Monomeric β-Diketiminato Group 13 Metal Dipnictogenide Complexes with Two Terminal EH2 Groups (E=P, As).

The pnictogenyl Group 13 compounds (Dipp2 Nacnac)M[E(SiMe3 )2 ]Cl and (Dipp2 Nacnac)M(EH2 )2 (Dipp2 Nacnac=HC[C(Me)N(Ar)]2 , Ar: Dipp=2,6-iPr2 C6 H3 ; M=Al, Ga, In; E=P, As) were successfully synthesized. The salt metathesis between (Dipp2 Nacnac)MCl2 and LiE(SiMe3 )2 only led to monosubstituted compounds (Dipp2 Nacnac)M[E(SiMe3 )2 ]Cl [E=P, M=Ga(1), In (2); E=As, M=Ga (3), In (4)], regardless of the stoichiometric ratios used. In contrast to the steric effect of the SiMe3 groups in 1-4, the reactions of the corresponding halides with LiPH2 ⋅DME (or KAsH2 ) facilely yielded the dipnictogenide compounds (Dipp2 Nacnac)M(EH2 )2 (E=P, M=Al (5), Ga (6), In (7); E=As, M=Al (8), Ga (9)), avoiding the use of flammable and toxic PH3 and AsH3 for their synthesis. The compounds 5-9 are the first examples of monomeric Group 13 diphosphanides and diarsanides in which the metal center is bound to two terminal PH2 and AsH2 groups, respectively. In contrast to the successful synthesis of the indium diphosphanide (Dipp2 Nacnac)In(PH2 )2 , the reaction of (Dipp2 Nacnac)InCl2 with KAsH2 led to an indium mirror due to the instability of the target product.

n class="Chemical">Pnictogenyl‐substituted Group 13 pan> class="Chemical">metal compounds have emerged as an important type of molecules, having enormous potential to be used in material science of for example, nanoparticles,1 optoelectronic layers, or semiconductors.2 Moreover, the highly reactive EH2 groups (E=N, P, and As) probably enable further substitution to obtain heterobimetallic species containing M−E(H)−M′ moieties. Given the Lewis acidity of the Group 13 metal center and the Lewis basicity of the EH2 groups, these types of complexes readily undergo oligomerization to give dimers (R2AlNH2)2 [R=SiMe3, N(SiMe3)2] and [Ph*MH(NH2)]2 (M=Al, Ga, Ph*=terphenyl ligands),3 trimers (R2AlNH2)3 (R=CH3, tBu),4 or pseudo oligomers Al[R2Al(NH2)2]3 (R=SiMe3, N(SiMe3)2) (A, B, C, Scheme 1).3b, 5 Therefore, steric crowding is essential to isolate stable monomeric species with parent EH2 substituents.6 In 2004, the first monomeric aluminum diamide supported by a β‐diketiminate substituent was synthesized from the corresponding chloride derivative with NH3 in the presence of an N‐heterocyclic carbene (D, Scheme 1).7 Subsequently, the corresponding gallium diamide was obtained through a similar procedure.8 Nonetheless, no progress on its heavier analogues has been made until now. This is caused by the extreme flammability and toxicity of PH3 and AsH3, respectively. To date, only very few of the heavier pnictogenyl Group 13 compounds with parent EH2 (E=P, As) substituents have been reported. The lithium phosphanylalanate LiAl(PH2)4 is known as the first aluminum phosphanide representing a convenient PH2 − transfer reagent although it is only stable in ethereal solution.9 In 2000, Driess et al. reported the synthesis of diisobutyl(phosphanyl)alane [iBu2AlPH2]3, which gives in situ in THF a monomolecular iBu2Al(THF)PH2 species (E, Scheme 1).10 Power and co‐workers reported phosphanyl alanes and ‐gallanes ring compounds stabilized by bulky terphenyl ligands (F, Scheme 1).3c, 3d Previously, our group was able to obtain aluminum and gallium monophosphanide complexes [(CO)5W]PH2MH2(NMe3) (M=Al, Ga), which were prepared through H2 elimination between Me3N⋅MH3 (M=Al, Ga) and PH3[W(CO)5] (G, Scheme 1).11 Similarly, versatile oligomers can be obtained by using different conditions and different Lewis bases during the synthesis.12 Moreover, lithium arsanylalanate LiAl(AsH2)4 was also synthesized by the reaction of LiAlH4 with AsH3.13 In general, far less research has been made in arsanyl (AsH2) chemistry because of its toxicity, light sensitivity, and instability.

Scheme 1

Selected examples of Group 13 metal pnictogenide compounds with EH2 (E=N, P) substituents.

Selected examples of Group 13 n class="Chemical">metal pnictogenide compounds with pan> class="Chemical">EH2 (E=N, P) substituents.

In addition to the n class="Chemical">aluminum anpan>d pan> class="Chemical">gallium diamide compounds (D, Scheme 1), the heavier analogues of Group 13 metal dipnictogenides are still unknown. Moreover, the preparation of pnictogenyl Group 13 compounds with an EH2 moiety generally requires the pnictogen hydrogen compoundsEH3, which, in addition to their flammability and toxicity, is invalid for stoichiometrically controlled reactions. Therefore, the quest for the stabilization of heavier Group 13 metal dipnictogenide species of type H is still open, and an appropriate novel synthetic route should be developed. In 1993, Cowley prepared the base‐stabilized phosphanyl‐ and arsanylalanes monomers Me3N⋅AlH2E(Mes)2 (E=P, As; Mes=2,4,6‐Me3‐C6H2) by metathesis reactions.14 More recently, we reported on the synthesis of Lewis base‐stabilized pnictogenylboranes based on metathesis,15 demonstrating the feasibility of this type of reaction in the preparation of pnictogenyl metal compounds. In addition, the Lewis base‐stabilized parent phosphanyl‐ and arsanylboranes H2EBH2⋅Me3N (E=P, As) also were obtained by the alcoholysis of the corresponding (SiMe3)2EBH2⋅Me3N (E=P, As) with methanol,15g which seems to be an alternative synthetic route. Herein, we report on the synthesis and characterization of first Group 13 metal diphosphanide and diarsanide compounds through salt metathesis, in which the sterically bulky β‐diketiminate ligand was employed for the stabilization of the target products.

Initin class="Chemical">ally, we explored the approach of Group 13 pan> class="Chemical">metal dipnictogenide compounds with SiMe3 substituents. Treatments of (Dipp2Nacnac)MCl2 (Dipp2Nacnac=HC[C(Me)N(Ar)]2, Ar=2,6‐iPr2C6H3; M=Ga, In)16 with lithium bis(trimethylsilyl)phosphanide [LiP(SiMe3)2]17 or arsanide [LiAs(SiMe3)2]18 in toluene only gave the monosubstituted compounds 1–4, respectively (Scheme 2). In 1–4, one chlorine atom has been replaced by a P(SiMe3)2 or As(SiMe3)2 group. However, even by using an excess amount of the lithium pnictogenide reagents, it was not possible to substitute both chlorine atoms, which is attributed to the steric hindrance of the bulky β‐diketiminate ligand as well as the size of trimethylsilyl groups. The compounds 1–4 are well soluble, for example in toluene. In the 1H NMR spectra of 1–4, characteristic resonances of the β‐diketiminate ligand are observed, for example, the singlet at δ=4.97 ppm for 1 [2: 4.84, 3: 4.93, and 4: 4.81 ppm] corresponds to the γ‐H in each compound. In addition, the 1H NMR spectra of 1 and 2 also show a doublet at δ=0.09 and 0.02 ppm, respectively, with a 3 J PH coupling constant of 4.6 Hz, corresponding to the trimethylsilyl groups. In comparison, the singlets assigned to the trimethylsilyl groups in the 1H NMR spectra of 3 and 4 were observed at δ=0.10 and 0.07 ppm, respectively. Moreover, the 31P{1H} NMR spectra of 1 and 2 show a singlet at δ=−255.0 and −252.9 ppm, respectively.

Scheme 2

Synthesis of compounds 1–4. Yields are given in parenthesis.

Synthesis of compounds 1–4. Yields n class="Chemical">are givenpan> inpan> ppan> class="Chemical">arenthesis.

Single‐crystn class="Chemical">al X‐ray diffractionpan> wpan> class="Chemical">as carried out to confirm the structures of 1–4. All four compounds crystallize in the monoclinic space group P21/m, the molecular structure of 1 is depicted in Figure 1. The metal center is in a tetrahedral coordination mode binding to a β‐diketiminate ligand, a chlorine atom as well as a bis(trimethylsilyl)phosphanide or arsanide group, respectively. The N‐M‐N angles (M=Ga, In) in 1–4 were determined to 96.03(13) (1) and 95.95(9)° (3) for the gallium compounds, and 90.33(10) (2) and 90.20(9)° (4) for the indium compounds. Compared with the N‐M‐N angles (M=Ga, In) in the starting materials (Dipp2Nacnac)GaCl2 (93.92(7)°) and (Dipp2Nacnac)InCl2 (92.5(1)°),16 respectively, the angles in 1–4 are significantly smaller, implying the steric effect of the substituents. The Ga−P bond length in 1 is 2.3310(9) Å, which is shorter than the In−P bond length in 2 (2.4806(8) Å). The Ga−As distance of 2.4196(4) Å in 3 is also shorter than the In−As distance in 4 (2.5632(3) Å).

Figure 1

Molecular structure of 1 with thermal ellipsoids at 30 % probability level. Carbon‐bound hydrogen atoms are omitted for clarity.

Molecular structure of 1 with thermpan> class="Chemical">al ellipsoids at 30 % probability level. Carbon‐bound hydrogen atoms are omitted for clarity.

Ren class="Chemical">gardless of the steric hinpan>dranpan>ce, the above‐menpan>tionpan>ed results inpan>dicate the promisinpan>g synpan>thesis of pan> class="Chemical">pnictogenyl metal complexes through salt metathesis. Thus, we further investigated the targeted synthesis of metal dipnictogenides with two terminal EH2 groups (E=P, As). The reaction of (Dipp2Nacnac)MX2 (X=I, M=Al; X=Cl, M=Ga, In)16, 19 with two equivalents of LiPH2⋅DME20 were carried out in diethyl ether or THF (Scheme 3). After workup, the expected metal diphosphanides 5–7 were isolated in yields of 32, 34.3, and 1 %, respectively. The quite low yield of isolated material for 7 is imputed to the extreme photosensitivity of indium compounds because the solution of 7 immediately became turbid upon exposure to light.

Scheme 3

Synthesis of compounds 5–9.

Compounds 5–7 n class="Chemical">are air anpan>d pan> class="Chemical">moisture sensitive and soluble in almost all common organic solvents. They were fully characterized by multinuclear NMR spectroscopy and X‐ray diffraction. In the 1H NMR spectra of 5–7, typical resonances of β‐diketiminate backbones are observed. The resonances of γ‐H are detected as a singlet at δ=4.97 (5), 4.83 (6), and 4.75 ppm (7), which are comparable to those of (Dipp2Nacnac)Al(NH2)2 (δ=4.88 ppm)7 and (Dipp2Nacnac)Ga(NH2)2 (δ=4.76 ppm).8 Moreover, the 1H NMR spectrum of 5 shows a doublet at δ=0.74 ppm (J PH=172.0 Hz), which was assigned to the PH2 moieties, whereas in the spectra of 6 and 7, the signal splits into a doublet of doublets at δ=1.03 ppm (1 J PH=175.7, 3 J PH=5.1 Hz) for 6 and 0.90 ppm (1 J PH=168.0, 3 J PH=4.6 Hz) for 7 (Table 1). The AB spin systems in 6 and 7 are a result of the nonplanarity of the C3N2M ring, because the gallium and indium atoms are relatively oversized. The same coupling constants also were observed in the 31P NMR spectra. In contrast, singlets at δ=−273.4 (5), −262.1 (6), and −285.5 ppm (7) were detected in the 31P{1H} NMR spectra, falling in the range of the typical resonances of terminal PH2 groups.3d, 10, 12a In addition, the LIFDI mass spectra of 5 and 6 show the most intense peak at m/z 477.3 and 519.4, respectively, assigned to the ionic fragment of [M+−PH2]. However, mass spectroscopy could not be used for the characterization of 7 beacuse of its light and air sensitivity.

Table 1

Resonances of the PH2 groups in 1H NMR spectra of compounds 5–7.

Complex	δ [ppm]	multiplicity	1 J PH [Hz]	3 J PH [Hz]
5	0.74	d	172.0	Not observed
6	1.03	dd	157.7	5.1
7	0.90	dd	168.0	4.6

Resonances of the n class="Gene">PH2 groups inpan> pan> class="Chemical">1H NMR spectra of compounds 5–7.

The structures of 5–7 were further determined by single‐crystn class="Chemical">al X‐ray diffractionpan>. Compounpan>ds 5 anpan>d 6 crystpan> class="Chemical">allize in the triclinic space group P , whereas 7 crystallizes in the monoclinic space group P21/n. The molecular structure of 5 is depicted in Figure 2 and those of 6 and 7 are included in the Supporting Information. These three compounds are isostructural because their metal centers adopt a distorted‐tetrahedral geometry in the coordination with two phosphorus and two nitrogen atoms, respectively. The phosphorus atoms adopt a pyramidal geometry with different orientations. The N‐M‐N angles (M=Al, Ga, In) decrease following the order of Group 13 as 97.47(5) (5), 95.81(5) (6), and 89.10(6)° (7). This indicates the nonplanarity of the C3N2M rings (M=Al, Ga, In), which is in agreement with the coupling in the 31P NMR spectra. In contrast, the P‐M‐P angles slightly increase in the sequence of 115.26(2) (5), 118.88(19) (6), and 122.55(2)° (7). In compound 5, the Al−P bond lengths are 2.3474(5) and 2.3718(5) Å, thus being comparable to those containing parent PH2 units in [{(CO)5W}H2PAlH2(NMe3)] (n=1, 2.367(1); n=2, 2.432(2) Å),11, 21 [{(CO)4Cr}H2PAlH2(NMe3)] (2.383(1) Å),22 [{(CO)5WPH2}(Me3N)AlPH{W(CO)5}]2 (2.368(6) Å),12a and [(Ar′Al)3(μ‐PH2)3(μ‐PH)PH2] [Ar′=C6H3‐2,6(C6H2‐2,4,6‐Me3)2] (2.378(2) Å).3d The Ga−P distances of 6 (2.3286(5) and 2.3532(5) Å) are close to those observed in [{(CO)5W}H2PGaH2(NMe3)] (2.349(2) Å)11 and 1 (2.3310(9) Å), whereas 7 has slightly longer In−P bonds (2.5255(7) and 2.5052(6) Å) compared with that of 2 (2.4808(8) Å).

Figure 2

Molecular structure of 5 with thermal ellipsoids at 30 % probability level. Carbon‐bound hydrogen atoms were omitted for clarity.

Molecular structure of 5 with thermpan> class="Chemical">al ellipsoids at 30 % probability level. Carbon‐bound hydrogen atoms were omitted for clarity.

Furthermore, it wn class="Chemical">as of inpan>terest if this synpan>thetic procedure could pan> class="Chemical">also be used for the synthesis of the more sensitive bis‐AsH2‐substituted derivatives. The treatment of (Dipp2Nacnac)MX2 (X=I, M=Al; X=Cl, M=Ga)16, 19 with two equivalents of KAsH2 23 resulted in the new aluminum and gallium diarsanide complexes 8 and 9 (Scheme 3). However, the reaction of (Dipp2Nacnac)InCl2 with KAsH2 led to an indium mirror as well as [(Dipp2Nacnac)H] as the only product detectable by 1H NMR spectroscopy,24 due to the high sensitivity, even though the compounds were handled in the absence of light. Compound 9 is also quite unstable in solution to give [(Dipp2Nacnac)H], whereas the aluminum derivative 8 shows a higher stability. Similarly, the 1H NMR spectra of 8 and 9 display characteristic signals for the β‐diketiminate ligands such as the singlets for γ‐H at δ=4.99 and 4.86 ppm and the septets for CHMe2 at δ=3.43 and 3.44 ppm, respectively. In addition, the resonances for AsH2 groups are observed as a singlet at δ=0.28 and 0.65 ppm, respectively. In the LIFDI mass spectrum of 8, the peak of the [M+−AsH2] fragment is observed at m/z 521.2, whereas the EI mass spectrum of 9 shows not only the [M+−AsH2] but also the [M+−2 AsH2] fragments at m/z 563.1(95) and 487.2(22), respectively.

Compound 8 wn class="Chemical">as further chpan> class="Chemical">aracterized by single crystal X‐ray diffraction and represents the first monomeric aluminum diarsanide that has been structurally characterized so far (Figure 3). Compound 8 crystallizes in the monoclinic space group P21/n, in which the fourfold‐coordinated aluminum center is bound to a β‐diketiminate ligand and two AsH2 substituents. Both arsenic atoms adopt a pyramidal geometry, indicating the existence of the lone pairs of electrons. The N(1)‐Al(1)‐N(2) angle in 8 (97.77(4)°) is similar to those of 5 (97.47(5)°) and (Dipp2Nacnac)Al(NH2)2 (95.7(1)°).7 The average As−Al bond length of 2.474(3) Å in 8 is comparable to those observed in lithium arsanylalanate [Li(DME)2]2[(AlH2AsR)3Li(DME)] (DME=1,2‐dimethoxyethane) (av. 2.472(3) Å),13 whereas they are longer than the corresponding bond in 3 (2.4196(4) Å).

Figure 3

Molecular structure of 8 with thermal ellipsoids at 30 % probability level. Carbon‐bound hydrogen atoms and the distorted part of the AsH2 units were omitted for clarity.

Molecular structure of 8 with thermpan> class="Chemical">al ellipsoids at 30 % probability level. Carbon‐bound hydrogen atoms and the distorted part of the AsH2 units were omitted for clarity.

Computationn class="Chemical">al DFT studies (see the Supplemenpan>tpan> class="Chemical">ary Information) indicate that the reactions leading to 1–4 and 5–9 are exergonic and that the formation of a solid salt is the driving force for the reaction. The computed thermodynamic data indicate that the stability of Group 13 metal diphosphanides and diarsanides decreases in the order Al>Ga>In. The processes of the decomposition of In‐containing derivatives into indium metal with the formation of [(Dipp2Nacnac)H], E4, and H2 as byproducts are exergonic at room temperature by 10 and 53 kJ mol−1 for E=P or As, respectively, which is in agreement with the experimentally observed low stability of 7 and the absence of its diarsanide analog.

The ann class="Chemical">alysis of the electronpan>ic structure revepan> class="Chemical">als that the HOMOs of 5–9 are essentially lone pairs of electrons located at pnictogen atoms. Thus, 5–9 can potentially serve as bidentate Lewis bases. However, the large E‐M‐E angles (115–123°, see above), which are unfavorable for the formation of 4‐membered cycles, suggest that such compounds could be rather bridging than chelating ligands.

In summn class="Chemical">ary, we provided a smooth synpan>thetic route to pan> class="Chemical">metal phosphanide and arsenide compounds through salt‐metathesis reactions and a series of β‐diketiminate ligand‐stabilized pnictogenyl Group 13 complexes (Dipp2Nacnac)M[E(SiMe3)2]Cl (E=P, As; M=Ga, In) and (Dipp2Nacnac)M(EH2)2 (E=P, M=Al, Ga, In; E=As, M=Al, Ga) have been synthesized. The utilization of alkali‐metal pnictides (LiE(SiMe3)2, LiPH2, and KAsH2) avoids the use of flammable and toxic PH3 and AsH3 for their synthesis. Due to steric hindrance, reactions of LiE(SiMe3)2 (E=P and As) with the corresponding metal chlorides only led to monosubstituted products 1–4. When using LiPH2⋅DME and KAsH2 as starting materials, the monomeric Group 13 diphosphanides and diarsanides 5–9 were obtained. For the first time, these complexes contain two terminal PH2 and AsH2 groups, respectively. Notably, the reaction of (Dipp2Nacnac)InCl2 with KAsH2 led to an indium mirror and [(Dipp2Nacnac)H] due to the intrinsic instability, implying the high photolability of such species. The good solubility and stability of aluminum diphosphanide and diarsenide compounds promise an interesting subsequent reactivity pattern, which is in the focus of current investigations. Moreover, the rather low stability of the M−E bonds might result in potential PH2 and AsH2 transfer reagents.25

Conflict of interest

The authors decln class="Chemical">are no conpan>flict of interest.

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Supplementn class="Chemical">ary

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