. 1. Selected bond lengths and angles are given in Table 2. Despite the disorder in the axially bound tetrahydrothiophenes, it is clear that binding to the metal occurs through the sulfur atom. The degree of disorder is reflected in the S(1A)Ru(1)S(1B) angle of 8.7(11)°. The THT rings are arranged in an anti fashion with respect to each other, which can clearly be seen in the unit cell diagram (Fig. 2). The RuRu bond length of 2.285(4) A, falls into the range of other diadduct complexes as do the RuO bond lengths of 2.013(9) to 2.032(10) A, [1,2]. Due to the disorder of the tetrahydrothiophene there are two RuRuS bond angles, Ru(1)Ru(1)S(1A) =174.1(6)° and Ru(1)Ru(1) S(1B)=175.8(6)°. These values are at the low end of most of the other diadducts but are close enough to being linear to not need further comment. They are certainly not as acute as the RuRuP angle of 160.4° seen in [Ru2(m-O2CCH3)4(PCy3)2](PF6), where Cy= cyclohexyl [5] (in that case the stronger p-acid phosphine would prefer the equatorial position but due to the steric bulk imposed it can only partially migrate). The
RuS bond lengths, Ru(1)S(1A) = 2.627(13) A, and Ru(1)S(1B) = 2.624(12) A, , are substantially longer than any other RuL(axial) bond in [Ru2(mO2CCH3)4L2]X type complexes. Only in the halide bridged polymer, Ru2(m-O2CH)4Br, where RuBr is 2.717–2.731 A, is the axial bond longer [15]. In that case, the long bond is not surprising given the size of Br− and the fact that it is bridging two adjacent Ru2 units. The RuS bond in 1 is also longer than it is in the dirhodium(II,II) analogue, [Rh2(m-O2CCH3)4(THT)2], where RhS = 2.517 [16]. While it was concluded by Cotton that the RhRh bond exerted a large trans effect on the RhS bond lengths in both Rh2(mO2CCH3)4(DMSO)2, RhS = 2.451(1) A, , and Rh2(mO2CCH3)4(THT)2, RhS = 2.517(1) (normal RhS bond lengths being in the range of 2.23–2.26 A, such as in fac-RhCl3(DMSO)3 [17]), they implied that there was a certain degree of p-backbonding in both of these adducts with the former manifesting it to a greater extent than the latter due to the shorter RhS bond. The RuS bond in 1 is 0.12 A, longer than in the dirhodium THT adduct and it would appear that p-backbonding is, at best, very weak. Previous theoretical [18] and experimental [4,19] studies have pointed to weak or non-existent p-backbonding in the axial direction in the Ru25 + complexes with significantly greater p interaction in the equatorial (carboxylate) positions, whereas the ho-
H.J. Gilfoy et al. / Inorganica Chimica Acta 331 (2002) 330–335
movalent Ru24 + species (with the additional p* electron) seem to show a modest degree of p-bonding to axial ligands such as pyrazine or 4-cyanopyridine [19]. A recent study in our lab [5] has shown the initial preference of phosphines, which are good s-donors and good p-acceptors, for the axial positions (driven kinetically by s-bond making). Except in the case of a sterically bulky phosphine, this is followed by subsequent migration to the thermodynamically favorable equatorial sites (partially displacing the carboxylates) to achieve the synergism of s-donation/p-accepting. In the present case, due to the long RuS bond, the saturated nature of THT and the fact that no axial– equatorial migration is seen, axial binding of THT would appear to be driven by s-bonding with minimal p-interactions from the metal centers. (Preliminary X-ray data on a complex where L=tetramethylthiourea, which is arguably a better p-acceptor than THT, reveals an RuS(axial) bond length of 2.610(2) A, . This is comparable to what is seen in complex 1).
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3.3. Physico-chemical properties The infrared spectrum of 1 shows the characteristic symmetric, wsym(COO), and asymmetric, wasym(COO), carboxylate stretches at 1393 and 1457 cm − 1, respec−1 tively, and the w(PF− [1]. Bands for the 6 ) at 843 cm −1 THT are seen at 2935 cm , due to the w(CH) mode, and at 1257 cm − 1, most likely arising from a ring vibration [20]. The electronic spectrum shows a band in the visible at 445 nm (m= 570 M − 1 cm − 1) which we assign to the p(RuO, Ru2) p*(Ru2) transition in accordance with the O and N-donor adducts we have studied in the past [3,4] but note that the band is red-shifted somewhat from the usual 423–430 nm. The magnetic moment of 4.1 B.M. at 300 K is consistent with three unpaired electrons. Finally, cyclic voltammetry measurements in 1,2dichloroethane show a one-electron reduction, corresponding to reduction of the Ru25 + species to the
Fig. 2. View of the unit cell looking down the b-axis of 1 clearly showing the anti configuration of the axially bound THT ligands.
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Table 2 Selected bond lengths (A, ) and angles (°) for [Ru2(m-O2CCH3)4(THT)2](PF6) (1) Bond lengths Ru(1)O(3) Ru(1)O(1) Ru(1)*O(4) Ru(1)*O(2) Ru(1)Ru(1)* Ru(1)S(1A) Ru(1)S(1B) O(1)C(1) O(2)C(1) O(3)C(3) O(4)C(3) C(1)C(2)
2.013(9) 2.025(10) 2.024(8) 2.032(10) 2.285(4) 2.627(13) 2.624(12) 1.24(2) 1.30(2) 1.26(2) 1.25(2) 1.45(2)
C(3)C(4) S(1A)C(5A) S(1A)C(8A) C(5A)C(6A) C(6A)C(7A) C(7A)C(8A) S(1B)C(5B) S(1B)C(8B) C(5B)C(6B) C(6B)C(7B) C(7B)C(8B)
1.49(2) 1.81(2) 1.81(2) 1.48(2) 1.47(2) 1.49(2) 1.81(2) 1.83(2) 1.49(2) 1.45(2) 1.50(2)
Bond angles O(3)Ru(1)O(4)* O(3)Ru(1)O(1) O(4)*Ru(1)O(1) O(3)Ru(1)Ru(1)* O(4)*Ru(1)Ru(1)* O(3)Ru(1)S(1A) O(1)Ru(1)S(1A) Ru(1)*Ru(1)S(1A) O(3)Ru(1)S(1B) O(1)Ru(1)S(1B) Ru(1)*Ru(1)S(1B) S(1A)Ru(1)S(1B) C(1)O(1)Ru(1) C(1)O(2)Ru(1)* C(3)O(3)Ru(1) C(3)O(4)Ru(1)* O(1)C(1)O(2) O(1)C(1)C(2)
178.1(4) 89.3(4) 91.1(4) 89.1(3) 89.0(3) 89.9(6) 85.3(7) 174.1(6) 87.6(6) 93.6(7) 175.8(6) 8.7(11) 120.7(10) 118.5(9) 119.1(9) 118.9(10) 122.5(14) 119.5(14)
O(2)C(1)C(2) O(4)C(3)O(3) O(4)C(3)C(4) O(3)C(3)C(4) C(5A)S(1A)C(8A) C(5A)S(1A) Ru(1) C(8A)S(1A)Ru(1) C(6A)C(5A)S(1A) C(7A)C(6A)C(5A) C(6A)C(7A)C(8A) C(7A)C(8A)S(1A) C(5B)S(1B)C(8B) C(5B)S(1B)Ru(1) C(8B)S(1B)Ru(1) C(6B)C(5B)S(1B) C(7B)C(6B)C(5B) C(6B)C(7B)C(8B) C(7B)C(8B)S(1B)
118.0(13) 123.8(14) 117(2) 118.9(13) 93.3(11) 112.2(2) 98.0(13) 107.8(14) 113.1(12) 112.9(12) 108.3(13) 93.9(12) 100(2) 111(2) 108.6(13) 113.4(13) 112(2) 106.3(13)
Ru24 + species, occurring at E1/2 = −322 mV versus the ferrocene/ferrocenium (Fc/Fc+) couple. The oxidation and reduction peak separations (DEp) are somewhat scan rate dependent, ranging from 110 mV at 20 mV s − 1 to 153 mV at 200 mV s − 1, making this process formally quasi-reversible. The anodic to cathodic Table 3 Electrochemical data for various [Ru2(m-O2CCH3)4L2]+/0 couples L
E1/2 (mV) a
PCy3 4-MePy Py DMSO THT DMF Unligated d
−730 −537 −519 −483 −322 −302 c −233
a
D.N. b
26.6 1e +
4. Conclusions We have isolated and structurally characterized the first sulfur-donor adduct of a diruthenium tetracarboxylate. Complex 1 has a very long RuS axial bond but nonetheless displays a one-electron reduction at a comparable potential to other, moderate Lewis base adducts of diruthenium(II,III) tetracarboxylate. This study provides further experimental evidence that the axial binding in these diruthenium(II,III) compounds is predominated by s-donation with minimal or no pbackdonation. 5. Supplementary material Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Center, CCDC No. 165304 for compound 1. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: + 44-1223-336 033; e-mail: [email protected] or www: http:// www.ccdc.cam.ac.uk).
References [5] [4] [4] [4] this work [4] [21]
35.1 33.1 29.8
current ratios are close to unity with some deviation at lower scan rates presumably due to axial ligand loss as has been observed for these types of systems in the past [4]. Table 3 lists some representative axial donor adducts (including the present complex) along with the E1/2 values for their [Ru2(m-O2CCH3)4L2] + /0 couple and the donor number of L where available. As we are not aware of a donor number value for THT and assuming the axial positions in these complexes involve predominantly ligand to metal electron donation with minimal metal to ligand p-backdonation, we can draw the conclusion that THT is acting as a moderate Lewis base towards the diruthenium core. It lies somewhere between DMF (D.N.=26.6) and DMSO (D.N.=29.8) in Table 3. This is somewhat surprising given the long RuS bond and, presumably, weak axial interaction.
Acknowledgements M.A.S.A. thanks NSERC (Canada) for financial support.
References
+
In 1,2-dichloroethane vs. Fc/Fc (Fc/Fc couple measured at 392 mV vs. Ag/AgCl, DE= 75–80 mV). b Refs. [22,23]. c Cathodic potential only. d For the [Ru2(m-octanoate)4]0/+ couple in dichloromethane. e Value for dichloromethane.
[1] M.A.S. Aquino, Coord. Chem. Rev. 170 (1998) 141. [2] F.A. Cotton, R.A. Walton, Multiple Bonds Between Metal Atoms, second ed., Clarendon Press, Oxford, 1993. [3] K.D. Drysdale, E.J. Beck, T.S. Cameron, K.N. Robertson, M.A.S. Aquino, Inorg. Chim. Acta 256 (1997) 243.
H.J. Gilfoy et al. / Inorganica Chimica Acta 331 (2002) 330–335 [4] G. Vamvounis, J.F. Caplan, T.S. Cameron, K.N. Robertson, M.A.S. Aquino, Inorg. Chim. Acta 304 (2000) 87. [5] G.G. Briand, M.W. Cooke, T.S. Cameron, H.M. Farrell, T.J. Burchell, M.A.S. Aquino, Inorg. Chem. 40 (2001) 3267. [6] R.W. Mitchell, A. Spencer, G. Wilkinson, J. Chem. Soc., Dalton Trans. (1973) 846. [7] B.N. Figgis, R.S. Nyholm, J. Chem. Soc. (1958) 4190. [8] SIR-92: A. Altomare, M. Cascarano, C. Giacovacco, A. Guagliardi, J. Appl. Cryst. 26 (1993) 343. [9] P.T. Beurskens, G. Admiraal, G. Beurskens, W.P. Bosman, R. de Gelder, R. Israel, J.M.M. Smits, DIRDIF-94, DIRDIF94 program system, Technical Report of the Crystallography Laboratory, University of Nijmegen, The Netherlands, 1994. [10] TEXSAN for Windows: Crystal Structure Analysis Package, Molecular Structure Corporation, 1997. [11] G.M. Sheldrick, SHELX-97, 1997. [12] C. Sudha, S.K. Mandal, A.R. Chakravarty, Inorg. Chem. 37 (1998) 270.
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[13] C. Sudha, S.K. Mandal, A.R. Chakravarty, Inorg. Chem. 32 (1993) 3801. [14] M.C. Barral, R. Jimenez-Aparicio, E.C. Royer, F.A. Urbanos, A. Monge, C. Ruiz-Valero, Polyhedron 10 (1991) 113. [15] T. Kimura, T. Sakurai, M. Shima, T. Togano, M. Mukaida, T. Nomura, Bull. Chem. Soc. Jpn. 55 (1982) 3927. [16] F.A. Cotton, T.R. Felthouse, Inorg. Chem. 19 (1980) 323. [17] V.I. Sokol, M.A. Porai-Koshits, Sov. J. Coord. Chem. 1 (1975) 476. [18] J.G. Norman Jr., G.E. Renzoni, D.A. Case, J. Am. Chem. Soc. 101 (1979) 5256. [19] J.L. Wesemann, M.H. Chisholm, Inorg. Chem. 36 (1997) 3258. [20] K.G.R. Pachler, F. Matlok, H.U. Gremlich, Merck FT-IR Atlas, Merck, Darmstadt, 1988. [21] M.H. Chisholm, G. Christou, K. Folting, J.C. Huffman, C.A. James, J.A. Samuels, J.L. Wesemann, W.H. Woodruff, Inorg. Chem. 35 (1996) 3643. [22] V. Gutman, The Donor– Acceptor Approach to Molecular Interactions, Plenum Press, New York, 1978. [23] Y. Marcus, J. Solution Chem. 13 (1984) 599.
Note
Synthesis and structure of the first sulfur-donor adduct of a diruthenium tetracarboxylate Heather J. Gilfoy a, Katherine N. Robertson b, T. Stanley Cameron b, Manuel A.S. Aquino a,* a
Department of Chemistry, St. Francis Xa6ier Uni6ersity, PO Box 5000, Antigonish, NS, Canada B2G 2W5 b Department of Chemistry, Dalhousie Uni6ersity, Halifax, NS, Canada B3H 4J3 Received 22 June 2001; accepted 29 August 2001 Dedicated to Professor A.G. Sykes
Abstract The first sulfur-donor adduct of a diruthenium tetracarboxylate, [Ru2(m-O2CCH3)4(THT)2]PF6 (1), where THT =tetrahydrothiophene, has been synthesized and characterized by X-ray diffraction, IR and UV– Vis spectroscopy, magnetic susceptibility and electrochemistry. Complex 1 displays a typical RuRu bond length of 2.285(4) A, but a very long RuS axial bond of 2.627(13) A, . The redox potential for the [Ru2(m-O2CCH3)4(THT)2] + /0 couple is − 322 mV (vs. Fc/Fc+) which is comparable to other moderately strong Lewis base adducts. Axial ligand binding is driven by s-donation with little p-backdonation from the metal centers. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Diruthenium carboxylate; Crystal structures; Sulfur-donor; Electrochemistry
1. Introduction Axial diadducts of diruthenium tetracarboxylates, either of the homovalent form, Ru2(m-O2CR)4L2, or valence-averaged form, [Ru2(m-O2CR)4L2]X, (where R= alkyl or aryl group, L=Lewis base and X= anion) have received significant attention over the past 25 years due to their interesting magnetic and spectroscopic properties [1,2]. Recent work in our lab has centered on the synthesis and structural characterization of various [Ru2(m-O2CCH3)4L2]PF6 adducts, with L= N, O and P-donor bases, in order to assess the effect of varying the s-donor strength of L on the RuRu and RuL bond lengths, the metal-centered redox potentials and the magnetic susceptibility [3–5]. We report here the synthesis of the first sulfur-donor diadduct of a diruthenium tetracarboxylate, its crystal structure and electrochemistry, and how its structural
* Corresponding author. Tel.: +1-902-867 5336; fax: + 1-902-867 2414. E-mail address: [email protected] (M.A.S. Aquino).
and physical properties compare with previously studied adducts.
2. Experimental
2.1. Starting reagents All reagents and solvents were purchased from commercial sources and used without further purification. Ru2(m-O2CCH3)4Cl and [Ru2(m-O2CCH3)4(H2O)2](PF6) were prepared using literature methods [3,6].
2.2. Physico-chemical measurements IR spectra were obtained as KBr pellets on a BioRad FTS175 FT-IR spectrophotometer. UV –Vis spectra were recorded on a Varian Cary 100 UV –Vis spectrophotometer. Cyclic voltammetry was carried out on a BAS CV-50 Voltammetric Analyzer using a cell setup as described previously [4]. The ferrocene/ferrocenium (Fc/Fc+) couple was consistently measured at 392 mV versus Ag/AgCl with an anodic to cathodic peak to
0020-1693/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 0 1 ) 0 0 7 5 2 - 6
H.J. Gilfoy et al. / Inorganica Chimica Acta 331 (2002) 330–335
peak separation of 75–80 mV at 100 mV s − 1. Room temperature (r.t.) magnetic susceptibility measurements were carried out using a Johnson Matthey MSB-1 balance using HgCo(SCN)4 as calibrant (g =16.17× 10 − 6 c.g.s. units at 25 °C) [7]. Elemental analysis was performed by Canadian Microanalytical Services Ltd., Delta, BC.
2.3. Synthesis of [Ru2(v-O2CCH3)4(THT)2](PF6) (1) A 0.100 g (0.16 mmol) quantity of [Ru2(mO2CCH3)4(H2O)2](PF6) was dissolved in 5 ml of 2propanol. To this solution was added a fourfold excess of tetrahydrothiophene (THT) (0.056 g, 0.64 mmol) and the resulting solution was allowed to react, with stirring, for 5 min at r.t. The brown– yellow precipitate, which formed almost immediately, was filtered off, washed with a small amount of cold 2-propanol and dried in vacuo overnight. Crystals of (1) could be grown by slow evaporation from 1,2-dichloroethane. Yield, 82 mg (67%), veff (295 K)=4.1 B.M. Anal. Found: C, 24.96; H, 3.70; S, 8.30. Calc. C, 25.30; H, 3.71; S, 8.44%.
2.4. X-ray crystallography All measurements were made on a Rigaku AFC5R diffractometer with graphite monochromated Cu Ka radiation and a rotating anode generator. Cell constants and an orientation matrix for data collection, Table 1 Crystal Data for [Ru2(m-O2CCH3)4(THT)2](PF6) (1) Empirical formula Formula weight Crystal color/habit
C8H14F3O4P0.5RuS 379.81 Reddish–brown, rectangular plate 0.15×0.30×0.35 monoclinic C2/c
Crystal size (mm) Crystal system Space group Lattice parameters a (A, ) 22.77(2) b (A, ) 7.56(2) c (A, ) 16.55(3) i (°) 107.18(8) V (A, 3) 2723(8) Z 8 Dcalc (g cm−3) 1.853 v (Cu Ka) (cm−1) 117.2 Temperature (°C) 23(1) Scan method –2q Total reflections 2470 Observed reflections [I\2.00|(I)] 2212 R1 a 0.0549 wR2 b 0.1439 Goodness-of-fit 1.010 a b
R1 =_ Fo − Fc /_ Fo . wR2 = [_(w (Fo 2−Fc 2)2)/_w(Fo 2)2]1/2.
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obtained from a least-squares refinement using setting angles of 24 carefully centered reflections in the range 43.17B 2q B 71.48°, corresponded to a C-centered monoclinic cell. Based on the systematic absences of hkl: h + k9 2n and h0l: l9 2n, packing considerations, a statistical analysis of intensity distribution, and the successful refinement of the structure, the space group was determined to be C2/c (no. 15). All pertinent crystallographic data are summarized in Table 1. A total of 2470 reflections were collected of which 2212 were unique. The intensities of three representative reflections were measured after every 150 reflections. The linear absorption coefficient, v, for Cu Ka radiation is 117.2 cm − 1. An empirical absorption correction based on azimuthal scans of several reflections was applied which resulted in transmission factors ranging from 0.34 to 1.00. The data were corrected for Lorentz and polarization effects. A correction for secondary extinction was applied. The structure was solved by direct methods [8] and expanded using Fourier techniques [9]. The non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included but not refined. The five-membered tetrahydrothiophene ring was disordered. Its atoms were split over two positions, each with occupancy of one-half. The bond lengths within the ring were restrained to reasonable values. The A/B distances in each pair of atoms were also restrained and each atom of the A/B pairs, C(5), C(6) and C(7), was assigned equal thermal parameters. A similar approach was adopted for the PF− 6 anion which was also found to be disordered. The final cycle of full-matrix least-squares refinement on F2 was based on the 2212 observed reflections and 197 variable parameters. All calculations were performed using the TEXSAN [10] crystallographic software package of Molecular Structure Corporation except for refinement, which was performed using SHELXL-97 [11].
3. Results and discussion
3.1. Synthesis To synthesize complex 1, we employed a ‘rapid precipitation’ reaction from 2-propanol which has been used successfully in our lab in the past for N and P-donor p-acid ligands [4,5]. One advantage of using 2-propanol is that both starting materials are soluble in it and the product is not, leading to immediate precipitation of product with negligible axial to equatorial migration of the substituting ligand. This migration is often a problem for moderate to strong p-acid ligands such as 1-methylimidazole and triphenylphosphine [12– 14]. A second advantage, particularly in the case of relatively weak binding ligands such as we have here, is
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Fig. 1.
ORTEP
drawing of [Ru2(m-O2CCH3)4(THT)2](PF6) (1). The PF− 6 counterion has been omitted for clarity.
that 2-propanol does not readily compete with the incoming ligand due to its modest steric constraints.
3.2. X-ray structure of [Ru2(v-O2CCH3)4(THT)2](PF6) (1) The ORTEP diagram of complex 1 is shown in Fig. 1. Selected bond lengths and angles are given in Table 2. Despite the disorder in the axially bound tetrahydrothiophenes, it is clear that binding to the metal occurs through the sulfur atom. The degree of disorder is reflected in the S(1A)Ru(1)S(1B) angle of 8.7(11)°. The THT rings are arranged in an anti fashion with respect to each other, which can clearly be seen in the unit cell diagram (Fig. 2). The RuRu bond length of 2.285(4) A, falls into the range of other diadduct complexes as do the RuO bond lengths of 2.013(9) to 2.032(10) A, [1,2]. Due to the disorder of the tetrahydrothiophene there are two RuRuS bond angles, Ru(1)Ru(1)S(1A) =174.1(6)° and Ru(1)Ru(1) S(1B)=175.8(6)°. These values are at the low end of most of the other diadducts but are close enough to being linear to not need further comment. They are certainly not as acute as the RuRuP angle of 160.4° seen in [Ru2(m-O2CCH3)4(PCy3)2](PF6), where Cy= cyclohexyl [5] (in that case the stronger p-acid phosphine would prefer the equatorial position but due to the steric bulk imposed it can only partially migrate). The
RuS bond lengths, Ru(1)S(1A) = 2.627(13) A, and Ru(1)S(1B) = 2.624(12) A, , are substantially longer than any other RuL(axial) bond in [Ru2(mO2CCH3)4L2]X type complexes. Only in the halide bridged polymer, Ru2(m-O2CH)4Br, where RuBr is 2.717–2.731 A, is the axial bond longer [15]. In that case, the long bond is not surprising given the size of Br− and the fact that it is bridging two adjacent Ru2 units. The RuS bond in 1 is also longer than it is in the dirhodium(II,II) analogue, [Rh2(m-O2CCH3)4(THT)2], where RhS = 2.517 [16]. While it was concluded by Cotton that the RhRh bond exerted a large trans effect on the RhS bond lengths in both Rh2(mO2CCH3)4(DMSO)2, RhS = 2.451(1) A, , and Rh2(mO2CCH3)4(THT)2, RhS = 2.517(1) (normal RhS bond lengths being in the range of 2.23–2.26 A, such as in fac-RhCl3(DMSO)3 [17]), they implied that there was a certain degree of p-backbonding in both of these adducts with the former manifesting it to a greater extent than the latter due to the shorter RhS bond. The RuS bond in 1 is 0.12 A, longer than in the dirhodium THT adduct and it would appear that p-backbonding is, at best, very weak. Previous theoretical [18] and experimental [4,19] studies have pointed to weak or non-existent p-backbonding in the axial direction in the Ru25 + complexes with significantly greater p interaction in the equatorial (carboxylate) positions, whereas the ho-
H.J. Gilfoy et al. / Inorganica Chimica Acta 331 (2002) 330–335
movalent Ru24 + species (with the additional p* electron) seem to show a modest degree of p-bonding to axial ligands such as pyrazine or 4-cyanopyridine [19]. A recent study in our lab [5] has shown the initial preference of phosphines, which are good s-donors and good p-acceptors, for the axial positions (driven kinetically by s-bond making). Except in the case of a sterically bulky phosphine, this is followed by subsequent migration to the thermodynamically favorable equatorial sites (partially displacing the carboxylates) to achieve the synergism of s-donation/p-accepting. In the present case, due to the long RuS bond, the saturated nature of THT and the fact that no axial– equatorial migration is seen, axial binding of THT would appear to be driven by s-bonding with minimal p-interactions from the metal centers. (Preliminary X-ray data on a complex where L=tetramethylthiourea, which is arguably a better p-acceptor than THT, reveals an RuS(axial) bond length of 2.610(2) A, . This is comparable to what is seen in complex 1).
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3.3. Physico-chemical properties The infrared spectrum of 1 shows the characteristic symmetric, wsym(COO), and asymmetric, wasym(COO), carboxylate stretches at 1393 and 1457 cm − 1, respec−1 tively, and the w(PF− [1]. Bands for the 6 ) at 843 cm −1 THT are seen at 2935 cm , due to the w(CH) mode, and at 1257 cm − 1, most likely arising from a ring vibration [20]. The electronic spectrum shows a band in the visible at 445 nm (m= 570 M − 1 cm − 1) which we assign to the p(RuO, Ru2) p*(Ru2) transition in accordance with the O and N-donor adducts we have studied in the past [3,4] but note that the band is red-shifted somewhat from the usual 423–430 nm. The magnetic moment of 4.1 B.M. at 300 K is consistent with three unpaired electrons. Finally, cyclic voltammetry measurements in 1,2dichloroethane show a one-electron reduction, corresponding to reduction of the Ru25 + species to the
Fig. 2. View of the unit cell looking down the b-axis of 1 clearly showing the anti configuration of the axially bound THT ligands.
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Table 2 Selected bond lengths (A, ) and angles (°) for [Ru2(m-O2CCH3)4(THT)2](PF6) (1) Bond lengths Ru(1)O(3) Ru(1)O(1) Ru(1)*O(4) Ru(1)*O(2) Ru(1)Ru(1)* Ru(1)S(1A) Ru(1)S(1B) O(1)C(1) O(2)C(1) O(3)C(3) O(4)C(3) C(1)C(2)
2.013(9) 2.025(10) 2.024(8) 2.032(10) 2.285(4) 2.627(13) 2.624(12) 1.24(2) 1.30(2) 1.26(2) 1.25(2) 1.45(2)
C(3)C(4) S(1A)C(5A) S(1A)C(8A) C(5A)C(6A) C(6A)C(7A) C(7A)C(8A) S(1B)C(5B) S(1B)C(8B) C(5B)C(6B) C(6B)C(7B) C(7B)C(8B)
1.49(2) 1.81(2) 1.81(2) 1.48(2) 1.47(2) 1.49(2) 1.81(2) 1.83(2) 1.49(2) 1.45(2) 1.50(2)
Bond angles O(3)Ru(1)O(4)* O(3)Ru(1)O(1) O(4)*Ru(1)O(1) O(3)Ru(1)Ru(1)* O(4)*Ru(1)Ru(1)* O(3)Ru(1)S(1A) O(1)Ru(1)S(1A) Ru(1)*Ru(1)S(1A) O(3)Ru(1)S(1B) O(1)Ru(1)S(1B) Ru(1)*Ru(1)S(1B) S(1A)Ru(1)S(1B) C(1)O(1)Ru(1) C(1)O(2)Ru(1)* C(3)O(3)Ru(1) C(3)O(4)Ru(1)* O(1)C(1)O(2) O(1)C(1)C(2)
178.1(4) 89.3(4) 91.1(4) 89.1(3) 89.0(3) 89.9(6) 85.3(7) 174.1(6) 87.6(6) 93.6(7) 175.8(6) 8.7(11) 120.7(10) 118.5(9) 119.1(9) 118.9(10) 122.5(14) 119.5(14)
O(2)C(1)C(2) O(4)C(3)O(3) O(4)C(3)C(4) O(3)C(3)C(4) C(5A)S(1A)C(8A) C(5A)S(1A) Ru(1) C(8A)S(1A)Ru(1) C(6A)C(5A)S(1A) C(7A)C(6A)C(5A) C(6A)C(7A)C(8A) C(7A)C(8A)S(1A) C(5B)S(1B)C(8B) C(5B)S(1B)Ru(1) C(8B)S(1B)Ru(1) C(6B)C(5B)S(1B) C(7B)C(6B)C(5B) C(6B)C(7B)C(8B) C(7B)C(8B)S(1B)
118.0(13) 123.8(14) 117(2) 118.9(13) 93.3(11) 112.2(2) 98.0(13) 107.8(14) 113.1(12) 112.9(12) 108.3(13) 93.9(12) 100(2) 111(2) 108.6(13) 113.4(13) 112(2) 106.3(13)
Ru24 + species, occurring at E1/2 = −322 mV versus the ferrocene/ferrocenium (Fc/Fc+) couple. The oxidation and reduction peak separations (DEp) are somewhat scan rate dependent, ranging from 110 mV at 20 mV s − 1 to 153 mV at 200 mV s − 1, making this process formally quasi-reversible. The anodic to cathodic Table 3 Electrochemical data for various [Ru2(m-O2CCH3)4L2]+/0 couples L
E1/2 (mV) a
PCy3 4-MePy Py DMSO THT DMF Unligated d
−730 −537 −519 −483 −322 −302 c −233
a
D.N. b
26.6 1e +
4. Conclusions We have isolated and structurally characterized the first sulfur-donor adduct of a diruthenium tetracarboxylate. Complex 1 has a very long RuS axial bond but nonetheless displays a one-electron reduction at a comparable potential to other, moderate Lewis base adducts of diruthenium(II,III) tetracarboxylate. This study provides further experimental evidence that the axial binding in these diruthenium(II,III) compounds is predominated by s-donation with minimal or no pbackdonation. 5. Supplementary material Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Center, CCDC No. 165304 for compound 1. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: + 44-1223-336 033; e-mail: [email protected] or www: http:// www.ccdc.cam.ac.uk).
References [5] [4] [4] [4] this work [4] [21]
35.1 33.1 29.8
current ratios are close to unity with some deviation at lower scan rates presumably due to axial ligand loss as has been observed for these types of systems in the past [4]. Table 3 lists some representative axial donor adducts (including the present complex) along with the E1/2 values for their [Ru2(m-O2CCH3)4L2] + /0 couple and the donor number of L where available. As we are not aware of a donor number value for THT and assuming the axial positions in these complexes involve predominantly ligand to metal electron donation with minimal metal to ligand p-backdonation, we can draw the conclusion that THT is acting as a moderate Lewis base towards the diruthenium core. It lies somewhere between DMF (D.N.=26.6) and DMSO (D.N.=29.8) in Table 3. This is somewhat surprising given the long RuS bond and, presumably, weak axial interaction.
Acknowledgements M.A.S.A. thanks NSERC (Canada) for financial support.
References
+
In 1,2-dichloroethane vs. Fc/Fc (Fc/Fc couple measured at 392 mV vs. Ag/AgCl, DE= 75–80 mV). b Refs. [22,23]. c Cathodic potential only. d For the [Ru2(m-octanoate)4]0/+ couple in dichloromethane. e Value for dichloromethane.
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