Tris( µ 2 -carbonyl)octacarbonyl(triphenylphosphite)tetracobalt

: In toluene solution, the reaction of [Co 2 (CO) 8 ] with an equimolar amount of [P(OPh) 3 ] yields first [Co 2 (CO) 7 {P(OPh) 3 }] 1 . Following heating of 1 , an alternative synthetic access to the tetranuclear cluster [{Co 4 ( µ 2 -CO) 3 (CO) 8 {P(OPh) 3 }] 2 is provided in a condensation reaction. Compound 2 has been characterized by IR and 31 P NMR spectroscopy. The tetranuclear cluster framework has been ascertained by a single-crystal X-ray diffraction study performed at 100 K.

It is worth noting that P(OMe) 3 and P(OPh) 3 can even form tetra-substituted clusters [Co 4 (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 2 of 8 to synthetize the monosubstituted dinuclear complex [Co 2 (CO) 7 {P(OPh) 3 }] 1 by adding a stoichiometric amount of P(OPh) 3 to a solution of [Co 2 (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 [Co 4 (µ 2 -CO) 3 (CO) 8 {P(OPh) 3 }] 2 by serendipity as the major component.  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 [Co 2 (CO) 6 {P(OPh) 3 } 2 ] (1978 cm −1 , very strong). Formation of this dinuclear bisphosphite complex is also corroborated by an NMR 31 P{ 1 H} 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 [Co 4 (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].

Marko
Molbank 2022, 2022, x FOR PEER REVIEW 2 of 9 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. 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. After workup, a product in the form of dark crystals was isolated, and elemental analysis revealed a composition of [Co 4 (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 [Co 4 (µ 2 -CO) 3 (CO) 8 (PPh 3 )] 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 PPh 3 .

Marko
The proton-decoupled 31 P-NMR recorded in CDCl 3 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 13 C 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 31 P or 13 C NMR data recorded at ambient temperature on related PR 3 and P(OR) 3 clusters are available in the literature. There is only one report on [Co 4 (CO) 11 {P(OMe) 3 }] at low temperature using 13 C-enriched CO, which allows for the differentiation between bridging and terminal carbonyls [19]. The proton-decoupled 31 P-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 13 C 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 31 P or 13 C 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 13 C-enriched CO, which allows for the differentiation between bridging and terminal carbonyls [19].   The proton-decoupled 31 P-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 13 C 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 31 P or 13 C 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 13 C-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.  In order to check whether [{Co 4 (µ 2 -CO) 3

Experimental Section
P(OPh) 3 (0.26 mL, 1.0 mmol) was added to a stirred solution of Co 2 (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 C 29 H 15  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.

Conclusions
We have demonstrated that direct addition of P(OR) 3 to [Co 2 (CO) 8 ] provides an alternative route to [{Co 4 (CO) 11 {P(OAr) 3 }] species, avoiding the use of quite expensive [Co 4 (CO) 12 ] as starting material. We have crystallographic evidence that cluster 2 adopts a structure quite reminiscent to that reported for [{Co 4 (µ 2 -CO) 3 (CO) 8 (PPh 3 )] and bears the P(OPh) 3 ligand at the axial site of a Co vertex.
Supplementary Materials: The following supporting information can be downloaded at online. Figure S1: IR ATR spectrum of compound 2; CIF file and Check-CIF report.
Author Contributions: Compound preparation, I.J.; X-ray data collection and data analysis, C.S. and R.S.; conceptualization, data analysis, and manuscript-writing, I.J. and M.K. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.

Data Availability Statement:
The X-ray data are available at CCDC as stated in the paper.