Article pubs.acs.org/Organometallics
A Ruthenium Dihydrogen Germylene Complex and the Catalytic Synthesis of Digermoxane Katharine A. Smart,†,‡ Emmanuelle Mothes-Martin,†,‡ Laure Vendier,†,‡ Robin N. Perutz,§ Mary Grellier,*,†,‡ and Sylviane Sabo-Etienne*,†,‡ †
CNRS, LCC (Laboratoire de Chimie de Coordination), BP44099, 205 route de Narbonne, F-31077 Toulouse cedex 4, France Université de Toulouse, UPS, INPT, F-31077 Toulouse, France § Department of Chemistry, University of York, Heslington, York YO10 5DD, U.K. ‡
S Supporting Information *
ABSTRACT: A germylene complex with a labile η2-H2 coligand, [RuH2(GePh2)(η2-H2)(PCy3)2] (2), was isolated in very good yield from the reaction of the bis(dihydrogen) complex [RuH2(η2-H2)2(PCy3)2] (1) with Ph2GeH2. The hydrolytic synthesis of 1,1,3,3-tetraphenyldigermoxane, (HPh2Ge)2O, was catalyzed by 2, 1, and the tricyclopentylphosphine analogue 1Cyp. Coordination of digermoxane by ruthenium led to a complex best formulated as [RuH2({η2-HGePh2}2O)(PCy3)2] on the basis of X-ray, NMR, IR, and DFT studies.
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INTRODUCTION The enormous resurgence of interest in main-group chemistry has focused attention on the properties of the heavier maingroup elements that make them distinct from the more familiar light elements. Germanium is, in particular, attracting more and more attention due to applications in electronic and optical devices.1 Easily accessible germanium building blocks are thus desirable for the introduction of germanium-doping additives. Recent years have seen the discovery of new Ge-containing ligands and catalytic phenomena in transition-metal germanium chemistry.2 However, the chemistry of organogermanium compounds remains unexplored in comparison to organosilicon chemistry. Silicon and germanium belong to the same group of the periodic table, but differences in behavior of analogous Ge and Si species have been illustrated in germylene and silylene chemistry.3 Compounds bearing at least one Ge−H bond are interesting building blocks for more complex germanium networks.2f,4 Moreover, σ complexes of the heavier group 14 complexes are limited in number and their role in catalysis is illdefined.5 In this context, we show that the novel combination of a RuGe bond with a labile η2-H2 coligand offers opportunities for new reactivity and catalytic applications as a result of preorganized H−Ge units.
The molecular structure identifies the new complex as a ruthenium dihydride dihydrogen germylene complex, [RuH2(GePh2)(η2-H2)(PCy3)2] (2) (Scheme 1 and Figure Scheme 1. Activation of Ph2GeH2 To Form 2 via 3/3′
1). The structure reveals two axial phosphine ligands with two hydrides, a dihydrogen, and the GePh2 moiety in the equatorial plane. The Ru−Ge distance is 2.3162(2) Å by X-ray,6,7 and the sum of the angles around germanium is 359.5°, as expected for sp2-hybridized germanium with three coplanar ligands. The hydride and dihydrogen ligands were located from the Fourier difference map and refined freely. The structure was also optimized by DFT calculations to support the hydrogen location. The H−H distance of the dihydrogen ligand is 1.06(5) Å by X-ray and 0.924 Å by DFT/B3PW91-D3, at the upper limit for unstretched dihydrogen complexes.5 The
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RESULTS AND DISCUSSION Synthesis of a Dihydrogen Germylene Complex. The reaction of [RuH2(η2-H2)2(PCy3)2] (1) with 1 equiv of Ph2GeH2 in THF at 236 K gave rise to a homogeneous orange solution from which a crystalline solid was isolated (see the Supporting Information). Orange crystals suitable for X-ray crystallography were grown from a pentane solution at 236 K. © XXXX American Chemical Society
Received: July 1, 2015
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DOI: 10.1021/acs.organomet.5b00570 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
Synthesis of a Digermoxane Complex. Isolation of the germylene complex 2 proved to be quite challenging, as the system is very sensitive to any traces of water. Tilley and coworkers reported that the germylene complex [Cp*RuH( GeH(Trip)(PMeiPr2)] reacted smoothly with degassed water in C6D6 to form the hydroxy germyl complex [Cp*RuH2{GeH(OH) (Trip)}(PMeiPr2)].7b NMR monitoring of the reaction of 2 with 1 equiv of degassed H2O indicated formation of several species, including a ruthenium digermoxane complex, identified below. When the reaction of 1 with Ph2GeH2 (2 equiv) was conducted on a Schlenk line rather than in a drybox, a mixture resulted from which a complex of 1,1,3,3tetraphenyldigermoxane ((HPh2Ge)2O) could be isolated, [RuH4{(Ph2Ge)2O}(PCy3)2] (4·Et2O), even though the solvents had been dried (Scheme 2, route a). A closely related
Figure 1. X-ray molecular structure of 2.
separation Ge···H1 of 2.53(2) Å by X-ray diffraction and 2.627 Å by DFT, associated with a Wiberg index value of 0.0875 (see below), rules out a residual interaction between germanium and hydrogen. The 1H NMR spectrum of 2 in THF-d8 at 298 K displays a triplet in the hydride region at δ −7.67 (2JPH = 14 Hz) integrating for 4 hydrogens relative to 10 aromatic protons between δ 8.02 and 7.38. There are no hydrogens in the region for GeH resonances around δ 5. The presence of a dihydrogen ligand is substantiated by the T1,min of 55 ms (500 MHz and 253 K). A singlet is observed at δ 84.5 in the 31P{1H} NMR spectrum at 298 K. No decoalescence of the hydride or the phosphorus resonances was detected down to 163 K. We investigated the reaction of 1 with Ph2GeH2 leading to 2 by following the reaction from 213 K using 1H and 31P NMR spectroscopy (Figures S3 and S4 in the Supporting Information). The reaction occurred quickly, yielding a new species characterized by a triplet at δ 5.28 (3JPH = 13 Hz, 1H) and a singlet at δ −7.88 (5H) in the 1H NMR spectrum which correlate with one another and to a singlet at δ 67.4 in the 31 1 P{ H} NMR spectrum. The chemical shift of δ 5.28 and value of JPH are in agreement with assignment to a GeH proton. There was no decoalescence of the hydride or 31P signals down to 163 K. These resonances can be assigned to the η2diphenylgermane dihydrogen complex [RuH2(η2-H-GeHPh2)(η 2 -H 2 )(PCy 3 ) 2 ] (3) 8 or to the diphenylgermyl bis(dihydrogen) complex [RuH(GeHPh2)(η2-H2)2(PCy3)2] (3′) arising from oxidative addition of one Ge−H bond (Scheme 1). Additionally, a singlet at δ 4.55 in the 1H NMR spectrum indicates evolution of dihydrogen. The coordination of dihydrogen in 3/3′ is supported by the T1,min value of the hydride signal of 35 ms (500 MHz and 213 K), which is significantly lower than that of 2. However, we are unable to distinguish structures 3 and 3′ on the basis of NMR spectroscopy because they are both prone to dynamic rearrangements. Indeed, complex 3 could, in principle, be converted to 3′ through a σ-CAM rearrangement.9 When the reaction mixture was warmed to 273 K, signals corresponding to 2 appeared, as well as signals corresponding to complex 4 (see below) as a result of the extreme sensitivity of the reaction mixture. The 1H NMR EXSY spectrum acquired at 273 K (Figure S5 in the Supporting Information) indicated exchange between the hydride of 2 at δ −7.65 and the GeH proton of 3/ 3′ at δ 5.38. On this basis, we propose that 2 is formed from 3′ by α-hydrogen migration to ruthenium of the remaining hydrogen attached to germanium. The EXSY spectrum suggested that interconversion of 2 and 3/3′ is reversible. We therefore reacted 2 with H2 (3 bar) in C6D6 solution at room temperature and observed almost quantitative formation of 1.
Scheme 2. Two Pathways Leading to the Digermoxane Complex 4a
a
In Et2O or pentane for 4·Et2O and 4·pent, respectively.
species (4·pent) was obtained by direct reaction of 1 with (HPh2Ge)2O in pentane (Scheme 2, route b). The two species were characterized by 1H and 31P NMR and infrared spectroscopy together with X-ray crystallography (Table 1, Figure 2, and the Supporting Information). They display identical NMR data. The two structures differ, however, in the orientation of the digermoxane with respect to the phosphines. DFT calculations led indeed to the optimization of two isomeric forms very close in energy, with 4·pent/DFT lying 1.8 kJ mol−1 lower in energy than the more symmetric structure corresponding to 4·Et2O/DFT (Table 1). The structures display Ru−H distances in a relatively narrow range of 1.57(3)−1.64(4) Å (X-ray) and 1.60−1.65 Å (DFT). The closest Ge−H distances have a greater spread, varying from 2.16(3) to 2.50(6) Å (X-ray) and from 2.25 to 2.50 Å (DFT). Siloxane and silazane analogues of 4 have been reported: [RuH 2 {(η 2 -HSiPh 2 ) 2 O}(PCy 3 ) 2 ] (5) and [RuH 2 {(η 2 HSiMe2)2NH}(PCy3)2] (6).10,11 A classical description of the ligand bonding in 4 with four hydrides and direct Ru−Ge σ bonds determines the formal oxidation state as Ru(VI), a highly unusual situation, while coordination of the digermoxane by two η2-Ge−H interactions sets the oxidation state as Ru(II). Even the shortest Ge−H distances, 2.16(3) Å (X-ray) and 2.25 Å (DFT), are greater than those assigned previously to η2-Ge− H interactions, which range from 1.65 to 2.1 Å by either X-ray or DFT;3b,8b−e for comparison, the distance in free germanes is ca. 1.53 Å. We note that Tilley regarded a distance of 2.3 Å as indicative of complete oxidative addition (the van der Waals radii are 2.19 and 2.11 Å for Ge and Si, respectively).7a,12 On the other hand, the similarity of the NMR and IR spectra to those of 5 and 6 support the presence of η2-Ge−H interactions (see Figures S12 and S13 in the Supporting Information). As can be seen in Figure 3 for 4·Et2O/DFT, the HOMO and HOMO-1 clearly identify significant interactions between the B
DOI: 10.1021/acs.organomet.5b00570 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics Table 1. X-ray and DFT (B3PW91-D3) Data for 4·Et2O and 4·penta 4·Et2O Ru−Ge1 Ru−Ge2 Ge1−O Ge2−O Ru−H Ge−H Ge1−Ru−Ge2 Ge1−O−Ge2 P−Ru−P a
4·pent
X-ray (Pn)
DFT
X-ray (P1̅)
DFT
2.4715(4) 2.4706(4) 1.807(2) 1.814(3) 1.58(4)−1.63(6) 2.25(3)−2.50(6) 68.391(11) 100.17(10) 108.43(3)
2.4947 2.4947 1.846 1.846 1.61−1.64 2.25−2.50 68.97 99.84 109.06
2.4492(4) 2.4546(4) 1.810(2) 1.812(2) 1.57(3)−1.64(4) 2.16(3)−2.44(4) 69.013(14) 100.17(10) 105.72(3)
2.4706 2.4683 1.858 1.849 1.60−1.65 2.25−2.46 69.72 99.21 106.77
Distances are given in Å and angles in deg.
thus avoiding as much as possible any decomposition of the catalyst precursor. When the reaction was repeated with the tricyclopentylphosphine analogue of 1, [RuH 2 (η 2 H2)2(PCyp3)2] (1Cyp), the reaction could be achieved in only 1 day (Scheme 3). The digermoxane was characterized by NMR, IR, and mass spectrometry.14 We checked that there is no reaction between Ph2GeH2 and H2O in THF without a catalyst (Ph2SiH2 behaves similarly under identical conditions),15 and both 1 and 1Cyp are stable in the presence of water. One can postulate the intermediacy of the dihydrogen germylene complex 2 in the catalytic cycle, as similar activity and selectivity were obtained on starting from 2 as the catalyst precursor (see the Supporting Information).
Figure 2. X-ray molecular structures of 4·pent (left) and 4·Et2O (right).
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CONCLUSION In summary, the reaction of Ph2GeH2 with [RuH2(η2H2)2(PCy3)2] with strict exclusion of water gives rise to the ruthenium germylene complex [RuH2(GePh2)(η2-H2)(PCy3)2] (2). This reaction contrasts strongly with the corresponding reaction of Ph2SiH2 with 1, which was dominated by silane redistribution.16 Complex 2 proved exceptionally sensitive to water, and we isolated the digermoxane complex 4 by reaction of 1 with Ph2GeH2 in the presence of traces of moisture or by direct reaction with (HPh2Ge)2O. Defining the coordination mode of a ligand stabilized by multiple interactions can be a matter of debate.17 In the case of 4, the data point to a RuH2({η2-H-GePh2}2O)(PCy3)2 formulation with multiple secondary interactions between the germanium and hydrogen atoms, reminiscent of secondary interactions in silane chemistry (SISHA),10c,11 even though the Ge−H distances are relatively long.12 Finally, we showed that catalytic hydrolysis of Ph2GeH2 to form digermoxane proceeded readily and was noticeably faster with 1Cyp than with 1. Metal-catalyzed formation of the Ge−O−Ge motif is unusual: Pannell et al. reported the reaction of Et3GeH and DMF catalyzed by Mo(CO)6, yielding Et3Ge−O−GeEt3,2d and Nakazawa et al. described the iron-catalyzed coupling of secondary germanes with DMF, giving rise to cyclic or linear germoxanes.2f As far as we are aware, this is the first report of catalytic hydrolysis preserving a Ge−H bond.
Figure 3. Representations (top) of the HOMO-1 (left) and HOMO (right) of 4·Et2O/DFT and their schematic representations (bottom) with the phosphine ligands removed for clarity (note change of viewing planes).
hydrogen and germanium atoms. To support these interactions, Wiberg bond indices can be used as good indicators.13 Values around 0.16 are consistent with the presence of a remaining interaction between Ge and H, each germanium interacting with two hydrides: Ge1 with H2 and H3 and Ge2 with H1 and H4 (see the Supporting Information). Catalytic Synthesis of Digermoxane. When Ph2GeH2 (0.13 M) was reacted with water (0.067 M) in THF at 248 K in the presence of a catalytic amount of 1 (10 mol %) and the mixture was stirred at room temperature for 5 days, one major new species (92%) with Ge−H bonds was formed, identified as (HPh2Ge)2O, together with 5% 1,3-diphenyldigermoxane ((H2PhGe)2O).14 The catalysis was initiated at low temperature to favor clean formation of the germylene complex and
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EXPERIMENTAL SECTION
General Procedures. Manipulations were carried out following standard Schlenk line and glovebox techniques, with O2 and H2O
