Cyclo‐Dipnictadialanes

Abstract

Using the Al^I precursor Cp^3tAl in conjunction with triphosphiranes (PAr)₃ (Ar=Mes, Dip, Tip) we have succeeded in preparing Lewis base‐free cyclic diphosphadialanes with both the Al and P atoms bearing three substituents. Using the sterically more demanding Dip and Tip substituents the first 1,2‐diphospha‐3,4‐dialuminacyclobutanes were obtained, whereas with Mes substituents [Cp^3tAl(μ‐PMes)]₂ is formed. This divergent reactivity was corroborated by DFT studies, which indicated the thermodynamic preference for the 1,2‐diphospha‐3,4‐dialuminacyclobutane form for sterically more demanding groups on phosphorus. Using Cp*Al we could extend this concept to the corresponding cyclic diarsadialanes [Cp*Al(μ‐AsAr)]₂ (Ar=Dip, Tip) and additionally add the phosphorus variants [Cp*Al(μ‐PAr)]₂ (P=Mes, Dip, Tip). The reactivity of one variant [Cp^3tAl(μ‐PPh)]₂ towards NHCs was tested and resulted in double NHC‐stabilised [Cp^3t(IiPr₂)Al(μ‐PPh)]₂.

Pnictaalenes with a pnictogen−aluminium double bond are inherently unstable and prone to dimerization. The synthesis of Lewis‐base‐free dipnictadialenes is revealed. It is shown that depending on the steric profile of the substituents either the alternating [Cp ^x Al(μ‐PAr)]₂ dimers or the head‐to‐head species [ArPnAlCp^3t]₂ are formed. DFT studies corroborate that the different outcomes are thermodynamically driven.

Introduction

Heterocycles composed of phosphorus and the group 13 elements have been first reported by Davidson and Brown, who accidentally synthesised the trimer of Me₂Al−PMe₂ (Type A′, Scheme 1), [Me₂AlPMe₂]₃. ^[1] A growing interest in the use of single‐source precursors for metalorganic chemical vapor deposition (MOVCD), for making group 13/15 semiconducting materials, sparked the development of cyclic heteroatomic group 13/15 compounds. Among the early examples, Cowley and Jones reported the aluminium and gallium cycles [iBu₂AlP(H)SiPh₃]₂ (Type A, Scheme 1) ^[2] and [Me₂GaPtBu₂]₂, ^[3] respectively. In these systems both phosphorus and the group 13 element are four‐coordinate, and these are therefore the dimers of the respective phosphinoalanes and ‐gallanes. Pioneering work by Scheer et al. has revealed that the parent phosphinoalane H₂Al−PH₂ can be intercepted by using Lewis bases (LB) and Lewis acids (LA) on aluminium and phosphorus, respectively. ^[4] Dehydrogenative trimerisation in CH₂Cl₂ solution afforded the six‐membered species [(CO₅W)P(H)−Al(H)NMe₃)]₃, which transforms into a bicyclic species upon further loss of H₂. ^[5] By judicious choice of a sterically demanding (kinetically stabilising) LB, e.g., IDip₂ (IDip₂=(HCNDip)₂C, Dip=2,6‐iPr₂C₆H₃), the first only LB‐stabilised parent compounds IDip₂⋅H₂EPnH₂ (E=Al, Ga; Pn=P, ^[6] As ^[7] ) have recently been realised. In general, dimeric species of the type [R₂Al−PR′₂]₂ (A, Scheme 1) are obtained by condensation reactions. The driving force in these reactions is the formation of volatile by‐products with thermodynamically stable bonds (e.g., H₂, alkanes, silanes, halosilanes). ^[8] In particular, the extrusion of H₂ takes advantage of the protic and hydridic nature of the P−H^δ+ and Al−H^δ− bonds, respectively. ^[9]

Scheme 1.

Known Al/P ring systems (A and B) and their respective monomers (A′ and B′) (left). Two potential dimers of phosphaalumenes investigated in this study (right).

In contrast, the formally doubly bonded pnictatrielenes RE=PnR′ (E=Al, Ga; Pn=P, As, Sb) have eluded facile synthesis until recently. Phospha‐ and arsaalumenes were prepared using a Cp*Al for PMe₃ exchange starting from the pnicta‐Wittig reagents ^DipTerPnPMe₃ (^DipTer=2,6‐Dip₂C₆H₃, Pn=P, As) and (Cp*Al)₄ to give ^DipTerPnAlCp* (Pn=P (C1), As (C2), Scheme 2) as base‐free monomeric compounds. ^[10] The corresponding phosphagallenes were synthesised using phosphanyl‐ or gallaphosphaketenes in the reaction with (^DipNacnac)Ga (^DipNacnac=HC[C(Me)NDip]₂) facilitating CO cleavage and formation of [(S)P]−P=Ga(^DipNacnac) ([(S)P]=(H _n CNDip)₂P; n=1, 2) (C3, Scheme 2), ^[11] or (^DipNacnac)Ga=P−Ga(Cl)(^DipNacnac) (C4, Scheme 2), ^[12] respectively. The arsagallene (^DipNacnac)Ga=AsCp* (C6, Scheme 2) was obtained when Cp*AsCl₂ was reacted with two equiv of (^DipNacnac)Ga. ^[13] Moreover, the stibagallene (^DipNacnac)Ga=Sb−Ga(Cl)(^DipNacnac) (C5, Scheme 2) has been reported. ^[14] In all these doubly bonded species the E−Pn (E=Al, Ga; Pn=P, As, Sb) multiple bond is highly polarised towards the group 15 element.

Scheme 2.

Known base‐free pnictatrielenes (C1–C6).

Due to their Lewis‐acidic group 13 centre and electron‐rich pnictogen centre, E−Pn multiple bonds are prone to oligomerisation. LB‐stabilised variants of cyclo‐1,3‐diphospha‐2,4‐dialanes (B, Scheme 1), formally the dimers of LB‐coordinated RP=AlR′ (B′, Scheme 1), are known and were synthesised from silyl‐substituted phosphanes R₃Si−PH₂ and H₃AlNMe₃, giving after dehydrocoupling the four‐membered heterocycle [RPAlH(NMe₃)]₂ (R=SitBuPh₂, iPr₃Si, Me₂(iPrMe₂C)Si). Attempts to generate the NHC‐adduct of the corresponding phosphaalumene (RP=Al(NHC)H) by addition of a free carbene to the four‐membered ring resulted in base‐exchange and formation of the respective bis‐carbene adducts [RPAlH(NHC)]₂ (NHC=(HCNiPr)₂C, IiPr₂). ^[15] The reaction of Mes*AlH₂ (Mes*=2,4,6‐tBu₃C₆H₂) with H₂EPh (E=P, As) in a 1:1 ratio at 160 °C afforded under H₂‐elimination the trimers of Mes*Al=EPh (Mes*AlPPh)₃ and (Mes*AlAsPh)₃, ^[16] respectively, which are formal heavier analogues of borazine. Four‐membered 1,3‐diphospha‐2,4‐diboretanes, the boron congeners of B (Scheme 1), have been reported, ^[17] mostly originating from unsuccessful attempts to access monomeric RP=BR species. ^[18] These phosphorus–boron heterocycles contain pyramidalised phosphorus atoms, making them potential ligands for transition metals.[ ^17e , ¹⁹ ] The related ring systems [RE(μ‐PR′₂)]₂ (E=B, ^[20] R=tBu, R′=iPr; E=Al, ^[21] R=PtBu₂, R′=tBu) with formally three coordinate B and Al centres, respectively, have been shown to be biradicals. The cyclo‐1,3‐dipnicta‐2,4‐dialanes in which the group 13 centre is not stabilised by a Lewis‐base have eluded facile synthesis to date. Herein, we close this gap and show that using cyclo‐tripnictanes of the type Pn₃Ar₃ (Pn=P, ^[22] As; ^[23] Ar=2,4,6‐Me₃C₆H₂, Mes; Dip; 2,4,6‐iPr₃C₆H₂, Tip) in conjunction with the Al^I synthons (Cp*Al)₄ ^[24] and Cp^3tAl (Cp^3t=1,2,4‐tBu₃C₅H₂), ^[25] four‐membered heterocycles with group 13 and 15 centres bearing three substituents, respectively, become synthetically feasible. Interestingly, two distinct forms were obtained, for example, the expected cyclo‐1,3‐dipnicta‐2,4‐dialanes [Cp ^x Al(μ‐PnAr)]₂ (x=*, 3t) and the head‐to‐head dimers 1,2‐diphospha‐3,4‐dialuminacyclobutanes. The experimental findings were corroborated by DFT‐studies shedding light on the divergent reactivity of differently substituted precursors.

Results and Discussion

Diphosphadialanes from Cp^3tAl and (PR)₃/(PAr)₅

Triphosphiranes (PR)₃ and cyclo‐oligophosphanes are, in general, the formal oligomers of phosphinidenes. ^[26] Especially (PhP)₅ has been shown to react with NHCs to give NHC phosphinidene adducts of the type NHC=PPh, ^[27] by formal phosphinidene transfer. We thus hypothesised that the combination of three equiv of Cp^3tAl with (ArP)₃ (Ar=Mes, Dip, Tip) would facilitate formation of Cp^3tAl=PAr, which might exist either in its monomer form or as a dimeric cyclo‐diphosphadialane (Scheme 3).

Scheme 3.

Reactivity of the alanediyl Cp^3tAl towards triphosphiranes (reactions i, ii, and iv) and a cyclo‐pentaphosphane (iii).

At first, we monitored the reaction of (MesP)₃ with three equiv of Cp^3tAl in C₆D₆ at room temperature. ^[28] This resulted in an initial colour change to orange and after a few minutes a yellow solution was obtained, which showed one signal in the ³¹P NMR spectrum at δ(³¹P)=−174.3 ppm and full consumption of (MesP)₃ was noted. In the ¹H NMR spectrum four characteristic signals were detected in the alkyl region, indicative of a Mes to Cp^3t ratio of 1:1. X‐ray quality crystals of this compound were grown from a saturated toluene solution at −30 °C and confirmed the formation of [Cp^3tAl(μ‐PMes)]₂ (1 a). It needs to be pointed out that 1 a precipitates from C₆D₆ solutions after 30 minutes, therefore precluding collection of satisfactory ¹³C NMR data. We therefore switched to more polar NMR solvents, but even using C₆D₅Br the compound could not be redissolved. We next turned to (DipP)₃ and (TipP)₃, with bulkier aryl groups. When combining the respective triphosphirane with three equiv of Cp^3tAl, a gradual colour change of the reaction mixture to orange was observed, accompanied by the precipitation of a microcrystalline red solid. In both cases broad, unresolved multiplet resonances at ca. −117 ppm were detected in the ³¹P NMR spectrum. Recrystallisation of the red microcrystalline solid from C₆H₆ (Ar=Dip) or slow evaporation of a saturated toluene solution (Ar=Tip) afforded X‐ray quality crystals, which showed that [Cp^3tAlPAr]₂ (Ar=Dip (1 b), Tip (1 c)) had formed, although in this case the first 1,2‐diphospha‐3,4‐dialuminacyclobutanes were obtained. This is reminiscent of the reactivity of two equiv of ^DipTerGa with TolN=NTol (Tol=4‐Me‐C₆H₄), giving the corresponding 1,2‐diaza‐3,4‐digallacyclobutanes [^DipTerGaNTol]₂. ^[29] This was rationalised by initial addition of ArGa to give a three‐membered N₂Ga ring system followed by insertion of a second ArGa, which then affords [^DipTerGaNTol]₂. Consequently, diphosphenes were considered to be potential intermediates in the formation of 1 b and 1 c. In a related study we have shown that (TipP)₂ reacted with Cp₂Ti(btmsa) to give the corresponding formal diphosphene complexes [Cp₂Ti(P₂Tip₂)] selectively. ^[22] Moreover, we have recently shown that the diphosphene DipP=PDip can be obtained in the PEt₃‐catalysed reductive coupling reaction of DipPBr₂ using Zn as a sacrificial reductant. ^[30] We therefore treated (DipP)₂ with two equiv of Cp^3tAl and noted an immediate colour change of the reaction mixture to deep orange, and the formation of 1 b was ascertained by detection of the broad ³¹P NMR resonance at δ(³¹P)=−117 ppm, as well as by cell determination of X‐ray quality crystals precipitating from the reaction mixture. Similarly to 1 a, comprehensive characterisation of 1 b and 1 c by multi‐nuclear NMR spectroscopy is hampered by its poor solubility in common NMR solvents, such as C₆D₆, C₇D₈, thf‐d₈ or even C₆D₅Br. LIFDI–MS studies showed the expected molecular ion peaks. 1 a–c crystallise in the triclinic space group P $\bar{1}$ with two molecules in the unit cell (Figure 1). ^[28] 1 a is situated on a crystallographically imposed centre of inversion, resulting in a central Al₂P₂ rectangle with alternating P and Al atoms and P−Al distances of 2.3176(7) and 2.3317(7) Å, respectively, which agrees well with the six‐membered species (Mes*AlPPh)₃ [cf. d _avg.(Al−P)= 2.328(3) Å]. ^[16] The angles at aluminium [90.11(2)°] and phosphorus [89.89(2)°] are nearly identical and the Mes groups on P are trans‐oriented with respect to the Al₂P₂ plane. The Al−C_Cp3t distances range from 2.2455(17) to 2.3584(18) Å, which renders the Cp^3t group η⁵‐coordinated. In contrast to 1 a, 1 b and 1 c show a puckered 1,2‐P₂‐3,4‐Al₂ four‐membered ring which is folded along the Al1⋅⋅⋅P2 axis by ca. 17°. The P−P [1 b 2.1677(9) Å; 1 c 2.1676(4) Å] bonds are contracted and closer to a single bond [cf. (DipP)₂ 2.0293(7); (TipPBr)₂ 2.2402(8)]. In contrast the Al−Al distances [1 b 2.6947(11) Å; 1 c 2.6933(5) Å] are rather long [cf. [Al(CH(SiMe₃)₂)₂]₂ 2.660(1) Å; ^[31] (Cp^3tAlBr)₂ ^[32] 2.586(3) Å, (^DipTerAl)₂(CSiMe₃)₂ ^[33] 2.4946(9) Å] and almost equidistant P1−Al1 and P2−Al2 bonds [1 b 2.4057(9), 2.4090(9) Å; 1 c 2.4090(5), 2.3977(5) Å] are detected within the ring. The Dip and Tip substituents are trans‐oriented with respect to the P₂‐unit [∡(C_Ar‐P‐P‐C_Ar) 1 b 94.31; 1 c 93.73°], which agrees well with the structure of [^DipTerGaNTol]₂ [∡(C_Tol‐N‐N‐C_Tol) 77.5°]. ^[29] The angles at P within the four‐membered ring are larger than 90° [1 b 94.96(3), 94.66(3); 1 c 93.617(13), 96.732(13)°], while the angles at Al are rather acute [1 b 82.58(3), 82.47(3); 1 c 81.798(13), 83.846(13)°].

Figure 1.

Molecular structures of 1 a, 1 b, and 1 c. ORTEPs drawn at 50 % probability. For clarity, all H atoms have been omitted and the alkyl groups on the Cp^3t, Dip, and Tip substituents have been rendered as wireframe. Selected bond lengths (Å) and angles (°) of 1 a: P2–Al2 2.3176(7), P2–Al2′ 2.3317(7); Al2‐P2‐Al2′ 90.11(2), P2‐Al2‐P2′ 89.89(2); 1 b: P1–P1 2.1677(9), Al1–P1 2.4057(9), Al2–P2 2.4090(9), Al1–Al2 2.6947(11); P2‐P1‐Al1 94.66(3), P1‐P2‐Al2 94.96(3), P1‐Al1‐Al2 82.58(3), P2‐Al2‐Al1 82.47(3); 1 c: P1–P1 2.1676(4), Al1–P1 2.4090(5), Al2–P2 2.3977(5), Al1–Al2 2.6933(5); P2‐P1‐Al1 93.617(13), P1‐P2‐Al2 96.732(13), P1‐Al1‐Al2 83.846(13), P2‐Al2‐Al1 81.798(13).

In order to investigate the bonding situation of these distinct three‐coordinate phosphorus–aluminium heterocycles, we conducted DFT calculations for 1 a and 1 b at the PBE0‐D3(BJ)/def2‐SVP level of theory. ^[34] For 1 a, the Mayer bond orders of the P−Al bonds are ca. 0.88, ^[35] indicating that no double bond character is found for these bonds, in agreement with the X‐ray data. Charge analyses reveal that the phosphorus atoms are partially negative, whereas the aluminium centres are positive. Accordingly, natural bond orbital (NBO) ^[36] and intrinsic bond orbital (IBO) ^[37] calculations indicate that the Al−P bonds are polarised to phosphorus (see Figure S41). The HOMO of 1 a is composed majorly by contributions at the phosphorus centres, while the LUMO is located mainly on the π system of the Mes substituents (Figure 2, left). At the PBE0‐D3(BJ)/def2‐SVP level of theory, the HOMO–LUMO gap of 1 a is 4.50 eV.

Figure 2.

Canonical Kohn–Sham molecular orbitals of 1 a and 1 b at the PBE0‐D3(BJ)/def2‐SVP level of theory. Isovalues: 0.03 a.u.

According to the Mayer bond order calculations, P−P (1.06) and Al−Al (0.86) single bonds are found in 1 b, supporting the attribution based on the crystal structures. The HOMO of 1 b is composed of a linear combination of the phosphorus lone pairs, while the LUMO is located at the π space of the Al−Al motif (Figure 2, right). The HOMO of 1 b is destabilised by ca. 0.7 eV in comparison to that of 1 a, while the LUMO is stabilised by ca. 0.5 eV. As a consequence, the HOMO–LUMO gap of 1 b (3.25 eV) is significantly smaller than that of 1 a. This indicates that the alternating Al₂P₂ heterocycle is more stable than its head‐to‐head counterpart if steric hindrance caused by the substituents is negligible, which is confirmed by further calculations (see below).

We then set out to test alkyl‐substituted cyclo‐oligophosphanes as phosphinidene source to determine the influence of the P substituent. When (PtBu)₃ ^[38] was treated with three equiv of Cp^3tAl (Scheme 3, reaction iii), a new species with an AX₂ spin system was detected in the ³¹P NMR spectrum (δ(³¹P)=76.1, −60.6 ppm; J _PP=210.6 Hz) and in the ¹H NMR spectrum unreacted Cp^3tAl was detected as well, indicating the formation of the four‐membered ring [Cp^3tAl(PtBu)₃] (2). This is in analogy to the formation of [Cp*Al(PtBu)₃] ^[39] and [(SiMe₃)₃CGa(PtBu)₃], ^[40] which are formed in a ring‐expansion reaction starting from (PtBu)₃ and the respective E(I) source. Using cyclo‐pentaphosphane (PPh)₅, the reaction with five equiv of Cp^3tAl gave rise to the formation of a new species with a ³¹P NMR signal at δ(³¹P)=−130.6 ppm and a Ph to Cp^3t ratio of 1:1 according to ¹H NMR spectroscopy (Scheme 3, reaction iv). The formulation as the 1,3‐diphospha‐2,4‐dialane [Cp^3tAl(μ‐PPh)]₂ (1 d) was corroborated by X‐ray analysis of crystals grown by slow evaporation of a saturated C₆H₆ solution of 1 d. The metrical parameters of 1 d are nearly identical with 1 a and the Ph rings are trans‐oriented with respect to the Al₂P₂ plane. The AlP₃ ring in 2 is minimally folded along the P1⋅⋅⋅P3 axis by ca. 10.7° with an all‐trans orientation of the tBu groups at phosphorus and an η⁵‐coordinated Cp^3t ring on aluminium. The P−Al distances [P1−Al1 2.3764(11), P3−Al1 2.3829(12)] are minimally longer than in [Cp*Al(PtBu)₃] [2.359(1), 2.360(1) Å], ^[39] indicating the influence of the sterically more demanding Cp^3t group. Consequently, the fold angle in 1 d is smaller than in Cp*Al(PtBu)₃ [18.7°]. ^[39]

Dipnictadialanes from Cp^*Al and (PAr)₃

We next investigated whether Cp*Al, generated from (Cp*Al)₄ at 80 °C,[ ^24a , ⁴¹ ] would show a reactivity like Cp^3tAl. Firstly, (PAr)₃ was combined with 0.75 equiv of (Cp*Al)₄ in C₆D₆ and the mixtures were heated to 80 °C overnight resulting in colourless solutions (Scheme 4). Analysis by ³¹P NMR spectroscopy revealed full conversion of the starting triphosphiranes and species with a singlet at δ(³¹P)=−208.2 (Ar=Mes), −230.6 (Ar=Dip) and −231.6 ppm (Ar=Tip) were detected. In the ¹H NMR spectrum one sharp signal for the Cp* group, indicating η⁵‐coordination, and signals for the aryl groups were detected in a 1:1 ratio. It needs to be noted that for 3 a–c the aryl groups can rotate freely on the NMR time‐scale, as evident from a minimal set of signals in the ¹H NMR spectrum (e.g., no splitting observed for the o‐Me groups (3 a) or the o‐iPr groups (3 b–c)). After evaporation of the solvent and extraction with n‐hexane, X‐ray quality crystals were obtained from concentrated filtrate solutions at −30 °C. In all three cases the base‐free 1,3‐diphospa‐2,4‐dialanes [Cp*Al(μ‐PAr)]₂ (Ar=Mes 3 a, Dip 3 b, Tip 3 c; Figure 3, left; Table 1) had formed. 3 a–c crystallise in the triclinic spacegroup P $\bar{1}$ with one molecule in the unit cell, with the Al₂P₂ ring being situated on a centre of inversion. In agreement with 1 a and 1 d, the central planar Al₂P₂ ring is a parallelogram with two distinct Al−P distances [3 a 2.3218(16), 2.3226(15); 3 b 2.3068(11), 2.3448(12); 3 c 2.3099(6), 2.3395(6) Å] and the angles at P [3 a 86.06(4); 3 b 89.00(3); 3 c 91.205(18)] and Al [3 a 93.94(4); 3 b 91.00(3); 3 c 88.796(19)] deviate minimally from 90°. The phosphorus atoms are trigonal pyramidally coordinated, even though a considerable degree of planarisation is observed according to the sum of angles at P [3 a 326.01; 3 b 334.60; 3 c 332.816°].

Scheme 4.

Reactivity of Cp*Al towards aryl‐substituted cyclotripnictanes giving [Cp*Al(μ‐PnAr)]₂ (Pn=P, 3 a–c; Pn=As, 4 a–b).

Figure 3.

Molecular structures of 3 c (left), 4 a (middle), and 4 b (right). ORTEPs drawn at 50 % probability. All H atoms have been omitted and the η⁵‐coordniation mode of Cp* is indicated by a dotted line from Al to the centroids of the Cp ring. Selected bond lengths (Å) and angles (°) are summarised in Table 1.

Table 1.

Selected bond lengths and angles of the Cp*‐substituted ring systems 3 a–c, 4 a, and 4 b.

	3 a	3 b	3 c	4 a	4 b
Pn1–Al1 [Å]	2.3218(16)	2.3068(11)	2.3099(6)	2.4106(8)	2.4160(5)
Pn1–Al1′ [Å]	2.3226(15)	2.3448(12)	2.3395(6)	2.4462(16)	2.4445(5)
Al1‐Pn1‐Al1 [°]′	86.06(4)	89.00(3)	91.205(18)	87.27(5)	86.990(15
Pn1‐Al1‐Pn1′ [°]	93.94(4)	91.00(3)	88.796(19)	92.73(5)	93.009(15)
Σ(∡Pn) [°]	326.01	334.60	332.816	328.51	324.49

Four‐membered Al₂As₂ heterocycles are rare and, for example, the butterfly‐shaped cyclic species [(Et₃N)ClAl(μ‐AsSi(CMe₂ iPr)Me₂)]₂ ^[42] and [(Me₃N)HAl(μ‐AsR)]₂ (R=SiiPr₃, SiMe₂(CMe₂ iPr)) ^[43] have been described in which the arsenic centre is three‐coordinate and the aluminium centre is four‐coordinate due to Lewis‐base adduct formation. Other examples with four‐coordinate Al and As centres have been synthesised by the Power group through the combination of the kinetically stabilised primary alane ^MesTerAlH₂ with liquid PhAsH₂ in the bulk phase, giving [^MesTer(H)Al−As(H)Ph]₂. ^[44] In analogy to the synthesis of 3 a–c we reasoned that aryl‐substituted cyclo‐triarsanes (AsAr)₃ (Ar=Dip, Tip) ^[23] would give base‐free cyclo‐diarsadialanes in the reaction with (Cp*Al)₄ and combination of both in a 4:3 ratio in C₆D₆ and heating to 80 °C overnight afforded colourless solutions that showed ¹H NMR spectra, which are similar to those of 3 b and 3 c. X‐ray quality crystals grown from saturated n‐hexane solutions at −30 °C showed that [Cp*Al(μ‐AsAr)]₂ (Ar=Dip 4 a, Tip 4 b) had indeed formed. 4 a and 4 b are colourless solids, which crystallise in the triclinic space group P $\bar{1}$ , with one molecule on a crystallographically imposed centre of inversion in the unit cell (Figure 3, middle, right; Table 1). Again, the central [Al(μ‐As)]₂ ring is best described as a parallelogram with different As−Al distances [4 a 2.4106(8), 2.4462(16); 4 b 2.4160(5), 2.4445(5)] and intra‐ring angles at arsenic smaller than 90° [4 a 87.27(5); 4 b 86.990(15)°] and wider angles on aluminium [4 a 92.73(5); 4 b 93.009(15)°], with the arsenic atoms being considerably planarised [Σ(∡As) 4 a 328.51; 4 b 324.49°]. This agrees well with the six‐membered species (Mes*AlAsPh)₃ reported by Power and co‐workers [cf. d _avg.(Al−As)=2.430(5) Å; Σ(∡As)=319.7(3.0)°]. ^[16]

Stability of Diphosphadialanes from DFT Calculations

In this section, we analyse the steric and electronic factors dictating the preferential formation of Al₂P₂ (alternating or head‐to‐head) and AlP₃ heterocycles based on thermodynamic arguments. All calculations were performed at the SMD(solvent=benzene) ^[45] /PBE0‐D3(BJ)/def2‐TZVP ^[34] level of theory from gas‐phase‐optimised structures at the PBE0‐D3(BJ)/def2‐SVP level (see SI for more details).

Our experiments revealed that, while an alternating Al₂P₂ ring is formed from Cp^3tAl and (PAr)₃ (Ar=Mes), a head‐to‐head Al₂P₂ structure is achieved if sterically more demanding Ar groups (Dip, Tip) are used. The relative free energies of 1 a, 1 b, and their unobserved isomers, [Cp^3tAlPMes]₂ and [Cp^3tAl(μ‐PDip)]₂, respectively, are shown in Figure 4 a. The head‐to‐head compound 1 b (Ar=Dip) is 27.7 kcal mol⁻¹ more stable than its alternating isomer [Cp^3tAl(μ‐PDip)]₂. This indicates that for sterically more demanding substituents, the thermodynamic reaction product is the head‐to‐head isomer, and its isomerization to the alternating structure is thermodynamically unfavoured. However, if sterically less demanding substituents at the phosphorus atoms are used, such as Mes, the alternating isomer becomes the thermodynamic product (for Ar=Mes, a free energy of 18.6 kcal mol⁻¹ favouring the alternating isomer is found). These results are in excellent agreement with the experimental findings, revealing that thermodynamic reasoning is already enough to predict the preference of head‐to‐head or alternating isomers during the course of the reaction. Another important experimental finding is that the reaction of Cp^3tAl with (PR)₃ (R=tBu) leads to an AlP₃ heterocycle, while an alternating Al₂P₂ system is formed if R=Mes. In order to explain this distinct reactivity profile, in Figure 4 b we compare the free energy of reaction leading to compounds [Cp^3tAl(μ‐PMes)]₂ (1 a, Scheme 3, reaction i), [Cp^3tAl(PtBu)₃] (2, Scheme 3, reaction iii), and to the corresponding species [Cp^3tAl(PMes)₃] and [Cp^3tAl(μ‐PtBu)]₂. Our results show that for both R groups, the alternating Al₂P₂ heterocycle is the thermodynamic product. However, in the case of R=Mes, a larger thermodynamic driving force for forming the alternating Al₂P₂ system is found, as the free energy difference between the two reactions is ca. 15 kcal mol⁻¹. Conversely, the free energy difference for R=tBu is merely 4 kcal mol⁻¹. These results indicate that steric factors are responsible for the distinct reactivity patterns observed for different R groups, with the sterically demanding tBu groups precluding the formation of alternating Al₂P₂ heterocycles by decreasing their stabilities in comparison to the AlP₃ system, presumably the first intermediate formed after interaction of Cp^3tAl and (PR)₃.

Figure 4.

Thermodynamic profiles of the distinct phosphorus–aluminium heterocycles depicted in this work. a) Comparison of the alternating [Cp^3tAl(μ‐PAr)]₂ and head‐to‐head [Cp^3tAlPAr]₂ structures for Ar=Mes, Dip. b) Comparison of the free energies of reaction forming the [Cp^3tAl(PR)₃] and [Cp^3tAl(μ‐PR)]₂ (R=tBu, Mes) heterocycles. c) Comparison of the alternating [R′Al(μ‐PDip)]₂ and head‐to‐head [R′AlPDip]₂ structures for R′=Cp^3t, Cp*. All energies are in kcal mol⁻¹.

Finally, we also performed DFT calculations to investigate the reactivity trends observed in our experiments if the Cp*Al species is considered. As shown in Scheme 4, if Cp*Al is used, the alternating [Cp*Al(μ‐PR)]₂ isomer is found for R=Mes, Dip, Tip, whereas for Cp^3t the head‐to‐head [Cp^3tAlPDip]₂ isomer is found for R=Dip, Tip. The relative free energies of the diphosphadialane systems with R=Dip and Cp^3tAl and Cp*Al fragments are shown in Figure 4 c. Our results indicate that while 1 b is preferred over its alternating isomer if R=Dip, reduction of the steric demands in the Al substituents inverts the free energy trends, with the head‐to‐head [(Cp*Al)₂(PDip)₂] isomer lying 13.1 kcal mol⁻¹ above 3 b. Similarly to the previous cases, steric demands on the substituents drastically influence the free energy trends of the corresponding heterocycles, ultimately dictating the reactivity profile of the diphosphadialane systems studied herein. The preferred products from the reactions depicted in this work are effectively predicted by thermodynamic reasoning.

Reactivity of Diphosphadialanes with Lewis Bases

Lewis‐base‐stabilised dipnictadialanes have been described,[ ¹⁵ , ⁴³ ] and as an entry, 1 d, with a rather small Ph group on the phosphorus atoms, was combined with two equiv of the NHC IiPr₂ (IiPr₂=(HCNiPr)₂C:). The ³¹P NMR spectrum showed one new species at δ(³¹P)=−123.3 ppm, which is minimally deshielded compared to 1 d and indicates formation of the bis‐NHC adduct [Cp^3t(IiPr₂)Al(μ‐PPh)]₂ (5, Scheme 5), which was corroborated by X‐ray analysis of crystals grown from slow evaporation of a C₆H₆ solution (Figure 5). The molecular structure revealed that IiPr₂ is coordinated to Al and in trans‐arrangement with respect to the Al₂P₂ ring resulting in a haptotropic shift from η⁵ to η¹ of the Cp^3t group. ^[46] This is reminiscent of the cluster compound [(Cp*Al(IMe₄))(μ,η³:η⁴‐P₅)FeCp*] with an η¹ Cp* group on aluminium, which displays an Al−C_NHC distance [2.017(6) Å] shorter than those of 5 [2.100(2), 2.083(2)]. ^[47] In the related compound [(IiPr₂)HAl(μ‐PSiPh₂ tBu)]₂ the NHCs are in a cis‐arrangement. ^[15] Surprisingly, one of the ring phosphorus atoms is now in a nearly planar coordination environment (Σ(∡P2)=347.87°), whereas the other is now closer to an ideal trigonal pyramidal coordination environment (Σ(∡P1)=299.83°). This is accompanied by deformation of the formerly planar near‐rectangular Al₂P₂ ring, which is now folded by ca. 16° along the P1⋅⋅⋅P2 axis giving a butterfly structure, in accord with [(IiPr₂)HAl(μ‐PSiPh₂ tBu)]₂. The Al−P bonds show a pair of longer [Al3−P4 2.4206(6), Al1−P4 2.4009(5) Å] and shorter [P2−Al1 2.3371(7), P2−Al3 2.3335(5) Å] bonds, with the shorter distances to the more planar phosphorus atom. The molecular structure would imply two chemically and magnetically distinct phosphorus atoms and, consequently, a set of two doublets in the ³¹P NMR spectrum. Thus, a solution of 5 in [D₈]toluene was cooled to −80 °C, which resulted in splitting of the singlet resonance at room temperature into two doublets at δ(³¹P)=−122.6 and −129.7 ppm (J _PP=67.1 Hz). This indicates rapid exchange in solution between the phosphorus positions at room temperature, but even at lower temperatures, as a significant deshielding of the planarised P atom would be expected.

Scheme 5.

Reaction of 1 d with the NCH IiPr₂, giving rise to the formation of the bis‐NHC adduct 5.

Figure 5.

Molecular structure of 5. ORTEPs drawn at 50 % probability, all H atoms omitted and alkyl substituents on Cp^3t and IiPr₂ rendered as wireframe. Selected bond lengths (Å) and angles (°) of 5: P2–Al1 2.3371(7), P2–Al3 2.3335(5), Al3–P4 2.4206(6), Al1–P4 2.4009(5), P2–C1 1.825(2), P4–C1 1.846(2), Al1–C1_4 2.084(1), Al3–C1_5 2.092(1), Al1–C1_6 2.100(2), Al3–C1_7 2.083(2); Al1‐P2‐Al3 91.49(2), P2‐Al3‐P4 87.98(2), Al3‐P4‐Al1 87.87(2), P4‐Al1‐P2 88.37(2), Al1‐P2‐C1 128.61(6), Al1‐P4‐C1 104.08(5).

Conclusion

Pnictaalumenes are characterised by alternating Lewis acidic group 13 and electron‐rich group 15 atoms, which results in a propensity to oligomerise. To date the corresponding cyclo‐dipnictadialanes have only been synthesised as their Lewis base adducts. Herein, we show that using cyclo‐tripnictanes (ArPn)₃ (Ar=Mes, Dip, Tip; Pn=P, As) in conjunction with Cp ^x Al (x=*, 3t) afforded the first examples of base‐free cyclo‐dipnictadialanes. With small aryl substituents on the pnictogen, Cp^3tAl (1 a, 1 d) and with Cp*Al (3 a–c, 4 a–b) in all cases the rings with alternating P and Al atoms [Cp ^x Al(μ‐PnAr)]₂ are thermodynamically favoured. Interestingly, the head‐to‐head‐connected 1,2‐diphospha‐3,4‐dialuminacyclobutanes, 1 b and 1 c, are preferred when both the substituents on phosphorus and aluminium are sterically demanding. This study clearly demonstrates (i) the potential of cyclo‐tripnictanes as building blocks to implement PnAr units into unusual small molecules and (ii) that base‐free cyclo‐dipnictadialanes are synthetically feasible by judicious choice of the substituents on aluminium and the pnictogen. Studies on the reactivity of the ring systems presented in here with respect to their potential to act as a source of the monomeric pnictaalumenes are currently underway.

Conflict of interest

The authors declare no conflict of interest.

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Supporting Information

S. Nees, F. Fantuzzi, T. Wellnitz, M. Fischer, J.-E. Siewert, J. T. Goettel, A. Hofmann, M. Härterich, H. Braunschweig, C. Hering-Junghans, Angew. Chem. Int. Ed. 2021, 60, 24318.