Untersuchungen und Vergleich des Reaktionsverhaltens von N-heterocyclisch Carben stabilisierten Kalium-Arsenideniden zu ihren Phosphoranaloga

In the first part of the present work, the unsaturated N-heterocyclic carbene stabilized “parent arsenide” [(SIDipp)AsH] 1 was successfully synthesized according to the slightly modified literature known synthesis by TAMM et al. 1 differs from its unsaturated analogue [(IDipp)AsH] mainly in the higher ratio of double bond character at the C1–As bond. The latter is given by a higher electronic shielding of the proton at the AsH unit in the 1H NMR spectrum as well as hindered rotation around the C1–As bond, which is indicated by the splitting of the methyl protons at the Dipp substituents into two sets of signals in the 1H NMR spectrum. In addition, the ?-donor strength of 1 was determined in direct comparison with the phosphorus analogue [(SIDipp)PH] by determination of the electronic Tolman parameter. This is based on the position of the CO vibrational modes of the totally symmetric A1 stretching of the [(SIDipp)ENi(CO)3] complexes 2 and 3 (E = As, P) (Figure 1). When comparing the two complexes 2 (?̃ = 2044 cm-1 ) and 3 (?̃ = 2048 cm-1 ), the position of these CO bands shows that the arsinidene ligand 2 is the slightly stronger ?-donor. Compared to phosphines (?̃ ≥ 2056 cm-1 ) and N-heterocyclic carbenes (?̃ = 2045–2089 cm-1 ), the totally symmetric A1 stretching frequencies for 2 and 3 are shifted to lower wavenumbers and to stronger ?-donor properties. Structurally, the two nickel complexes differ only slightly within the measurement differences of arsenic and phosphorus. The significantly longer C1–As and C1–P bond lengths compared to 1 and [(SIDipp)PH] indicate the loss of double bond character in these compounds (E = As, P), which is consistent with the sterically demanding strong electron donating abilities of Ni(CO)3 substituents by weakening the ?-backbonding. These electronic properties are also reflected in a downfield shift of the protons of the E–H units in the 1H NMR spectra in combination with an upfield shift of the carbene carbon atoms in the 13C NMR spectra. A similar behavior of the structural and spectroscopic data was also observed for the Lewis acid-base adducts [(SIDipp)AsHW(CO)5] 4 and [(SIDipp)AsFe(CO)4] 5 (Figure 1). As expected, the structural comparison with the nickel complex 2 showed an increase in the As–M bond lengths and a broadening of the C1–As–M bond angles with increasing atomic radius of the transition metal. Selected structural and spectroscopic data in Table 1 provide an overview. Furthermore, based on the reactions described above, the Lewis basic compound [(SIDipp)AsH] 1 as such and its affinity to use only one of the two lone pairs to form mononuclear arsenide complexes (2, 4, and 5) was determined. The position of the A1 stretches of the CO bands of 4 and 5 in the IR spectrum compared to similar known compounds showed that the arsinidene ligand has a higher back-donating ability than the CO ligand and is therefore a stronger ?-donor. This confirmed the TEP values of the arsenide nickel complex 2. Later on in this work, the potassium arsinidenide [(SIDipp)AsK] 6 was synthesized by deprotonation of 1 with the strong base benzylpotassium according to a slightly modified literature known synthesis by LEMP and BALMER. It is a red solid that is insoluble in common aromatic/aliphatic solvents and decomposes upon reaction with ethers at temperatures above −70 °C. The characterization of 6 was initially carried out using common solid-state analyzes (CHN, IR). By reaction with [18]-crown-6 at low temperatures (−76 °C) in toluene and constant handling and storage at the same temperature, the soluble crown ether complex [K([18]-crown-6)][(SIDipp)As] 6a was received as the first time example in this compound class. This also enabled characterization by means of crystal structure analysis. The molecular structure of [K([18]-crown-6)][(SIDipp)As] 6a is shown in Figure 2. The molecular structure of 6a shows a slight shortening of the C1–As bond length compared to the starting compound [(SIDipp)AsH] 1. As a result to the increase in electron density on the arsenic atom by binding to the less electronegative potassium atom and the resulting stronger ?-backbonding. The second part of this work deals with the applicability of the previously synthesized potassium arsinidene 6 as an arsenic precursor in reactions with element halides and -bis(trimethylsilyl)amides of groups 2 or 12, 14, 15 and 16, for the preparation of main-group substituted low valent arsinidenes. Furthermore, a direct structural and analytical comparison of these main-group substituted arsinidenes with the phosphinidene analogous compounds was drawn, which were either literature known or prepared within this work. Reactions with Group 15 halides As starting point in this chapter, the reaction behavior of 6 towards the di-tert-butylelement chlorides of group 15 tBu2ECl (E = P, As, Sb, Bi) is examined. The compounds of the type (SIDipp)AsEtBu2 7–10 obtained in salt elimination reactions were characterized by means of 1H, 13C, 31P NMR, IR spectroscopy, elemental analysis and crystal structure analysis and the measured values obtained were related to those of the phosphorus compounds. Analogous to the latter, identical molecular structures in the crystal as well as similar trends of the structural and analytical parameters were observed. As expected, these results show that with an increasing radius of E, the As–E bond lengths and consequently the C1–As–E bond angles also increase. Furthermore, the 1H NMR spectra of compounds 7–10 show a minimal downfield shift of the protons of the tBu2 units and of the hydrogen atoms in the carbene backbone with increasing atomic number. The C1–As bond lengths of compounds 7-10, identically to the C–P bonds of their phosphorus analogues are unaffected by substitution at the arsenic atom. This is in contrast to the Lewis acid–base adducts 2,4, and 5. Reactions with Group 16 halides and bis(trimethylsilyl)amides Reactions of [(SIDipp)AsK] 6 and [(SIDipp)PK] with the Group 16-chlorides S2Cl2, Se2Cl2, SeCl4, and TeCl4 turned out to be ineffective for reactions with potassium pnictinides because of the high residual water content of the Group 16 reactants. In addition to the product species that cannot be defined due to the lack of crystallization, renewed protonation of the arsenide or phosphinidene was observed. Subsequent comparative reactions of the potassium pnictinidenides with Group 16 bis(trimethylsilyl)amides (Se(hmds)2 & Te(hmds)2) led to the selenium-substituted arsenide 12, as well as the two chalcogen-substituted phosphinidenes 11 and 13 were received. With these three compounds it was possible to expand the rare class of (organoseleno)pnictinidenides by two derivatives (11 and 12) and to prepare the first ionic ?-tellurium-bridged phosphinidene (13). The subsequent full characterization of these compounds and the comparison with literature known compounds showed that the selenium-substituted pnictinidenides (SIDipp)ESe(hmds) 11 and 12 have an identical structural configuration. The C–E–Se–N torsion angles (E = As, P) in both compounds close to 180° indicate an overlap of the ?*-orbital of the respective C–E units with the vacant p*-orbital of Se(hmds) unit. The central motif of the cation of the ionic phosphinide compound 13 is a TeP2K distorted diamond shape, in which the potassium ion interacts with the ?-system of the Dipp substituents to saturate its coordination sphere. Two hmds units bonded via a hydrogen atom are present as the anion. The C=P double bond is not affected by the substitution with tellurium. Reactions with Group 2 and Group 12 halides and bis(trimethylsilyl)amides For the synthesis of ?-bridged arsinidenes of groups 2 and 12 and the analogous phosphinide compounds, the reaction behavior towards dichlorides of group 2 was investigated. These, identical to [(SIDipp)AsK] and [(SIDipp)PK], showed insufficient solubility in toluene and n-pentane, so no reactions were observed. Hence, reactions of 6 and [(SIDipp)PK] with the easily soluble group 2 bis(trimethylsilyl)amides M(hmds)2 were performed. Owing to the affinity of the latter for the formation of trisamides ([M(hmds)3] ‒ ) favored by the HSAB concept, the ionic NHC-stabilized K/P or K/As clusters were synthesized instead of the ?-bridged species [{(SIDipp)E}4K5] + [M(hmds)3] ‒ (14–17, 19, and 20) (Figure 5). The central motif of the cation of these clusters - a distorted window. It is noticeable that reactions of 6 with Mg(hmds)2 and Ba(hmds)2 along with C=As bond cleavage are leading to ionic ?-potassium-bridged dicarbenes [(SIDipp)2K]+ [M(hmds)3] ‒ , identical to the literature compounds synthesized by HILL et al. (18 and 21, Figure 5). In addition, the clusters 14–20 shown only insufficient solubility in the usual aromatic/aliphatic solvents, which is probably due to the trisamide anion. The structural and spectroscopic data of the ionic compounds 14–21 agree well with those of the analogous literature compounds. Only the phosphorus signals of 14–17 show a steadily increasing upfield shift with increasing metal character of the alkaline earth metal in the trisamide anion. Based on the similar electronic properties of magnesium(II) and zinc(II), the linear zinc compounds 22 [{(SIDipp)As}2Zn], 23 [{(SIDipp )P}2Zn] and 24 are [K2{(SIDipp)P}2Zn]2+ 2 [Zn(hmds)3] ‒ were obtained from reactions of the potassium pnictinidenides with ZnCl2 and Zn(hmds)2. In the crystal, 22 and 23 show equivalent molecular structures to the mercury(II) bis(phosphinide) compounds of BISPINGHOFF et al. Remarkably, the ?-zinc-bridged bis(phosphinidenide) 23 shows insufficient solubility in common aromatic/aliphatic solvents, suggesting that this is caused by the presence of the trisamide anion. The structural comparison of the compounds with each other and with analogous literature known compounds revealed an identical molecular structure with linear (~180°) E–Z–E bond angles (E = As, P). The other structural values also are in good agreement with the compounds presented abouve and known from the literature.