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Synthesis of Tris(arylphosphite)-ligated Cobalt(0) Complexes [Co2(CO)6{P(OAr)3}2], and their Reactivity towards Phenylacetylene and Diphenylacetylene

Isabelle Jourdain 1,*, Michael Knorr 1,*, Lukas Brieger 2, Carsten Strohmann 2

1 . Institut UTINAM UMR 6213 CNRS, Université Bourgogne Franche-Comté, 16, Route de Gray, 25030 Besançon, France

2 . Anorganische Chemie, Technische Universität Dortmund, Otto-Hahn Straße 6, 44227 Dortmund, Germany

Correspondence: Isabelle Jourdain and Michael Knorr

Academic Editor: Maxim L. Kuznetsov

Special Issue: Coordination Chemistry and Metal Complexes

Received: September 07, 2020 | Accepted: November 06, 2020 | Published: November 25, 2020

Advances in Chemical Research 2020, Volume 2, Issue 4, doi:10.21926/acr.2004009

Recommended citation: Jourdain I, Knorr M, Brieger L, Strohmann C. Synthesis of Tris(arylphosphite)-ligated Cobalt(0) Complexes [Co2(CO)6{P(OAr)3}2], and their Reactivity towards Phenylacetylene and Diphenylacetylene[1]. Advances in Chemical Research 2020;2(4):20; doi:10.21926/acr.2004009.

© 2020 by the authors. This is an open access article distributed under the conditions of the Creative Commons by Attribution License, which permits unrestricted use, distribution, and reproduction in any medium or format, provided the original work is correctly cited.

Abstract

The dinuclear complexes [Co2(CO)6{P(OAr)3}2] (1a, Ar = Ph; 1b, Ar = o-Tol) were prepared and reacted with PhCCH and PhCCPh to yield dimetallatetrahedranes [{(ArO)3P}(OC)2Co-Co(μ-RCCPh)(CO)2{P(OAr)3}] (3a, Ar = Ph, R = H; 3b, Ar = o-Tol, R = H; 3c, Ar = Ph, R = Ph; 3d, Ar = o-Tol, R = Ph). The main reaction of diphenylacetylene was accompanied by a side reaction, leading to the dissociation of a P(OAr)3 ligand for the formation of mono(phosphite) complexes [{(ArO)3P}(OC)2Co-Co(μ-PhCCPh)(CO)3] (2b, R = Ph; 2d, R = o-Tol). The crystal structures of 1b, 3a, and 3d were determined along with that of the mono(phosphite) complex [{(PhO)3P}(OC)2Co-Co(μ-PhCCPh)(CO)3] 2b.

Graphical abstract

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Keywords

Cobalt; alkyne; phosphite; crystal structure; dimetallatetrahedrane; metal-metal bond

1. Introduction

The organometallic reaction of dicobalt octacarbonyl with alkynes to yield dicobaltatetrahedranes [Co2(CO)6(µ-RC≡CR′)] [1,2] has been extensively studied ever since its first discovery in the fifties [3]. The studies are not only aimed to understand the synthetic aspects, spectroscopic and structural characterization of these cage-like species but also to utilize these compounds as precursors for further modifications. The Nicholas reaction is an example, where a dicobalt hexacarbonyl-stabilized propargylic cation [Co2(CO)6(µ-R1C≡C-CR2R3)]+, generated by the reaction of [Co2(CO)8] with a propargylic ether R1C≡C-C(OR)R2R3 or an alkynol R1C≡C-C(OH)R2R3, followed by subsequent treatment with a Lewis acid, reacts with a nucleophile (Nuc) to form [Co2(CO)6(µ-R1C≡C-C(Nuc)R2R3)] [4,5,6,7]. The mild oxidation of such functionalized dimetallatetrahedranes results in the formation of substituted alkynes, which finds application in organic chemistry [8]. The Pauson-Khand reaction is, however, the most common transformation, in which [Co2(CO)8] is reacted with an alkyne, alkene, and carbon monoxide to form an α,β-cyclopentenone [9,10,11,12,13,14,15,16]. This [2+2+1] cycloaddition is often used in synthetic organic chemistry. The addition of bridging diphosphines, such as bis(diphenylphosphino)methane (dppm) or bis(diphenylphosphino)amine (dppa), stabilizes the moderately labile metal-metal bond of the air-sensitive [Co2(CO)8] complex [7,13,17,18,19]. Besides, more stabilization may be achieved by the replacement of one or more carbonyls by monodentate P-donors such as PR3 and P(OR)3. A literature-survey revealed considerable studies on the dicobalt species [Co2(CO)n-x{P(OR3)}x] [20] coordinated with up to four alkyl phosphite ligands and their reaction with alkynes [21,22,23,24,25,26,27]. However, the structural characterization of [Co2(CO)n-x{P(OAr)3}x] complexes is limited, comprising of the structures of [Co2(CO)6{P(OPh)3}2] and [Co2(CO)6{P(O-2,4-t-Bu2C6H3)3}2] alone [28,29]. These compounds were probed for hydroformylation studies. However, there are no reports on the structural features of aryl phosphite-substituted dimetallatetrahedranes [{(ArO)3P}(OC)2Co-Co(µ-RC≡CR)(CO)2{P(OAr)3}]. In this study on the reactivity of metal-metal bonded dinuclear complexes with alkynes [30,31,32,33,34,35,36], an unknown complex, namely [Co2(CO)6{P(OTol-o)3}2] 1b, was synthesized along with [Co2(CO)6{P(OPh)3}2] 1a and reacted with phenylacetylene and diphenylacetylene. The resultant Co(0) complexes were characterized in solution by IR and multinuclear NMR spectroscopy as well as four single-crystal X-ray diffraction studies.

2. Materials and Methodologies

2.1 General procedure

All the reactions were performed in Schlenk tubes under an argon atmosphere. Based on previous literature, the solvents were dried and distilled before use, which were further stored under argon. The reagents diphenylacetylene, phenylacetylene, triphenyl phosphite, tri-o-tolylphosphite, and dicobaltoctacarbonyl were used as received from Sigma-Aldrich or Acros. The Co2(CO)6{P(OPh)3}2 complex was prepared as described in the literature [28]. The reactions were monitored using IR spectroscopy in the n(CO) region. The infrared spectra were recorded in solution using the Shimadzu IR Affinity-1 spectrometer or Bruker Vertex 70 FT-IR spectrometer equipped with a Platinum ATR accessory containing a diamond crystal. The standard IR abbreviations used are as follows: s = strong, m = medium, sh = shoulder, vs = very strong, and w = weak. The elemental analyses were performed at Service d’Analyses Elémentaires, UMR 7053 CNRS, Vandoeuvre-Les-Nancy, France. The 31P{1H}, 13C{1H}, and 1H NMR spectra were recorded in CDCl3 at 161.99, 100.62, and 400.13 MHz, respectively, using the Bruker Avance 400HD spectrometer. The solvent was used as the standard reference for calibrating the chemical shift. The chemical shift δ is given in ppm and J in Hz. The phosphorus chemical shifts are externally referenced using 85% H3PO4 in H2O. The standard NMR abbreviations used are as follows: br = broad, m = multiplet, s = singlet, d = doublet, and Cq = quaternary carbon.

2.2 Synthesis of the Complexes

[Co2(CO)6{P(OTol-o)3}2] (1b) This complex was prepared by following the same procedure as that reported in the literature for the synthesis of [Co2(CO)6{P(OPh)3}2] [28]. A mixture of [Co2(CO)8] (1.00 g, 2.93 mmol) and 2.1 eq of P(OTol-o)3 (1.90 ml, 6.15 mmol) were stirred in toluene (10 ml) under 90°C. The reaction was monitored using IR spectroscopy. After 3h, the solution was concentrated under vacuum, and the product 1b was crystallized at 4°C as a reddish-brown solid. About 2.1 g of the product was obtained with a yield of 73%. The calculated values for C48H42Co2O12P2 (990.67) were C, 58.20, and H, 4.27, against which the observed values were C, 58.11, and H, 4.22. The 1H NMR spectrum showed peaks at δ 2.14 (br, 18H, CH3) and 7.10-7.29 (m, 24H, Ar). The 13C{1H} NMR spectrum was found to possess peaks at δ 16.3 (CH3), 120.7, 125.2, 126.9, 131.6 (CH), 130.4 (C-Me), 149.7 (C-OP), and 199.8 (CO). The 31P{1H} NMR peak was observed at δ 159.9 (s). The IR (ATR) spectrum showed peaks at 1993 (w) and 1957 cm-1 (vs) n(CO).

Synthesis of [{(ArO)3P}(OC)2Co-Co(μ-RCCPh)(CO)2{P(OAr)3}]

[{(PhO)3P}(OC)2Co-Co(μ-HCCPh)(CO)2{P(OPh)3}] (3a) Method a: The reagent phenylacetylene (21.9 ml, 0.2 mmol) was added to 5 ml solution of Co2(CO)6{P(OPh)3}2 (182.0 mg, 0.2 mmol) mixed in toluene. The reaction mixture was heated at 60°C. After 2h, the resultant solution was cooled to room temperature prior to reducing its temperature to 4°C. The reddish-brown needles formed on crystallization were collected by filtration. The product yield was found to be 89 %. The observed values for C48H36Co2O10P2 (952.63) were C, 60.38; H, 3.78 against the calculated values of C, 60.52; H, 3.81.

Method b: Prior to the addition of phenylacetylene (32.9 ml, 0.3 mmol), the [Co2(CO)8] complex (100.0 mg, 0.3 mmol) was dissolved in 5 ml of heptane. The solution was stirred overnight at room temperature. To this mixture, 2 eq. of P(OPh)3 (153 ml, 0.6 mmol) was added and stirred under 80°C. After 4h, the resulting solution was allowed to reach room temperature. The temperature of the mixture was further reduced to 4°C. The resultant reddish-brown needles were collected by filtration. The product yield was found to be 83 %. 1H NMR: d 4.57 (br, 1H, CH), 6.90-7.46 (m, 35H, Ar). 13C{1H} NMR: d 69.7 (CH), 87.0 (Cq), 121.2, 124.8, 126.6, 128.0, 129.6, 131.9 (CH), 140.0 (Cq), 151.2 (C-OP), 203.8 (CO). 31P{1H} NMR: d 153.0 (s). IR (ATR): 2032(s), 1978(vs) n(CO). IR (cyclohexane): 2045 (s), 2000 (s), 1992 (s) n(CO).

[{(o-TolO)3P}(OC)2Co-Co(μ-HCCPh)(CO)2{P(OTol-o)3}] (3b) This derivative was prepared by following the same procedures as described above and was, in turn, isolated as a reddish brown solid. Method a: Yield: 85 %. Calculated values for C54H48Co2O10P2 (1036.79): C, 62.56; H, 4.67; Observed values for C54H48Co2O10P2 (1036.79): C, 62.64; H, 4.72. Method b: Yield: 81 %. 1H NMR: d 2.01 (br, 18H, CH3), 5.04 (br, 1H, CH), 6.90-7.46 (m, 29H, Ar). 13C{1H} NMR: d 16.4 (CH3), 68.6 (CH), 89.1 (Cq), 120.2, 124.4, 125.6, 126.6, 127.3 (CH), 129.9 (C-Me), 131.4, 131.5, (CH), 139.3 (Cq), 150.0 (C-OP), 203.6 (CO). 31P{1H} NMR: d 148.8 (s). IR (ATR): 2032 (s), 1976 (vs) n(CO). IR (cyclohexane): 2045 (s), 1999 (s), 1993(s), 1937 (w) n(CO).

[{(PhO)3P}(OC)2Co-Co(μ-PhCCPh)(CO)2{P(OPh)3}] (3c) This compound was prepared by following the same procedures as described above and was, in turn, isolated as a reddish black solid. Method a: Yield: 81 %. Using method b, the slow concentration of the mother liquors was achieved resulting in the formation of black needles of 2b (Yield: 5%). Method b: Yield: 86 %. Calculated values for C54H40Co2O10P2 (1028.72): C, 63.05; H, 3.92; Observed values for C54H40Co2O10P2 (1028.72): C, 62.85; H, 3.79. 1H NMR: d 6.65-7.93 (m, Ar). 13C{ 1H} NMR: d 86.8 (Cq), 121.0, 124.7, 126.2, 128.2, 129.6, 131.1 (CH), 141.6 (Cq), 151.3 (C-OP), 203.1 (CO). 31P{1H} NMR: d 147.4 (s). IR (ATR): 2033 (s), 1981 (vs) n(CO). IR (cyclohexane): 2046 (s), 2006 (s), 1994 (s), 1946 (w) n(CO).

[(OC)3Co-Co(μ-PhCCPh)(CO)2{P(OPh)3}] (2b) Calculated values for C37H25Co2O8P (746.44): C, 59.54; H, 3.38; Observed values for C37H25Co2O8P (746.44): C, 59.63; H, 3.45. 1H NMR: d 6.59-7.67 (m, Ar). 13C{1H} NMR: d 89.4 (Cq), 121.0 (d, 3JPC = 4 Hz) , 125.1, 127.1, 128.7, 129.9, 130.2 (CH), 140.0 (Cq), 151.2 (Cq-O, d, 2JPC = 9 Hz), 200.6, 202.5 (CO) 31P{1H} NMR: d 147.0 (s). IR (ATR): 2062 (s), 2017 (s), 2001 (s), 1978 (s) n(CO).

[{(o-TolO)3P}(OC)2Co-Co(μ-PhCCPh)(CO)2{P(OTol-o)3}] (3d) This compound was prepared by following the same procedures as described above and was further isolated as black-purple plates. Method a: Yield: 88 %. The slow concentration of the mother liquors via method b afforded 2d as a brown powder (Yield: 8%). Method b: Yield: 86 %. Calculated values for C60H52Co2O10P2 (1112.89): C, 64.76; H, 4.71; Observed values for C60H52Co2O10P2 (1112.89): C, 64.64; H, 4.72. 1H NMR: d 1.89 (br, 18H, CH3), 6.77-8.04 (m, 34H, Ar). 13C{1H} NMR: d 16.1 (CH3), 85.8 (Cq), 120.3, 124.1, 126.3, 126.5, 128.3 (CH), 129.7 (C-Me), 130.9, 131.1 (CH), 141.2 (Cq), 150.3 (C-OP), 203.9 (CO). 31P{1H} NMR: d 142.1 (s). IR (ATR): 2033 (s), 1979 (vs) n(CO). IR (cyclohexane): 2043 (s), 2000 (s), 1991 (s), 1932 (w) n(CO).

[(OC)3Co-Co(μ-PhCCPh)(CO)2{P(OTol-o)3}] (2d) Calculated values for C40H31Co2O8P (788.53): C, 60.93; H, 3.96; Observed values for C40H31Co2O8P (788.53): C, 61.09; H, 3.88. 1H NMR: d 2.12 (br, 9H, CH3), 6.65-7.66 (m, 22H, Ar). 13C{1H} NMR: d 16.2 (CH3), 88.7 (Cq), 120.0, 125.5, 126.5, 127.2, 128.6 (CH), 128.6 (C-Me), 131.5, 131.4 (CH), 139.7 (Cq), 149.2 (Cq-O), 200.4, 203.4 (CO). 31P{1H} NMR: d 142.2 (s). IR (ATR): 2062 (s), 2021 (s), 1998 (s) n(CO).

2.3 Crystallographic Studies

The crystal structures of the compounds 1b, 2b, 3a, and 3d were determined using the Bruker D8 Venture four-circle diffractometer equipped with a PHOTON II CPAD detector by Bruker AXS GmbH. The X-ray radiation was generated by the IµS/IμS 3.0 microfocus source (for compound 3a) Mo (l = 0.71073 Å) from Incoatec GmbH equipped with HELIOS mirror optics and a single-hole collimator by Bruker AXS GmbH. The selected crystals of 1b, 3a, 2b, and 3d were covered with an inert oil (perfluoropolyalkyl ether) and mounted on the MicroMount from MiTeGen.

The APEX 3 Suite (v.2018.7-2) software integrated with SAINT (integration) and SADABS (adsorption correction) programs by Bruker AXS GmbH were used for data collection. The processing and finalization of the crystal structure were performed using the Olex2 program [37]. The crystal structures were solved by the ShelXT [38] structure solution program using the Intrinsic Phasing option, which were further refined by the ShelXL [39] refinement package using Least Squares minimization. The non-hydrogen atoms were anisotropically refined. The C-bound H atoms were placed in geometrically calculated positions, and a fixed isotropic displacement parameter was assigned to each atom according to the riding-model: C–H = 0.95–0.98 Å with Uiso(H) = 1.5Ueq(CH3) and 1.2Ueq(CH) for other hydrogen atoms. In compound 3a, the hydrogen atom on C1 was located on the difference Fourier map and refined independently. In compound 1b, two of the three ortho-cresyl groups were positionally disordered and refined using free variables. The oxygen atom O4 and carbon atoms C4-C10 were disordered with a ratio of 52:48, while the carbon atoms C18-C24 were disordered with a ratio of 55:45. The carbon atoms C8 and C18 were excluded from the disorder. In compound 2b, two of the three -OPh groups were positionally disordered. The –OPh group at the P4 atom with the oxygen atom O26 was disordered with a ratio of 77:23. The carbon atom C123 was excluded from the disorder. The–OPh group at the P4 phosphorus atom with the oxygen atom O27 was disordered with a ratio of 83:17. The carbon atom C125 was excluded from the disorder. The crystallographic data and structural refinement are listed in Table 1. The crystallographic data for the structures of 1b, 3a, 2b, and 3d have been published as supplementary publication number 2016769(1b), 2016778(3a), 2016782(2b), and 2016780(3d) in the Cambridge Crystallographic Data Centre. A copy of these data can be obtained for free by applying to CCDC, 12 Union Road, Cambridge CB2 IEZ, UK, fax: 144-(0)1223-336033 or e-mail: deposit@ccdc.cam.ac.uk.

Table 1 Crystallographic data and structure refinement of 1b, 2b, 3a, and 3d.

3. Results and Discussion

3.1 Synthesis and Characterization of [Co2(CO)6{P(OTol-o)3}2] (1b)

The synthesis of dimer 1b is similar to that of the previously reported P(OPh)3 derivative, namely [Co2(CO)6{P(OPh)3}2] 1a. On heating [Co2(CO)8] with 2.1 equivalents of tris(o-tolyl)phosphite in hot toluene at 90°C, the complex 1b was isolated as a reddish-brown air-stable crystalline solid with a yield of 73% (Scheme 1).

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Scheme 1 Preparation of bis(tris(o-tolyl)phosphite)hexacarbonyldicobalt complex 1b

The D3h symmetry of the complex was deduced from the IR spectrum exhibiting intense n(CO) vibrations for the six carbonyls at 1957 cm-1 and a low-intensity peak at 1993 cm-1. The magnetically equivalent phosphorus nuclei resulted in a broadened singlet resonating at d 159.9 in the 31P NMR spectrum. The molecular structure of 1b is in line with the solution data, as shown in Figure 1. The symmetrically equivalent Co atoms adopt a trigonal pyramidal coordination sphere consisting of three equatorially oriented carbonyls, a metal-metal bond to the adjacent Co(CO)3P moiety, and a Co-bound phosphite ligand. The length of the Co-Co bond (2.6688(5) Å) is quite comparable to that of [Co2(CO)6{P(OPh)3}2] (2.6722(4) Å), but remarkably shorter than that of [Co2(CO)6{P(O-2,4-t-Bu2C6H3)3}2] bearing two bulky (2,4-tert-butylphenyl)phosphite ligands (2.706(5) Å) [28,29]. For comparison, the Co-Co distance reported for [Co2(CO)6{P(OiPr)3}2] was 2.6544(12) Å, reflecting the steric and electronic impacts of the OR substituents on the metal-metal separation [25]. It is noted that the Co-Co bond length of [Co2(CO)8] itself shrinks to a value of 2.522(1) Å [40,41]. The Co-P distances in the nearly trans-oriented P(OAr)3 ligands (175.33(3)°) of 1b were close to those reported for 1a (2.1218(6) vs. 2.1224(4) Å).

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Figure 1 Molecular structure of [Co2(CO)6{P(OTol-o)3}2] (1b). The H atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]: Co-Co 2.6688(5), Co-P 2.1218(6), Co-C1 1.777(2), Co-C2 1.795(2), Co-C3 1.795(2), P-Co-Co 175.33(3), C1-Co-P 91.32(8), C1-Co-C2 122.48(12), C1-Co-C3 116.26(11), C2-Co-P 95.40(7), C3-Co-P 98.21(7). A thermal ellipsoid plot of 1b at the probability level of 50% is shown in Figure S16 of the Supporting Information.

3.2 Synthesis and Characterization of Dimetallatetrahedranes [{(ArO)3P}(OC)2Co-Co(m-PhCCR)(CO)2{P(OAr)3}]

The reaction of equimolar quantities of [Co2(CO)6{P(OAr)3}2] and PhC≡CR in hot toluene was monitored by IR spectroscopy until the disappearance of the carbonyl bands of the initial phosphite cobalt complex, indicating the formation of dimetallatetrahedrane complexes. Alternatively, these compounds were isolated after a two-step sequence involving the acetylene complexation of the dicobaltoctacarbonyl complex, followed by the substitution of two CO groups by the aryl phosphite ligands (Scheme 2). The IR monitoring indicated the formation of minor amounts of a monosubstituted phosphite compound characterized by the carbonyl stretch at ca. 2065 cm-1 [22]. The crystallization of the reaction mixture in toluene or heptane afforded 3a-d as dark red stable solids in high yields (near 85%). The reaction of diphenylacetylene with [Co2(CO)6{P(OAr)3}2] leads to small amounts of monosubstituted derivatives [{(ArO)3P}(OC)2Co-Co(µ-PhC≡CPh)(CO)3]. The compounds 2b (Ar = Ph) and 2d (Ar = o-Tol) were isolated as black crystals and brown powder, respectively. Thus, it was presumed that the coordination of the alkyne implies the first-step elimination of carbonyl and phosphite groups in this side reaction, leading to the formation of monophosphite tetrahedrane. The latter reacts with the in situ liberated phosphate, causing the substitution of a second carbonyl ligand (Scheme 2). Another possible mechanistic pathway initially implies the formation of bisphosphite tetrahedrane 3, followed by the partial dissociation of a phosphite ligand. Further mechanistic studies using the other P(OR)3 ligands should be performed to clarify this point. All the complexes described therein were characterized by IR and NMR spectroscopy.

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Scheme 2 Synthesis of (arylphosphite)dicobalt tetrahedrane complexes

The IR (ATR) spectra of [{(ArO)3P}(OC)2Co-Co(µ-PhC≡CR)(CO)2{P(OAr)3}] displayed similar two strong absorptions at about 2030 and 1980 cm-1. Another weak CO vibration appeared near 1950 cm-1 as a shoulder-type peak. The representative spectrum of compound 3d is illustrated in Figure 2. This IR spectroscopic observation in the solid state was in agreement with the structural characterization of compounds 3a and 3d (see below). In solution, four bands in the CO stretching region were generally observed for the diaxially phosphane-substituted tetrahedrane complexes such as [(Ph3P)(OC)2Co-Co(µ-PhC≡CPh)(CO)2(PPh)3}] [22]. This characteristic pattern was observed in the IR spectra of the complexes in cyclohexane, and the corresponding wavenumbers are given in the experimental part. In the lower-symmetry monophosphite complexes 2b and 2d, the five n(CO) bands appeared in the terminal carbonyl region (Figure 2). The IR spectra matched with those reported by Manning et al. for [{(MeO)3P}(OC)2Co-Co(µ-PhC≡CPh)(CO)3] and [(Ph3P)(OC)2Co-Co(µ-PhC≡CPh)(CO)3], which discussed the substitution of an axial carbonyl ligand by the P-donor ligand, regardless of the cone angle [22].

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Figure 2 IR (ATR) spectra of diphenylacetylene P(OTol-o)3-dicobalt derivatives 2d (left) and 3d (right) in the n(CO) region.

The NMR data are reported in the experimental part. The 1H NMR spectra show a line-broadening of all the peaks probably due to the quadrupolar relaxation of the 59Co nuclei [20]. The acetylenic proton of [Co2(µ-PhC≡CH)(CO)6] was reported to resonate at d 6.38 [12], while those of the phenylacetylene derivatives 3a and 3b were shifted towards high-field as a broad signal (due to unresolved 3J P-H coupling) near 5 ppm. Compared to free phenylacetylene (d 3.06), the downfield shift of the terminal proton is in agreement with the reduction of the triple bond character after complexation. The observation of a single 31P NMR resonance peak for 3a-d is consistent with the magnetically equivalent phosphorus nuclei in the disubstituted compounds. The chemical shifts were quite affected by the nature of the coordinated alkyne. An upfield shift of ca. 7 ppm was observed for the diphenylacetylene derivatives compared to that of the phenylacetylene derivatives. The 31P NMR spectroscopy showed no differentiation between the mono- and disubstituted complexes due to the presence of singlet resonance peak of identical chemical shifts in both cases. However, the 13C NMR spectra of the complexes were significantly different in the carbonyl region (Figure 3). In the disubstituted complexes 3a-d, a single resonance was observed for the carbonyls around 200 ppm, while two distinct broad singlets were observed for the terminal carbonyls in [{(ArO)3P}(OC)2Co-Co(µ-PhC≡CPh)(CO)3] (Ar = Ph, 2b; Ar = o-Tol, 2d). Besides, a downfield shift of about 3 ppm was observed for the acetylenic carbon atoms (Figure 3).

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Figure 3 13C{1H} NMR spectra of 2b (bottom) and 3d (top) in the range of 80-210 ppm recorded in CDCl3.

The molecular structure for the dicobaltatetrahedrane scaffold of 3a is shown in Figure 4. The PhC≡CH moiety was nearly orthogonally oriented with respect to the vector of the Co-Co bond (89.436(16)°). The two crystallographically non-equivalent P(OPh)3 ligands lost their ideal linear trans-disposition of 180° observed in the precursor [Co2(CO)6{P(OPh)3}2] 1a complex with the P1-Co1-Co2-P2 dihedral angle of 11.96°. In contrast, the mean Co-P distance of 3a was close to that of the precursor 1a complex (2.1303(7) vs. 2.1224(4) Å). There are no crystallographic data for the parent compound [Co2(µ-PhC≡CH)(CO)6]. However, several entries were reported in the CSD base for the phosphane-ligated phenylacetylene dicobaltatetrahedranes, as listed in Table 2. Generally, the Co-Co bond distances lie in the range of 2.4287(8) to 2.5140(5) Å. However, the Co-Co bond distance of 3a was observed in the mid-range with a value of 2.4665(6) Å.

 

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Figure 4 Molecular structure of 3a. All the H atoms apart from the acetylenic ones are omitted for clarity. The C-bound hydrogen atom on C1 was located in the difference Fourier map, and refined independently. Selected bond lengths [Å] and angles [°]: Co1-Co2 2.4666(5), Co1-P1 2.1292(7), Co2-P2 2.1313(7), Co1-C1 1.951(2), Co1-C2 1.994(2), Co2-C1 1.970(2), Co2-C2 1.970(2), Co1-C9 1.795(2), Co1-C10 1.794(3), Co2-C11 1.798(2), Co2-C12 1.797(2), C1-C2 1.337(3), P1-Co1-Co2 147.52(2), P2-Co2-Co1 150.93(2), P1-Co1-C1 105.06(7), P1-Co1-C2 96.45(7), P2-Co2-C1 105.08(7), P2-Co2-C2 99.54(7), P1-Co1-C10 100.50(8), P1-Co1-C9 99.34(8), P2-Co2-C11 101.70(8), P2-Co2-C12 94.19(8), Co2-C1-Co1 77.96(9), Co2-C2-Co1 76.97(8), C1-C2-C3 140.3(2). The thermal ellipsoid plot of 3a at the probability level of 50% is shown in Figure S17 of the Supporting Information.

Table 2 Relevant bond lengths (Å) in crystallographically characterized phosphane- or diarsane-substituted tetrahedrane scaffolds featuring a µ-PhCCH-Co2 fragment.

According to Scheme 2, the compounds 1a and 1b were treated with diphenylacetylene PhC≡CPh to study the impact of an internal alkyne on the structural features. Although the targeted bis(phosphite) complexes 3b and 3d were readily isolated, the presence of another minor species featuring an n(CO) stretch at about 2060 cm-1 was observed by IR monitoring attributing to the [{(PhO)3P}(OC)2Co-Co(µ-PhC≡CPh)(CO)3] complex (2b). As mentioned above, the monophosphite complex 2b was successfully isolated by fractional crystallization, allowing complete spectroscopic characterization. The complete characterization of 2b crystallizing in the monoclinic space group P21/c was performed by the X-ray diffraction study. It is noted that the corresponding unit cell is composed of four independent molecules. The following values used in the structural discussion are related to the unit presented in Figure 5. The dicobaltatetrahedrane scaffold bears the P(OPh)3 ligand at the axial position of the Co1(CO)2 moiety. The Co-P bond length was slightly elongated with respect to that of the [Co2(CO)6{P(OPh)3}2] 1a complex (2.1404(5) vs. 2.1224(4) Å). The comparison of the resultant complex with the parent compound [(OC)3Co-Co(µ-PhC≡CPh)(CO)3] revealed that the Co-Co bond length is (within the experimental error) not affected by the substitution of a carbonyl group by P(OPh)3 (2.4760(4) vs. 2.476(2) Å) [48]. The mean Co-µ(C) bond distance of 1.9679(18) was in agreement with that reported for [(OC)3Co-Co(µ-PhC≡CPh)(CO)3] (1.97(1) Å). The consistency was observed in the C=C bond length values (1.351(3) vs. 1.36 (1) Å) of the complex and bridging diphenylacetylene unit as well. Table 3 gives the relevant bond lengths of the previously reported phosphane-substituted dicobaltatetrahedrane scaffolds bridged by PhC≡CPh.

The molecular structure of the dicobaltatetrahedrane 3d complex is shown in Figure 6. There were no significant variations in the bond lengths and angles relative to similar compounds.

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Figure 5 Molecular structure of [{(PhO)3P}(OC)2Co-Co(μ-PhC≡CPh)(CO)3] (2b). The H atoms are omitted for clarity. Only one of the four independent molecules in the unit cell is shown. Selected bond lengths [Å] and angles [°]: Co1-Co2 2.4760(4), Co1-P1 2.1404(5), Co1-C19 1.809(2), Co1-C20 1.808(2), Co1-C24 1.9608(18), Co1-C31 1.9533(18), Co2-C21 1.817(2), Co2-C22 1.812(2), Co2-C23 1.795(2), Co2-C24 1.9704(18), Co2-C31 1.9871(18), C24-C31 1.351(3); P1-Co1-Co2 145.494(18), P1-Co1-C24 96.83(6), P1-Co1-C31 96.56(), P1-Co1-C19 100.60(6), P1-Co1-C20 102.75(7), Co1-C24-Co2 78.08(7), Co1-C31-Co2 77.85(6), C25-C24-C31 143.91(13), C24-C31-C32 143.11(17). The thermal ellipsoid plot of 2b at the probability level of 50% is shown in Figure S18 of the Supporting Information.

Table 3 Relevant bond lengths (Å) in crystallographically characterized phosphane-substituted tetrahedrane scaffolds featuring a µ-PhCCPh-Co2 fragment.

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Figure 6 Molecular structure of [{(o-TolO)3P}(OC)2Co-Co(μ-PhC≡CPh)(CO)2{P(OTol-o)3}]. (3d). The H atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]: Co1-Co2 2.4642(5), Co1-P1 2.1525(5), Co1-P2 2.1447(6), Co1-C22 1.7961(14), Co1-C23 1.8035(14), Co1-C26 1.9937(13), Co1-C33 1.9953(13), Co2-C24 1.7991(14), Co2-C25 1.8036(14), Co2-C26 1.9593(13), Co2-C33 1.9953(13), C24-C31 1.3461(17); P1-Co1-Co2 151.652(13), P1-Co1-C22 98.43(5), P1-Co1-C23 99.32(5), P1-Co1-C26 101.86(4), P1-Co1-C33 103.26(4), Co1-C26-Co2 77.12(5), Co1-C33-Co2 77.03(4), C27-C26-C34 141.64(12), C26-C33-C34 142.23(112), P2-Co2-Co1 151.923(14), P2-Co2-C24 97.25(5), P2-Co2-C25 98.84(4), P2-Co2-C26 101.67(6), P2-Co2-C33 103.61(4). The thermal ellipsoid plot of 3d at the probability level of 50% is shown in Figure S19 of the Supporting Information.

4. Conclusions

In this study, the structural analyses of four dicobalt compounds were performed to crystallographically characterize the arylphosphite-substituted dicobalt complexes. Besides, the conversion of [{Co2(CO)6{P(OAr)3}2] to dicobaltatetrahedranes is accompanied by the partial dissociation of one of the P(OAr)3 ligands, leading to the formation of [{(ArO)3P}(OC)2Co-Co(μ-PhC≡CPh)(CO)3] as a minor component. The current studies are focused on phosphite-substituted dicobaltatetrahedranes bearing functionalized N-heterocyclic alkynes, intending to utilize these complexes as substrates for the Pauson-Khand reactions and use thioether-functionalized compounds [{(R’O)3P}(OC)2Co-Co(μ-RSCH2C≡CCH2SR)(CO)2{P(OR’)3}] as precursors for the development of polynuclear heterometallic assemblies by the coordination of other metal fragments to the S-donor sites of the alkyne.

Acknowledgments

We thank Stephanie Boullanger for recording the NMR spectra.

Additional Materials

The following additional materials are uploaded at the page of this paper.

1. Figure S1: IR(ATR) spectrum of Co2(CO)6{P(OTol-o)3}2 (1b).

2. Figure S231P{1H} NMR spectrum of Co2(CO)6{P(OTol-o)3}2 (1b) recorded in CDCl3.

3. Figure S313C{1H} NMR spectrum of Co2(CO)6{P(OTol-o)3}2 (1b) recorded in CDCl3.

4. Figure S4: IR(ATR) spectrum of Co2(CO)4{P(OPh)3}2(μ-PhCCH) (3a).

5. Figure S531P{1H} NMR spectrum of Co2(CO)4{P(OPh)3}2(μ-PhCCH) (3a) recorded in CDCl3.

6. Figure S6: IR(ATR) spectrum of Co2(CO)4{P(OTol-o)3}2(μ-PhCCH) (3b).

7. Figure S731P{1H} NMR spectrum of Co2(CO)4{P(OTol-o)3}2(μ-PhCCH) (3b) recorded in CDCl3.

8. Figure S8: IR(ATR) spectrum of Co2(CO)4{P(OPh)3}2(μ-PhCCPh) (3c).

9. Figure S931P{1H} NMR spectrum of Co2(CO)4{P(OPh)3}2(μ-PhCCPh) (3c) recorded in CDCl3.

10. Figure S10: IR(ATR) spectrum of Co2(CO)5{P(OPh)3}(μ-PhCCPh) (2b).

11. Figure S1131P{1H} NMR spectrum of Co2(CO)5{P(OPh)3}(μ-PhCCPh) (2b) recorded in CDCl3.

12. Figure S12: IR(ATR) spectrum of Co2(CO)4{P(OTol-o)3}2(μ-PhCCPh) (3d).

13. Figure S1331P{1H} NMR spectrum of Co2(CO)4{P(OTol-o)3}2(μ-PhCCPh) (3d) recorded in CDCl3.

14. Figure S14: IR(ATR) spectrum of Co2(CO)5{P(OTol-o)3}(μ-PhCCPh) (2d).

15. Figure S1531P{1H} NMR spectrum of Co2(CO)5{P(OTol-o)3}(μ-PhCCPh) (2d) recorded in CDCl3.

16. Figure S16: A thermal ellipsoid plot of 1b at the 50% probability level.

17. Figure S17: A thermal ellipsoid plot of 3a at the 50% probability level.

18. Figure S18: A thermal ellipsoid plot of 2b at the 50% probability level.

19. Figure S19: A thermal ellipsoid plot of 3d at the 50% probability level.

Author Contributions

I. Jourdain and M. Knorr performed the synthetic work and wrote the manuscript, L. Brieger and C. Strohmann determined the crystal structures.

Funding

We are grateful to the Université de Franche-Comté for financial support via a Chrysalide funding .

Competing Interests

The authors have declared that no competing interests exist.

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