Molecular Structure of Co 2 ( μ-Alkyne ) Complex Containing Ph 2 PC 5 F 6 PPh 2 Ligand

Oxidative-decarbonylation of Co2(CO)6(μ-PhC≡CH) with Me3NO in the presence of an electron deficient ligand, Ph2PC5F6PPh2(F6FOS), produces Co2(CO)4(μ-PhC≡CH)(F6FOS), (1). The metrical values of 1 have been compared to those of the closely related cobalt carbonyl alkyne compound A containing (Z)-Ph2PCH=CHPPh2 (Z-dppe) ligand. Strikingly anomalous is an alkyne C≡C bond (1.34(1) Å) in 1, which is somewhat elongated compared to A (1.31(1) Å). When taking a strong electron-withdrawing power of fluoride atom into account, F6FOS ligand appeared to reduce the π-back-donation ability of cobalt atom, making this bond shortened in comparison to the same bond in A. Bond lengthening in the alkyne C≡C bond in 1 is attributed to the enhanced electron donor ability of F6FOS compared to Z-dppe and can be understood by examining resonance structures of F6FOS ligand.


Introduction
We have become interested in ligands of the type R 2 PC 5 F 6 PR 2 (R = Ph, cyclo-C 6 H 11 ), which are reported by Cullen (Chart 1) [1].
These ligands seem to display unique electronic properties in comparison to typical diphosphine ligands such as DIPHOS (Ph 2 PCH 2 CH 2 PPh 2 ) because they possess a low-lying π* orbital due to a bridging perfluorocyclopentenyl ring, which is recognized to be a strong electron-withdrawing group [2].Therefore an organometallic compound with this type of ligand is anticipated to serve as an effective electron reservoir by stabilizing electron counts in excess of 18-electrons [3] [4].To establish the electronic influence of F 6 FOS (R = Ph, 1,2-bis(diphenylphosphino)hexafluorocyclopentene) ligand, we wish to report the synthesis and single-crystal X-ray diffraction study of an alkyne-bridged cobalt complex containing F 6 FOS.
We reckoned that a comparison of the metrical values for 1 and A served to elucidate the electronic effect of F 6 FOS ligand.In addition, no crystallographic study on F 6 FOS appears to have been recorded, and consequently data on this ligand are also briefly reported.

Synthesis of Complex 1
To 0.080 g (0.21 mmol) of Co 2 (CO) 6 (μ-PhC≡CH) and 0.11 g (0.20 mmol) of F 6 FOS in 15 mL of ethanol was added 0.030 mg (0.40 mmol) of Me 3 NO 2 .The solution was stirred for 24 h at ambient temperature.The darkredprecipitate was formed and the supernatant was removed.The resulting solid was washed with hexane and pumped dry to yield 1 (0.11 g, 62%).Crystals suitable for an X-ray structural analysis were obtained by recrystallization from dichloromethane/n-heptane.

Single Crystal X-Ray Diffraction
Selected structural details for the structures of F 6 FOS ligand and complex 1 are included in Table 1 and full crystallographic data have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication numbers CCDC 1052995 and 1052994, respectively.Copies of these data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif

Results and Discussion
The ligand, F 6 FOS, was synthesized from HPPh 2 and octafluorocyclopentene.Complex 1 was synthesized in a Scheme 1.The reactions of the alkyne-bridged binuclear Co complex with F 6 FOS and Z-dppe.2.
Complex 1 crystallizes in the triclinic space P-1 (#2).The alkyne portion of 1 is coordinated through its π-bond to Co1 and Co2, while F 6 FOS ligand is coordinated to Co1.In the structure, there is a distorted octahedral geometry about each cobalt atom, the two alkyne carbons and two cobalt atoms forming a tetrahedral unit.
It is instructive to compare the geometry about the F 6 FOS fragment in 1 to that of the parent F 6 FOS ligand.Upon coordination to cobalt atom, the bond lengths and angles of the ligand itself alter significantly.The mean P-C bond distance in 1 at 1.834 Å is somewhat longer than the corresponding value of 1.824 Å in F 6 FOS while the C=C bond distance at 1.34(1) Å in the C 5 F 6 ring of 1 is slightly shorter than that observed in F 6 FOS ligand itself, 1.370(8) Å.This may be explained by a substantial decrease of degree of π-bonding between the phos-   The structure I or II makes a considerable contribution to the hybrid because the strong electron-withdrawing effect of fluoride substituents decreases the electron density at the charge-bearing carbons.This structural feature accommodates some degree of P-C multiple bonding.Consequently, this gives the C=C bond of F 6 FOS a partial single-bond character and lengthens it.Then, coordination of the ligand to cobalt atom would lead to reduce π-donation from the electron pairs on phosphorus atoms.As expected, the mean endocyclic P-C=C bond angle of 119.0˚ in 1 is smaller than that of 123.4˚ in F 6 FOS ligand.
Our major objective of this study is to establish how the substitution of Z-dppe for F 6 FOS affects the structure of the rest of the complex.Since complexes 1 and A were expected to be sterically very similar with respect to the polyhedral Co 2 core, it could be argued that any observed structural differences would have their origin in the electronic properties inherent to each ancillary diphosphine ligand.
To begin with, we verified the bonding geometry about Co 1 atoms in 1 and A. Table 3 compares pertinent distances and bond angles of 1 and A. The P 2 -Co 1 -P 1 bite angle in 1 is 90.4(1)˚ and, the Co 1 -P 1 bond with a bond length of 2.216(3) and Co 1 -P 2 bond with a bond length of 2.175(3) average at 2.196 Å.These values are analogous to those reported for A (88.39(8)˚ and 2.188 Å, respectively).Furthermore, the Co 1 -CO bond distance of 1.769(9) Å in 1 is essentially consistent with the value that observed in A (1.769(8) Å).It may be inferred from these facts that A and 1 are sterically very similar with respect to coordination around Co 1 , and therefore, the steric effects from the substituents (H versus C 3 F 6 frame) of the C 3 =C 4 bonds on the nature of the rest of the molecules are rather weak.As a result, only the electronic effect will be at the origin of any variations in the measured parameters.
Interestingly, the C 3 =C 4 bond distance at 1.34(1) Å in the C 5 F 6 ring of 1 is somewhat longer than the corresponding bond distance in A (1.328(9) Å).This behavior confirms that the C 3 =C 4 bond in 1 still has partial single-bond character, as stated above.One might think the alkyne C 1 ≡C 2 bond length is a better criterion for Co-(π*C) interaction due to the bond weakening resulting from back donation of d-electrons on the Co atom into an empty π* orbital.The alkyne C 1 ≡C 2 distance of 1 is found to be 1.34(1) Å. Contrary to our expectation, this distance is slightly longer than that in A (1.31(1) Å) and is equal to a value found for the C 3 =C 4 bond in 1.We envisaged that electron-withdrawing C 5 F 6 ring in F 6 FOS should decrease in the amount of electron density of Co 1 atom in 1, which reduces the π*-back-donation ability of the atom, making this bond closer to the triple bond in free PhC≡CH.As a consequence, in the case of 1 the alkyne C 1 ≡C 2 bond was expected to be shortened in comparison to the same bond in A. However, the lengthening of the alkyne C 1 ≡C 2 distance in 1 relative to A is observed.The result provides evidence that the Co 1 atom in 1 may be acting as a better donor of electron density than the corresponding cobalt atom in A. As described above, the C 3 =C 4 bond distance in the C 5 F 6 ring of 1 has partial single-bond character, suggesting that the C 3 and C 4 atoms have sp 3 character to some extend.It is well known that electronegativity values vary with the bond order [13].So the phosphorus atoms in 1 may have a greater σ-donating ability than those in A because carbon atoms with sp 2 bonding generally have a greater electron-withdrawing power than those with sp 3 bonding.Hence, it can be concluded that by substitution of Z-dppe by F 6 FOS the electron density on the cobalt atom has not indeed decreased, but rather resulting in more  π-back-donation to the alkyne ligand.This lengthened the alkyne C 1 ≡C 2 distance in 1 is counter-balanced by the short Co 1 -Co 2 distance of 2.471(2) Å, somewhat shorter than the corresponding bond length of 2.495(2) Å found in A. In addition, the Co-C (alkyne) distances in 1 are meaningfully longer than the same bond distances in A (1.92(1) versus 1.920(8), 1.962(8) versus 1.929(7), 2.000(8) versus 1.983(8), 2.004(7) versus 1.991(9) Å, respectively).In comparison to A, it thus appears that in 1 the alkyne ligand remains directed away from Co 1 atom, whereas the Co 2 (CO) 3 fragment is located more close to Co 1 .The Co 2 -CO bond lengthsin 1 average to 1.773 Å, which is close to the value reported in (A) (1.765Å).

Concluding Remarks
The bidentate ligand F 6 FOS displaces two carbonyl groups from Co 2 (CO) 6 (μ-PhC≡CH) to give Co 2 (CO) 4 (μ-PhC≡CH) (F 6 FOS) (1), the structure of which is compared with that of a closely related cobalt carbonyl alkyne compound (A).The crystal structure of 1 confirms that the electron-withdrawing C 5 F 6 group in F 6 FOS makes the alkyne C≡C lengthened in comparison to the same bond in A.

2 .
straightforward fashion from Co 2 (CO) 6 (μ-PhC≡CH) and F 6 FOS in the presence of the oxidative-decarbonylation reagent Me 3 NO.The IR spectrum of 1 shows three strong CO absorptions in the range of 1900 -2100 cm −1 .In the 1 HNMR spectrum of 1, the alkyne proton resonance occurs at δ 5.0 ppm, which is 0.8 ppm upfield approximately from that of the parent complex Co 2 (CO) 6 (μ-PhC≡CH).The 31 P{1 H}NMR spectrum of 1 displays two phosphorus signals at δ 69 ppm and δ 80 ppm, showing a downfield coordination shift (δ −15 ppm in F 6 FOS).In contrast to F 6 FOS ligand (giving two sets of fluorine resonances in a 2:1 intensity ratio), 1 shows five types of inequivalent fluorine atoms in the 19 F NMR spectrum due to the asymmetry imposed by the formation of the chelate ring.There are two doublets of pseudotriplets at δ −132.61(J (FF) = 236 and 11 Hz) and −131.29 ppm (J (FF) = 236 and 11 Hz), which may be assigned to the central two fluorine atoms in the C 5 F 6 ring.In addition, the three pseudodoublets at δ −109.53 (J (FF) = 267 Hz), −109.60 (J (FF) = 271 Hz), and −111.34 ppm (J (FF) = 267 Hz) are attributable to rest of the four fluorine atoms in the ring.This spectrum also indicates strong germinal coupling between fluorine atoms above and below the plane of the ring.The molecular structures of 1 and F 6 FOS are shown in Figure 1 and Figure Pertinent structural parameters for F 6 FOS and 1 are compared in Table

Figure 1 .
Figure 1.The molecular structure of complex 1.

Figure 2 .
Figure 2. The molecular structure of ligand F 6 FOS.

Scheme 2 .
Scheme 2. The resonance forms of F 6 FOS.

Table 2 .
Comparison of structural parameters for F 6 FOS ligand and Complex 1.