Triple-decker sandwich complexes with a bent cyclo-P 5 middle-deck †

New types of triple-decker complexes with an organo-substituted P5 middle-deck were synthesized by the reaction of [Cp*Fe(g-P5R)] (1a: R = CH2SiMe3; 1b: R = NMe2) with halogeno-bridged transition metal dimers [Cp0 0 MX]2 (M = Cr, Fe, Co, Ni; X = Cl, Br). By oxidation of [(Cp*Fe)(Cp0 0 Co)(l,g-P5CH2SiMe3)] 2a with [Cp2Fe][PF6], the cationic complex [(Cp*Fe)(Cp0 0 Co)(l,g-P5CH2SiMe3)] + was isolated. The electronic structure of the synthesized complexes was elucidated by DFT calculations.

Ferrocene is one of the most frequently used organometallic reagents in chemistry, with very broad applications. [1][2][3] This 65 year old molecule 4,5 does not only show a fascinating chemistry in redox processes but particularly as a starting material for subsequent reactions. Its ability to be metalated 6,7 or to undergo Friedel-Craft reactions 8,9 renders it a valuable starting material in organometallic synthesis. The isolobal analogue of ferrocene is pentaphosphaferrocene and its Cp* derivative [Cp*Fe(Z 5 -P 5 )] (I) was first discovered in 1987. 10 The majority of reactivity studies of I is dedicated to the coordination chemistry towards Lewis acidic coordination moieties, forming 1D and 2D coordination polymers [11][12][13][14][15] or spherical supramolecular clusters. [16][17][18][19][20] In cothermolysis or cophotolysis reactions with organometallic reagents, fragmentations and deformations of the cyclo-P 5 ring of I occur. [21][22][23][24] A new direction for the reactivity of pentaphosphaferrocene opened up when I was used in redox processes [25][26][27] and especially when it was converted by nucleophiles. 28 In the latter case, a selective functionalization of the P 5 ring in I was achieved. As a result, monoanionic complexes of the type [Cp*Fe(Z 4 -P 5 R)] À (R = CH 2 SiMe 3 , NMe 2 , PH 2 ) were isolated, leading to new perspectives in the chemistry of I. These monoanionic complexes raised the question, whether reacting them with electrophiles leads to a reformation of the initial cyclo-P 5 ring (by the retention of the former substitution), or if a rearrangement takes place to give products with novel structures. Moreover, so far only few tripledecker complexes exhibiting a cyclo-P 5 middle deck are known. Starting from P 4 in thermolysis reactions, the compounds [(Cp BIG Mn) 2 (m,Z 5:5 -P 5 )] (Cp BIG = C 5 (4-nBuC 6 H 4 ) 5 ) and [(Cp R Cr) 2 -(m,Z 5:5 -P 5 )] (Cp R = Cp, Cp*) are obtained. [29][30][31] Starting from I, some cationic triple-decker compounds [(Cp*M)(Cp R M 0 )(Z 5:5 -P 5 )] + (M, M 0 = Fe, Ru; Cp R = Cp, Cp*), containing group 8 elements, have been reported. 32,33 Furthermore, mixed-metal lanthanideiron compounds with a cyclo-P 5 middle-deck 26 and triple-decker complexes consisting of I and a [M(CO) 3 ] fragment (M = Cr, Mo, W) are known. 34 Herein, we report the synthesis of the first neutral complexes with a functionalized P 5 middle-deck under mild conditions. By using different transition metal halides [Cp 0 0 0 MX] 2 (M = Cr, Fe, Co, Ni; X = Cl, Br), a broad variety of different tripledecker complexes are easily accessible. Their bonding situation has been investigated by quantum chemical computations.
The reaction of 1a/1b with the transition metal dimers [Cp 0 0 0 MX] 2 (M = Cr, Fe, Co, Ni; X = Cl, Br) leads to the tripledecker complexes 2-5, containing the whole 3d element series from Cp 0 0 0 Cr to Cp 0 0 0 Ni decks (Scheme 1). Unfortunately, despite several efforts, we were not able to synthesize the missing manganese containing triple-decker complex in this series. 35  the minor influence of the substituent of the cyclo-P 5 ring on the features of the SOMO (Fig. 1). 36 All SOMOs are delocalized, but the analysis of the spin density reveals that the metal centre bonded to the Cp 0 0 0 ligand exhibits the highest spin density. The calculated atomic spin densities of 2a/2b show that the Co atom possesses the highest positive spin density (about 62%), followed by the Fe atom (about 19%). The 31 P NMR spectrum of the diamagnetic nickel/iron tripledecker complex 3 shows an AXX 0 ZZ 0 spin system, with one triplet of triplets centered at 40.6 ppm and two multiplets centered at À30.29 and À53.9 ppm. For the iron/iron tripledecker complex 4, the 1 H NMR spectrum shows only sharp signals. However, in contrast to the triple-decker complex 3, in the 31 P NMR spectrum of 4 one sharp signal at 73.8 ppm, one broad signal at À131.1 ppm and one very broad signal at À150.8 ppm are observed at room temperature. By cooling down the sample to 193 K, five broad signals in a 1 : 1 : 1 : 1 : 1 integral ratio are monitored in the 31 P NMR spectrum, centered at 65.8, 42.2, À104.5, À195.0 and À344.9 ppm, revealing a dynamic behavior of the P 5 ring. In this process, the phosphorus atoms adjacent to the substituted P atom coordinate alternatingly to the Cp*Fe fragment, and the Cp 0 0 0 Fe fragment slips over the middle-deck. At 193 K, the signals of the Cp 0 0 0 ligand in the 1 H NMR spectrum of 4 become very broad, showing that the free rotation of the cyclopentadienyl ligand is slowed down.
When 1a is reacted with [Cp 0 0 0 CrCl] 2 , the 31 P NMR spectrum of the reaction solution shows a broad doublet of doublets at 281.1, one broad doublet at 9.5 ppm and a triplet of triplets at À66.7 ppm for the chromium/iron triple-decker complex 5. Also, one set of signals of an unidentified byproduct is observed (in about 13%), but 5 can be isolated and purified further by recrystallization. By cooling down the sample of 5 in the 31 P NMR spectrum at 253 K, the signals become sharp and a fine structure is determined. 36 The simulation of this spectrum reveals unusual coupling constants: the 1 J P-P coupling (P2-P3) between the P 2 dumbbell and the P 3 allylic moiety is remarkably small (22.65 Hz). This is consistent with the corresponding elongated P-P distance (vide infra). The 2 J P-P coupling (P1-P3) is comparably large with a value of about 100 Hz. Usually the absolute value of a 1 J P-P coupling is significantly higher compared to a 2 J P-P coupling, as it is observed in 3 or the starting materials 1a/1b. This unusual behaviour may originate from the orbital interaction between the phosphorus atoms via the metal centres (as it is seen in the HOMOÀ3, 36 which has contributions from the atomic orbitals of the P1, Cr and Fe atom, respectively, and the orbital of the P3-P4 unit).
Compounds 2-5 could all be characterized by X-ray structure analysis. The triple-decker complexes 2a, 2b and 3 exhibit a similar structural motif, in which the P 5 ligand adopts a Z 4 :Z 3 -coordination mode (Fig. 2). Compared to 1a/1b, in 2a/2b and 3 the enveloped conformation of the P 5 unit becomes more distinct, with three of the five phosphorus atoms (P1, P3, P4; labeling according to Fig. 2) coordinating to the Cp 0 0 0 metal fragment. The phosphorus atom, which does not lie in the Z 4 -P 4 plane, still bears the organic rest. In 1a/1b, all P-P bonds    The elongation of this P-P bond, by going from 2a to 3, is in line with the increased electron density in the P 5 moiety (compound 2a features a CH 2 SiMe 3 group exhibiting a +I effect; compound 2b features an NMe 2 group exhibiting +M effect; compound 3 contains one additional electron due to the exchange of the cobalt with a nickel atom).  36 By comparing 2a, 2b and 3 with the iron/iron complex 4, the conformation of the P 5 ring changes (Fig. 3). Due to a distortion of the phosphorus ring, the Fe2-P5 bond (2.6313(5) Å) is prolonged (remaining Fe-P bond lengths in 4: 2.1362(5)-2.4690(5) Å, Fig. 3), thus the coordination mode of the P 5 ligand is best described as Z 4 :Z 4 . Furthermore, the enveloped P 5 ring of 4 is bent towards the Cp*Fe fragment -instead towards the Cp 0 0 0 M fragment, as observed in the triple-decker sandwich complexes 2a,b. The phosphorus atom, which bears the dimethylamine rest, lies within a distorted Z 4 -P 4 plane. In comparison to 2a, 2b and 3, the P2-P3 (2.3587(6) Å) and P4-P5 (2.3187(6) Å) bonds are considerably longer in 4 and the P3-P4 bond exhibits double bond character in 4 (2.1054(6) Å).
The iron/nickel and iron/iron containing complexes 3 and 4 are stable and formally only differ by one electron in comparison to the iron/cobalt containing triple-decker complexes 2a/2b. Therefore, the electrochemical properties of 2a/2b were investigated. The cyclic voltammogram of 2a in CH 2 Cl 2 shows two oxidations and one reduction (Fig. 1). The first oxidation occurs at a half potential of À0.79 V and exhibits a reversible character (i p(reverse) /i p(forward) = 0.82). 39 The second oxidation at 0.39 V is considered irreversible. At À1.67 V, a reversible reduction is observed (i p(reverse) /i p(forward) = 0.98). The cyclic voltammogram of 2b exhibits similar features, 36 with one reversible oxidation at À0.88 V (i p(reverse) /i p(forward) = 0.98) and a following irreversible one at 0.39 V. 39 A reversible reduction is observed at À1.61 V (i p(reverse) /i p(forward) = 0.97).
Based on these studies, we chose the oxidizing agents [Cp 2 Fe][PF 6 ] for the chemical oxidization of 2a, which has a half potential of À0.59 V against fc/fc + in MeCN. 40 Contrary to the expectation that [(Cp*Fe)(Cp 0 0 0 Co)(m,Z 4:3 -P 5 CH 2 SiMe 3 )] + (6) should be diamagnetic in analogy to 4, in the 31 6 ] were obtained from a Et 2 O solution (Scheme 1). The X-ray structure analysis reveals that [6] + is not just isoelectronical to the triple-decker complex [(Cp*Fe)(Cp 0 0 0 Fe)(m,Z 4:4 -P 5 NMe 2 )] (4), but that 2a undergoes a structural rearrangement during the oxidation, resulting in [6] + to be isostructural to 4 (Fig. 4). The phosphorus atom, which bears the organic rest and was out of the Z 4 -P 4 plane in 2a, interchanges hereby the position with an unsubstituted phosphorus atom from the Z 4 -P 4 plane. Unfortunately, we did not succeed in isolating any reduced products of 2a or 2b, by using K or KH as reducing agents despite many attempts.
In summary, we showed a subsequent chemistry of the anionic functionalized pentaphosphaferrocenes, by reacting  them with transition metal halide dimers. That way, several unique neutral triple-decker sandwich complexes with unprecedented functionalized cyclo-P 5 middle-decks were obtained. The integrity of the initial cyclo-P 5 middle-deck depends strongly on the electronic situation of the coordinating metal fragments, which leads from a structural rearrangement of the enveloped P 5 moiety in 2a/2b, 3 and 4 to a complete fragmentation of the P 5 ring, as seen for [(Cp*Fe)(Cp 0 0 0 Cr)(m,Z 4:5 -P 5 CH 2 SiMe 3 )] (5). In addition, the triple-decker complexes 2a and 2b show interesting electrochemical properties and a change of conformation of the P 5 moiety is observed upon oxidation. The successful salt elimination of the anionic pentaphosphaferrocene derivatives opens new avenues for the chemistry of pentaphosphaferrocene. Further functionalization of the P 5 ring should now be possible, which will lead to transfer reactions of the P 5 moiety or to the isolation of uncoordinated organo-substituted phosphorus derivatives.