ARTICLE pubs.acs.org/Organometallics
Synthesis, Characterization, and Reactivity of an N-Heterocyclic Germanium(II) Hydride: Reversible Hydrogermylation of a Phosphaalkyne Sam L. Choong, William D. Woodul, Christian Schenk, Andreas Stasch, Anne F. Richards, and Cameron Jones* School of Chemistry, Monash University, P.O. Box 23, Clayton, Victoria 3800, Australia
bS Supporting Information ABSTRACT: The β-diketiminato germanium(II) chloride complex [(ButMesNacnac)GeCl] (ButMesNacnac = [{N(Mes) C(But)}2CH] , Mes = mesityl) has been prepared and spectroscopically characterized. Treating this compound and a less bulky system, [(MeMesNacnac)GeCl] (MeMesNacnac = [{N(Mes)C(Me)}2CH] ), with K[HBEt3] led to differing outcomes, namely, the formation of the novel diamido germylene [(ButMesNacnacH)Ge:] (ButMesNacnacH = [N(Mes)C(H) (But)C(H)C(But)N(Mes)]2‑) and the germanium(II) hydride [(MeMesNacnac)GeH], both of which were crystallographically characterized. The former product forms via a 1,3-hydrogen migration reaction involving [(ButMesNacnac)GeH] as an unstable reaction intermediate. Reactions of [(MeMesNacnac)GeH] with CO2 and PtCBut proceed at or below ambient temperature, in the absence of a catalyst, to give the crystallographically characterized hydrogermylation products [(MeMesNacnac)GeOC(dO)H] and [(MeMesNacnac)GeC(But)dPH], with complete regioselectivity. When a solution of the latter was gently heated, the hydrogermylation reaction was partially reversed, regenerating [(MeMesNacnac)GeH] and PtCBut. Heating to higher temperatures led to the irreversible isomerization of [(MeMesNacnac)GeC(But)dPH] to (E)-[(MeMesNacnac)GePdC(H)(But)], theoretical studies of which indicate is the thermodynamic product of the original hydrogermylation reaction.
’ INTRODUCTION The chemistry of low-oxidation-state main-group complexes has rapidly expanded since the turn of the millennium.1 Although this is primarily a result of significant fundamental interest in the unusual structural, bonding, and other properties of such compounds, their generally high reactivity has led to increasing explorations into their applications potential in recent times. In this respect, it is emerging that low-oxidation-state p-block compounds can participate in reactions (e.g., the activation of saturated and unsaturated small molecules) that are normally considered the realm of transition-metal complexes.2 Perhaps the most poorly studied low-oxidation-state p-block systems are the hydrides, which almost exclusively require sterically very demanding coligands to be capable of ambient existence.3 If a greater range and understanding of these compounds could be accessed, their reactivity could be further developed and compared to that of d-block metal hydride complexes, which are of undoubted importance to many areas of synthesis and catalysis.4 Most of the recent progress in this area has come from group 14 element(II) hydride complexes,3a a considerable number of which have been kinetically stabilized by incorporation of bulky monodentate (e.g., terphenyl5 or N-heterocyclic carbene6) or bidentate (e.g., amidinate7 or β-diketiminate8) ligands. Of these, the monomeric germanium(II) hydride complex [(DipNacnac)GeH] (1) (Chart 1; DipNacnac = [{N(Dip)C(Me)}2CH] , Dip = C6H3Pri2r 2011 American Chemical Society
2,6)8b has been the most studied as a reagent. It has been shown to readily hydrogermylate alkynes, CO2, ketones, and diazoalkenes under mild conditions and in the absence of a catalyst.9 In addition, it has been used as a precursor to terminal germanium(II) azide and hydroxide compounds,10 a monomeric germanium dithiocarboxylic acid analogue,9 and the diamido germylene 2.11 Given the synthetic versatility of 1, it is surprising that related three-coordinate germanium heterocycles incorporating β-diketiminate ligands of varying steric bulk have not been reported. Very recently, we attempted the preparation of a bulkier example, [(ButNacnac)GeH] (ButNacnac = [{N(Dip)C(But)}2CH] ), either via the reaction of the germanium(I) radical [(ButNacnac)Ge:]• (3) with Bun3SnH or by treating [(ButNacnac)GeCl] with the hydride source K[HBEt3].12 In both cases, the thermally unstable diamido germylene 4 was isolated instead. It was proposed that this was formed via the intended product [(ButNacnac)GeH], which is unstable and undergoes a 1,3-hydrogen migration to give 4. The instability of [(ButNacnac)GeH] is thought to result from the unsaturated backbone of its very bulky β-diketiminate ligand being significantly more localized than that of 1.12 This in turn leads to its imine C-center being more susceptible to intramolecular nucleophilic attack.13 Herein, we report on the less bulky and more reactive analogues Received: September 1, 2011 Published: September 29, 2011 5543
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Scheme 1
of 1 and 4 and the reversible hydrometalation of a phosphaalkyne by the germanium(II) hydride compound.
’ RESULTS AND DISCUSSION The β-diketiminate ligands chosen for this study, [{N(Mes) C(R)}2CH] (Mes = mesityl, R = Me (MeMesNacnac), But (ButMesNacnac)),14 differ from those used in the preparation of 1 4 in that they include smaller mesityl substituents at their N centers. It was believed this would lead to germanium(II) hydride complexes derived from the ligands displaying a greater, and potentially broader, reactivity than 1. The germanium(II) chloride precursors to the target complexes, [(MeMesNacnac)GeCl]15 and [(ButMesNacnac)GeCl] (5), were initially prepared via reactions of the appropriate β-diketiminato lithium compounds16 with GeCl2 3 (dioxane). The complex [(ButMesNacnac)GeCl] (5) has not been previously reported, though its characterization data are consistent with it possessing a monomeric, three-coordinate structure similar to those of [(MeMesNacnac)GeCl],15 [(DipNacnac)GeCl],17 and [(ButNacnac)GeCl].18 The reactions of [(MeMesNacnac)GeCl] and [(ButMesNacnac)GeCl] with 1 equiv of K[HBEt3] in toluene/THF led to differing outcomes, namely the isolation of moderate (65%) and low (32%) yields of orange [(MeMesNacnac)GeH] (6) and yellow [(ButMesNacnacH)Ge:] (7) (ButMesNacnacH = [N(Mes) C(H)(But)C(H)C(But)N(Mes)]2 ), respectively (Scheme 1). That said, spectroscopic analyses of both total reaction mixtures prior to workup suggested that 6 and 7 are the major (>80%) germanium-containing products. The low isolated yield of
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crystalline 7 results from its high solubility in aliphatic organic solvents. Despite its sterically less demanding ligand, compound 6 has thermal stability in the solid state (dec pt 160 162 °C) similar to that of 1 (dec pt 170 °C)8b and is indefinitely stable in solution at room temperature.19 In contrast, the diamido germylene 7 is less stable in solution than its bulkier counterpart 4 and decomposes over ca. 30 h at 20 °C to cleanly give the new azabutadiene MesNdC(But)C(H)dC(H)But via a C N cleavage process. Compound 4 similarly decomposes, but over ca. 48 h, to give DipNdC(But)C(H)dC(H)But.13 It is likely that the reasons for the differing formations of 6 and 7 from seemingly very similar reactions are equivalent to those mentioned above for the reactions that gave 1 and 4. However, in the present case, following the reaction that ultimately gave 7 by 1H NMR spectroscopy confirmed that [(ButMesNacnac)GeH] (8) is an intermediate in the reaction and that this compound rapidly isomerizes to 7 at 20 °C. What should be noted from these results is that relatively small differences in the steric bulk of βdiketiminate ligands can lead to major differences in their ability to stabilize low-oxidation-state p-block systems. The characterization data for 6 and 7 are comparable with those for 1 and 4. For example, a hydride resonance is seen in the 1 H NMR spectrum of 6 (δ 8.25 ppm) at a field similar to that of 1 (δ 8.08 ppm). In addition, both compounds exhibit Ge H stretching bands in their infrared spectra at wavenumbers (6, ν 1722 cm 1; 1, ν 1733 cm 1) lower than are normally seen for germanium(IV) hydrides (1953 2175 cm 1).8b There is no spectroscopic evidence for the presence of a hydride ligand in 7; instead, its 1H NMR spectrum displays an AB coupling pattern for its two ligand backbone protons. The asymmetrical nature of the diamido ligand of 7 is also reflected in its 13C{1H} NMR spectrum. The differences between 6 and 7 are very evident from their molecular structures, which are depicted in Figure 1. The structure of 6 is similar to that for 1 and shows it to be monomeric with a close to planar NC3N fragment, the bond lengths within which imply significant π delocalization. The Ge atom is displaced from the ligand plane by 0.65 Å and is coordinated by two N centers and a hydride ligand, the latter of which was located from difference maps and refined isotropically. The angles about the Ge atom of the molecule are all close to 90°, which possibly indicates that the Ge lone pair has high s character. It is clear from the interatomic distances within the NC3N fragment of 7 that C(2) C(3) is a localized double bond, while the other three interactions are single bonds. This situation has arisen from protonation of C(1), which also causes the ligand backbone of 7 to deviate significantly away from planarity, as was the case for 4. It is of note that C(1) is a chiral center. Both the Ge N distances in 7 are considerably smaller than those in 6, while its N Ge N angle is more obtuse (cf. Ge N = 1.865 Å mean and N Ge N = 95.84(8)° for the related diamidogermylene 2).11 These differences can be explained by the lower coordination number of the Ge center in 7 and the likely higher degree of s character for the Ge N interactions that would be expected for this compound. Germanium(IV) hydrides have been widely used in synthetic transformations, including hydrogermylations of unsaturated substrates.20 To the best of our knowledge, the only reports of hydrogermylations involving a germanium(II) hydride are those mentioned above for 1. So as to compare the reactivity of 6 with 1, we chose to investigate its reactions with CO2 and the phosphaalkyne PtCBut. It has previously been shown that 1 5544
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Figure 1. Thermal ellipsoid plot (25% probability surface) of the molecular structures of (a) 6 and (b) 7. Hydrogen atoms (except the hydride ligand of 6 and H(1) and H(2) of 7) are omitted. Selected interatomic distances (Å) and angles (deg) for 6: Ge(1) N(2) = 1.977(2), Ge(1) N(1) = 1.991(2), Ge(1) H(1) = 1.569(10), N(1) C(2) = 1.329(3), N(2) C(4) = 1.336(3), C(2) C(3) = 1.394(4), C(3) C(4) = 1.394(4); N(2) Ge(1) N(1) = 90.70(9), N(2) Ge(1) H(1) = 90.5(13), N(1) Ge(1) H(1) = 91.6(12). Selected interatomic distances (Å) and angles (deg) for 7: Ge(1) N(1) = 1.8297(12), Ge(1) N(2) = 1.8635(11), N(1) C(1) = 1.4789(18), N(2) C(3) = 1.4242(18), C(1) C(2) = 1.510(2), C(2) C(3) = 1.344(2); N(1) Ge(1) N(2) = 98.16(5), N(1) C(1) C(2) = 112.30(12), C(3) C(2) C(1) = 131.29(13), C(2) C(3) N(2) = 119.71(12).
Scheme 2
reacts with CO2 slowly at room temperature to give the germylene ester of formic acid, [(DipNacnac)GeOC(dO)H], in quantitative yield.9b In the current study we found that 1 does not react with the phosphaalkyne PtCBut up to temperatures of 50 °C. The reactions of the less hindered 6 with both substrates were carried out, and that with CO2 proceeded at temperatures above 30 °C to give [(MeMesNacnac)GeOC(dO)H] (9) regioselectively (Scheme 2). Although the isolated yield of this compound was 75%, monitoring the reaction by 1H NMR spectroscopy indicated that it is essentially quantitative. When 6 was treated with an excess of PtCBut at 20 °C, hydrogermylation of the phosphaalkyne slowly occurred (over ca. 3 days) to give the C-phosphaalkenyl complex 10 exclusively as its Z isomer in a good isolated yield as a dark red crystalline solid. When the reaction was carried out at 35 °C, it was complete within 15 h, though in that case the product cocrystallized with ca. 10% of its
E isomer. It is clear from both reactions that 6 is significantly more reactive than 1 toward hydrogermylation reactions, as might be expected on steric grounds. The clean formation of the germanium phosphaalkenyl complex 10 is unusual for a number of reasons. First, germylenes have been shown to readily undergo cycloaddition reactions with phosphaalkynes on a number of previous occasions,21 yet there is no evidence for such reactivity here. Indeed, as was the case with the CO2 reaction, the +2 oxidation state of germanium is retained post-hydrogermylation of the phosphaalkyne. Second, although hydrometalation reactions of phosphaalkynes have been reported,22 they are rare and always give the P-phosphaalkenyl product [LnM PdC(H)R], despite the established polarity of alkyl-substituted phosphaalkynes, viz. δ+PtCδ R.23 It should also be noted that while hydrogermylations of phosphaalkynes are unknown, one report on their hydrostannylation with tin(IV) hydrides has suggested the formation of C-phosphaalkenyl intermediates, R13SnC(R2)dPH, though these were never isolated and, instead, underwent further reactions.24 The spectroscopic data for 9 and 10 are compatible with their proposed formulations. Of most note are the 31P NMR spectra of the Z and E isomers of 10, which exhibit low-field resonances at δ 253.9 and 190.0 ppm, respectively: i.e., in the known region for phosphaalkenes and metallophosphaalkenes.23,25 The JPH coupling constants for the compounds (144 and 96 Hz) are in the normal range for one-bond interactions,23 and identical couplings were observed for the PH resonances in the 1H NMR spectra of a cocrystallized mixture of the complexes (at δ 8.22 and 8.02 ppm, respectively). Moreover, a band in the infrared spectrum of a pure sample of (Z)-10 at 2357 cm 1 (Nujol mull) was assigned to its P H stretching mode. The molecular structures of 9 and (Z)-10 are depicted in Figures 2 and 3, respectively. The structure of compound 9 is very similar to that previously reported for [(DipNacnac)GeOC 5545
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Figure 2. Thermal ellipsoid plot (25% probability surface) of the molecular structure of 9. Hydrogen atoms (except H(24)) are omitted. Selected interatomic distances (Å) and angles (deg): Ge(1) O(1) = 1.9514(18), Ge(1) N(1) = 1.963(2), Ge(1) N(2) = 1.965(2), O(1) C(24) = 1.286(3), O(2) C(24) = 1.202(3), N(1) C(2) = 1.324(4), N(2) C(4) = 1.338(4), C(2) C(3) = 1.393(4), C(3) C(4) = 1.382(4); N(1) Ge(1) N(2) = 90.52(9), O(1) Ge(1) N(1) = 88.79(8), O(1) Ge(1) N(2) = 88.36(8), C(24) O(1) Ge(1) = 120.87(17), O(2) C(24) O(1) = 128.0(3).
(dO)H], while the structure of (Z)-10 represents the first for a C-metallophosphaalkene derived from a hydrometalation reaction. The NC3N backbones of the β-diketiminate ligands of both 9 and (Z)-10 appear to be largely delocalized. Their Ge centers have similar coordination geometries with Ge N distances and N Ge N angles which are comparable with each other and to those in the structure of 6. The phosphaalkenyl fragment of (Z)-10 exhibits a P C distance (1.676(2) Å) which is fully consistent with a localized double-bonded interaction,23,25 while the location of the hydride ligand from difference maps, and its subsequent unrestrained isotropic refinement, allowed the assignment of the stereochemistry of the compound. It is clear from Figure 3 that, for steric reasons, the Z isomer of 10 should be the preferred form, though even in this case the hydride ligand, H(1), has a relatively close interaction with Ge(1) (3.13(3) Å). The molecular structure of 10 could provide an explanation for an unusual observation that was made when a crystalline sample of the cocrystallized major (Z) and minor (E) isomers of the compound was dissolved in C6D6 and gently heated (see Figure 4). Even at temperatures as low as 30 °C a small resonance (D) appeared at δ 67 ppm in the 31P{1H} NMR spectrum, in addition to the signals for (Z)-10 (B) and (E)-10 (C). Resonance D corresponds to the free phosphaalkyne PtCBut. This, in combination with the fact that the 1H NMR spectrum of the sample at that temperature showed the generation of [(MeMesNacnac)GeH] (6), implies that the hydrogermylation reaction that gave 10 is reversible under mild conditions. Indeed, as the sample was heated to 60 °C, the signal for the phosphaalkyne grew at the expense of that for (Z)-10. Interestingly, the relative intensity of the signal for (E)-10 changed little over the temperature range. Moreover, when the sample was cooled back to 20 °C and let stand for 1 h, the signal for the phosphaalkyne vanished and that for (Z)-10 increased in intensity, while no 9b
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Figure 3. Thermal ellipsoid plot (25% probability surface) of the molecular structure of (Z)-10. Hydrogen atoms (except H(1)) are omitted. Selected interatomic distances (Å) and angles (deg): Ge(1) N(2) = 2.0041(18), Ge(1) N(1) = 2.0259(17), Ge(1) C(24) = 2.058(2), P(1) C(24) = 1.676(2), P(1) H(1) = 1.30(3), N(1) C(2) = 1.328(3), N(2) C(4) = 1.337(3), C(2) C(3) = 1.395(3), C(3) C(4) = 1.399(3); N(2) Ge(1) N(1) = 90.58(7), N(2) Ge(1) C(24) = 101.14(8), N(1) Ge(1) C(24) = 96.04(8), C(24) P(1) H(1) = 99.9(13), P(1) C(24) Ge(1) = 125.99(12).
[(MeMesNacnac)GeH] was seen in the 1H NMR spectrum of the sample after cooling. Given these observations, it can be proposed that upon heating the close proximity of the hydride ligand of (Z)-10 to its germanium center allows an intramolecular nucleophilic attack at that center to give PtCBut and [(MeMesNacnac)GeH] via a β-hydrogen elimination-like process. The two compounds then react to regenerate (Z)-10 upon cooling. It seems that the elimination of PtCBut from (E)-10 is not facile, probably because its hydride ligand is directed away from its germanium center. When the sample of 10 was heated to 80 °C, a number of other changes to its 31P{1H} NMR spectrum occurred. After 40 min at this temperature, the signals corresponding to PtCBut and both isomers of 10 had almost vanished and a new signal (A) appeared at δ 334.4 ppm (N.B. a trace of this signal is present at 60 °C). In the corresponding proton-coupled spectrum, this appears as a doublet with a JPH coupling constant of 21 Hz, which is within the two-bond range for phosphaalkenes.23 We propose that this signal corresponds to the P-phosphaalkenyl germanium complex [(MeMesNacnac)GePdC(H)(But)] (11), which on the basis of steric arguments should preferentially exist in its E isomeric form. In fact, the 2JPH coupling constant observed for this compound would be expected to be larger (ca. 30 50 Hz) for its Z isomer,23 in which the alkenic proton and P lone pair are cis to each other. When the solution containing (E)-11 was cooled to room temperature and let stand for 1 h, there was no change in its 31 1 P{ H} NMR spectrum, which indicates that it is stable to isomerization back to 10. It is unknown how (E)-11 is formed, but one possibility is that it is the thermodynamic product of the original phosphaalkyne hydrogermylation, whereas 10 is the kinetic product. If so, at temperatures above 60 °C the free phosphaalkyne PtCBut and [(MeMesNacnac)GeH] react to give (E)-11, thereby altering the equilibrium of the system, which leads to the eventual consumption 5546
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Figure 4. Solution 31P{1H} NMR spectra of a cocrystallized mixture of (Z)-10 and (E)-10 recorded over the temperature range 30 80 °C (spectra recorded after ca. 35 min at each temperature).
Table 1. Calculated Relative Energies (kcal/mol) and Calculated (Gas-Phase) and Observed (C6D6 Solution) 31P NMR Chemical Shifts (ppm) for (Z/E)-10 and (Z/E)-11 31
P NMR chemical shift
compd
rel energy
calcd
measd
(Z)-10
+19.45
267.1
253.9
(E)-10
+20.18
201.5
190.0
(Z)-11
+6.09
359.0
not obsd
(E)-11
0.00
327.0
334.4
of all (Z)-10. One explanation for the loss of (E)-10 from the system at 80 °C is that it thermally isomerizes to (Z)-10 at that temperature, which then converts to (E)-11 by the process mentioned above. Similar thermal isomerizations of phosphaalkenes, at even lower temperatures, have been reported.26 Although these proposals are compelling, above 60 °C the integration of signal A is much lower than would be expected for the clean conversion of 10 to (E)-11. This, combined with the fact that the 1 H NMR spectrum of the sample revealed several unidentifiable products, suggests that other processes are involved at higher temperatures. In order to ascertain the relative energies of the four isomers (Z/E)-10 and (Z/E)-11, their gas-phase structures were optimized using DFT calculations (RI-DFT/BP86/def2-SVP). Once optimized, the isotropic 31P NMR chemical shift was calculated for each isomer using the Turbomole program. The results of these calculations are summarized in Table 1. It is first worth noting that the calculated optimized geometry of (Z)-10 is close to that of the compound in the solid state (see the Supporting Information). From Table 1 it can be seen that the two isomers of the C-germaphosphaalkene 10 are significantly higher in energy than those of the P-germaphosphaalkene 11. Within these two sets of isomers, (Z)-10 and (E)-11 lie at the lowest energies. Both of these results are consistent with our proposal that (Z)-10 is the kinetic product of the original phosphaalkyne hydrometalation, whereas (E)-11 is the thermodynamic product. The calculated
31
P NMR chemical shifts for the three experimentally observed isomers are reassuringly close to those recorded in the variabletemperature NMR experiment, whereas no signal was seen in those spectra at a field similar to that calculated for (Z)-11. This strongly suggests that this isomer is not formed in significant quantities during the thermal isomerization of 10.
’ CONCLUSIONS In summary, the second example of a monomeric N-heterocyclic germanium(II) hydride compound, [(MeMesNacnac)GeH], has been prepared and shown to be more reactive than the previously reported and sterically more bulky system [(DipNacnac)GeH]. Attempts to prepare and isolate the slightly bulkier system [(ButMesNacnac)GeH] were not successful. This compound was, however, spectroscopically observed and found to rearrange via a 1,3-hydrogen migration to give the novel diamido germylene [(ButMesNacnacH)Ge:]. These results show that small differences in the steric bulk of β-diketiminate ligands can lead to major differences in their ability to stabilize low-oxidation-state germanium systems. Preliminary studies of the reactivity of [(MeMesNacnac)GeH] toward the hydrogermylation of unsaturated substrates have been carried out. It has been shown that reactions with CO2 and PtCBut proceed at or below ambient temperature, in the absence of a catalyst, to give the hydrogermylation products [(MeMesNacnac)GeOC(dO)H] and [(MeMesNacnac)GeC(But)dPH] with complete regioselectivity. When a solution of the latter product was gently heated, the hydrogermylation reaction was partially reversed, regenerating [(MeMesNacnac)GeH] and PtCBut. Heating to higher temperatures led to the irreversible isomerization of [(MeMesNacnac)GeC(But)=PH] to its P-germaphosphaalkenyl regioisomer (E)-[(MeMesNacnac)GePdC(H)(But)], theoretical studies of which indicate is the thermodynamic product of the original hydrometalation reaction. This study has highlighted the potential versatility of [(MeMesNacnac)GeH] as an effective hydrogermylation reagent. This potential is currently being further developed in our laboratory. 5547
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’ EXPERIMENTAL SECTION General Methods. All manipulations were carried out using standard Schlenk and glovebox techniques under an atmosphere of high-purity dinitrogen. Hexane and toluene were distilled over potassium, while diethyl ether was distilled over Na/K alloy. 1H, 13C{1H} and 31 1 P{ H} NMR spectra were recorded on either Bruker DPX300 or AvanceIII 400 spectrometers and were referenced to the resonances of the solvent used or external H3PO4 (85% solution in D2O). Mass spectra were obtained from the EPSRC National Mass Spectrometric Service at Swansea University. IR spectra were recorded using a Perkin-Elmer RX1 FT-IR spectrometer as Nujol mulls between NaCl plates. Microanalyses were carried out by Campbell Microanalytical, Otago, New Zealand, or at the Science Centre, London Metropolitan University. Melting points were determined in sealed glass capillaries under dinitrogen and are uncorrected. The compounds [(MeMesNacnac)GeCl]15 and (ButMesNacnac)H14 were prepared by variations of the literature procedures. The phosphaalkyne PtCBut was synthesized by the [Li{N(SiMe3)2}]-catalyzed elimination of hexamethyldisiloxane from (Me3Si)PdC(But) (OSiMe3).27 All other reagents were used as received. Preparation of [(ButMesNacnac)GeCl] (5). To as solution of But ( MesNacnac)H (1.0 g, 2.37 mmol) in diethyl ether (15 mL) at 78 °C was added a 1.6 M solution of n-butyllithium in hexane (1.6 mL, 2.56 mmol), whereupon the reaction solution was warmed to room temperature and stirred for 1 h. The resultant solution was added over 5 min to a precooled ( 78 °C) suspension of GeCl2.(dioxane) (0.55 g, 2.37 mmol) in diethyl ether (15 mL). The reaction mixture was warmed to ambient temperature and stirred for 15 h. All volatiles were subsequently removed in vacuo, the residue was extracted into hexane (20 mL), the extract was filtered, and the filtrate was stored at 30 °C. Pale yellow crystals of 5 deposited overnight (1.02 g, 82%). Mp: 173 175 °C. 1H NMR (300 MHz, C6D6, 298 K): 1.09 (s, 18H, C(CH3)3), 2.00, 2.06, 2.70 (3 s, 3 6H, o-CH3, p-CH3), 6.25 (s, 1H, NCCHCN), 6.63 (s, 2H, Ar-H), 6.67 (s, 2H, Ar-H). 13C{1H} NMR (400 MHz, C6D6, 300 K): 19.9, 20.6, 21.4 (o-CH3, p-CH3), 31.1 (C(CH3)3), 41.4 (C(CH3)3), 104.9 (NCCHCN), 128.9, 130.3, 133.6, 134.7, 135.4, 142.2 (Ar-C), 171.3 (NCCH); IR ν/cm 1 (Nujol): 1377 s, 1260 s, 861 w, 722 w, 682 w, 660 w. MS (EI 70 eV) m/z (%): 526.2 (M+, 45), 491.3 (M+ Cl, 28), 202.1 (MesNCBut+, 100). Anal. Calcd for C29H41ClGeN2: C, 66.25; H, 7.86; N, 5.33. Found: C, 66.23; H, 7.94; N, 5.24. Synthesis of [(MeMesNacnac)GeH] (6). To a solution of [(MeMesNacnac)GeCl] (0.20 g, 0.45 mmol) in toluene (20 mL) at 80 °C was added a 1.0 M solution of K[HBEt3] in THF (0.45 mL, 0.45 mmol) over 5 min. The resultant mixture was warmed to ambient temperature and stirred overnight. All volatiles were subsequently removed in vacuo, the residue was extracted into hexane (30 mL), and the extract was filtered and stored at 30 °C overnight to yield orange crystals of 6 (0.12 g, 65%). Mp: 160 162 °C dec. 1H NMR (300 MHz, C6D6, 298 K): 1.52 (s, 6H, NCCH3), 2.09, 2.29, 2.31 (3 s, 3 6H, o-CH3, p-CH3), 4.92 (s, 1H, NCCHCN), 6.67 (s, 2H, Ar-H), 6.70 (s, 2H, Ar-H), 8.25 (s, 1H, GeH). 13C{1H} NMR (300 MHz, C6D6, 300 K): 18.3, 18.6, 20.7 (o-CH3, p-CH3), 22.0 (NCCH3), 97.5 (NCCHCN), 129.3, 129.7, 132.7, 134.9, 135.2, 142.0 (Ar-C), 166.3 (NCCH). IR ν/cm 1 (Nujol): 1722 s (Ge H), 1525 s, 1376 s, 1147 m, 1015 m, 859 m, 706 m. MS (EI, 70 eV) m/z (%): 407.2 (M+, 100). HREI m/z: acc mass calcd for C23H3070GeN2, 403.1571; found, 403.1568. Anal. Calcd for C23H30GeN2: C, 67.86; H, 7.43; N, 6.88. Found: C, 67.71; H, 7.34; N, 6.82. Preparation of [(ButMesNacnacH)Ge:] (7). A solution of [(ButMesNacnac)GeCl] (0.20 g, 0.38 mmol) in toluene (20 mL) at 78 °C was treated with a 1.0 M solution of K[HBEt3] in THF (0.4 mL, 0.40 mmol). The resultant mixture was warmed to room temperature, whereupon all volatiles were removed in vacuo. The yellow residue was
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extracted with hexane (10 mL), the extract was filtered, and the filtrate was placed at 30 °C to yield pale yellow crystals of 7 (0.06 g, 32%). Mp: 164 167 °C dec. 1H NMR (300 MHz, C6D6, 298 K): 1.06 (s, 9H, C(CH3)3), 1.09 (s, 9H, C(CH3)3), 2.11, 2.16, 2.25, 2.41, 2.52, 2.57 (6 s, 6 3H, 6 Ar CH3), 4.03 (d, 1H, 3JHH = 6.9 Hz, CHBut), 4.87 (d, 1H, 3JHH = 6.9 Hz, NCCHCN), 6.75 7.03 (m, 4H, Ar-H). 13C{1H} NMR (400 MHz, C6D6, 300 K): 20.0, 20.8, 20.9, 21.1, 21.2, 21.8 (6 Ar-CH3), 27.3, 31.8 (2 C(CH3)3), 38.4, 40.2 (2 C(CH3)3), 72.6 (CHBut), 102.2 (NCCHCN), 129.6, 129.7, 130.2, 131.0, 132.5, 133.2, 135.3, 136.7, 139.2, 143.5, 147.6, 149.2 (Ar-C), NC(But)CH signal not observed. IR ν/cm 1 (Nujol): 1377 s, 1260 s, 864 m, 721 m, 665 m, 660 m. MS (EI 70 eV) m/z (%): 491.2 (M+, 3), 228.1 (MesNC (But)C(H)C(H)+, 100). Anal. Calcd for C29H42GeN2: C, 70.90; H, 8.62; N, 5.70. Found: C, 70.81; H, 8.68; N, 5.80. N.B. An aliquot of the reaction mixture was taken when it reached 30 °C. Volatiles were removed from this in vacuo, and the residue was dissolved in C6D6. A 1H NMR spectrum of this was sample was immediately acquired and showed an approximately 80:20 mixture of [(ButMesNacnac)GeH] (8) and [(ButMesNacnacH)Ge:] (7). After a further 10 min at 20 °C all [(ButMesNacnac)GeH] had converted to [(ButMesNacnacH)Ge:]. Attempts to isolate and crystallize [(ButMesNacnac)GeH] were not successful. Reliable 1H NMR data for the compound were, however, extractable from the aforementioned initial spectrum. Data for [(ButMesNacnac)GeH] (8): 1H NMR (300 MHz, C6D6, 298 K) 1.10 (s, 18H, C(CH3)3), 2.08, 2.25, 2.42 (3 s, 3 6H, o-CH3, p-CH3), 5.94 (s, 1H, NCCHCN), 6.67 (br s, 4H, Ar-H), 7.94 (s, 1H, GeH). N.B. When solutions of 7 in C6D6 were left to stand at ambient temperature for 30 h, they decomposed to give mixtures of the new azabutadiene MesNdC(But)C(H)dC(H)But and other unidentifiable products. The 1H and 13C{1H} NMR spectroscopic data for this compound could be reliably assigned from analyses of the spectra of the mixture and are comparable to those previously reported for DipNdC(But)C(H)dC(H)But.13 Data for MesNdC(But)C(H)dC(H)But: 1H NMR (300 MHz, C6D6, 298 K) 0.66 (s, 9H, C(CH3)3), 1.27 (s, 9H, C(CH3)3), 1.98 (s, 6H, o-CH3), 2.15 (s, 3H, p-CH3), 5.47 (d, 1H, 3 JHH= 17.8 Hz, CHBut), 5.65 (d, 1H, 3JHH= 17.8 Hz, NCCHCN), 6.72 (s, 2H, Ar-H); 13C{1H} NMR (400 MHz, C6D6, 300 K) 16.8 (o-CH3), 19.5 (p-CH3), 27.2, 27.5 (2 C(CH3)3), 31.8, 38.9 (2 C(CH3)3), 116.2 (CHBut), 123.5 (NCCHCN), 127.3, 129.4, 146.2, 147.2 (Ar-C), 172.9 (NC(But)CH). Preparation of [(MeMesNacnac)GeOC(dO)H] (9). A solution of [(MeMesNacnac)GeH] (0.12 g, 0.30 mmol) in toluene (10 mL) at 80 °C in a 250 mL Schlenk flask was placed under an atmosphere of CO2 gas (predried by passage over P2O5). The orange reaction solution was stirred and warmed slowly to ambient temperature, during which time (at ca. 30 °C) the solution become colorless. The resultant colorless solution was concentrated in vacuo to ca. 4 mL and layered with hexane (15 mL) to afford colorless crystals of 9 (0.10 g, 75%); Mp: 175 178 °C. 1H NMR (400 MHz, C6D6, 298 K): 1.52 (s, 6H, NCCH3), 2.06, 2.09, 2.36 (3 s, 3 6H, o-CH3, p-CH3), 5.04 (s, 1H, NCCHCN), 6.72 (s, 2H, Ar-H), 6.75 (s, 2H, Ar-H), 8.64 (1H, OCH). 13 C{1H} NMR (400 MHz, C6D6, 300 K): 18.5, 19.0, 20.7 (o-CH3, p-CH3), 22.3 (NCCH3), 99.1 (NCCHCN), 129.4, 130.4, 132.5, 135.1, 136.2, 140.2 (Ar-C), 164.5 (NCCH), 164.7 (O(CO)H). IR ν/cm 1 (Nujol): 1654 m (CdO), 1376 s, 1260 s, 862 w, 722 m. MS (EI 95 eV) m/z (%): 452.2 (M+, 20), 407.2 (M+ CO2H, 100). Anal. Calcd for C24H30GeN2O2: C, 63.90; H, 6.70; N, 6.21. Found: C, 63.64; H, 7.08; N, 6.20. Preparation of [(MeMesNacnac)GeC(But)dPH] (10). Neat PtCBut (0.17 mL, 1.03 mmol) was added to a solution of [(MeMesNacnac)GeH] (0.143 g, 0.35 mmol) in toluene (10 mL) at 20 °C. The reaction solution was warmed to 35 °C and stirred for 15 h, during which time the mixture slowly turned deep red. Volatiles were removed in vacuo, and 5548
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Table 2. Summary of Crystallographic Data for 6, 7, 9, and (Z)-10 6
7
9
(Z)-10 C28H39GeN2P
empirical formula
C23H30GeN2
C29H42GeN2
C24H30GeN2O2
formula wt
407.08
491.24
451.09
507.17
cryst syst
monoclinic
triclinic
orthorhombic
monoclinic
space group
P21/n
P1
P212121
P21/n
a (Å)
7.1930(14)
9.1645(3)
7.5831(4)
11.739(2)
b (Å)
24.351(5)
11.2956(2)
14.1863(5)
7.4260(15)
c (Å)
12.147(2)
12.9117(4)
20.696(2)
31.153(6)
α (deg.) β (deg)
90 99.41(3)
87.869(2) 88.142(2)
90 90
90 94.95(3)
γ (deg)
90
85.141(2)
90
90
vol (Å3)
2099.0(7)
1330.30(6)
2226.4(3)
2705.6(9)
Z
4
2
4
4
F (calcd) (g cm 3)
1.288
1.226
1.346
1.245
μ (mm 1)
1.468
1.169
1.398
1.209
F(000)
856
524
944
1072
T (K) no. of rflns collected
150(2) 8572
123(2) 21 157
123(2) 8233
123(2) 11 114
no. of unique rflns
4627
5812
4352
5960
Rint
0.0505
0.0249
0.0306
0.0288
R1 indices (I > 2σ(I))
0.0467
0.0267
0.0313
0.0339
wR2 indices (all data)
0.1116
0.0681
0.0636
largest peak and hole (e Å 3)
0.67,
CCDC no.
842376
0.81
the residue was extracted into hexane (5 mL). The extract was filtered and the filtrate stored at 30 °C overnight to yield dark red crystals of 10 (0.12 g, 67%). Mp: 140 142 °C. Data for (Z)-10: 1H NMR (300 MHz, C6D6, 298 K) 1.29 (s, 9H, C(CH3)3), 1.53 (s, 6H, NCCH3), 2.08, 2.38, 2.48 (3 s, 3 6H, o-CH3, p-CH3), 4.65 (s, 1H, NCCHCN), 6.75 (s, 2H, Ar-H), 6.78 (s, 2H, Ar-H), 8.22 (d, 1H, 1JPH = 144 Hz, PH); 13 C{1H} NMR (400 MHz, C6D6, 300 K): 18.7, 19.5, 20.0 (o-CH3, p-CH3) 21.8 (NCCH3), 29.9 (C(CH3)3), 44.4 (d, 2JCP = 17 Hz, C(CH3)3), 94.2 (NCCHCN), 128.7, 128.8, 131.9, 133.9, 134.1, 140.4 (Ar-C), 164.6 (NCCH), PdC resonance not observed; 31P NMR (400 MHz, C6D6, 300 K) δ 253.9 (d, 1JPH = 144 Hz); IR ν/cm 1 (Nujol) 2357 m (P H), 1524 s, 1377 s, 1260 s, 862 m, 722 m, 660 w; MS (EI 70 eV) m/z (%) 508.2 (M+, 5), 407.1 (M+ HPCBut, 100). Anal. Calcd for C28H39GeN2P: C, 66.30; H, 7.75; N, 5.52. Found: C, 66.42; H, 8.05; N, 5.32. N.B. Some NMR spectroscopic data for (E)-10 and (E)-11 could be reliably extracted from the spectra of the compound mixtures resulting from the variable-temperature NMR experiment. These compounds could not be isolated, despite several attempts. Data for (E)-10: 1H NMR (300 MHz, C6D6, 298 K) 1.25 (s, 9H, C(CH3)3), 1.59 (s, 6H, NCCH3), 2.06, 2.42, 2.45 (3 s, 3 6H, o-CH3, p-CH3), 4.95 (s, 1H, NCCHCN), Ar-H signals hidden below that of (Z)-10, 8.02 (d, 1H, 1JPH = 96 Hz, PH); 31P NMR (400 MHz, C6D6, 300 K) δ 190.0 (d, 1JPH = 96 Hz). Data for (E)11: 31P NMR (400 MHz, C6D6, 300 K) δ 334.4 (2JPH = 21 Hz). X-ray Crystallography. Crystals of 6, 7, 9, (Z)-10, [(MeMesNacnac) Li(tmeda)], and [(MeMesNacnac)SnCl] suitable for X-ray structural determination were mounted in silicone oil. Crystallographic measurements were made with an Oxford Gemini Ultra diffractometer using a graphite monochromator with Mo Kα radiation (λ = 0.710 73 Å). The structures were solved by direct methods and refined on F2 by full-matrix least squares (SHELX97)28 using all unique data. All non-hydrogen atoms are anisotropic with hydrogen atoms included in calculated positions (riding model), except the hydride ligands of 6 and (Z)-10, which were refined isotropically. The absolute structure factor for the crystal structure
0.34,
0.27
842377
0.37,
0.0895 0.27
842378
0.64,
0.41
842379
of 9 is 0.027(9). Crystal data and details of data collection and refinement for 6, 7, 9 ,and (Z)-10 are given in Table 2. Theoretical Studies. Quantum-chemical calculations were carried out with the Turbomole program package,29 employing the Becke Perdew 86 functional30 at the RI-DFT level31 with the def2-SVP basis set.32 Isotropic NMR shieldings were calculated with the NMR script of Turbomole; PH3 (calculated isotropic shielding 599.7 ppm; chemical shift δ 238 ppm) was used as a standard for the 31P NMR calculations.
’ ASSOCIATED CONTENT Supporting Information. CIF files giving crystallographic data for 6, 7, 9, (Z)-10, [(MeMesNacnac)Li(tmeda)], and [(MeMesNacnac)SnCl] tables and figures giving crystal data and details of data collection and refinement and ORTEP diagrams for [(MeMesNacnac)Li(tmeda)] and [(MeMesNacnac)SnCl], and tables giving further details of the calculations. This material is available free of charge via the Internet at http://pubs.acs.org.
bS
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT C.J. thanks the Australian Research Council (No. DP0665057), the donors of The American Chemical Society Petroleum Research Fund, and the U.S. Air Force Asian Office of Aerospace Research and Development for financial support. C.S. thanks the Alexander von Humboldt Foundation for a Feodor-Lynen Fellowship. The EPSRC Mass Spectrometry Service at Swansea University is also thanked. 5549
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’ REFERENCES (1) Selected recent reviews: (a) Wang, Y.; Robinson, G. H. Inorg. Chem. DOI:10.1021/ic200675u. (b) Asay, M.; Jones, C.; Driess, M. Chem. Rev. 2011, 111, 354. (c) Stasch, A.; Jones, C. Dalton Trans. 2011, 40, 5659. (d) Fischer, R. C.; Power, P. P. Chem. Rev. 2010, 110, 3877. (e) Schn€ockel, H. Chem. Rev. 2010, 110, 4125. (f) Jones, C. Coord. Chem. Rev. 2010, 254, 1273.(g) Lee, V. Y.; Sekiguchi, A. Organometallic Compounds of LowCoordinate Si, Ge, Sn and Pb; Wiley: Chichester, U.K., 2010. (h) Mizuhata, Y.; Sasamori, T.; Tohkitoh, N. Chem. Rev. 2009, 109, 3479. (i) Nagendran, S.; Roesky, H. W. Organometallics 2008, 27, 457. (2) Selected recent reviews: (a) Power, P. P. Acc. Chem. Res. 2011, 44, 627. (b) Yao, S.; Xiong, Y.; Driess, M. Organometallics 2011, 30, 1748. (c) Power, P. P. Nature 2010, 463, 171. (3) Selected reviews: (a) Rivard, E.; Power, P. P. Dalton Trans. 2008, 4336. (b) Aldridge, S.; Downs, A. J. Chem. Rev. 2001, 101, 3305. (c) Jones, C. Chem. Commun. 2001, 2293. (4) Selected books: (a) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals, 5th ed.; Wiley: Hoboken, NJ, 2009. (b) The Activation of Small Molecules; Tolman, W. B., Ed.; Wiley-VCH: Weinheim, Germany, 2006. (c) Kubas, G. J. Metal Dihydrogen and σ-Bonded Complexes: Structure, Theory and Reactivity, 1st ed.; Kluwer Academic/ Plenum Publishers: London, 2001. (5) See for example: (a) Rivard, E.; Steiner, J.; Fettinger, J. C.; Giuliani, J. R.; Augustine, M. P.; Power, P. P. Chem. Commun. 2007, 4919. (b) Rivard, E.; Fischer, R. C.; Wolf, R.; Peng, Y.; Merrill, W. A.; Schley, N. D.; Zhu, Z.; Pu, L.; Fettinger, J. C.; Teat, S. J.; Nowik, I.; Herber, R. H.; Takagi, N.; Nagase, S.; Power, P. P. J. Am. Chem. Soc. 2007, 129, 16197. (c) Spikes, G. H.; Fettinger, J. C.; Power, P. P. J. Am. Chem. Soc. 2005, 127, 12232. (d) Richards, A. F.; Phillips, A. D.; Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc. 2003, 125, 3204. (6) See for example: (a) Al-Rafia, S. M. I.; Malcolm, A. C.; McDonald, R.; Ferguson, M. J.; Rivard, E. Angew. Chem., Int. Ed. 2011, 50, 8354. (b) Al-Rafia, S. M. I.; Malcolm, A. C.; Liew, S. K.; Ferguson, M. J.; Rivard, E. J. Am. Chem. Soc. 2011, 133, 777. (c) Abraham, M. Y.; Wang, Y.; Xie, Y.; Wei, P.; Schaefer, H. F., III; Schleyer, P. v. R.; Robinson, G. H. J. Am. Chem. Soc. 2011, 133, 8874. (d) Thimer, K. C.; Al-Rafia, S. M. I.; Ferguson, M. J.; McDonald, R.; Rivard, E. Chem. Commun. 2009, 7119. (7) (a) Jana, A.; Leusser, D.; Objartel, I.; Roesky, H. W.; Stalke, D. Dalton Trans. 2011, 40, 5458. (b) Zhang, S.-H.; Yeong, H.-X.; Xi, H.-W.; Lim, K. H.; So, C.-W. Chem. Eur. J. 2010, 16, 10250. (8) See for example: (a) Arii, H.; Nakadate, F.; Mochida, K. Organometallics 2009, 28, 4909. (b) Pineda, L. W.; Jancik, V.; Starke, K.; Oswald, R. B.; Roesky, H. W. Angew. Chem., Int. Ed. 2006, 45, 2602. (c) Leung, W.-P.; So, C.-W.; Chong, K.-H.; Kan, K.-W.; Chan, H.-S.; Mak, T. C. W. Organometallics 2006, 25, 2851. (d) Ding, Y.; Hao, H.; Roesky, H. W.; Noltemeyer, M.; Schmidt, H.-G. Organometallics 2001, 20, 4806. (9) (a) Jana, A.; Tavcar, G.; Roesky, H. W.; John, M. Dalton Trans. 2010, 39, 9487. (b) Jana, A.; Ghoshal, D.; Roesky, H. W.; Objartel, I.; Schwab, G.; Stalke, D. J. Am. Chem. Soc. 2009, 131, 1288. (c) Jana, A.; Sen, S. S.; Roesky, H. W.; Schulzke, C.; Dutta, S.; Pati, S. K. Angew. Chem., Int. Ed. 2009, 48, 4246. (10) Jana, A.; Roesky, H. W.; Schulzke, C. Dalton Trans. 2010, 39, 132. (11) Jana, A.; Objartel, I.; Roesky, H. W.; Stalke, D. Inorg. Chem. 2009, 48, 7645. N.B. The original preparation of compound 2 was reported several years earlier: Driess, M.; Yao, S.; Brym, M.; van Wullen, C. Angew. Chem., Int. Ed. 2006, 45, 4349. (12) Woodul, W. D.; Carter, E.; M€uller, R.; Richards, A. F.; Stasch, A.; Kaupp, M.; Murphy, D. M.; Driess, M.; Jones, C. J. Am. Chem. Soc. 2011, 133, 10074. (13) N.B. A very similar process has been proposed for the rearrangement of unstable [(ButNacnac)Sc(H)Cl] to [(ButNacnacH)ScCl]: Conroy, K. D.; Piers, W. E.; Parvez, M. Organometallics 2009, 28, 6228. (14) Budzelaar, P. H. M.; van Oort, A. B.; Orpen, A. G. Eur. J. Inorg. Chem. 1998, 1485. (15) Ayers, A. E.; Klapotke, T. M.; Dias, H. V. R. Inorg. Chem. 2001, 40, 1000.
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(16) From one preparation of [(MeMesNacnac)GeCl], which involved an initial in situ lithiation of MeMesNacnacH in the presence of tmeda, several crystals of the monomeric complex [(MeMesNacnac) Li(tmeda)] were isolated. See the Supporting Information for details of its crystal structure. (17) Ding, Y.; Roesky, H. W.; Noltemeyer, M.; Schmidt, H.-G.; Power, P. P. Organometallics 2001, 20, 1190. (18) Woodul, W. D.; Richards, A. F.; Stasch, A.; Driess, M.; Jones, C. Organometallics 2010, 29, 3655. (19) Attempts to prepare the corresponding tin(II) hydride complex [(MeMesNacnac)SnH] were unsuccessful. From one such attempt crystals of a previously unknown polymorph of the precursor complex [(MeMesNacnac)SnCl] were isolated. See the Supporting Information for details of its crystal structure. For details of the structure of the known polymorph of this compound, see ref 15. (20) See for example: (a) Marciniec, B.; yawicka, H. Appl. Organomet. Chem. 2008, 22, 510. (b) Miura, K.; Ootsuka, K.; Hosomi, A. J. Organomet. Chem. 2007, 692, 514. (c) Trost, B. M.; Ball, Z. T. Synthesis 2005, 853. (d) Schwier, T.; Gevorgyan, V. Org. Lett. 2005, 7, 5191. (e) Faller, J. W.; Roman, G. K. Organometallics 2003, 22, 199. (f) Kinoshita, H.; Nakamura, T.; Kakiya, H.; Shinokubo, H.; Matsubara, S.; Oshima, K. Org. Lett. 2001, 3, 2521. (g) Esteruelas, M. A.; Martin, M.; Oro, L. A. Organometallics 1999, 18, 2267. (h) Widenhoefer, R. A.; Vadehra, A.; Cheruvu, P. K. Organometallics 1999, 18, 4614. (i) Wada, F.; Abe, S.; Yonemaru, N.; Kikukawa, K.; Matsuda, T. Bull. Chem. Soc. Jpn. 1991, 64, 1701. (j) Nozaki, K.; Ichinose, Y.; Wakamatsu, K.; Oshima, K.; Utimoto, K. Bull. Chem. Soc. Jpn. 1990, 63, 2268 and references therein. (21) (a) Jones, C.; Schulten, C.; Stasch, A. Inorg. Chem. 2008, 47, 1273. (b) Cowley, A. H.; Hall, S. W.; Nunn, C. M.; Power, J. M. J. Chem. Soc., Chem. Commun. 1988, 753. (c) Meiners, F.; Saak, W.; Weidenbruch, M. Chem. Commun. 2001, 215. (22) (a) Brym, M.; Jones, C. Dalton Trans. 2003, 3665. (b) Hill, A. F.; Jones, C.; Wilton-Ely, J. D. E. T. Chem. Commun. 1999, 451. (c) Bedford, R. B.; Hill, A. F.; Jones, C.; White, A. J. P.; Williams, D. J.; Wilton-Ely, J. D. E. T. Organometallics 1998, 4744. (d) Bedford, R. B.; Hill, A. F.; Jones, C.; White, A. J. P.; Williams, D. J.; Wilton-Ely, J. D. E. T. Chem. Commun. 1997, 179. (e) Benvenutti, M. H. A.; Cenac, N.; Nixon, J. F. Chem. Commun. 1997, 1327. (f) Bedford, R. B.; Hill, A. F.; Jones, C. Angew. Chem., Int. Ed. Engl. 1996, 35, 547. (23) See for example: (a) Dillon, K. B.; Mathey, F.; Nixon, J. F. In Phosphorus: The Carbon Copy; Wiley: Chichester, U.K., 1998. (a) Multiple Bonds and Low Coordination in Phosphorus Chemistry; Regitz, M., Scherer, O. J., Eds.; Thieme: Stuttgart, Germany, 1990. (24) Schmitz, M.; Goller, R.; Bergstraßer, U.; Leininger, S.; Regitz, M. Eur. J. Inorg. Chem. 1998, 2, 227. (25) (a) Weber, L. Coord. Chem. Rev. 2005, 249, 741. (b) Weber, L. Angew. Chem., Int. Ed. Engl. 1996, 35, 271. (26) Goerlich, J. R.; Schmutzler, R. Phosphorus, Sulfur Silicon Relat. Elem. 1995, 101, 245. (27) Francis, M. D. Ph.D. Thesis, University of Wales, Swansea, U.K., 1998. (28) Sheldrick, G. M. SHELX-97; University of G€ottingen, G€ottingen, Germany, 1997. (29) Treutler, O.; Ahlrichs, R. J. Chem. Phys. 1995, 102, 346. (30) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (b) Perdew, J. P. Phys. Rev. B 1986, 33, 8822. (31) Eichkorn, K.; Treutler, O.; Ohm, H.; H€aser, M.; Ahlrichs, R. Chem. Phys. Lett. 1995, 240, 283. (32) Sch€afer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571.
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