Tris(μ₂-carbonyl)octacarbonyl(triphenylphosphite)tetracobalt

Abstract

In toluene solution, the reaction of [Co₂(CO)₈] with an equimolar amount of [P(OPh)₃] yields first [Co₂(CO)₇{P(OPh)₃}] 1. Following heating of 1, an alternative synthetic access to the tetranuclear cluster [{Co₄(μ₂-CO)₃(CO)₈{P(OPh)₃}] 2 is provided in a condensation reaction. Compound 2 has been characterized by IR and ³¹P NMR spectroscopy. The tetranuclear cluster framework has been ascertained by a single-crystal X-ray diffraction study performed at 100 K.

1. Introduction

The tetrahedral carbonyl cluster [Co₄(μ₂-CO)₃(CO)₉] was first described by Hieber in 1932 and is commonly prepared by decarbonylation of dicobaltoctacarbonyl [1,2]. This 60-electron cluster, which contains both terminal and bridging carbonyls, has been the subject of several crystallographic investigations [3,4] and has served as the starting material for a number of organic (for example, the formation of arene clusters [Co₄(CO)₉(arene)]) and inorganic transformations [2,5,6]. Among the latter reaction, several papers have been devoted to the kinetics and mechanistic aspects of substitution reactions with various phosphine PR₃, diphosphine (such as bis(diphenylphosphino)amine dppa and bis(diphenylphosphino)methane dppm), and phosphite P(OR)₃ ligands have been published [7,8,9,10,11,12,13]. Some examples of structurally characterized mono- and di-substituted derivatives [Co₄(μ₂-CO)₃(CO)_9-n(PR₃)_n] (n = 1, 2) are depicted in Scheme 1. There is also a report on the crystal structure of [Co₄(CO)₁₀(PMe₂Pr)₂] with one PR₃ ligand at the apical and a second one at the axial position (see Figure 4) [14].

It is worth noting that P(OMe)₃ and P(OPh)₃ can even form tetra-substituted clusters [Co₄(CO)₈{P(OR)₃}₄] [8]. In the context of our research on P(OPh)₃-substituted Co-Co carbonyl complexes towards alkynes producing dicobaltatetrahedranes [15,16], we attempted to synthetize the monosubstituted dinuclear complex [Co₂(CO)₇{P(OPh)₃}] 1 by adding a stoichiometric amount of P(OPh)₃ to a solution of [Co₂(CO)₈]. The existence of this substitution product has been mentioned in the literature, but apart from its IR spectrum, no further characterization data have been communicated [17]. We repeated this reaction under similar conditions with the goal of isolating this species, but upon heating, we isolated the title compound [Co₄(μ₂-CO)₃(CO)₈{P(OPh)₃}] 2 by serendipity as the major component.

2. Results

Marko et al. first mentioned the title compound 2 in 1975, and it was obtained by treating [Co₄(CO)₉(arene)] with triphenylphosphite at 20 °C under CO atmosphere, along with [Co₄(CO)₁₀{P(OPh)₃}₂] and [Co₄(CO)₉{P(OPh)₃}₃] [5]. There was no additional characterization data presented aside from a detailed IR analysis in solution. To prepare the compound [Co₂(CO)₇{P(OPh)₃}], we first treated a solution of [Co₂(CO)₈] in toluene at ambient temperature in a 1:1 ratio (Scheme 2). The formation of 1 (2086, 2034, 1999, and 1978 cm⁻¹) was revealed by IR monitoring as well as traces of [Co₂(CO)₆{P(OPh)₃}₂] (1978 cm⁻¹, very strong). Formation of this dinuclear bisphosphite complex is also corroborated by an NMR ³¹P{¹H} analysis performed on a sample of the reaction mixture, which shows a singlet at 167.3 ppm [18]. To complete the reaction, the mixture was then heated for 5 h at 60 °C. Surprisingly, the IR bands attributed to 1 had disappeared and replaced by novel ones at 2089, 2050, 2042, 2032, 2012 and 1882, 1850, and 1839 cm⁻¹, the latter being in the characteristic region of bridging carbonyl ligand. The formation of minor amounts of [Co₄(CO)₁₀{P(OPh)₃}₂] is suggested by a CO stretching vibration at 2073 cm⁻¹ in the IR spectrum, in accordance with value reported by Marko et al. [5].

After workup, a product in the form of dark crystals was isolated, and elemental analysis revealed a composition of [Co₄(CO)₁₁{P(OPh)₃}]. The IR spectrum of this moderate air-stable product in cyclohexane, shown in Figure 1, reveals that in addition to the ν(CO) vibrations at 2088, 2049, 2043, 2032, and 2011 cm⁻¹, three further absorptions at 1885, 1856, and 1842 cm⁻¹, are attributed to bridging carbonyls. These values fit well with those reported in heptane by Marko et al. [5]. The infrared band pattern is similar to that reported for [Co₄(μ₂-CO)₃(CO)₈(PPh₃)] in heptane, the CO vibrational frequencies being slightly shifted to higher wavenumbers due to the weaker electron-donating propensity exerted by P(OPh)₃ with respect to PPh₃.

The proton-decoupled ³¹P-NMR recorded in CDCl₃ reveals a strongly broadened singlet at δ 130.4 due to the coordinated triphenylphosphite ligand, suggesting a fluxional behavior in solution (Figure 2). In line with this hypothesis, we were unable to identify at ambient temperature distinct carbonyl resonances in the proton-decoupled ¹³C NMR spectrum despite long data acquisition overnight. Only a broad hump centered at about δ 196 could be observed for the 11 carbonyl groups (Figure 3). It should be noted that no ³¹P or ¹³C NMR data recorded at ambient temperature on related PR₃ and P(OR)₃ clusters are available in the literature. There is only one report on [Co₄(CO)₁₁{P(OMe)₃}] at low temperature using ¹³C-enriched CO, which allows for the differentiation between bridging and terminal carbonyls [19].

In order to check whether [{Co₄(μ₂-CO)₃(CO)₈{P(OPh)₃}] 2 is isostructural to [{Co₄(μ₂-CO)₃(CO)₈(PPh₃)], we examined the product by an X-ray diffraction study performed at 100 K. Indeed, cluster 2 crystallizes like its PPh₃ analogue in the monoclinic crystal system but has been refined with space group P2₁/c instead of the P2₁/n employed for the latter. Darensbourg and Incorva proposed that a monosubstituted [Co₄(μ₂-CO)₃(CO)₈L] skeleton can have a priori three isomeric motifs (Figure 4) [11].

As shown in Figure 5, the tetrahedral nido-shaped cluster (according to the Wade–Mingos rules) [20,21] exhibits an isomer III-type structure since the phosphite ligand occupies an axial site with respect to the basal triangular plane formed by Co1, Co2, and Co3. The Co1 center, which bears the P(OPh)₃ ligand is also ligated by one terminal CO ligand and shares two symmetrically bridging carbonyls with the adjacent Co2 and Co3 centers. Co2 and Co3 in turn bear each of the two terminal Cos and have one shared edge bridged by a μ₂-CO ligand. The fourth vertex of the tetrahedral core is composed of the Co4 fragment, bearing exclusively three terminal COs. The Co1–P bond is quite colinear with the Co1–Co4 vector, the angle Co4–Co1–P being 166.108(11)°. Overall, the molecular structures of 2 and [{Co₄(μ₂-CO)₃(CO)₈(PPh₃)] are very similar, the corresponding Co–Co–PPh₃ angle of 174.98(4)° being slightly more linear.

Table 1. Relevant bond lengths (Å) in 2 and crystallographically characterized monosubstituted [Co₄(μ₂-CO)₃(CO)₈(PR₃)] clusters.

	P(OPh)3	PPh3	PMe3
Co1–Co4	2.5037(4)	2.542(1)	2.532(2)
Co1–Co2	2.4937(4)	2.491(1)	2.485(2)
Co1–Co3	2.4568(3)	2.487(3)	2.474(2)
Co2–Co3	2.4494(3)	2.468(1)	2.449(2)
Co2–Co4	2.5153(3)	2.523(1)	2.529(2)
Co3–Co4	2.5129(3)	2.526(1)	2.530(2)
Average Co–Co	2.4886	2.5062	2.4998
Basal-basal	2.4666	2.482	2.469
Basal-apical	2.5106	2.530	2.530
Co1–P	2.1546(4)	2.246(1)	2.222
Co1-μC19	1.9181(12)	1.908(4)	1.926
Co3-μC19	1.9610(12)	1.976(4)	1.989
Co2-μC24	1.9365(11)	1.929(4)	1.970
Co3-μC24	1.9436(12)	1.951(4)	1.918
Co1-μC21	1.9153(12)	1.887(5)	1.852
Co2-μC21	1.9712(11)	1.971(5)	1.927
Average Co-μC	1.941	1.937	1.930
Co1-C20	1.7828(13)	1.758(5)	1.677
Co2-C22	1.8000(13)	1.794(5)	1.759
Co2-C25	1.7928(13)	1.778(5)	1.736
Co3-C23	1.8033(12)	1.789(7)	1.849
Co3-C29	1.7852(12)	1.776(5)	1.760
Co4-C26	1.8147(12)	1.832(5)	1.606
Co4-C27	1.8315(13)	1.827(5)	1.800
Co4-C28	1.8325(14)	1.822(5)	1.779
Average Co-C	1.805	1.797	1.746
Apical	1.826	1.827	1.728
Equatorial	1.7869	1.771	1.724
Axial	1.8017	1.792	1.804
CSD reference	This work	BAFFET [11]	MSTCOB [12]

compares the most relevant bond lengths of 2 to those of [Co₄(μ₂-CO)₃(CO)₈(PR₃)]. As a result, the mean Co–Co bond length of 2 is slightly shorter than that of its PPh₃ and PMe₃, analogues, as well as that reported for its parent compound [Co₄(μ₂-CO)₃(CO)₉] (2.4837 vs. 2.492 Å). When compared to the reported Co-P distances for [Co₄(μ₂-CO)₃(CO)₈(PR₃)], that of 2 is shortened and matches the average Co–P distances of [Co₄(CO)₁₀{P(OMe)₃}₂] shown in Figure 1 (2.1546(4) vs. 2.158(2) Å). The mean length of the Co–C bond decreases in the following order: P(OPh)₃ > PPh₃ > PMe₃. Notable is the observation that the Co4–C distances are elongated compared to the other ones.

Inspection of the crystal structure reveals the existence of various other weak intermolecular contacts. A partial view of the crystal packing is shown in Figure 6. The shortest contact implies two carbonyl ligands, one axial and one bridging [d(C23···O9′) = 3.029 Å; symmetry code (‘) 1 − x, −½ + y, ½ − z]. An apical carbonyl also has two weak interactions with phenyl ring atoms [d(O11···C4″) = 3.073 Å; symmetry code (‘’) −1+x, y, z] and [d(H11···O11′) = 2.611 Å; symmetry code (‘) 1 − x, −½ + y, ½ − z]. There are two intermolecular C-H·π interactions observed, but because all hydrogen atoms were not freely re-fined, a discussion is not appropriate.

3. Discussion

Several methods have been described in the past to synthesize phosphine and phosphite-substituted tetranuclear clusters [Co₄(μ₂-CO)₃(CO)_9-n(PR₃)_n]. [{Co₄(μ₂-CO)₃(CO)₈(PMe₃)] has been obtained in low yield by reaction of Me₂PCl₃ with Na[Co(CO)₄] (12). More common is the use of a preformed tetranuclear scaffold, such as [Co₄(CO)₁₂] or [Co₄(CO)₉(arene)], followed by substitution with P(OR)₃ or PR₃ [5,8,9,10,11]. Since the elucidation of the mechanism of the formation of 2 was not the objective of this short note, we did not investigate this in detail. However, we can rule out the initial formation of [Co₄(μ₂-CO)₃(CO)₉] followed by CO/P(OPh)₃ exchange, since the formation of [Co₂(CO)₇{P(OPh)₃}] 1 is detectable by IR monitoring. We believe that cluster 2 is formed by the initial formation of [Co₂(CO)₇{P(OPh)₃}] 1, which reacts in a thermal cluster condensation reaction with [Co₂(CO)₈] yielding 2. There are also IR vibrations in the reaction mixture that can be attributed to the disubstituted cluster [{Co₄(μ₂-CO)₃(CO)₃{P(OPh)₃}₂] [5], but we were unable to isolate the compound in pure form.

In addition, we attempted to condensate [Co₂(CO)₆{P(OPh)₃}₂] with [Co₂(CO)₈] in hot toluene. The formation of cluster 2 as well as [Co₄(CO)₁₀{P(OPh)₃}₂] and minor amounts of unreacted [Co₂(CO)₆{P(OPh)₃}₂] was revealed by spectroscopic examination of the reaction mixture. No attempts were undertaken to separate the mixture.

4. Experimental Section

P(OPh)₃ (0.26 mL, 1.0 mmol) was added to a stirred solution of Co₂(CO)₈ (342 mg, 1.0 mmol) in toluene (5 mL). An immediate evolution of gas was observed. The reaction mixture was heated to 60 °C for 5 h. The solution was cooled to room temperature prior to lowering its temperature to 4 °C. The product 2 crystallized as dark plates collected by filtration. Yield: 39%. Anal. Calc. for C₂₉H₁₅Co₄O₁₄P (M.W = 854.14 g.mol⁻¹): C, 40.78; H, 1.77%. Found: C, 40.93; H, 1.84%. ¹³C{¹H} NMR (CDCl₃) at 298 K: δ 121.0 (Co), 125.6 (Cp), 129.9 (Cm), 151.2 (Cipso-O, d, ²J_PC = 12 Hz), 196 (br, CO) ppm.

For the refinement of the crystallographic data, a disorder model was applied for the heavy atoms Co2, Co3, and Co4 using an occupation ratio of 96:4.

Crystal data for C₂₉H₁₅Co₄O₁₄P, M = 854.10 g.mol⁻¹, black crystals, crystal size 0.377 × 0.194 × 0.15 mm³, monoclinic, space group P2₁/c; a = 12.5987(10) Å, b = 8.9773(9) Å, c = 27.953(2) Å, α = 90°, β = 96.829(3)°, γ = 90°; V = 3139.2(5) ų, Z = 4, D_calc = 1.807 g/cm³, T = 100 K, GOF = 1.034; R₁ = 0.0236, wR2 = 0.0546 for 11,958 reflections with I > = 2σ (I) and 11,958 independent reflections. Largest difference in peak/hole = Å⁻³ 0.58/−0.41. Data were collected using graphite monochromated MoK_α radiation with l = 0.71073 Å and deposited at the Cambridge Crystallographic Data Centre as CCDC 2174841. (Supplementary Materials). The data can be obtained free of charge from the Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/getstructures. The structure was solved by direct methods and refined by full-matrix least-squares against F² (SHELXL, 2015 [22,23,24]).

5. Conclusions

We have demonstrated that direct addition of P(OR)₃ to [Co₂(CO)₈] provides an alternative route to [{Co₄(CO)₁₁{P(OAr)₃}] species, avoiding the use of quite expensive [Co₄(CO)₁₂] as starting material. We have crystallographic evidence that cluster 2 adopts a structure quite reminiscent to that reported for [{Co₄(μ₂-CO)₃(CO)₈(PPh₃)] and bears the P(OPh)₃ ligand at the axial site of a Co vertex.