Portal de Periódicos da CAPES

Ge 2 Trapped by Triple Bonds between Two Metal Centers: The Germylidyne Complexes trans , trans ‐[Cl(depe) 2 MGeGeM(depe) 2 Cl] (M=Mo, W) and Bonding Analyses of the MGeGeM Chain

2005; Wiley; Volume: 44; Issue: 37 Linguagem: Inglês

10.1002/anie.200501968

1521-3773

Alexander C. Filippou, Gregor Schnakenburg, Athanassios I. Philippopoulos, N. Weidemann,

Coordination Chemistry and Organometallics

Going straight: Thermolysis of the germylidyne complexes trans-[Cl(depe)2MGe(η1-Cp*)] (M=Mo, W; depe=Et2PCH2CH2PEt2, Cp*=C5Me5) leads upon cleavage of the GeCp* bond to 1, the first compounds in which a Ge2 unit is trapped between two transition-metal centers by triple bonds. Quantum chemical analyses suggest a weak conjugation of the M−Ge triple bonds in the linear chain MGeGeM. Complexes, in which sp-hybridized carbon chains span two transition-metal centers, [LnMCxM′L′n]y+, (Ln, L′n=ligand sphere; x, y=integer) have attracted particular attention in recent years.1 These compounds form an important class of carbon-based molecular wires,2 which are appealing for studies of the electronic communication between redox-active metal termini,3 and are of potential practical importance in molecular electronics4 and in nonlinear optics.5 Moreover, these compounds can be viewed, at large carbon-chain lengths, as polymeric sp-hybridized carbon allotropes, revealing a conceptual relationship to the classical polymeric sp2- and sp3-hybridized carbon allotropes, graphite and diamond.6 For x=2, three classes of metal complexes have been reported to date (I a–I c, Scheme 1). The ethynediyl complexes I a are by far the most prevalent.1a,1b,1d,1f, 7 In contrast, analogous compounds of the heavier Group 14 elements Si–Pb are not known to date.8 This situation is not surprising given the reluctance of these elements to form triple bonds.9, 10 A rich elimination and substitution chemistry of main-group elements was elaborated in recent years using the heterolytic or homolytic cleavage of Cp*–element bonds (Cp*=C5Me5).11 A successful implementation of this method in the synthesis of the first transition-metal complexes with triply bonded Ge2 chains is presented herein. Valence bond structures of C2 bridged complexes. For the synthesis, the dichloro complexes trans-[MCl2(depe)2] (depe=Et2PCH2CH2PEt2; M=Mo (1-Mo-Cl), M=W (1-W-Cl)) were employed. Complex 1-Mo-Cl was obtained upon reduction of mer-[MoCl3(thf)3]12 with zinc in THF in the presence of 2.1 equivalents of depe, and 1-W-Cl upon heating of trans-[WCl2(PMe3)4]13 with 2.1 equivalents of depe in toluene at reflux. Both compounds were isolated as slightly air-sensitive, orange-yellow solids in 90 % yield and decompose upon melting at 211 and 205 °C, respectively.14 Reduction of 1-M-Cl with sodium powder in THF under N2 atmosphere afforded selectively the dinitrogen complexes trans-[M(depe)2(N2)2] (M=Mo (2-Mo), M=W (2-W)), which were isolated as air-sensitive, orange (2-Mo) and bright red (2-W) solids in 98 and 72 % yields, respectively (Scheme 2).14, 15 Stepwise synthesis of the germylidyne complexes 4-M-X and 5-M-Cl starting from the dichlorides 1-M-Cl. Treatment of 2-Mo with a stoichiometric amount of the germanium(II) halides [{Ge(Cp*)X}n] (X=Cl, n=1 (3-Cl); X=Br, n=2 (3-Br))16 in toluene at reflux gave the brown-orange, air-sensitive molybdenum germylidyne complexes 4-Mo-Cl and 4-Mo-Br in 32 % and 44 % yields, respectively (Scheme 2).14, 17 Both compounds are very soluble in pentane and melt in sealed capillaries under vacuum at 152 and 155 °C, respectively. Simultaneous thermal analysis (thermogravimetry/differential thermal analysis/mass spectrometry (TG-DTA-MS)) of 4-Mo-Cl revealed that the germylidyne complex melts at the extrapolated onset temperature =144 °C (peak temperature, Tp=149 °C) and then decomposes at 181 °C releasing C5Me5H.14, 18 These results prompted us to study the thermolysis of 4-Mo-Cl on a preparative scale. Upon heating the germylidyne complex 4-Mo-Cl at 180–185 °C under static vacuum, the brown-orange sample of 4-Mo-Cl melted and then turned progressively to a red-brown mass. Analysis of the molten mass by 1H and 31P{1H} NMR spectroscopy revealed a gradual conversion of 4-Mo-Cl into the digermanium complex 5-Mo-Cl, which after completion of the reaction and work-up was obtained as an air-sensitive, brown solid in 27 % yield (Scheme 2).14 During the thermolysis of 4-Mo-Cl, a yellow volatile material deposited on the cold top part of the reaction tube and was identified by NMR spectroscopy to be a mixture of depe, Cp*H,19 1,2,3,4-tetramethylpentafulvene,20 and some [GeCp*2].21 These results suggest that homolysis of the GeCp* bond occurs upon thermal activation of 4-Mo-Cl to generate [MoCl(Ge)(depe)2] and Cp* radicals. The [MoCl(Ge)(depe)2] radicals dimerize to form 5-Mo-Cl, whereas the Cp* radicals disproportionate under the reaction conditions to give Cp*H and 1,2,3,4-tetramethylpentafulvene.22 Heating of an equimolar mixture of complex 2-W and 3-Cl in toluene at reflux afforded a 7.7:1 mixture of the tungsten germylidyne complexes 4-W-Cl and 5-W-Cl (Scheme 2). Thermolysis of this mixture at 190 °C under static vacuum transformed 4-W-Cl into the digermanium complex 5-W-Cl, which was isolated as a very air-sensitive, green-brown solid in 30 % overall yield. Complex 5-W-Cl decomposes upon melting at 240 °C.14 The molecular structures of 4-Mo-Br and 5-W-Cl were determined by single-crystal X-ray diffraction (Figures 1 and 2). The trans-configured octahedral complex 4-Mo-Br has a very short MoGe triple bond (2.2798(5) Å) and an almost linear coordinated germanium atom (Mo-Ge-CCp* 177.46(8)°) which has an η1-bonded Cp* substituent as shown by the GeC1/C5 distances, the alternating (CC)ring bond lengths, and the bond angles at the C1 atom (Figure 1).23 The MoGe bond of 4-Mo-Br is even shorter than those of the dppe complexes trans-[X(dppe)2MoGe-(η1-Cp*)] (dppe=Ph2PCH2CH2PPh2, X=Cl, 2.3185(6) Å; X=Br, 2.3103(6) Å).9c This situation suggests that sterically less-demanding phosphine ligands of higher σ-basicity strengthen the metal–germanium triple bonds. Complex 5-W-Cl features a crystallographically imposed center of symmetry at the midpoint of the GeGe bond, an almost linear Ge2 chain spanning the two trans-configured, octahedral tungsten centers (W-Ge-Ge# 175.13(3)°), and two WGe triple bonds (2.3087(5) Å) that are linked by a short GeGe single bond (2.362(1) Å) (Figure 2). The WGe bonds of 5-W-Cl compare well with those of the germylidyne complexes trans-[X(dppe)2WGe-(η1-Cp*)] (X=H, Cl, Br, I, NCO, CN; W-Ge 2.293(1)–2.3184(6) Å).9c, 9g The GeGe distance (2.362(1) Å) is the shortest known for a GeGe single bond.24, 25 This distance can be rationalized with the increased s character of the σ hybrid orbitals forming the GeGe bond and the π conjugation of the WGe bonds (see below). Note the GeGe bond length of 5-W-Cl lies close to the range of lengths calculated for Ge2 (2.36–2.46 Å).26 The 1H, 13C{1H}, and 31P{1H} NMR spectra corroborate the structures of complexes 4-M-X and 5-M-Cl (X=Cl, Br; M=Mo, W). Thus, the 1H and 13C{1H} NMR spectra display a double set of resonances for the diastereotopic ethyl groups and methylene protons of the depe ligands, as expected for diamagnetic complexes of the general formula trans-[M(depe)2(L)(L′)].14 In addition, the NMR spectra of 4-M-X reveal a fast haptotropic shift of the Cp* substituent. This shift gives rise to one singlet signal for the methyl protons in the 1H NMR spectra, and two singlet resonances in the 13C{1H} NMR spectra, one for the methyl carbon and one for the ring carbon nuclei of the Cp* group.14 The 31P{1H} NMR spectra display a singlet resonance confirming the trans-configuration of 4-M-X and 5-M-Cl. The 31P NMR signals of the Ge2-bridged complexes (5-Mo-Cl, δ=56.3 ppm; 5-W-Cl, δ=38.2 ppm) appear slightly upfield of those of the mononuclear complexes (4-Mo-Cl, δ=57.2 ppm; 4-W-Cl, δ=39.2 ppm), and for the tungsten compounds are flanked by a pair of satellites arising from coupling with the 183W nucleus (4-W-Cl, 1J(W,P)=259 Hz; 5-W-Cl, 1J(W,P)=249 Hz).14 DIAMOND plot of the molecular structure of 4-Mo-Br, thermal ellipsoids set at 30 % probability. Hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and bond angles [°]: Mo-Ge 2.2798(5), Ge-C1 2.046(3), Mo-Br 2.6899(5), Mo-P1 2.479(1), Mo-P2 2.498(1), Mo-P3 2.465(1), Mo-P4 2.4812(9), C1-C2 1.504(4), C2-C3 1.356(5), C3-C4 1.466(5), C4-C5 1.353(4), C1-C5 1.485(4); Mo-Ge-C1 177.46(8), Ge-Mo-Br 177.94(2), Ge-Mo-P1 95.38(2), Ge-Mo-P2 95.30(2), Ge-Mo-P3 96.86(2), Ge-Mo-P4 92.80(2), Ge-C1-C2 99.8(2), Ge-C1-C5 99.9(2), Ge-C1-C6 114.0(2). DIAMOND plot of the molecular structure of 5-W-Cl, thermal ellipsoids set at 30 % probability. Hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and bond angles [°]: W-Ge 2.3087(5), Ge-Ge# 2.362(1), W-Cl 2.518(1), W-P1 2.455(1), W-P2 2.459(1), W-P3 2.470(1), W-P4 2.461(1); W-Ge-Ge# 175.13(3), Ge-W-Cl 176.66(3), Ge-W-P1 92.96(3), Ge-W-P2 94.47(3), Ge-W-P3 95.59(3), Ge-W-P4 90.28(3). Density functional theoretical (DFT) calculations of the model complex trans,trans-[Cl(PH3)4WGe2W(PH3)4Cl] (5-PH3-W-Cl) were performed without symmetry restraints using the exchange correlation functionals B3LYP and BP86 and various basis sets.14, 27 Geometry optimization of the model compound at the B3LYP/IIa level of theory led to six minimum structures on the ground-state singlet potential-energy surface, which can be classified into one pair of isomers featuring an end-on coordinated Ge2 ligand (A), and two pairs of isomers displaying a side-on bonded Ge2 ligand (B, C; Figure 3). Each pair consists further of an eclipsed (e) and a staggered conformer (s) differing in the orientation of the [WCl(PH3)4] fragments. The eclipsed isomer A-e of D4h symmetry and the corresponding D4d symmetric staggered isomer A-s, which feature a linear end-on bonded Ge2 chain, are the most stable compounds and differ in energy by less than 1 kJ mol−1. The isomers B displaying a planar W2Ge2 bicyclic ring lie at slightly higher energy than A (10–13 kJ mol−1), whereas the species C with a W2Ge2 butterfly structure are considerably less stable than A (Figure 3). The calculated bond lengths and angles of the eclipsed isomer A-e compare well with the experimental values of 5-W-Cl (Table 1). Selected bond lengths [Å] and bond angles [°], symmetries, and relative energies [kJ mol−1, ZPVE corrected] of the minimum structures of 5-PH3-W-Cl (B3LYP/IIa). Hydrogen atoms are omitted for clarity. Method/basis set WGe [Å] GeGe [Å] WCl [Å] P-W-Cl [°] P-W-Pcis [°] B3LYP/IIa 2.327 2.405 2.518 86.1 89.7 RI-BP86/I 2.357 2.453 2.541 86.9 89.8 BP86/IIb 2.328 2.389 2.456 86.2 89.7 X-ray (5-W-Cl) 2.3087(5) 2.362(1) 2.518(1) 86.7[a] 89.8[a] Analysis of the electronic charge distribution of A-e using the natural bond orbital (NBO) partitioning scheme28 gives an optimal Lewis structure with two WGe triple bonds connected by a GeGe single bond. Both WGe triple bonds are composed of one σ component, which is polarized towards germanium (64.8 %), and two degenerate π bonds, which result from the overlap of pure tungsten d orbitals and germanium p orbitals and are polarized towards tungsten (71.6 %) (Table 2). The polarity of the WGe bonds is reflected in the partial charges of the W and Ge atoms (−1.25 and +0.30, respectively) and the Wiberg bond index of 1.71.29 Further evidence for the considerable ionic character of the WGe triple bonds of A-e was provided by the energy-partitioning analysis of the interaction between the closed-shell fragments trans-[WCl(η1-Ge2)(PH3)4]+ and [WCl(PH3)4]− using the energy decomposition analysis (EDA) method,30 which revealed a considerable contribution of the electrostatic term ΔEelstat (−622.6 kJ mol−1) to the total attractive interactions (ΔEelstat+ΔEorb=−1374.4 kJ mol−1), leading to a 55 % covalent character of the WGe triple bonds (Table 2). The EDA analysis also showed that the degenerate WGe π bonds make the largest contribution (78 %) to the orbital interaction energy ΔEorb indicating the same high π-bonding character of the WGe bonds in A-e as in trans-[Cl(CO)4WGeH].31 Taking into account the polarization of the WGe σ and π bonds, an extreme view of isomer A-e is that of a Ge22+ ion, which is embedded in an η1:η1 bonding mode between two d6 [WCl(PH3)4]− fragments by two σ ([Ge2]2+→[WCl(PH3)4]−) and four π [WCl(PH3)4]−→[Ge2]2+) donor–acceptor bonds. Ge22+ has a singlet ground state with a very weak and long GeGe bond (3.099 Å) owing to Coulomb repulsion.32 Metal complexation is accompanied by a considerable electron drift from the metal fragments to Ge22+, which decreases the partial charges on the germanium atoms from +1 to +0.3, reduces thereby the Coulomb repulsion, and shortens the GeGe bond by 0.694 Å (B3LYP/IIa, Table 1). Bond NBO analysis[a] EDA[b] [kJmol−1] AB occ. % (A) hyb. % (B) hyb. q(A) WBI ΔEPauli ΔEelstat ΔEorb ΔEint WGe σ: 1.90 35.2 sd2.1 64.8 sp0.5 −1.25 1.71 507.4 −622.6 −751.8 (55 %) −866.5 π1: 1.85 71.6 d 28.4 p ΔEσ(a1): −144.1 π2: 1.85 71.6 d 28.4 p ΔEπ(e): −588.4 GeGe 1.96 50.0 sp1.9 50.0 sp1.9 +0.30 1.10 909.4 −398.4 −795.3 (67 %) −283.8 ΔEσ(a1): −730.1 ΔEπ(e): −65.0 The π bonding and π* antibonding orbitals of the W-Ge-Ge-W chain form four doubly degenerate levels (HOMO−3, HOMO, LUMO, and LUMO+3; Figure 4) resulting from the in-phase or out-of-phase combination of the tungsten dxz(dyz) orbitals and the germanium px(py) orbitals (the z axis of the Cartesian coordinate system and the C4 symmetry axis of A-e are collinear).33 Four electrons occupy the lowest energy e1u symmetric pair of molecular orbitals (HOMO−3), which are bonding with respect to the WGe and GeGe linkages, and four electrons occupy the e1g symmetric pair of orbitals (HOMO), which are bonding with respect to the WGe linkages but antibonding with respect to the GeGe linkage (Figure 4).33, 34 This molecular orbital picture corresponds in valence bond terms to the digermanehexayl formula [LnWGeGeWLn] and suggests some π conjugation of the WGe triple bonds.35 This proposal is supported by the NBO and EDA analyses, which indicate the presence of a GeGe σ bond with low π-bonding character (WBI (GeGe)=1.10; ΔEπ is 8.2 % of ΔEorb; Table 2), and the natural resonance theoretical (NRT) analysis of A-e,36 which provides 21 different resonance structures with a resonance weight greater than 1 % leading to a natural bond order of 2.76 for the WGe bonds and 1.06 for the GeGe bond.37 π and π* Kohn–Sham orbitals of the W-Ge-Ge-W chain in A-e. Only one of the doubly degenerate π and π* levels is depicted. Analysis of the wave function using the electron localization function (ELF)38 at the B3LYP/IIa level of theory shows, that the density of localized electrons between tungsten and germanium has a toroidal shape typical for triple bonds.9f, 14, 39 Volume integration of the electron density over the valence disynaptic basins connecting the W with the Ge nuclei and linking the two Ge nuclei, leads to a population of 5.7 electrons in the WGe bonding region and 2.6 electrons in the GeGe bonding region, which supports the bonding picture of two conjugated WGe triple bonds in A-e.40 This result is further confirmed by the quantification of conjugation in A-e using the anisotropy of the induced current density (AICD) of the GeGe bond.41 The AICD isosurface plot of A-e at an isosurface value of 0.015 displays strong delocalization over the atoms (Figure 5). The continuous boundary surface enclosing the conjugating Ge atoms breaks at an isosurface value of 0.041 (Figure 5), which suggests that the extent of conjugation in A-e is lower that in benzene (0.08) or butadiene (0.069), but higher than in 1,3-butadiyne (0.013) at the same level of theory.42 Conjugation in A-e might be caused by the small HOMO–LUMO gap of 2.7 eV which leads to an excitation energy of 2.67 eV (λ=464 nm) for the allowed one-electron transition (TD-RIDFT/TZVPP).43 Isosurface plots of the AICD in A-e at 0.015 (top) and the critical value of 0.041 (bottom), at which the topology of the AICD boundary surface changes. The GeGe bond fragmentation energy of A-e (ΔEfrag=221.0 kJ mol−1) is lower than that of Ge2H6 (288.5 kJ mol−1) calculated at the same level of theory (B3LYP/IIa, Table 3).44 Inclusion of relaxation and zero-point vibrational effects (ZPVE) leads to a theoretical value of 217.1 kJ mol−1 for the GeGe bond dissociation energy of A-e (D°0), which is also lower than that of Ge2H6 (D°0=270.4 kJ mol−1; Table 3),44 the latter value comparing well with the experimental value (D°=276 kJ mol−1).45 In comparison, the GeGe bond length of A-e (2.405 Å) is shorter than that of Ge2H6 (2.439 Å, Table 3) leading to an opposite bond length/dissociation energy correlation to that usually observed. This result suggests that the radicals trans-[WCl(Ge)(PH3)4] formed upon GeGe bond homolysis are stabilized by delocalization of the unpaired electron in the electronic ground state (2A1).46 This electronic stabilization provides a rationale for the preferred formation of the radicals trans-[WCl(Ge)(depe)2] during thermolysis of 4-W-Cl. The energy (ZPVE corrected) required to cleave the WGe bond of A-e into the fragments trans-[WCl(η1-Ge2)(PH3)4]47 and [WCl(PH3)4] in the respective 2A1 and 4A electronic ground states and minimum geometries, was calculated to be slightly lower (333.9 kJ mol−1) than that of the germylidyne complex trans-[Cl(PH3)4WGe-Me] (354.8 kJ mol−1) (Table 3). The species trans-[WCl(η1-Ge2)(PH3)4] is a local minimum on the energy potential surface and relaxes to a C2v symmetric species featuring a side-on bonded Ge2 ligand.47 The released relaxation energy of −52.2 kJ mol−1 reduces the WGe bond dissociation energy of A-e to 281.7 kJ mol−1 (Table 3). Compound Bond Length [Å] ΔEfrag[a] ΔErelax[b] ΔEZPVE[c] D°0[d] ΔEtherm[e] ΔG°[f] 5-PH3-W-Cl (A-e) GeGe 2.405 +221.0 −0.2 −3.7 +217.1 −55.4 +161.7 Ge2H6 GeGe 2.439 +288.5 −2.3 −15.8 +270.4 −45.8 +224.6 5-PH3-W-Cl (A-e) WGe 2.327 +476.6 −133.1 −9.6 +333.9 −74.3 +259.6 +281.7[g] −69.6[g] +212.1[g] [Cl(PH3)4WGe-Me] WGe 2.304 +622.3 −251.9 −15.6 +354.8 −56.4 +298.4 Several experimental and theoretical studies have been carried out on the diatomic molecule Ge2.48 Ge2 can be regarded as the lightest member in the family of germanium clusters Gen, which are of importance owing to their potential applications in nanoelectronics.48 Encapsulation of this very reactive species in the complexes 5-M-Cl provides a new example for the ability of stereoelectronically well-defined transition-metal fragments to stabilize high temperature molecules in unprecedented bonding patterns. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2002/2005/z501968_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.