DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1998, Pages 2625.2633 2625 Syntheses, crystal structures and (spectro)electrochemical studies of novel clusters [Ru4(I-H)4(CO)10(L)] [L 5 2,29-bipyrimidine, 2,3-bis(pyridin-2-yl)pyrazine, 2,29-bipyridine] ¢Ó Jos NijhoV,a Maarten J. Bakker,a Frantis¢§ek Hartl,*,a Gideon Freeman,b Scott L. Ingham b and Brian F. G. Johnson *,c a Anorganisch Chemisch Laboratorium, Institute of Molecular Chemistry, Universiteit van Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands b Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh, UK EH9 3JJ c University Chemical Laboratory, University of Cambridge, Lensfield Road, Cambridge, UK CB2 1EW Three tetrahedral clusters [Ru4(m-H)4(CO)10(L)] [L = 2,29-bipyrimidine 1, 2,3-bis(pyridin-2-yl)pyrazine 2 and 2,29-bipyridine 3] were obtained from a substitution reaction of [Ru4(m-H)4(CO)12] with L, involving the monosubstituted intermediates [Ru4(m-H)4(CO)11(h1-L)].The solid-state structures of 1 and 3 have been elucidated by single-crystal X-ray analysis. In solution, dynamic behaviour of the hydride ligands in 1.3 was apparent from 1H NMR spectra. UV/VIS and resonance-Raman spectroscopy established a significant Ru(dp)�¡L(p*) charge transfer (MLCT) character of the lowest electronic transition of 1.3 in the visible region. Consistent with the dominantly L(p*)-localized LUMO, one-electron reduction of 1 and 2 produced corresponding radical anions 1b and 2b which could be characterized by IR, UV/VIS and EPR spectroscopy.Subsequent one-electron reduction of 1b and 2b yielded unstable dianions [Ru4(m-H)4(CO)10(L)]22 which were found to eliminate H2. The dihydrido dianionic products 1c, 2c were also formed via slow disproportionation of 1b and 2b. The radical anion 3b, containing the stronger s-donor 2,29-bipyridine anion, was detectable only on the subsecond time-scale of cyclic voltammetry.The electrochemically produced dihydrido dianion 3c is similar but diVerent from [Ru4(m-H)2(CO)10(bpy)]22 3c9 obtained by deprotonating 3 in reaction with NEt4OH. Tetranuclear hydridoruthenium carbonyl clusters have received considerable attention in homogeneous catalysis, in particular for their hydrogenation potential.1 Promising research opportunities exist in the field of their selective photo- and electrochemical activation under substantially milder conditions of temperature and pressure.The particularly well known example is hydrogenation of ethylene photoinduced by [Ru4- (m-H)4(CO)12] which proceeds without cluster fragmentation.2 Elimination of hydrogen from the latter cluster also occurs on its electrochemical reduction. The intimate mechanism of this process has recently been reported by Osella et al.3 Another approach to redox activation of low-nuclearity transition-metal clusters involves co-ordination of redox active ligands. Their protecting role as electron reservoirs oVers the possibility of (spectro)electrochemical characterization of otherwise short-lived reactive transients, in particular radicals, generated along the reduction path.This strategy has recently been applied for the clusters [Os3(CO)10(a-diimine)]. The introduction of the reducible a-diimine ligand permitted detailed investigation of the primary one-electron cathodic step followed by temperature-controlled splitting of an Os]Os- (a-diimine) bond in [Os3(CO)10(a-diimine)]~2 and concomitant electron transfer producing a reactive open-structure dianionic species.4 In consequence of the aforementioned investigations, we ¢Ó Supplementary data available: interpretation of the 1H NMR decoupling experiments carried out on complex 2 to assign the proton resonances of the dpp ligand: oV resonance irradiation at d 11.00, and irradiation at d 8.89, 8.67 and 7.34.For direct electronic access see http://www.rsc.org/suppdata/dt/1998/2625/, otherwise available from BLDSC (No.SUP 57401, 5 pp.) or the RSC Library. See Instructions for Authors, 1998, Issue 1 (http://www.rsc.org/dalton). Non-SI unit employed: G = 1024 T. became interested in the synthesis and electrochemistry of a-diimine-substituted derivatives of [Ru4(m-H)4(CO)12]. This cluster is known5,6 to undergo an eYcient thermal substitution of carbonyl ligands by tertiary phosphines, producing [Ru4- (m-H)4(CO)12 2 n(PR3)n] (n = 1.4), where n depends on the reaction conditions and the amount of phosphine added.A mixture of [Ru4(m-H)4(CO)12 2 n(PPh3)n] (n = 1.3) was also obtained under mild conditions via electrocatalytic (ETC) substitution reactions.3 In this paper we report on the targeted syntheses and characterization of the disubstituted derivatives [Ru4(m-H)4(CO)10- (L)] [L = 2,29-bipyrimidine (bpym), 2,3-bis(pyridin-2-yl)pyrazine (dpp) and 2,29-bipyridine (bpy)], with bpy as the strongest s-donor and bpym as the strongest p-acceptor in the series.Next, we have performed a (spectro)electrochemical study of their redox properties and reduction paths, aimed at comparison with the unsubstituted precursor [Ru4(m-H)4(CO)12] and with the related clusters [Os3(CO)10(a-diimine)]. The ligands L and the adopted numbering schemes are shown in Fig. 1. Experimental Materials The cluster [Ru4(m-H)4(CO)12] was synthesized from [Ru3- (CO)12] in a magnetically stirred BurghoV (250 ml) autoclave according to the literature method.7 2,29-Bipyrimidine (Alfa and Lancaster Chemicals), 2,3-bis(pyridin-2-yl)pyrazine and 2,29-bipyridine (both Aldrich) were used as purchased.Trimethylamine N-oxide dihydrate, Me3NO?2H2O (Alfa), was carefully dried by first refluxing the sample (15 g) in benzene (250 ml), typically overnight, to remove the water of crystallisation via Dean.Stark distillation. The benzene was then decanted oV2626 J. Chem. Soc., Dalton Trans., 1998, Pages 2625–2633 and the sample dried under vacuum on a Schlenk line, and sublimed prior to use.The supporting electrolyte NBu4PF6 (Aldrich) was dried in vacuo at 80 8C for 10 h. The salt NEt4OH was purchased from Fluka as a 25% solution in MeOH. Ferrocene (Fc) was used as received from BDH. Solvents (all Acros Chimica, analytical grade) were dried using a benzophenone– sodium mixture (THF, benzene), sodium wire (hexane), P2O5 (CH2Cl2) or CaH2 (MeCN), and freshly distilled under nitrogen.Silica gel (Kieselgel 60, Merck, 70–230 mesh) for column chromatography was dried and activated by heating overnight in vacuo at 160 8C. Preparative TLC was carried out using glass plates (20 × 20 cm) supplied by Merck, coated with a 0.25 cm layer of Kieselgel 60 F254. Syntheses of [Ru4(Ï-H)4(CO)10(L)] (L 5 bpym 1, dpp 2 or bpy 3) All clusters 1–3 were prepared according to the following general procedure. A solution of 200 mg [Ru4(m-H)4(CO)12] and 2.5 equivalents of L (L = bpym, dpp or bpy) in dichloromethane (150 ml) was cooled to 195 K in a dry ice–acetone bath.The reaction was initiated by the dropwise addition of 2.2 equivalents of Me3NO in dichloromethane (20 ml) over a 20 min period under stirring. After stirring for an additional 10 min the mixture was allowed to warm gradually to room temperature and was observed to change colour from yellow-orange to green (for L = bpym or bpy) and finally to deep red-brown. The solvent was evaporated and the crude product was separated from unreacted [Ru4(m-H)4(CO)12] by using either TLC with hexane–dichloromethane (3 : 7 v/v, for L = bpy or dpp) or 100% dichloromethane (for L = bpym) as eluent, or by column chromatography by gradual elution with hexane–dichloromethane mixtures (initially 1 : 9 v/v).All preparations and purifications were performed under a nitrogen atmosphere using Schlenk techniques. The purity of the cluster 1–3 was checked with elemental analysis or mass spectroscopy.All compounds were characterized by IR, 1H NMR (see Fig. 1 for the adopted numbering schemes) and UV/VIS spectroscopy. [Ru4(Ï-H)4(CO)10(bpym)] 1. IR [n(CO)/cm21; THF]: 2073m, 2042s, 2018s, 2000m, 1979m, 1945w. 1H NMR (CDCl3): d 9.16 (dd, J = 2.17, 4.74, 2 H, H2, H8), 9.07 (dd, J = 2.15, 5.66, 2 H, H4, H6), 7.62 (dd, J = 4.80, 5.61 Hz, 2 H, H3, H7), 215.9 (s, 2 H), 221.5s, 2 H). UV/VIS [lmax/nm (emax/M21 cm21); CH2Cl2]: 495 (3350), 359 (13 700) [Found (calc.for C18H10N4O10Ru4): C, 25.68 (25.66); H, 1.37 (1.20); N, 6.18 (6.65%)]. [Ru4(Ï-H)4(CO)10(dpp)] 2. IR [n(CO)/cm21; THF]: 2073m, 2041s, 2018vs, 2000m, 1979m, 1946w. 1H NMR (CDCl3): d 8.92 (d, J = 2.96, 1 H, H7), 8.89 (d, 1 H, H2), 8.67 (d, J = 2.9, 2 H, H6, H11), 8.04 (td, J = 1.75, 7.71, 1 H, H9), 7.94 (ddd, J = 1.11, 1.21, 7.90, 1 H, H8), 7.56 (m, 2 H, H4, H10), 7.34 (ddd, J = 1.45, 5.45, 7.62, 1 H, H3), 7.04 (d, J = 8.37 Hz, 1 H, H5), 215.8 (vw, br), 221.3 (w br).UV/VIS [lmax/nm (emax/M21 cm21); CH2Cl2]: 513 Fig. 1 Structures of the ligands L and adopted numbering schemes for clusters 1–3 N N H2 H3 H4 N N H8 H7 H6 Ru N H2 H3 H4 H5 N N H7 H6 N H8 H9 H10 H11 Ru N H2 H3 H4 H5 N H11 H10 H9 H8 Ru 1 L = bpym 3 L = bpy 2 L = dpp (3640), 357 (15 500) and 276 (26 700). Mass (FAB1): (m/z)1 925 (922 calc.) [M]1 [Found (calc. for C24H14N4O10Ru4): C, 31.12 (31.24); H, 1.62 (1.53); N, 5.91 (6.07%)]. [Ru4(Ï-H)4(CO)10(bpy)] 3. IR [n(CO)/cm21; THF]: 2071m, 2039vs, 2015vs, 1996s, 1974m, 1947sh. 1H NMR (CDCl3): d 8.92 (d, J = 5.78, 2 H, H2, H11), 8.11 (d, J = 6.6, 2 H, H5, H8), 7.96 (dt, J = 7.52 and 1.55, 2 H, H4, H9), 7.44 (ddd, J = 7.49, 5.61, 1.49 Hz, H3, H10), 215.68 (s, 2 H), 221.45 (s, 2 H). UV/ VIS [lmax/nm (emax/M21 cm21); CH2Cl2]: 476 (3210), 357 (12 100) and 301 (32 600). Mass (FAB1): (m/z)1 845 (844 calc.) [M]1 [Found (calc. for C20H12N2O10Ru4): C, 28.27 (28.44); H, 1.49 (1.43); N, 3.14 (3.32%)]. Attempted synthesis of [Ru4(Ï-H)4(CO)8(bpym)2] A solution of 155 mg [Ru4(m-H)4(CO)12] in 150 ml CH2Cl2 was cooled to 278 8C before addition of 2,29-bipyrimidine (91 mg, 2.8 equivalents, dissolved in 10 ml CH2Cl2). Trimethylamine N-oxide (66 mg, 4.2 equivalents, in 25 ml CH2Cl2) was then added dropwise to the reaction mixture over a 20 min period.The solution turned from dark green to deep red on gradual warming to room temperature. A large amount of unassigned insoluble material was formed in addition to a red product which was purified by TLC (100% CH2Cl2 as eluent) and identified by IR and mass spectroscopy as 1 and not [Ru4(m-H)4- (CO)8(bpym)2].X-Ray crystallography Crystals of the clusters 1 and 3 suitable for single-crystal X-ray analysis were grown from a dichloromethane–hexane solution at 220 8C (1) or 5 8C (3). Despite repeated attempts crystals of 2 have not been obtained. Data collection was performed on a Stoë Stadi four-circle diVractometer, equipped with an Oxford Cryosystems low-temperature device.The intensities were reduced to Fo 2 and an empirical absorption correction based on semiempirical y-scan data was applied. The structures were solved by direct methods, followed by Fourier-diVerence and full-matrix least-squares refinements on F 2 using the computer programs SHELXS 86 and SHELXL 93 and Figs. 2 and 3 generated using ORTEP.8 Crystal data for [Ru4(Ï-H)4(CO)10(bpym)] 1. C18H10N4O10Ru4? CH2Cl2, M = 929.49, monoclinic, a = 8.474(2), b = 18.897(4), c = 18.400(5) Å, b = 92.15(2)8, U = 2944.4(1) Å3, T = 150(2) K, space group P21/c, Z = 4, m(Mo-Ka) = 2.248 mm21, 4549 reflections collected, 3842 reflections independent (Rint = 0.0208) which were used in all calculations.The final wR(F 2) was 0.1024; R1 = 0.0431. Crystal data for [Ru4(Ï-H)4(CO)10(bpy)] 3. C20H12N2O10Ru4? 0.25 CH2Cl2, M = 865.83, triclinic, a = 8.6204(10), b = 9.9099(12), c = 16.431(2) Å, a = 73.625(10), b = 84.345(10), g = 88.371(9)8, U = 1340.1(3) Å3, T = 293(2) K, space group P1� , Z = 2, m(Mo-Ka) = 2.314 mm21, 4728 reflections collected, 4728 reflections independent which were used in all calculations.The final wR(F2) was 0.0994; R1 = 0.0347. CCDC reference number 186/1056. Spectroscopic measurements The UV/VIS absorption spectra were recorded on softwareupdated Perkin-Elmer Lambda 5 or Varian Cary 4E spectrophotometers, FTIR spectra on Bio-Rad FTS-7 or Perkin-Elmer 1600 spectrometers, and 1H NMR spectra on Bruker WH 250 MHz or AMX 300 spectrometers. X-Band EPR spectra were recorded on a Varian Century E-104A spectrometer. 2,29- Diphenyl-1-picrylhydrazyl (DPPH) was employed as an external ‘g mark’. Mass spectra (FAB1) were measured on a Kratos MS50TC spectrometer, calibrated with CsI.TheJ. Chem. Soc., Dalton Trans., 1998, Pages 2625–2633 2627 samples were run in a matrix solution (m-nitrobenzyl alcohol, 3-NOBA). Resonance-Raman measurements were performed using a Dilor Modular XY spectrometer which employs a backscattering geometry and a multichannel diode array detection system.Spectra were taken from KNO3 pellets at room temperature. Excitation lines of wavelengths 457.9, 476.5, 496.5 and 514.4 nm were obtained from an SP model 2016 Ar1 laser. (Spectro)electrochemical measurements Cyclic voltammograms were recorded in a gastight cell under a nitrogen atmosphere. The cell was equipped with Pt disc working (apparent surface area of 0.42 mm2), Pt gauze auxiliary, and Ag wire pseudo-reference electrodes.The working electrode was carefully polished with a 0.25 mm grain diamond paste. The scan rate was varied between 0.02 and 2 V s21. All redox potentials are reported against the ferrocene–ferrocenium (Fc/Fc1) redox couple used as an internal standard9 [E2� 1 (Fc/Fc1) = 10.58 V vs. SCE in THF]. The solutions for cyclic voltammetric experiments were typically 2 × 1023 M in the cluster compounds and 3 × 1021 M in NBu4PF6. The potential control was achieved with a PAR model 283 potentiostat equipped with positive feedback for ohmic-drop compensation.Bulk electrolyses were carried out in a gastight cell that consisted of three chambers separated at the bottom by S4 frits, with a Pt-flag working electrode (120 mm2 surface) in the middle, and Ag wire pseudo-reference and Pt gauze auxiliary electrodes in the lateral chambers. The concentrations used were 5 × 1023 M 1 and 2 and 3 × 1021 M NBu4PF6.Infrared and UV/VIS spectroelectrochemical experiments at room temperature were performed with an optically transparent thin layer electrode (OTTLE) cell,10 equipped with a Pt minigrid working electrode (32 wires per cm) and CaF2 optical windows. For IR and UV/VIS spectroelectrochemistry at low temperatures another OTTLE cell 11 was employed which fitted into a liquid-nitrogen cryostat. 12 The spectroelectrochemical samples were typically 1022 M in the cluster compounds.A PA4 potentiostat (EKOM, Czech Republic) was used to carry out the controlled-potential electrolyses. Results and Discussion Syntheses of [Ru4(Ï-H)4(CO)10(L)] In contrast to the availability of [Ru4(m-H)4(CO)12 2 n(PR3)n] (n = 1–4),5,6 the substitution reaction of [Ru4(m-H)4(CO)12] with an excess of the a-diimine ligand (L), initiated by an addition of 2.2 equivalents of Me3NO, resulted in the overall substitution of two carbonyl ligands, producing stable [Ru4(m-H)4(CO)10(L)] (L = bpym 1, dpp 2, bpy 3) in 10–20% yield. Attempts to synthesize the tetrasubstituted clusters [Ru4(m-H)4(CO)8(L)2] using 4.2 equivalents of Me3NO were unsuccessful.A similar situation applies for related [Os3(CO)12] which only aVords the disubstituted cluster [Os3(CO)10(L)].4,13 Irrespective of whether the reaction of [Ru4(m-H)4(CO)12] with L was carried out using 1.1 or 2.2 equivalents of Me3NO, only two carbonyl ligands were removed and substituted by the a-diimine.Addition of 2.2 equivalents of Me3NO only increased the yield of [Ru4(m-H)4(CO)10(L)]. For L = bpy or bpym, however, a green intermediate was observed during the reaction course at low temperatures. For L = bpym, this species was stable enough at T < 195 K to permit recording of its IR spectrum which showed n(CO) bands at 2080m, 2066s, 2053s, 2029m, 2022m and 2005w cm21 (in CH2Cl2). The spectrum is virtually identical to that of [Ru4(m-H)4(CO)11(py)] [n(CO) at 2087vw (sh), 2080m, 2066s, 2056m, 2029m, 2022m (sh), 2006w, 1989vw (br) cm21] 14 and closely resembles that of [Ru4- (m-H)4(CO)11{P(OMe)3}].15 The green intermediate is therefore most likely the monosubstituted cluster [Ru4(m-H)4(CO)11- (h1-L)] where L co-ordinates in a monodentate fashion.We therefore cude that the formation of [Ru4(m-H)4(CO)10(L)] is a stepwise process, as depicted in Scheme 1. It is probably the chelate eVect which favours this substitution pattern.The COextrusion reaction of the h1-bound a-diimine was discussed in detail by Lees and co-workers for [M(CO)n(a-diimine)] (n = 4 or 5; M = Cr, Mo or W).16 Solid-state structure of [Ru4(Ï-H)4(CO)10(L)] (L 5 bpym 1 or bpy 3) The isotropic solid-state structures of 1 and 3 are depicted in Fig. 2A and 2B, respectively. Selected bond lengths and angles of 1 and 3 are listed in Tables 1 and 2, respectively. The crystal structures of 1 and 3 confirm that the a-diimine ligand is bound in a bidentate fashion through the nitrogen lone pairs to a single ruthenium atom bearing only one terminal CO group.All ten carbonyl ligands are terminal, as could also be inferred from the IR spectra (see below). The bite angle N]Ru]N is 77.48 for both 1 and 3. Notably, the Ru]N bond lengths of 2.099(8) and 2.117(8) Å in 1 are considerably longer than those in 3 [2.095(5) and 2.090(5) Å]. This diVerence reflects the electron-withdrawing character of the unco-ordinated nitrogen atoms of the 2,29-bipyrimidine ligand, responsible for reduced NÆRu s-donation which results in weaker Ru]N bonds in 1.The apical bonds Ru(2)]C(21) in 1 and Ru(3)]C(31) in 3 are shorter than the other Ru]CO bonds in these clusters due to the increased electron density and hence stronger RuÆCO p-back bonding at the a-diimine-substituted site. The Ru4 core is edge-bridged by four hydride ligands, three of them spanning the Ru]Ru (L) bonds. The hydride positions, localized from Fourier-diVerence maps of the low-angle diVraction data, are also indicated by distortion of the tetrahedral cluster geometry due to lengthening of the four hydride- Scheme 1 Stepwise formation of the disubstituted clusters [Ru4(m-H)4- (CO)10(L)] Ru Ru Ru Ru C C C Ru Ru Ru Ru C N C Ru Ru Ru Ru N N C O N O O O O O + L –CO –CO N N = L = bpy, bpym or dpp Table 1 Selected bond lengths (Å) and angles (8) for [Ru4(m-H)4- (CO)10(bpym)] 1 Ru(1)]Ru(2) Ru(1)]Ru(3) Ru(1)]Ru(4) Ru(2)]Ru(3) Ru(2)]Ru(4) Ru(3)]Ru(4) Ru(2)]C(21) Ru(2)]N(1b) Ru(2)]N(4b) Ru(2)]H(1) 2.944(12) 2.787(13) 2.760(11) 3.035(12) 2.949(13) 2.931(12) 1.834(11) 2.099(8) 2.117(8) 1.74(3) Ru(2)]H(2) Ru(2)]H(3) Ru(3)]H(3) Ru(3)]H(4) Ru(4)]H(2) Ru(4)]H(4) mean Ru]C mean C]O N(1b)]Ru(3)]N(4b) 1.74(3) 1.75(3) 1.75(3) 1.75(3) 1.75(3) 1.74(3) 1.89 1.13 77.4(3) Table 2 Selected bond lengths (Å) and angles (8) for [Ru4(m-H)4(CO)10- (bpy)] 3 Ru(1)]Ru(2) Ru(1)]Ru(3) Ru(1)]Ru(4) Ru(2)]Ru(3) Ru(2)]Ru(4) Ru(3)]Ru(4) Ru(3)]C(31) Ru(3)]N(1) Ru(3)]N(2) Ru(1)]H(1) 2.925(7) 3.023(7) 2.795(8) 2.964(7) 2.780(8) 2.946(7) 1.844(7) 2.095(5) 2.090(5) 1.76(3) Ru(1)]H(4) Ru(2)]H(2) Ru(2)]H(4) Ru(3)]H(1) Ru(3)]H(2) Ru(3)]H(3) Ru(4)]H(3) mean Ru]C mean C]O N(1)]Ru(3)]N(2) 1.76(3) 1.77(3) 1.77(3) 1.76(3) 1.75(3) 1.76(3) 1.76(3) 1.90 1.13 77.4(2)2628 J.Chem. Soc., Dalton Trans., 1998, Pages 2625–2633 bridged Ru]Ru bonds [average Ru]Ru distance 2.964(6) Å in 1 and 2.964(5) Å in 3] compared to the two unbridged Ru]Ru bonds [average Ru]Ru distance 2.773(4) Å in 1 and 2.787(4) Å in 3].Similar deformation of the metal core has been reported for other tetrahedral Ru4 clusters with bridging hydride ligand(s).6,17 Fig. 2 A, An ORTEP drawing of the crystal structure of [Ru4(m-H)4- (CO)10(bpym)] 1. B, An ORTEP drawing of the crystal structure of [Ru4(m-H)4(CO)10(bpy)] 3 Table 3 Raman wavenumbers of co-ordinated 2,29-bipyrimidine from resonance-Raman spectra of [Ru4(m-H)4(CO)10(bpym)] 1 in KNO3 at room temperature, compared with those of related complexes [W(CO)4(bpym)] 19 and [Os3(CO)10(bpym)] 13 Compound n/cm21 1[ W(CO)4(bpym)] [Os3(CO)10(bpym)] 1575 1577 1578 1548 1548 1537 1466 1463 1463 1415 1417 1407 1335 1335 1340 1203 1198 1194 1025 1017 1018 Spectroscopic properties of [Ru4(Ï-H)4(CO)10(L)] IR and 1H NMR spectroscopy.The infrared spectra of 1–3 in the CO stretching region are very similar and correspond to terminal co-ordination of all carbonyl ligands.The slightly larger n(CO) wavenumbers of 1 and 2 relative to those of 3 reflect less pronounced RuÆCO p-back donation due to the larger p-acceptor capacity of the bpym and dpp ligands compared to that of the bpy ligand.18 The 1H NMR data of 1 and 3 (see Experimental section) confirm that the bpy and bpym ligands co-ordinate to Ru in a normal bidentate fashion. The two aromatic rings of these ligands remain magnetically equivalent upon co-ordination.For cluster 1, the D2h symmetry of the free bpym ligand is lost upon co-ordination. The 1H NMR resonances of the cluster 2 could be completely assigned with the aid of decoupling experiments described in detail in SUP 57401. The chemical shift of the ortho protons adjacent to the co-ordinated nitrogen atoms to lower values by 0.12–0.50 ppm in comparison with the resonances of the unco-ordinated ligands L is characteristic for sN, sN9-co-ordination of L to a low-valent metal centre.This eVect arises from a larger contribution of a resonance form in which the nitrogen atoms bear a partial negative charge and the adjacent carbon atoms a corresponding partial positive charge. Notably, the hydride resonances show a higher symmetry than expected from the solid-state structures. There are two hydride resonances around d 216 and 222, each accounting for two protons. The signals are rather weak and broad, indicating that a fluxional process may well be responsible for their pairwise equivalence. Electronic absorption and resonance-Raman spectra.The clusters 1–3 are orange to red-brown in colour. Their UV/VIS absorption maxima and corresponding molar absorption coe Ycients in CH2Cl2 are listed in the Experimental section. The position of the lowest energy absorption band depends on the electronic properties of the a-diimine ligands L and on the solvent polarity. For 3, the absorption maximum shifts from 486 nm in benzene to 463 nm in THF and 447 nm in acetonitrile.In contrast to this, the absorption maxima at higher energy hardly show any solvatochromism. The negative solvatochromism and the low-energy shift of lmax, resulting from replacement of 2,29-bipyridine by the stronger p-acceptor 2,29-bipyrimidine (from 463 to 484 nm in THF) point to a charge-transfer character of the lowest-energy electronic transition. Its nature was further investigated by resonance- Raman (rR) spectroscopy on visible excitation of 1 with four diVerent Ar1 laser lines (see Experimental section).The result is presented in Table 3. The main rR eVect is observed for bands in the 1000–1600 cm21 region which belong to internal stretching modes of the 2,29-bipyrimidine ligand. The presence of the Raman band at 1335 cm21, which is assigned to the inter-ring C]C stretching vibrations, indicates population of the lowest p*(b2u) orbital of 2,29-bipyrimidine in the charge-transfer excited state.19 Importantly, the peak due to the resonance enhanced ns(CO) vibration of 1 also showed up at 2070 cm21, indicating depopulation of the Ru(dp) orbitals involved in the RuÆCO p-back bonding.The combined resonance-Raman and UV/VIS features thus reveal that the lowest electronic transition of [Ru4(m-H)4(CO)10- (L)] is directed towards the a-diimine ligand L, having a signifi- cant Ru(dp)ÆL(p*) charge-transfer (MLCT) character. (Spectro)electrochemical studies of [Ru4(Ï-H)4(CO)10(L)] The redox properties of clusters 1–3 and their reduction paths were investigated by cyclic voltammetry, IR and UV/VIS spectroelectrochemistry, and by EPR spectroscopy.Redox potentials of 1–3 and their reduction products are presented in Table 4. Cyclic voltammograms of 2 and 3 are depicted in Fig. 3. Infrared n(CO) wavenumbers and UV/VIS spectroscopicJ. Chem. Soc., Dalton Trans., 1998, Pages 2625–2633 2629 Table 4 Redox potentials of the clusters 1–3 and their reduction products a Cluster 123 Epc (R1) b 21.54 21.57 21.88 DEp (R1/O1) 0.12 0.11 0.14 Epc (R2) c 22.23 22.23 22.35 Epc (R3) d 22.38 22.38 22.42 Epa (Om) e 10.39 10.41 10.35 Epa (O29) f 21.04 21.08 21.17 a Conditions: 2 × 1023 M 1–3 in THF–3 × 1021 M NBu4PF6, T = 298 K, Pt disc electrode, n = 100 mV s21; potentials given in V vs.E2� 1 (Fc/Fc1) (= 10.575 V vs. SCE); DEp(Fc/Fc1) = 0.11 V. b Reduction (1e) of 1–3. c Reduction (1e) of the corresponding radical anions 1b–3b. d Reduction (1e) of the dihydrido dianions 1c–3c. e Oxidation of 1–3.f Oxidation of the dihydrido dianions 1c–3c. Table 5 IR n(CO) wavenumbers of the clusters 1–3 and their electrochemical reduction products n/cm21 1 a,b 1b a,b 1c b 2 a,b 2 c 2b a,b 2b c 2c b 3 b 3c b 3e b 2073m 2065m 2006w 1770 (sh) 2073m 2074m 2063m 2066m 2008m 1762 (sh) 2071m 2025w 1879m 1985vw 1707w 2042s 2033s 1979m 1736m/w 2041s 2042s 2031s 2033s 1979m 1744m 2039vs 1990s 1862 (sh) 1948m 1680w 2018s 2008s 1956 (sh) 1712w 2018s 2017s 2006s 2008s 1957s 1724m/w 2015vs 1956 (sh) 1771 (sh) 1905s 2000m 1989m 1929s (br) 1677vw 2000m 1999m 1986m 1987m 1934s (br) 1678vw 1996s 1948vs 1735w 1892s 1979w 1970w 1889s 1979m 1979m 1966w 1966w 1892s 1974m 1923s 1712w 1848m 1945w 1943w 1867 (sh) 1946w 1942w 1942w 1941w 1875 (sh) 1947 (sh) 1910 (sh) 1670vw 1831m a In CH2Cl2–electrolyte at T = 298 K.b In THF–electrolyte at T = 298 K. c In CH2Cl2–electrolyte at T = 223 K. data of 1–3 and their reduction products are collected in Tables 5 and 6, respectively.Radical anions [Ru4(Ï-H)4(CO)10(L)]~2 (L 5 bpym or dpp). Reduction of the clusters 1 and 2 [cathodic peak R1 in Fig. Fig. 3 Cyclic voltammograms of 2 (A) and 3 (B). Oxidation of 3c generated via bulk electrolysis of 3 at E(R1) (B1). Conditions: Pt disc microelectrode (0.42 mm2 apparent surface), THF–NBu4PF6, T = 298 K, n = 0.1 V s21 3(A)] is both electrochemically and chemically reversible on the subsecond time-scale of cyclic voltammetry (defined by n > 0.02 V s21), satisfying the usual diagnostic tests.20 The product of this cathodic step was further studied in situ by IR and UV/VIS spectroelectrochemistry in a thin-layer cell.Fig. 4 shows the IR/UV/VIS spectral changes accompanying the reduction of 1. The n(CO) pattern of 1 remains preserved on reduction, being shifted by some 9–12 cm21 to smaller wavenumbers. The retention of isosbestic points indicates stability of the reduction product at room temperature over a period of ca. 10 min. After this period, however, a slow secondary reaction was evident from gradually increasing absorbances below 1900 cm21. This secondary reaction was not observed at T < 253 K, allowing characterization of the primary reduction product, denoted as 1b, by UV/VIS spectroscopy [see Fig. 4(a)]. The most prominent features of the UV/VIS spectrum of 1b are the two sharp absorption bands at 475 and 503 nm, accompanied by a lower-lying broad absorption band with lmax ª 850 nm.These bands can straightforwardly be attributed to intraligand (IL) electronic transitions of the co-ordinated radical anion [bpym]~2.19,21 From the cyclic voltammetric and spectroelectrochemical experiments it is concluded that the reduction of 1 is a one-electron process which produces the corresponding radical anion [Ru4(m-H)4(CO)10(bpym)]~2 1b with the odd electron dominantly localized on the lowest p* orbital of the 2,29-bipyrimidine ligand.This also applies for 2. In this case, however, the radical anion 2b (see Tables 5 and 6) partly decomposed in CH2Cl2 at room temperature in the course of the reduction of 2, i.e. more rapidly than 1b under the same conditions. The decomposition significantly slowed down in less polar THF. The UV/VIS spectra of 2b (see Table 6) were recorded in CH2Cl2 at T = 223 K where the radical anion remained inherently stable and could be fully reoxidized back to 2. The radical nature of 1b and 2b was unambiguously con- firmed by recording their EPR spectra (see Fig. 5). For this purpose 1b and 2b were generated in THF at T = 263 K by bulk electrolysis at the cathodic potential E(R1). The EPR spectra, found at g = 2.0015 for 1b and g = 2.0016 for 2b (i.e. close to the2630 J. Chem. Soc., Dalton Trans., 1998, Pages 2625–2633 Table 6 UV/VIS spectra of the clusters 1–3 and some of their reduction products lmax/nm (emax/M21 cm21) 1 a 1b a 2 b 2b b 3 c 3c c 256 (sh) (31 000) 278 (sh) (23 000) 274 (41 500) 266 (sh) (39 000) 301 (32 600) 336 (12 600) 358 (13 700) 362 (20 200) 357 (15 500) 349 (23 500) 354 (12 100) 484 (3350) 475 (7000) 486 (3700) 459 (sh) (9000) 463 (3200) ca. 500 (sh) 503 (7150) 763 (1500) 850 (650) 830 (1450) a In CH2Cl2 at T = 253 K. b In CH2Cl2 at T = 223 K. c In THF at T = 298 K. All solutions contained 3 × 1021 M NBu4PF6. free-electron value), are not well resolved, showing some hyper- fine structure which probably originates from splitting due to the 14N (I = 1, 99.63% abundance) and 1H (I = ��� , 99.98% abundance) nuclei of the a-diimine ligand. Additional splitting, partly responsible for the poorly resolved EPR signals, may arise from the 99Ru (I = 5 2 –, 12.7% abundance) and 101Ru (I = 5 2 –, 17.1% abundance) nuclei, and from the 1H nuclei bridging the cluster edges. Elimination of H2 from [Ru4(Ï-H)4(CO)10(L)]~2/22.Subsequent chemically irreversible one-electron reduction of the radical anions 1b and 2b at the cathodic potential E(R2) [see Table 4 and Fig. 3(A)] yielded 1c and 2c, respectively (see Tables 5 and 6). These compounds are identical with the species produced from 1b and 2b in a thermal, probably disproportionation 3 reaction (see above). The dianionic clusters 1c and 2c are also formed via the direct two-electron route, from unstable dianions [Ru4(m-H)4(CO)10(L)]22 (L = bpym or dpp) produced Fig. 4 Spectral changes in the IR n(CO) [(a), in THF at T = 298 K] and UV/VIS [(b), in CH2Cl2 at T = 253 K] regions, accompanying the reversible reduction of the cluster 1 producing the radical anion 1b during the one-electron reduction of 1b and 2b.The nature of the products 1c and 2c was elucidated by performing a (spectro)electrochemical study of the cluster 3. The reduction of 3 at E(R1) is shifted more negatively compared to the reduction potentials of 1 and 2 [see Fig. 3(B) and Table 4], being only partly chemically reversible at n = 0.1 V s21 (Ipa/Ipc ª 0.5).This result agrees with a predominantly bpy-localized one-electron cathodic step producing the unstable radical anion [Ru4(m-H)4(CO)10(bpy)]~2 3b. Obviously, the more basic bpy ligand with a higher-lying p* LUMO in comparison with the bpym and dpp ligands is less suited to accommodate the added electron in this case. The radical anion 3b, though still detectable on the reverse anodic scan due to its oxidation at E(O1) [see Fig. 3(B)], decomposes more rapidly than 1b and 2b and was not observed by IR, UV/VIS and EPR spectroscopy during spectroelectrochemical experiments at room temperature. Instead, the reduction of 3 on the time-scale of minutes yielded exclusively the dianion 3c, as was judged from the close similarity of IR spectra of 1c–3c (see Fig. 6, left, and Table 5). The UV/VIS spectrum of 3c is depicted in Fig. 6, right. The n(CO) bands between 1800 and 1670 cm21 in the IR spectra of 1c–3c are indicative of bridging CO ligands which may have replaced some of the edge-bridging hydrogen atoms on the reduction of 1b–3b.This behaviour is analogous to that reported for unsubstituted [Ru4(m-H)4(CO)12] whose irreversible two-electron reduction produces the dianion [Ru4(m-H)2- (CO)12]22 with three CO ligands bridging the basal Ru]Ru bonds.3,22 We could obtain the latter species neatly via stepwise deprotonation of [Ru4(m-H)4(CO)12] in THF on addition of 2 equivalents of NEt4OH in MeOH, in conformity with the Fig. 5 The EPR spectra of 1b (a) and 2b (b) in THF at 298 KJ. Chem. Soc., Dalton Trans., 1998, Pages 2625–2633 2631 recorded IR spectra attributed to the intermediate NEt4[Ru4- (m-H)3(CO)12] [n(CO) at 2074w, 2038s, 2032 (sh), 2015s, 1990s cm21] 23 and the yellow-orange final product [NEt4]2[Ru4(m-H)2- (CO)12] [n(CO) at 2031w, 1990s, 1951vs (br), 1904m, 1886m, 1814vw, 1761m, 1744m cm21] 22 (see Fig. 7). The salt NEt4OH was therefore chosen as a suitable reagent for the antideprotonation of 3.The reaction was performed in MeCN owing to poor solubility of the product(s) in THF. Initially a brown species 3c9 was formed whose IR spectrum closely resembled that of [NEt4]2[Ru4(m-H)2(CO)12] under the same conditions: n(CO) at 2004w, 1982m, 1941vs, 1900 (sh), 1887m, 1797vw, 1754m, 1737m cm21 (see Fig. 7). The smaller wavenumbers of 3c9 relative to those of [NEt4]2[Ru4(m-H)2(CO)12] (by ca. 10 cm21) point to co-ordination of the s-donor 2,29- bipyridine ligand, which was also evident from the 1H NMR spectrum of 3c9 in CD3CN.The signals of the bpy-protons of the parent cluster 3 [d 9.01 (d, 2 H), 8.44 (d, 2 H), 8.20 (dt, 2 H), 7.64 (dt, 2 H)] were replaced after addition of NEt4OH by a new set for 3c9 at d 8.80 (d, 2 H), 8.26 (d, 2 H), 8.07 (t, 2 H) and 7.56 (t, 2 H). These values deviate from those for the uncoordinated 2,29-bipyridine under identical conditions [d 8.80 (d), 8.52 (d), 7.99 (dt) and 7.50 (dt)].Notably, the cluster 3c9 could only be detected as an unstable intermediate. It transformed within a few minutes into another CO-bridged compound 3d Fig. 6 Left: infrared spectral changes [n(CO) region] accompanying the irreversible reduction of the cluster 3 producing the dianion 3c. Right: UV/VIS spectra of 3 (dot-dash line), 3c (full line) and the tetraanion 3e, the 2e reduction product of 3c (broken line). All spectra were recorded in THF at T = 298 K Fig. 7 The IR spectra [n(CO) region] of (a) [NEt4]2[Ru4(m-H)2(CO)12], (b) 3c9 in MeCN, and (c) 3d in MeCN. All spectra recorded at T = 298 K characterized by n(CO) bands at 2034 (sh), 2017m, 1992 (sh), 1967vs, 1955 (sh), 1926m, 1907m, 1821vw, 1770m cm21 (see Fig. 7) and, in the 1H NMR spectrum, by resonances due to the bpy ligand at d 8.68 (d, 2 H), 8.30 (d, 2 H), 8.09 (dt, 2 H), 7.56 (dt, 2 H) and broad hydride resonances at 216.8 (s, 1 H) and 223.7 (s, 1 H). Recall that the 1H NMR spectrum of [Ru4(m-H)2- (CO)12]22 exhibits at room temperature only a sharp hydride singlet at d 219.26.22 Importantly, IR spectroelectrochemical experiments revealed that 3d is identical to the product of electrochemical oxidation of 3c at the anodic potential E(O29) (see Fig. 3 and Table 4). The nature of the conversion of 3c9 into 3d, apparently demanding an oxidation step, was not investigated in detail. All attempts to crystallize 3d have been unsuccessful so far. In essence, the above IR and 1H NMR data show that deprotonation of 3 yields 3c9 which can be formulated as the tetrahedral dianion [Ru4(m-H)2(CO)10(bpy)]22, probably with three bridging CO ligands such as those established for the unsubstituted derivative [Ru4(m-H)2(CO)12]22.22 Successive electrochemical reduction of 3 and the radical anion 3b ultimately yields 3c exhibiting slightly lower n(CO) wavenumbers and a diVerent, more complex n(CO) pattern with regard to those of 3c9 (see Figs. 6 and 7).The question remains how much 3c corresponds to 3c9. According to the IR spectra, these species are not identical. There is also no evidence that 3c and 3c9 interconvert. The actual diVerence between them, however, is believed not to be significant as both identically produce 3d (see above). For comparison, both electrochemical reduction 3 and deprotonation 22 of [Ru4(m-H)4(CO)12] yielded the same product, viz. [Ru4(m-H)2(CO)12]22. At this stage of investigation we can conclude that, similarly to 3c9, the electrochemical reduction product 3c is also a dianionic Ru4-cluster with edgebridging hydride and carbonyl ligands, formed from the transient cluster [Ru4(m-H)4(CO)10(bpy)]22 via elimination of H2. The very similar averaged n(CO) wavenumbers and the almost a-diimine-independent oxidation potentials E(O29) of the dianions 1c–3c (see Tables 4 and 5) imply eVective delocalization of the negative charge over the cluster core, residing more at the CO-bridged basal Ru3(CO)9 moiety than at the apical Ru(a-diimine) fragment.The UV/VIS spectrum of 3c exhibits a broad band at 763 nm which may belong to Ru(bpy)- localized electronic transitions, for [Ru4(m-H)2(CO)12]22 does not absorb at such a low energy. Further reduction of 3c at the electrode potential E(R3), studied by IR/UV/VIS spectroelectrochemistry, yielded the cluster 3e (see Table 5). The CO-bridge absorptions in the IR spectrum of 3c shifted on the reduction to lower wavenumbers by approximately 30 cm21, which is slightly more than that found for [N(PPh3)2]2[Ru4(m-H)2(CO)12] 22 and [Ph4P]4[Ru4- (CO)12].24 The UV/VIS spectrum of 3e shows a broad structured band with maxima at 533 and 572 nm and shoulders at 505, 640, 700 and 760 nm (see Fig. 6).These features strongly resemble the visible absorption of the two-electron-reduced anion [Re(CO)3(bpy)]2 having the added two electrons strongly p-delocalized over the Re(bpy) chelate ring.25,26 We may thus assign 3e as [NBu4]4[Ru4(CO)10(bpy)], with the two extra electrons added to the dianion 3c predominantly residing on the p-system of the Ru(bpy) moiety.According to the cyclic voltammograms of 2 and 3, the reduction of 3c at E(R3) should be a one-electron process which aVords the radical [3c]~2. This transient species was indeed observed in the course of the UV/ VIS OTTLE experiment. Its UV/VIS spectrum exhibits a characteristic structured band with absorption maxima at 495 and 522 nm, belonging to an intraligand electronic transition of the one-electron-reduced ligand [bpy]~2.27 The radical [3c]~2 probably further disproportionated into 3c and 3e under liberation of H2.Recall that reduction of [Ru4(m-H)2(CO)12]22 is also initially a one-electron process; 3 although, deprotonation of the dianion with 1 equivalent of KH only produced a 1: 1 mixture of [Ru4(m-H)2(CO)12]22 and [Ru4(CO)12]42.22 In fact, the2632 J. Chem. Soc., Dalton Trans., 1998, Pages 2625.2633 only reported route to [Ru4(m-H)(CO)12]32 involved protonation of [Ru4(CO)12]42 with HBr.24 The reduction path of 1.3 is summarized in Scheme 2.Oxidation of [Ru4(I-H)4(CO)10(L)] 10. The cyclic voltammograms of 2 and 3 in Fig. 3 reveal that these clusters undergo chemically irreversible oxidation consuming more than one electron [Ia(Om) �£ 2Ic(R1)]. The very close values of the corresponding anodic peak potentials E(Om) (see Table 4) are indicative of an anodic process localized on the metal core, which may induce cleavage of Ru]Ru bonds.The oxidation of [Ru4- (m-H)4(CO)10(L)] was not further investigated. Comparison of the reduction paths of [Ru4(I-H)4(CO)10(L)] and [Os3(CO)10(L)]. Both [Ru4(m-H)4(CO)10(L)] and [Os3(CO)10- (L)] 4 (L = bpym, dpp or bpy) initially undergo one-electron reduction mainly localized on the lowest p* orbital of the adiimine ligand. The corresponding radical anionic products are therefore substantially more stable than those derived from the unsubstituted clusters [Ru4(m-H)4(CO)12] 3 and [Os3(CO)12],28 and can be detected by diverse spectroelectrochemical methods.The reactivity induced by reduction of these low-nuclearity clusters can conveniently be controlled by tuning the electronic properties of the a-diimine ligands L. Raising the s,p-donor character of the reduced a-diimine initiates secondary chemical/electron transfer reactions of the radical anions, whose nature is identical with the reactivity of the purely carbonyl precursors.In particular, the species [Os3(CO)12 2 2n- (L)n]~2 (n = 0 or 1) undergo cleavage of an Os]Os bond resulting ultimately in formation of open-structure dianionic clusters.4 Contrary to this, the species [Ru4(m-H)4(CO)12 2 2n- (L)n]~2 (n = 0 or 1) react via the loss of hydrogen to give the corresponding dianions [Ru4(m-H)2(CO)12 2 n(L)n]22. Notably, we have obtained no spectroscopic evidence for participation of the anion [Ru4(m-H)3(CO)10(L)]2 in the reduction path of [Ru4(m-H)4(CO)10(L)], as was reported 3 for the reduction of [Ru4(m-H)4(CO)12].The radical anions [Ru4(m-H)4(CO)10(L)]~2 are apparently more stable than the corresponding species [Os3(CO)10(L)]~2. In the latter case only [Os3(CO)10(bpym)]~2 could be characterized spectroscopically, becoming stable at T = 213 K. The radical [Os3(CO)10(dpp)]~2 is only stable on a subsecond time-scale of cyclic voltammetry while [Os3(CO)10(bpy)]~2 partly decomses at moderate scan rates even at T = 220 K and at room temperature it is not detectable at all.4 The order of increasing stability of the radical anions on co-ordination of stronger p-acceptor L, bpy ! dpp < bpym, applies also for [Ru4(m-H)4- (CO)10(L)]~2.All the latter species could be detected by conventional cyclic voltammetry at room temperature and, for L = dpp or bpym, they were also characterized by IR, UV/VIS Scheme 2 Reduction paths of the clusters [Ru4(m-H)4(CO)10(L)] [Ru4(m-H)4(CO)10(L)] 1.3 [Ru4(m-H)4(CO)10(L)].. 1b.3b [Ru4(m -H)4(CO)10(L)]2.+ e. (1b, 2b stable at T 253 K) .H2 1c, 2c 3c [Ru4(m-H)2(CO)10(L)]2. disprop. .H2 rapid (3b) slow (1b,2b) L = bpy 3d (.e. ?) 3e + e. 3c' .e. .H2 + 2e. + 2 OH. . 2 H2O and EPR spectroscopy. The large diVerence in the stability of [Ru4(m-H)4(CO)10(L)]~2 and the corresponding Os3 derivatives can be ascribed to a more delocalised bonding situation in the robust closely-packed tetrahedral Ru4(m-H)4 core.Conclusion The co-ordination of the reducible a-diimine ligand L in the novel clusters [Ru4(m-H)4(CO)10(L)] prevents rapid decomposition of the primary one-electron reduction products. The radical anions [Ru4(m-H)4(CO)10(L)]~2 (L = dpp or bpym) are stable on the time-scale of minutes and could be characterized by IR, UV/VIS and EPR spectroscopy. Regardless of this stabilizing influence, the overall reactivity remains unaVected.Either on uptake of another electron, or thermally via disproportionation, the radical anions [Ru4(m-H)4(CO)10(L)]~2 lose dihydrogen and transform to dianions [Ru4(m-H)2(CO)10(L)]22 which contain edge-bridging CO ligands. The reactivity is highest for [Ru4(m-H)4(CO)10(bpy)]~2 which could only be observed by cyclic voltammetry. 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