Regioselective Insertion of Aluminum(I) in the cyclo‐P5 Ring of Pentaphosphaferrocene

Abstract A route to directly access mixed Al–Fe polyphosphide complexes was developed. The reactivity of pentaphosphaferrocene, [Cp*Fe(η 5‐P5)] (Cp*=C5Me5), with two different low‐valent aluminum compounds was investigated. The steric and electronic environment around the [AlI] centre are found to be crucial for the formation of the resulting Al–Fe polyphosphides. Reaction with the sterically demanding [Dipp‐BDIAlI] (Dipp‐BDI={[2,6‐iPr2C6H3NCMe]2CH}−) resulted in the first Al‐based neutral triple‐decker type polyphosphide complex. For [(Cp*AlI)4], an unprecedented regioselective insertion of three [Cp*AlIII]2+ moieties into two adjacent P−P bonds of the cyclo‐P5 ring of [Cp*Fe(η5‐P5)] was observed. The regioselectivity of the insertion reaction could be rationalized by isolating an analogue of the reaction intermediate stabilized by a strong σ‐donor carbene.

The discovery of ferrocene [Cp 2 Fe] (Cp = h 5 -C 5 H 5 ) in 1951 led to a fundamental change in organometallic chemistry. [1] The isolobal analogy between Cp À and cyclo-P 5 À steered interest in using cyclo-P 5 À as a ligand for the synthesis of sandwich-type complexes. [2] In a seminal report in 1987, Scherer and Brück synthesized pentaphosphaferrocene, [Cp*Fe(h 5 -P 5 )] (Cp* = C 5 Me 5 ), by co-thermolysis of white phosphorous and [Cp*Fe(CO) 2 ] 2 . [3] Scheer and co-workers have engaged in using [Cp*Fe(h 5 -P 5 )] as a tool to access inorganic supramolecules and polymers by taking advantage of the phosphorus lone pairs on the cyclo-P 5 ring. [4] Apart from using [Cp*Fe(h 5 -P 5 )] in inorganic polymer chemistry, understanding the reactivity of [Cp*Fe(h 5 -P 5 )] towards nucleophiles and different reducing agents has also attracted recent attention. [5] The redox properties of [Cp*Fe(h 5 -P 5 )] were studied by cyclic voltammetry [6] and synthetically. [5a-c,e] It is interesting to explore the scope of air-stable starting materials as sources for poly-pnictogen species as alternatives to the conventionally used and highly reactive P 4 . [7] Recently, we have shown that the cyclo-P 5 ring of [Cp*Fe(h 5 -P 5 )] could be used as a polyphosphorous source. The reaction of [Cp*Fe(h 5 -P 5 )] with [LSiCl] (L = PhC(N t Bu) 2 ) resulted in the sila-phosphaferrocene, [h 4 -P 4 SiL-FeCp*], via substitution of one P atom by an isoelectronic [LSi] fragment. [8] Recently, the organometallic chemistry of mono-valent aluminum compounds, which was pioneered in the 1990s, [9] has witnessed renewed interest, [10] which can be attributed to their ability to activate small molecules and organic substrates featuring single, double, or triple bonds. [9l, 11] The reactivity of [Al I ] is not limited to organic substrates: main-group elements, such as S 8 , [12] Se, [9b] and Te, [9b] have been used to make aluminum heterocyclic complexes. Monovalent aluminum complexes have also been used to access rare Al-P cages and clusters by reducing white phosphorous. [12a, 13] In general, phosphorous containing heterocyclic compounds can be prepared by derivatization of the highly reactive P 4 cage. We were challenged to examine the reactivity of air stable [Cp*Fe(h 5 -P 5 )] with [Al I ] complexes to obtain Al polyphosphorous complexes. The reactivity pattern of [Al I ] is known to be highly dependent on the type of ligands used to stabilize the monovalent aluminum centre. [9l, 11a,c,d, 14, 15] Therefore, using different electronic and steric environments on [Al I ] complexes may lead to different types of activation of [Cp*Fe(h 5 -P 5 )], such as conformational changes or controlled fragmentation of the cyclo-P 5 ring.
Herein, we report on the reactivity of [Cp*Fe(h 5 -P 5 )] with two different monovalent aluminum complexes. We have isolated the first examples of Al-Fe-based neutral tripledecker polyphosphides. Also, the insertion of three [Cp*Al III ] 2+ moieties into PÀP bonds led to the isolation of an unprecedented Al-Fe polyphosphide complex containing four metal centres. The possible intermediate for the insertion of [Cp*Al III ] 2+ moieties in the cyclo-P 5 ring was trapped by using a nucleophilic carbene.
The reaction between equimolar amounts of [Cp*Fe(h 5 -P 5 )] and [Dipp-BDIAl I ] [9j] in toluene at room temperature resulted in the formation of [(Dipp-BDIAl III )(m,h 3 :h 4 -P 5 )FeCp*] (1) in 35 % yield (Dipp-BDI = {[2,6i Pr 2 C 6 H 3 NCMe] 2 CH} À ; Scheme 1). During the reaction, the aluminum atom is oxidized to give [Al III ] while [Cp*Fe(h 5 -P 5 )] is reduced twice. As a result, the cyclo-P 5 ring loses its 6 pelectron aromaticity, resulting in a conformational change from planar to envelope-shaped. [5b,e] The 1 H NMR spectrum of 1 showed one single resonance for the Cp* methyl protons, shifted downfield from d = 1.08 ppm (in [Cp*Fe(h 5 -P 5 )]) to d = 1.22 ppm. Also, three new broad resonances at d = 98.4, 60.7, and 32.1 ppm were observed in the 31 P{ 1 H} NMR spectrum at room temperature, suggesting a fluxional behaviour of the cyclo-P 5 ring. A well-resolved 31 P{ 1 H} NMR spectrum could be recorded at À40 8C showing an AA'MXX' spin system with multiplets at d = 97.2 (P XX' ), 60.6 (P M ), and 30.7 (P AA' ) ppm apparent for the formation of an envelope conformation of the cyclo-P 5 ring ( Figure 1 and Table S1 in the Supporting Information). The molecular structure of 1 in the solid state revealed the formation of a triple-decker type complex with a bent cyclo-P 5 ring ( Figure 2). To our knowledge, 1 is the first example of an Al-containing neutral triple-decker heterometallic polyphosphide complex. The Al À P1 (2.3231 (15) ) and Al À P4 (2.465-(2) ) bond lengths are in the reported range of Al À P single bonds (2.308(2) to 2.422(2) ), [12a, 13] whereas the Al-P5 separation (2.784 (2) ) is relatively long indicating only a weak coordination. [16] The P2ÀP3 (2.1647 (14) ) and P3ÀP4 (2.186(2) ) bond lengths are shorter than the P1 À P2 (2.223-(2) ), P1 À P5 (2.2145 (14) ) and P4 À P5 (2.2735 (14) ) analogues, which is a result of the elongation of P À P bonds upon coordination to aluminum. This is in line with the theoretically calculated shared electron numbers (SEN) given in the Supporting Information. A similar trend has been observed in a samarium polyphosphide complex.  [17] This anomalous trend in reactivity can be mainly attributed to the higher reductive ability of [Al I ] complexes as compared to [Ga I ] and [Tl I ] analogues.
As the reactivity of monovalent aluminum complexes highly depends on the coordination environment around the aluminum centre (see above), we decided to examine the reactivity of another low-valent aluminum complex, that is, [(Cp*Al I ) 4 ]. [9a,b] The reaction between [(Cp*Al I ) 4 ] and [Cp*Fe(h 5 -P 5 )] in toluene resulted in the formation of complex [(m 3 -P)(Cp*Al III ) 2 {P 4 (Al III Cp*)} (FeCp*)] (2) irrespective of the stoichiometric ratio and the reaction conditions. The solid-state structure of 2 confirmed the formation of an unprecedented Al-Fe polyphosphide complex containing four metal centres ( Figure 3). Formally, the cyclo-P 5 -ring has been six-fold reduced by three equivalents of [Cp*Al I ] forming one P 4 4À and one P 3À unit, which are charged balanced by three [Cp*Al III ] 2+ and one [Cp*Fe] + moiety. However, theoretical calculations show an electron distribution, which is more complex (see below). The [4+1] fragmentation of the cyclo-P 5 ring is very rare. [18] The reaction of [LSi-SiL] with [Cp*Fe(h 5 -P 5 )] showed a similar fragmentation, however, in this case, a seven-membered Si-P ring, [h 4 -P 5 (SiL) 2 -FeCp*], was obtained. [8] On the basis of the identity of 2, the yield of the complex could be increased to 47 % by using the optimised conditions (Scheme 2). The reaction mixture needs to be heated for 7 days at 80 8C to ensure the purity of complex 2. We have noticed that during prolonged heating all the minor sideproducts decompose and precipitate from the toluene solution, hence facilitating the isolation of complex 2 in a pure form. As illustrated in Figure 3, one [Cp*Fe] + unit is h 4coordinated to one cyclo-P 4 (AlCp*) moiety, where the average Fe À P bond length is slightly longer than that in [Cp*Fe(h 5 -P 5 )] (2.317 vs. 2.273 , respectively). [5b] The cyclo-P 4 (AlCp*) unit is bound in a h 4 -mode to [Cp*Fe] + as well as h 2 -coordinated to two [Cp*Al III ] 2+ units. In addition, a P atom   is bound to the ring. The Cp*-rings bind in a h 5 -fashion to Al1 and Al2 whereas Al3 is only h 3 -coordinated. Al1 and Al2 also bind in h 2 -mode to the cyclo-P 4 (AlCp*) moiety and are further coordinated to the terminal P5. The average Al À P5 bond length (2.316(2) ) is in the usual range of Al À P single bonds (2.308(2) to 2.422(2) ), [12a, 16] as are the Al3ÀP1 and Al3ÀP4 bonds. However, the Al1ÀP1 (2.476(2) ), Al1ÀP2 (2.676(2) ), Al2ÀP3 (2.678(2) ), and Al2ÀP4 (2.517(2) ) bonds are longer than usual Al À P single bonds, which suggests a weaker coordination. [13, 12a, 16] In addition, there are weak Al-Al interactions with short Al-Al separations (Al1 À Al3 (2.911(2)) and Al2ÀAl3 (2.919 (2) ). [13] The average PÀP bond length (2.191(2) ) in the cyclo-P 4 (AlCp*) unit is shorter than a PÀP single bond, indicating the presence of a partial double-bond character. [12a, 16] In contrast to the non-equivalent character of the three aluminum centres of 2 in the solid state, only two sharp singlets at d = 2.16 ppm (45 H, Cp* on Al atoms) and d = 1.30 ppm (15 H, Cp* on Fe atom) were observed in the 1 H NMR spectrum of 2 at room temperature suggesting a fluxional behaviour in solution of the Cp* ligands bound to Al. Surprisingly, the 31 P{ 1 H} NMR spectrum of 2 showed only two singlets at d = 73.4 ppm (cyclo-P 4 -(AlCp*)) and d = À202.9 ppm, respectively. No P-P coupling pattern for the cyclo-P 4 (AlCp*) unit was observed even at low temperatures, although a broadening and splitting of the signals was noticed with decreasing temperatures (Figure S11, Supporting Information).
The regioselectivity of the insertion reaction of [Cp*Al III ] 2+ in two adjacent PÀP bonds may arise by formation of the proposed intermediate 2 i (Scheme 2). The intermediate 2 i has an envelope-shaped cyclo-P 5 ring, in which the PÀP bonds, out of the planar P 4 fragment  (Figure 4). The Al centre is h 3 -coordinated to the cyclo-P 5 ring and h 1 -coordinated to the Cp* ring. The AlÀP bond lengths are similar to those in complex 1. The Al À C1(carbene) (2.017 (6) ) bond length is in line with previous reports. [19] The 1 H NMR spectrum of complex 3 (203 K) features only one singlet for the [AlCp*] methyl protons, indicating a fluxional behaviour in solution. The 31 P{ 1 H} NMR spectrum (203 K) showed five sets of multiplets at d(ppm) = À96.2, À58. 6, 53.2, 103.6, and 138.7 corresponding to the envelope-shaped cyclo-P 5 ring (details in Section 3.5 in the Supporting Information).
To obtain a better view in the energetics of the system under discussion, theoretical DFT calculations were performed (technical details are given in the Supporting Information). The results are as follows: Although probably resulting from combined steric and electronic effects, the larger energy gain in the formation of 1, in contrast to hypothetical 2 i, is qualitatively made plausible on the basis of the calculated localized MOs with prominent Al À P bonding character (see Figures S31 and S32 in the Supporting Information). For 1, two Al À P bonds in the range around 2.4 are found, while only one AlÀP bond is found for 2 i. Shared electron numbers (SEN) as reliable measures for covalent bonding obtained from Ahlrichs-Heinzmann population analyses [20] confirm these findings. The reaction step of 2 i with two equivalent units of [Cp*Al I ] leading to 2 is highly exothermic (À139.6 kJ mol À1 ) due to the formation of a stable Al/P framework. The high two-centre SEN(Al-P) and three-centre SEN(Al-Al-P) confirm this hypothesis and can explain the unexpectedly short Al-Al distances. A comparable situation has been intensively investigated in the compound [P 4 (AlCp*) 6 ]. [13] The highly exothermic reaction of hypothetical 2 i and the carbene ITMe leading to 3 (À111.5 kJ mol À1 ) is explained by substantial electron transfer away from the carbene (0.48 electron) accompanied by the formation of a strong AlÀC bond (SEN(Al-C) = 1.04).
In summary, we have studied the reactivity of [Cp*Fe(h 5 -P 5 )] with monovalent aluminum complexes. In case of monomeric [Dipp-BDIAl I ], a neutral triple-decker Al-Fe polyphosphide complex 1 was obtained. Complex 1 consists of a formally di-reduced anion [(m,h 3 :h 4 -P 5 )FeCp*] 2À coordi-nated to a [Dipp-BDIAl III ] 2+ moiety. In contrast, the reaction between tetrameric [(Cp*Al I ) 4 ] and [Cp*Fe(h 5 -P 5 )] in a molar ratio of 3:4 resulted in an unprecedented Al-Fe polyphosphide cluster 2 containing four metal atoms, which is formed by the regioselective insertion of three [Cp*Al III ] 2+ moieties in the cyclo-P 5 ring of [Cp*Fe(h 5 -P 5 )]. The formation of intermediate 2 i could explain the [4+1] fragmentation of the cyclo-P 5 ring. The possible intermediate for the insertion product was stabilized by using a strong s-donor carbene, resulting in the Al-Fe triple-decker type polyphosphide 3. Noticeably, these findings highlight the role of supporting ligands and donor groups in the reduction chemistry of polyphosphide systems. In addition, this work establishes a route to directly access Al containing heterometallic polyphosphide complexes.