First evidence for two different mu-eta¹-eta¹- and mu-eta¹-eta²- co-ordination modes of the P3C2Bu t2 ring of [Fe(eta5-P3C2Bu t2)(eta5-C5H5)] to a same cluster fragment: synthesis and characterisation of [Ir4(CO)10{[Fe(eta5-P3C2Bu t2)(eta5-C5H5)}] and x-ray molecular structure of the mu-eta¹-eta²- isomer

Abstracts

Investigation of the solution structures of [Ir4(CO)11L] (L = [Fe(eta5-P3C2Bu t2)(eta5-C5H 5)] (1) and [Fe(eta5-P3C2Bu t2)(eta5-P2C 3Bu t3)] (2) by 13C and 31P NMR spectroscopy showed that, at 163K, 1 exists in the form of two isomers with bridged and non-bridged structures, in a 1:0.15 ratio, respectively, whereas 2 exists only in the bridged form. At RT, 1,2 shift of the eta5-P3C2Bu t 2 ring was only observed for compound 2. Where as 2 loses CO readily in solution to give [Ir4(CO)10{mu-eta¹-eta¹-[Fe(eta5-P3C2 Bu t2)(eta5-P2 C3Bu t3)}] (3), activation with Me3NO was necessary to produce [Ir4(CO)10{[Fe(eta5-P 3C2Bu t2)(eta5-C5H5)}] (4), obtained in the form of two non-interconverting isomers 4a and 4b, which were not able to be separated. A single crystal X-ray diffraction study of isomer 4a established that the [Fe(eta5-P3C2Bu t2)(eta5-C5H 5)] ligand bridges one of the edges of the Ir4 tetrahedron, interacting via the lone electron pair of one of the adjacent P atoms and in an eta²- mode via the P-P double bond of the eta5-P3C2Bu t 2 ring and that all CO ligands are terminally bonded. Variable temperature 31P{¹H} NMR spectroscopy evidenced a fluxional process involving interactions between the Ir1 and Ir2 atoms and the lone pair on P1, the P1-P2 bond, and the lone pair on P2. According to multinuclear NMR, cluster 4b has similar structure to compound 3, with the eta5-P3C2Bu t 2 ring co-ordinated in a eta¹-eta¹- mode via the two adjacent P atoms, and all CO ligands bonded terminally.

iridium cluster; fluxional process; tri and pentaphosphaferrocenes

A investigação das estruturas em solução dos compostos [Ir4(CO)11L] [L = [Fe(eta5-P3C2Bu t2) (eta5-C5H5)] (1) e [Fe(eta5-P3C2Bu t2)(eta5-P2C 3Bu t3)] (2)], por espectroscopia de RMN de 13C e de 31P, mostrou que, a 163 K, o composto 1 existe na forma de dois isômeros, um deles contendo carbonilas em ponte e o outro, carbonilas terminais, na razão de 1:0,15, respectivamente, enquanto que o composto 2 existe somente na forma do isômero contendo carbonilas em ponte. À temperatura ambiente, somente no composto 2 ocorre um deslocamento 1,2 do anel eta5-P3C2Bu t 2. Enquanto o composto 2 sofre fácil dissociação de CO em solução para dar [Ir4(CO)10{mi-eta¹-eta¹-[Fe(eta5-P3C2 Bu t2)(eta5-P2 C3Bu t3)}] (3), foi necessário usar Me3NO para produzir [Ir4(CO)10{[Fe(eta5-P 3C2Bu t2)(eta5-C5H5)}] (4) a partir de 1. Este composto foi obtido sob a forma de dois isômeros interconversíveis, 4a e 4b, que não puderam ser separados. Um estudo de difração de raios-X do isômero 4a mostrou que o ligante [Fe(eta5-P3C2Bu t2) (eta5-C5H5)] encontra-se ligado em ponte a uma das arestas do tetraedro de Ir4, interagindo através do par de elétrons livres de um dos átomos de fósforo adjacentes e de modo eta²- através da ligação dupla P=P do anel eta5-P3C2Bu t 2 e que todos os ligantes CO encontram-se ligados de modo terminal. Um estudo de RMN de 31P{¹H} a várias temperaturas evidenciou um processo fluxional que envolve as interações entre os átoms de Ir1 e Ir2 e o par de elétrons livres no átomo P1, a ligação P1-P2 e o par de elétrons livres no átomo P2. De acordo com os dados de RMN multinuclear, a estrutura do cluster 4b é semelhante à do composto 3, com o anel eta5-P3C2Bu t 2 coordenado de modo eta¹-eta¹- através dos dois átomos de P adjacentes e com todos os ligantes CO ligados de modo terminal.

Article

First Evidence for two Different m-h¹-h¹- and m-h¹-h²- Co-ordination Modes of the P₃C₂Bu^t₂ Ring of [Fe(h⁵-P₃C₂Bu^t₂)(h⁵-C₅H₅)] to a Same Cluster Fragment: Synthesis and Characterisation of [Ir₄(CO)₁₀{[Fe(h⁵-P₃C₂Bu^t₂)(h⁵-C₅H₅)}] and X-ray Molecular Structure of the m-h¹-h²- Isomer

Maria Helena Araujo^a,b, Peter B. Hitchcock^a, John F. Nixon^*a and Maria D. Vargas^*b

^aSchool of Chemistry, Physics and Environmental Science, University of Sussex, Brighton, BN1 9QJ, UK.

^b Instituto de Química, Universidade Estadual de Campinas, CP 6154, 13083-970, Campinas - SP, Brazil.

A investigação das estruturas em solução dos compostos [Ir₄(CO)₁₁L] [L = [Fe(h⁵-P₃C₂Bu ^t₂) (h⁵-C₅H₅)] (1) e [Fe(h⁵-P₃C₂Bu ^t₂)(h⁵-P₂C ₃Bu^t₃)] (2)], por espectroscopia de RMN de ¹³C e de ³¹P, mostrou que, a 163 K, o composto 1 existe na forma de dois isômeros, um deles contendo carbonilas em ponte e o outro, carbonilas terminais, na razão de 1:0,15, respectivamente, enquanto que o composto 2 existe somente na forma do isômero contendo carbonilas em ponte. À temperatura ambiente, somente no composto 2 ocorre um deslocamento 1,2 do anel h⁵-P₃C₂Bu^t ₂. Enquanto o composto 2 sofre fácil dissociação de CO em solução para dar [Ir₄(CO)₁₀{m-h¹-h ¹-[Fe(h⁵-P₃C₂ Bu^t₂)(h⁵-P₂ C₃Bu^t₃)}] (3), foi necessário usar Me₃NO para produzir [Ir₄(CO)₁₀{[Fe(h⁵-P ₃C₂Bu^t₂)(h⁵-C₅H₅)}] (4) a partir de 1. Este composto foi obtido sob a forma de dois isômeros interconversíveis, 4a e 4b, que não puderam ser separados. Um estudo de difração de raios-X do isômero 4a mostrou que o ligante [Fe(h⁵-P₃C₂Bu ^t₂) (h⁵-C₅H₅)] encontra-se ligado em ponte a uma das arestas do tetraedro de Ir₄, interagindo através do par de elétrons livres de um dos átomos de fósforo adjacentes e de modo h²- através da ligação dupla P=P do anel h⁵-P₃C₂Bu^t ₂ e que todos os ligantes CO encontram-se ligados de modo terminal. Um estudo de RMN de ³¹P{¹H} a várias temperaturas evidenciou um processo fluxional que envolve as interações entre os átoms de Ir₁ e Ir₂ e o par de elétrons livres no átomo P₁, a ligação P₁-P₂ e o par de elétrons livres no átomo P₂. De acordo com os dados de RMN multinuclear, a estrutura do cluster 4b é semelhante à do composto 3, com o anel h⁵-P₃C₂Bu^t ₂ coordenado de modo h¹-h¹- através dos dois átomos de P adjacentes e com todos os ligantes CO ligados de modo terminal.

Investigation of the solution structures of [Ir₄(CO)₁₁L] (L = [Fe(h⁵-P₃C₂Bu ^t₂)(h⁵-C₅H ₅)] (1) and [Fe(h⁵-P₃C₂Bu ^t₂)(h⁵-P₂C ₃Bu^t₃)] (2) by ¹³C and ³¹P NMR spectroscopy showed that, at 163K, 1 exists in the form of two isomers with bridged and non-bridged structures, in a 1:0.15 ratio, respectively, whereas 2 exists only in the bridged form. At RT, 1,2 shift of the h⁵-P₃C₂Bu^t ₂ ring was only observed for compound 2. Where as 2 loses CO readily in solution to give [Ir₄(CO)₁₀{m-h¹-h ¹-[Fe(h⁵-P₃C₂ Bu^t₂)(h⁵-P₂ C₃Bu^t₃)}] (3), activation with Me₃NO was necessary to produce [Ir₄(CO)₁₀{[Fe(h⁵-P ₃C₂Bu^t₂)(h⁵-C₅H₅)}] (4), obtained in the form of two non-interconverting isomers 4a and 4b, which were not able to be separated. A single crystal X-ray diffraction study of isomer 4a established that the [Fe(h⁵-P₃C₂Bu ^t₂)(h⁵-C₅H ₅)] ligand bridges one of the edges of the Ir₄ tetrahedron, interacting via the lone electron pair of one of the adjacent P atoms and in an h²- mode via the P-P double bond of the h⁵-P₃C₂Bu^t ₂ ring and that all CO ligands are terminally bonded. Variable temperature ³¹P{¹H} NMR spectroscopy evidenced a fluxional process involving interactions between the Ir₁ and Ir₂ atoms and the lone pair on P₁, the P₁-P₂ bond, and the lone pair on P₂. According to multinuclear NMR, cluster 4b has similar structure to compound 3, with the h⁵-P₃C₂Bu^t ₂ ring co-ordinated in a h¹-h¹- mode via the two adjacent P atoms, and all CO ligands bonded terminally.

Keywords: iridium cluster, fluxional process, tri and pentaphosphaferrocenes

Introduction

Tri-, penta- and hexaphosphorus analogues of ferrocene ligands [Fe(h⁵-P₃C₂Bu ^t₂)(h⁵-R)] (R = P₂C₃Bu^t₃ , P₃C₂Bu^t₂ and C₅R'₅, R' = H or Me) have been reported in recent years¹. To date two types of co-ordination modes of these phosphaferrocene ligands, I and II, have been reported in the literature, as illustrated below.

Bonding mode I, consisting of h¹-co-ordination via the phosphorus lone pair of one of the adjacent P atoms, was observed in several species, e.g. [M(CO)₅{h¹-[Fe (h⁵-P₃C₂Bu^t ₂)(h⁵-C₅R₅)]}] (R = H, M = W; R = Me, M = Mo, W or Cr)^2,3,4 and [W(CO)₅{h¹-[Fe(h⁵-P ₃C₂Bu^t₂) (h⁵-R)]}] (R = P₂C₃Bu^t₃ or P₃C₂Bu^t₂ )⁵, whereas bonding mode II, involving h²-ring edge ligation through the P-P multiple bond was only found in [Rh(h⁵-C₅Me₅)CO{h ²-[Fe(h⁵-P₃C₂ Bu^t₂)(h⁵-C₅ H₅)]}]⁶. Although the interactions of dinuclear and cluster compounds with phosphaferrocenes have been much less investigated, their additional bonding capabilities have led to a number of other modes of interactions as shown below.

Thus, bonding mode I, found in [Ru₃(CO)₁₁{h¹-[Fe( h⁵-P₃C₂Bu^t ₂)(h⁵-C₅Me₅)]}] ⁷, was also observed in [Ru₃(CO)₁₀{m-h¹:h ¹-[Fe(h⁵-P₃C₂ Bu^t₂)₂]}]⁸ in which the two rings co-ordinate to adjacent ruthenium atoms. Bonding modes III and IV in which both lone pairs on adjacent P ring atoms are used was encountered in [Ni(CO)₂{h¹-h¹-[Fe( h⁵-P₃C₂Bu^t ₂)(h⁵-C₅Me₅)]}] ₂⁴ and [Ir₄(CO)₁₀{m-h¹-h ¹-[Fe(h⁵-P₃C₂ Bu^t₂)(h⁵-P₂ C₃Bu^t₃)]}] (3)⁹, respectively. Bonding mode V of two adjacent P atoms to three metal centres involving symmetrical side-on and end-on co-ordination has only been observed in [Ru₃(CO)₉{m-h²-[Fe( h⁵-P₃C₂Bu^t ₂)(h⁵-C₅Me₅)]}] ⁷, and bonding mode VI, involving an asymmetric m-h¹-h²- interaction of two adjacent P atoms, was reported for [Fe₂(CO)₇{m-h¹-h ²-[Fe (h⁵-P₂C₃(OSiMe ₃)₂(SiMe₃)(h⁵ -C₅H₃Bu^t₂ )]}]¹⁰.

We recently described the Ir₄ mono-substituted derivatives [Ir₄(CO)₁₁L] [L = [Fe(h⁵-P₃C₂Bu ^t₂)(h⁵-C₅H ₅)] (1) and [Fe(h⁵-P₃C₂Bu ^t₂)(h⁵-P₂C ₃Bu^t₃)] (2)] which were isolated in yields up to 90 % from NBu₄[Ir₄(CO)₁₁Br] by bromide abstraction with silver salts in the presence of L⁹. Their ³¹P NMR spectra were particularly informative and established different behaviour in solution. Whereas at room temperature the pentaphosphaferrocene derivative 2 exhibited a rapid 1,2 shift of the Ir₄ fragment between adjacent P atoms, as previously observed for [W(CO)₅{h¹-[M(h⁵-P ₃C₂Bu^t₂)(h ⁵-P₂C₃Bu^t ₃)]}] (M = Fe or Ru)¹¹, the room temperature spectrum of the triphosphaferrocene species 1 revealed the three different environments of the P-ring atoms. Furthermore, compound 2 was shown to undergo facile CO dissociation in solution with formation of [Ir₄(CO)₁₀{m-h¹-h ¹-[Fe(h⁵-P₃C₂ Bu^t₂) (h⁵-P₂C₃Bu^t ₃)]}] 3. Under similar conditions, however, the analogous compound 1 was found to be stable. These structural and reactivity differences were rationalised in terms of the slightly different basicities of the two phosphaferrocene ligands. In this paper we describe the Me₃NO induced transformation of compound 1 into two isomers of [Ir₄(CO)₁₀{Fe(h⁵-P ₃C₂Bu^t₂)(h ⁵-C₅H₅)}] (4a and 4b) in which the [Fe(h⁵-P₃C₂Bu ^t₂)(h⁵-C₅H ₅)] ligand exhibits different co-ordination modes VI and IV, respectively. We also present further solution characterisation of compounds 1, 2 and 3 by NMR spectroscopy.

Results and Discussion

Synthesis of [Ir₄(CO)₁₀{Fe(h⁵-P ₃C₂Bu^t₂)(h⁵-C₅H₅)}] (4a and 4b)

Thermolysis of the triphosphaferrocene derivative 1 in toluene at 60 ^oC for 48 h did not lead to the product analogous to the known species 3. Activation of dark orange compound 1 was only achieved by treatment with an equivalent amount of Me₃NO, at -78 ^oC, in CH₂Cl₂ and then by allowing the solution to warm up slowly to room temperature. The resulting brown product (78% yield after purification by TLC) was later identified by ³¹P{¹H} NMR spectroscopy to be a mixture of two isomeric forms of the expected product (4a:4b ~1:0.07). These isomers only differ slightly with respect to the co-ordination mode of the triphosphaferrocene and could not be separated by TLC under a variety of conditions. The mixture was characterised by analytical and spectroscopic data (Table 1 and Experimental). Crystals of isomer 4a were obtained from CH₂Cl₂/hexane at 4^o C and a single crystal X-ray analysis was carried out.

Molecular structure of [Ir₄(CO)₁₀{Fe(h⁵-P ₃C₂Bu^t₂) - (h⁵-C₅H₅)}] (4a)

The molecular structure of compound 4a is shown in Figure 1. Selected bond lengths and angles are given in Table 2.

The structure confirms the bonding mode of the h⁵-P₃C₂Bu^t ₂ ring to the cluster proposed on the basis of the ³¹P NMR data (Table 1) discussed below. The four iridium atoms exhibit a distorted tetrahedral Ir₄ core with Ir-Ir bond lengths lying in the range 2.665(1)-2.723(1) Å and the longest Ir-Ir edge [Ir(1)-Ir(2) 2.723(1) Å] spanned asymmetrically by the 4-electron donor [Fe(h⁵-P₃C₂Bu ^t₂)(h⁵-C₅H ₅)] ligand via the two adjacent P atoms. All the carbonyl ligands are terminally bonded. As in the other rare tetrahedral Ir₄ clusters containing only terminal CO ligands^12-14, the average Ir-Ir distance of 2.6945 Å is relatively short, comparable with the average Ir-Ir distance of 2.693 Å in the parent binary carbonyl [Ir₄(CO)₁₂]¹⁵. The Ir-C and C-O bond distances are in the normal range of values found for terminal CO groups. The marked differences in the Ir-P bond lengths [Ir(2)-P(1) 2.252(5) Å, Ir(1)-P(1) 2.455(5) Å and Ir(1)-P(2) 2.517(5) Å] can best be explained in terms of a h¹- interaction of P(1) with Ir(2) and a h²- interaction of the P(1)-P(2) bond with Ir(1) atom, as described in bonding mode VI. The Ir(2)-P(1) distance is slightly shorter than Ir-P bond lengths found in other triphosphaferrocene containing iridium clusters, i.e. [Ir₄(CO)₁₁{Fe(h⁵-P ₃C₂Bu^t₂)(h⁵-C₅H₅)}] 2.345(4) Å and [HIr₄(CO)₁₀{[Fe(h⁵ -P₃C(CMe₂CH₂)(CBu ^t)(h⁵-C₅H₅)} [Ir₄(CO)₁₁] [2.230(7) and 2.318(7) Å] ⁹, although it is within the values observed in Ir₄ clusters containing phosphines^9,16,17. The Fe atom maintains its h⁵- interaction with the P₃C₂Bu^t₂ ring [Fe-P(2) 2.350(6) and Fe(3) 2.328(6) Å], although the additional h²-side-on co-ordination of the P₃C₂Bu^t₂ ring leads to a significant P(1)-P(2) bond lengthening from 2.114(1) Å in the free ligand ⁶ to 2.268(8) Å in the title compound, as previously observed for the other complex with similar interaction, i.e. [Fe₂(CO)₇{m-h¹-h ²-[Fe(h⁵-P₂C₃ (OSiMe₃)₂(SiMe₃)(h⁵-C₅H₃Bu^t ₂)]}]¹⁰ [2.281(4) Å] or even for other compounds with analogous types of interaction, e.g. [Ru₃(CO)₉{m-h² -[Fe(h⁵-P₃C₂Bu ^t₂) (h⁵-C₅Me₅)]}] ⁷ [2.131(2) Å], [Rh(h⁵-C₅Me₅)(CO) {h²-[Fe(h⁵-P₃C ₂Bu^t₂)(h⁵-C ₅H₅)}]⁶ [2.306(2) Å] and [Ir(h⁵-C₅Me₅)(CO){ h²-[Fe(h⁵-P₅)(h ⁵-C₅Me₅)]}]¹⁸ [2.359(2) Å].

Solution behaviour of compounds 3, 4a, and 4b.

A variable temperature ³¹P{¹H} NMR study of the mixture containing 4a and 4b in CD₂Cl₂/CS₂ (Figure 2) revealed that compound 4a is fluxional in solution and also established that 4a and 4b do not undergo inter-conversion.

At 298 K the P nuclei of 4a appeared as broad signals at d 58 and between d -25 and 25. At 168 K a pattern of lines corresponding to an [AMX] spin system emerged indicating that there were three distinct phosphorus sites (Table 1). The large separation between the P₁ and P₂ resonances of 288 ppm and a decrease in the ¹J(P₁P₂) from 457 Hz in compound 1 to 351 Hz in 4a are in accord with the ³¹P NMR data reported for [Fe₂(CO)₇{m-h¹-h ²- [Fe(h⁵-P₂C₃(OSiMe ₃)₂(SiMe₃)(h⁵ -C₅H₃Bu^t₂ )]}]¹⁰ for which same bonding mode was established in the solid state and in solution. As shown in Figure 3, the lowest chemical shift observed in the spectrum of this iron compound was attributed to P₁.

The fluxional process exhibited by isomer 4a can be explained in terms of a rapid 1,2 shift of the Ir₁ and Ir₂ atoms between P₁ and the P₁-P₂ bond to the adjacent P₂ and P₁-P₂ bond, respectively, as illustrated in Figure 4. It is interesting to note that compound [Fe₂(CO)₇{m-h¹-h ²-[Fe(h⁵-P₂C₃ (OSiMe₃)₂(SiMe₃)(h ⁵-C₅H₃Bu^t ₂)]}]¹⁰ does not exhibit same fluxional behaviour.

The room temperature ³¹P{¹H} NMR spectrum of the mixture indicated that compound 4b (Figure 2 and Table 1) is very similar to [Ir₄(CO)₁₀{Fe(h⁵-P ₃C₂Bu^t₂) (h⁵-P₂C₃Bu^t ₃)}] 3⁹. It shows the equivalence of the two adjacent phosphorus nuclei of the h⁵-P₃C₂Bu^t ₂ ring, P₁ and P₂, which appear as a doublet at relatively low frequency, thus indicating that the [Fe(h⁵-P₃C₂Bu ^t₂) (h⁵-C₅H₅)] ligand is co-ordinated to the Ir₄ fragment via these two P atoms, as described in bonding mode IV. It is interesting that neither compound 4b nor compound 3 exhibit fluxionality of the phosphaferrocene ligand.

In the ¹³C{¹H} NMR spectrum of the mixture of 4a and 4b containing ¹³CO enriched carbonyl ligands in CD₂Cl₂/CS₂, at 168K, (Table 1) only signals due to terminal carbonyls of compound 4a were observed. Therefore the ground state geometry of isomer 4a is similar to the structure in the solid state. Due to the very low concentration of isomer 4b in the mixture, it was not possible to identify all carbonyl resonances of this isomer in the ¹³C{¹H} NMR spectra of the mixture, even at 168 K. An indication that this compound contains only terminal CO ligands came from the IR spectrum of the 4a and 4b mixture in hexane (Experimental), which only exhibited terminal n_CO bands, and also from the ¹³C{¹H} NMR spectra of the analogous ¹³CO enriched 3. At room temperature, broad resonances between d 170 and 155 could be spotted, but upon cooling the CD₂Cl₂/CS₂ solution of 3 to 168K, the 10 resonances due to terminal CO ligands were observed (Table 1).

The fact that in solution all three compounds 3, 4a, and 4b have a ground state geometry with all CO ligands terminal is rather interesting and merits a few considerations. Indeed, all [Ir₄(CO)₁₀(m-L-L)] clusters (L-L = diphosphine ligands) known to date exhibit the alternative ground state geometry with three edge bridging COs defining the basal plane of the metal tetrahedron^13,19. As has been pointed out before¹², the derivatives with unbridged structure always exhibit shorter Ir-Ir bonds than those with bridged structure. Consequently it might be that formation of a four-membered Ir-Ir-P-P ring upon co-ordination of the adjacent P atoms of the phosphaferrocene ligand in compounds 3 and 4a leads to a preference for the structure with short Ir-Ir bonds over the alternative bridged structure, whereas generation of larger rings with five or more members in diphosphine containing clusters might be responsible for the stabilisation of the bridged structure. Ring size is certainly not the only determining factor in the stabilisation of one of these structures over the other. Indeed, although co-ordination of L-L = Ph₂PC(H)MePPh₂ and MeSC(H)MeSMe in [Ir₄(CO)₁₀(m-L-L)] generates five- membered rings in both cases, X-ray diffraction and VT ¹³C NMR studies have established that the diphosphine compound exhibits the bridged structure, whereas the dithioethane derivative has the alternative unbridged structure, both in solution and in the solid state¹⁴.

Solution characterisation of compounds 1 and 2.

In our previous paper⁹ we reported that the solid-state structure of compound 1 exhibited only terminally bonded carbonyl ligands. However, its solution IR showed the presence of terminal and bridging CO ligands, implying that this compound existed as a mixture of the bridging and non-bridging isomers. A VT ³¹P{¹H} NMR study indicated that the [Fe(h⁵-P₃C₂Bu ^t₂)(h⁵-C₅H ₅)] ligand was not fluxional at room temperature and that at 165 K, the two isomers were present in solution in a 1:0.15 ratio.

In order to find out which was the highest concentration isomer, a variable-temperature ¹³C{¹H} NMR study of an enriched ¹³CO sample of compound 1 was undertaken in CD₂Cl₂/CS₂. At room temperature, broad resonances between d 170-165 and d 158-155 and a singlet at d 154 were observed, indicating fluxionality of the CO ligands. As shown in Table 1, the spectrum obtained at 165K exhibited the eleven resonances of the two isomers and established that the predominant isomer in solution is the bridging isomer.

In contrast, the low temperature ¹³C{¹H} NMR spectrum of an enriched ¹³CO sample of compound 2 (Table 1) only showed the presence of eleven CO ligands of the bridging isomer, thus indicating that this compound exists in solution only in the bridging form.

Conclusions

It is clear that the nature of the h⁵-C₅H₅ and h⁵-P₂C₃Bu^t ₃ rings in the L = [Fe(h⁵-P₃C₂Bu ^t₂)(h⁵-C₅H ₅)] and [Fe(h⁵-P₃C₂Bu ^t₂)(h⁵-P₂C ₃Bu^t₃)] ligands, respectively, influences markedly: i) the relative stabilities of the [Ir₄(CO)₁₁L] compounds and the activation energies for the 1,2 shift of the P₃C₂Bu^t₂ ring in the two species, the pentaphosphaferrocene derivative 2 being far more labile and reactive than cluster 1, and in a subtle way, the CO ligands distribution in the co-ordination sphere of these clusters, and ii) the mode of interaction of the P₃C₂Bu^t₂ ring in the [Ir₄(CO)₁₀(m-L)] clusters. Our results suggest that the h⁵-C₅H₅ ring exerts an inductive effect on the h⁵-P₃C₂Bu^t ₂ ring, rendering the [Fe(h⁵-P₃C₂Bu ^t₂)(h⁵-C₅H ₅)] ligand more basic than the pentaphosphaferrocene ligand, which explains the higher activation energy needed for CO loss for 1, compared to 2, and also the fact that electronic density along the P-P bond in [Fe(h⁵-P₃C₂Bu ^t₂)(h⁵-C₅H ₅)] becomes available for co-ordination and competes with the lone pairs of the adjacent P atoms. Thus, whereas in compound 3 the pentaphosphaferrocene ligand co-ordinates via the adjacent P lone pairs in a h¹-h¹- mode, in the main product (1:0.07) from CO loss in 1, 4a, the P₃C₂Bu^t₂ ring interacts in a h¹-h²- mode.

Experimental

All manipulations and reactions were performed under an atmosphere of dry argon, unless otherwise specified, by using Schlenk-type glassware, syringe and high vacuum-line techniques, with glassware dried in vacuum prior to use. Solvents were dried, degassed and redistilled before use, CH₂Cl₂ was dried over CaH₂, hexane over sodium wire and thf over sodium and benzophenone. The compound [Ir₄(CO)₁₁{Fe(h⁵-P ₃C₂Bu^t₂)(h ⁵-C₅H₅)}] 1 was prepared as described previously⁹, and Me₃NO.3H₂O was sublimed in vacuum. Purification of the products was carried out by preparative TLC (1 mm thickness glass-backed silica plates 20 x 20 cm, silica gel type GF₂₅₄, Fluka) using CH₂Cl₂/hexane (1:4 v/v) as eluent and the compound was extracted from silica with CH₂Cl₂. Compounds [Fe(h⁵-P₃C₂Bu ^t₂)(h⁵-C₅H ₅)]⁶ and [Fe(h⁵-P₃C₂Bu ^t₂) (h⁵-P₂C₃Bu^t ₃)]²⁰ were synthesised according to the literature. Samples of ¹³CO enriched clusters, prepared from NBu₄[Ir₄(CO)₁₁Br] ²¹, were used for the ¹³C NMR experiments. All Ir₄-clusters were stored under an inert atmosphere to avoid decomposition observed in some of the compounds stored for long time in the solid state.

Solution NMR spectra were recorded on a Bruker DPX 300 or AC 300P. Standard pulse sequences were used for the NMR experiments. They were carried out in CD₂Cl₂/CS₂ (CS₂d 191.75) solutions. Chemical shifts are given in ppm and coupling constants (J) in Hz. Deuterated solvents were used as lock and reference (¹H NMR relative to the proton resonance resulting from incomplete deuteration of the CD₂Cl₂ (5.32); ¹³C NMR relative to the carbon of the CD₂Cl₂ (53.7) and for ³¹P NMR external 85 % H₃PO₄). Infrared spectra were recorded on a Bomen (FT-IR Michelson) spectrophotometer scanning between 2200 and 1600 cm^-1 (n_CO) using a CaF₂ liquid cell.

Synthesis of [Ir₄(CO)₁₀{Fe(h⁵-C ₅H₅)(h⁵-P₃C ₂Bu^t₂)}] 4a and 4b.

An orange solution of 1 (200 mg, 0.14 mmol) in CH₂Cl₂ (30 mL) was cooled to - 78 ^oC and treated with a CH₂Cl₂ solution (2 mL) of Me₃NO (10.5 mg, 0.14 mmol). The reaction mixture was allowed to warm to room temperature, and the solution concentrated in vacuo. Separation of the products by TLC afforded 4a and 4b as a mixture (150 mg, 0.11 mmol, 78 %) and unreacted 1 (30 mg, 0.02 mmol, 14 %) along with some decomposition (base line on the TLC plates). IR n_max/cm^-1 (CO) 2085.2vs, 2053.5vs, 2034.7vs, 2027.8vs, 2006.7m, 1991.2s and 1968.3m (hexane). ¹H-NMR (25 ^oC, 300 MHz): 4a d 4.9 (s, 5H, Cp) and 1.6 (s, 18H,^tBu); 4b d 5.2 (s, 5H, Cp) and 1.2 (s, 18H, ^tBu).

Solution characterization of 1.

¹³C{¹H}-NMR spectrum except CO region (25 ^oC, 75.43 MHz): d 76.1 (s, 5C, Cp), 38.8 [dd, 1C, ²J(CP) 17.4, ³J(CP) 5.8 Hz, CMe₃], 38.3 [dd, 1C, ²J(CP) 15.3, ³J(CP) 4.4 Hz, CMe₃], 35.8 [dd, 3C, ³J(CP) 10.9, ³J(CP) 4.4 Hz, CCH₃], 35.5 [t, 3C, ³J(CP) 9.4 Hz, CCH₃].

Crystallographic data for 4a.

C₂₅H₂₃FeIr₄O ₁₀P₃: M = 1401.0; monoclinic; space group P2₁/n; a = 9.779(2), b = 14.396 (5), c = 22.783(6) Å; b = 96.32(2)^o; U = 3188(2) ų; Z = 4; D_calc. = 2.92 Mg/m³; crystal dimensions 0.4 x 0.4 x 0.3 mm; F(000) = 2528; T = 173(2) K; Mo-Ka radiation l = 0.71073 Å. Data were collected on an Enraf-Nonius CAD4 diffractometer and of the total 5331 independent reflections measured, 4320 having I > 2s(I) were used in the calculations. The structure was solved by direct methods and refined by full matrix least square on all F². The final indices (I > 2s(I)) were R₁ = 0.068, wR₂ = 0.180.

Acknowledgements

We acknowledge financial support from the Commission of European Communities (J.F.N. and M.D.V.), Conselho Nacional de Desenvolvimento Científico e Tecnológico - CNPq (M.D.V) and Fundação de Amparo à Pesquisa do Estado de São Paulo - FAPESP (M.H.A).

Supplementary Information

Crystallographic data (excluding structure factors) for the structures in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos. CCDC 140420. Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, (fax: [phone] or e-mail: [e-mail].)

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Received: February 15, 2000

FAPESP helped in meeting the publication costs of this article.

Publication Dates

• Publication in this collection
  17 Nov 2000
• Date of issue
  Aug 2000

History

• Received
  15 Feb 2000