Synthesis, characterisation and some reactions of alkynyl derivatives [Ni(η⁵-C₅H₅)(PPh₃)C≡CX]

Abstract

Oxalic acid catalyses the hydrolysis of the Ni(II) acetylide [Ni(η⁵-C₅H₅)(PPh₃)C≡CCH(OEt)₂] 1, to the alkynylaldehyde [Ni(η⁵-C₅H₅)(PPh₃)C≡CCHO] 2, in high yield. Condensation reactions of 2 with phenylhydrazine and dinitrophenylhydrazine in the presence of acetic acid, and with malononitrile and 3-phenyl-5-isoxazolone (C₉H₇NO₂) in the presence of triethylamine yield [Ni(η⁵-C₅H₅)(PPh₃)C≡CX] derivatives where X = CH=NNHC₆H₅3, CH=NNHC₆H₃(NO₂)₂-2,4 4, CH=C(CN)₂5, and CH=C₉H₅NO₂6. The reactivity of [Ni(η⁵-C₅H₅)(PPh₃)C≡CX] complexes towards [Co₂(CO)₈] is a function of X. Thus 1 and 2, where X = CH(OEt)₂ or CHO, react readily to give the bridging alkyne derivatives [{μ-η¹:η¹-Ni(η⁵-C₅H₅)(PPh₃)C≡CCH(OEt)₂}{Co₂(CO)₆}] 7, and [{μ-η¹:η¹-Ni(η⁵-C₅H₅)(PPh₃)C≡CCHO}{Co₂(CO)₆}] 8, but 5, where X is the strongly electron-withdrawing CH=C(CN)₂ group, does not react even after 24 h at room temperature. Furthermore, coordination of the alkyne to a Co₂(CO)₆ fragment appears to inhibit the normal reactions of the group X in 7 and 8. Thus the acetal grouping in 7 does not undergo oxalic acid-catalysed hydrolysis to an aldehyde in 8, and the aldehyde function in 8 does not undergo a Knoevenagel condensation with CH₂(CN)₂. The IR spectra of 1, and 3–6 show a single ν(C≡C) band the frequency of which decreases along the series X = CH(OEt)₂ > CH=NNHC₆H₅ > CH=NNHC₆H₃(NO₂)₂-2,4 > CH=C(CN)₂ ≈ CH=C₉H₅NO₂; that of 2 is anomalous in that it can show two ν(C≡C) bands. The UV-visible spectra of 1–6 show a strong charge transfer absorption band which increases in wavelength 1 < 3 < 2 < 4 < 5 < 6. These spectroscopic data and the ¹³C chemical shifts suggest that the (η⁵-C₅H₅)(Ph₃P)Ni moiety is a donor and, when X is an acceptor, charge separated cumulenic mesomers such as Ni⁺=C=C=X⁻ contribute to the description of the bonding in 1–6. This is not reflected in the molecular dimensions of 1, 2 and 5 as determined by X-ray diffraction. However, the crystal structure of [{μ-η¹:η¹-Ni(η⁵-C₅H₅)(PPh₃)C≡CCHO}{Co₂(CO)₆}], 8, shows that the C₂Co₂ cluster core is severely distorted because of the strong donor (Ni) and acceptor (CHO) substituents on the acetylenic carbon atoms.

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Introduction

Transition-metal σ-acetylide complexes have attracted considerable interest as precursors in the expanding field of organometallic derivatives that contain π-conjugated systems. These have potential applications in nonlinear optics,¹ liquid crystal technology, dendrimer science and nanostructured materials for molecular devices.^2,3 Attention has focused on the acetylide group because of its accessibility and the ease with which it can be incorporated into organometallic complexes and multimetallic assemblies, and because C≡C can provide a facile pathway for electron delocalization between interacting metal centres.^3–5 In this context, nickel cyclopentadienyl alkynyl derivatives having the general formulae [Ni(η⁵-C₅H₅)(PR₃)C≡CR]^6,7 are of current interest in donor–π-acceptor systems.^5,8–10

Here, we report studies into the synthesis, structures, and spectra of a series of [Ni(η⁵-C₅H₅)(PPh₃)C≡CX] complexes which were obtained from the alkynylaldehyde [Ni(η⁵-C₅H₅)(PPh₃)C≡CCHO] by condensation reactions with hydrazines and some active methylene compounds (Scheme 1). We have found that some, but not all of these alkynes, form [(μ-alkyne){Co₂(CO)₆}] complexes, and that the presence of the Co₂(CO)₆ moiety appears to affect some of the characteristic reactions of the group X. The structures of [Ni(η⁵-C₅H₅)(PPh₃)C≡CCHO] 2, [Ni(η⁵-C₅H₅)(PPh₃)C≡CCH=C(CN)₂] 5, and [{μ-η¹:η¹-Ni(η⁵-C₅H₅)(PPh₃)C≡CCHO}{Co₂(CO)₆}] 8, have been determined by single crystal X-ray diffraction and compared with those of related compounds.^8–12

Scheme 1 Synthetic route to the complexes 1–8 (CO ligands have been omitted from the Co₂(CO)₆ moiety for the sake of clarity). (i) H₂O/oxalic acid; (ii) PhNHNH₂/H⁺; (iii) C₆H₃(NO₂)₂NHNH₂/H⁺; (iv) CH₂(CN)₂/Et₃N; (v) C₉H₇NO₂/Et₃N; (vi) Co₂(CO)₈.

Experimental

[Ni(η⁵-C₅H₅)(PPh₃)Br] was prepared according to literature methods.¹³ Other chemicals were purchased from the usual sources.

All reactions were carried out under N₂ in dried and deoxygenated solvents unless stated otherwise. They were monitored by IR or NMR spectroscopy where appropriate.

IR spectra were recorded on a Perkin-Elmer 1710 FT spectrometer (peak positions are in cm⁻¹ with relative peak heights in parentheses) and UV-visible spectra were recorded on a UNICAM UV2 spectrometer (band positions are in nm with intensities ε in dm³ mol⁻¹ cm⁻¹). NMR spectra were obtained in CDCl₃ solution on a Jeol JNM-GX270 FT-NMR spectrometer. ¹H (270 MHz) and ¹³C (67.8 MHz) chemical shifts are reported downfield from tetramethylsilane as internal standard; ³¹P (109.3 MHz) spectra are referenced to 85% phosphoric acid with downfield shifts reported as positive. Analyses were carried out in the Microanalytical Laboratory, University College Dublin.

Preparation of [Ni(η⁵-C₅H₅)(PPh₃)C≡CCH(OEt)₂], 1

A solution of CuI (5 mg, 0.026 mmol), [Ni(η⁵-C₅H₅)(PPh₃)Br] (0.5 g, 1.1 mmol) and HC≡CCH(OEt)₂ (0.14 g, 1.1 mmol) in Et₃N (50 cm³) was stirred overnight. The solvent was removed under reduced pressure and the residue dissolved in Et₂O. This solution was filtered, chromatographed (basic alumina/Et₂O), and recrystallized from Et₂O–hexane to give green crystals of [Ni(η⁵-C₅H₅)(PPh₃)C≡CH(OEt)₂] 1 (0.43 g, 80%), mp 76–77 °C (Found: C, 70.6; H, 6.0%. C₃₀H₃₁NiO₂P requires C, 70.2; H, 6.1%). ν_max/cm⁻¹ (C≡C) 2112 (CH₂Cl₂); (C≡C) 2109 (KBr); λ_max/nm (CH₂Cl₂) 305 (ε/dm³ mol⁻¹ cm⁻¹ 15 200 ); λ_max/nm (CH₃CN) 303 (ε/dm³ mol⁻¹ cm⁻¹ 14 900); δ_H(CDCl₃) 7.26–7.72 [15 H, m, Ph], 5.16 [5 H, s, C₅H₅], 4.92 [1 H, s, C≡CCH], 3.10 [4 H, q, J(HH) 6.8 Hz, OCH₂] and 0.89 [6 H, t, J(HH) 6.8 Hz, CH₃]; δ_C(CDCl₃) 134.0 [d, J(CP) 11 Hz, o-Ph], 133.9 [d, J(CP) 48 Hz, i-Ph], 130.2 [d, J(CP) 2 Hz, p-Ph], 128.2 [d, J(CP) 10 Hz, m-Ph], 114.2 [s, NiC≡C], 92.7 [s, η⁵-C₅H₅], 82.0 (d, J(CP) 40 Hz, NiC≡C], 71.7 [s, C≡CCH], 59.3 s, OCH₂] and 15.1 [s, CH₃]; δ_P(CDCl₃) 41.24.

Preparation of [Ni(η⁵-C₅H₅)(PPh₃)C≡CCHO], 2

A solution of oxalic acid (0.03 g, 0.21 mmol) in H₂O (10 cm³) was added to one of [Ni(η⁵-C₅H₅)(PPh₃)C≡CH(OEt)₂] 1 (0.1 g, 0.2 mmol) in tetrahydrofuran (10 cm³). The reaction mixture was stirred for 1 h at room temperature and then shaken with CHCl₃ (100 cm³) and water (50 cm³). The CHCl₃ layer was washed with water, dried with magnesium sulfate, and chromatographed (alumina/CH₂Cl₂–Et₂O). The green band was evaporated to dryness and the residue crystallised from dichloromethane–hexane mixtures to give green crystals of [Ni(η⁵-C₅H₅)(PPh₃)C≡CCHO] 2 (0.07 g, 80%), mp 118–119 °C (Found: C, 71.6; H, 5.0%. C₂₆H₂₁NiOP requires C, 71.1; H, 4.8%); ν_max/cm⁻¹ (C≡C) 2079(7) and 2034(3), (CO) 1625(10), 1622(10) (CH₂Cl₂); (C≡C) 2081(7) and 2034(3), (CO) 1627(10) (KBr); λ_max/nm (CH₂Cl₂) 364 (ε/dm³ mol⁻¹ cm⁻¹ 16 100 ); λ_max/nm (CH₃CN) 359 (ε/dm³ mol⁻¹ cm⁻¹ 16 000); δ_H(CDCl₃) 8.41 [1 H, s, CHO], 7.37–7.70 [15 H, m, Ph] and 5.22 [5 H, s, C₅H₅]; δ_C(CDCl₃) 174.4 [s, CHO], 133.7 [d, J(CP) 11 Hz, o-Ph], 132.9 (d, J(CP) 49 Hz, i-Ph], 130.5 (d, J(CP) 2 Hz, p-Ph), 128.3 (d, J(CP) 11 Hz, m-Ph], 124.6 [s, NiC≡C], 122.8 (d, J(CP) 43 Hz, NiC≡C] and 93.27 [s, C₅H₅]; δ_P(CDCl₃) 42.64.

The reaction of [Ni(η⁵-C₅H₅)(PPh₃)C≡CCHO] with hydrazines

[Ni(η⁵-C₅H₅)(PPh₃)C≡CCHO] 2 (0.5 g, 1.1 mmol) and phenylhydrazine, H₂NNHC₆H₅ (0.11 g, 1.1 mmol), were dissolved in dichloromethane (50 cm³). A few drops of glacial acetic acid were added to the mixture which was then stirred for 2 h at 35 °C. The solution colour changed from brown to red. The solvent was removed in vacuo and the residue dissolved in CH₂Cl₂, layered with n-hexane and stored at −20 °C overnight. Filtration gave [Ni(η-C₅H₅)(PPh₃)C≡CCH=NNHC₆H₅] 3 as a brick-red powder (0.55 g, 90%), decomp. > 150 °C (Found: C, 72.1; H, 5.1; N, 5.4%. C₃₂H₂₇NiN₂P requires C, 72.6; H, 5.1; N, 5.3%); ν_max/cm⁻¹ (C≡C) 2090(4.5), (C=N) 1593(10) (CH₂Cl₂); (C≡C) 2091(4.3), (C=N) 1595(10) (KBr); λ_max/nm (CH₂Cl₂) 350 (ε/dm³ mol⁻¹ cm⁻¹ 12 200); λ_max/nm (CH₃CN) 354 (ε/dm³ mol⁻¹ cm⁻¹ 10 100); δ_H(CDCl₃) 6.51–7.76 [20 H, m, Ph and Ph₃P), 5.25 [5 H, s, C₅H₅], 5.09 [1 H, s, NNH] and 4.22 [1 H, s, CH]; δ_C(CDCl₃) 120.1–143.0 (Ph, PPh₃), 113.0 [s, CH=N], 93.1 [s, NiC≡C], 92.4 [s, η⁵-C₅H₅] and 91.4 [d, J(CP) 48 Hz, NiC≡C].

A similar procedure but with NH₂NHC₆H₅ replaced by NH₂NHC₆H₃(NO₂)₂-2,4 gave [Ni(η⁵-C₅H₅)(PPh₃)C≡CCH=NNHC₆H₃(NO₂)₂-2,4] 4 (0.67 g, 95%) decomp. > 150 °C (Found: C, 61.2; H, 4.0; N, 8.7%. C₃₂H₂₅NiN₄O₄P requires C, 62.1; H, 4.1; N, 9.0%); ν_max/cm⁻¹ (C≡C) 2082 and (NO₂) 1617, 1600 (CH₂Cl₂); (C≡C) 2082 and (NO₂) 1615, 1601, (KBr); λ_max/nm (CH₂Cl₂) 394 (ε/dm³ mol⁻¹ cm⁻¹ 19 600 ); λ_max/nm (CH₃CN) 394 (ε/dm³ mol⁻¹ cm⁻¹ 18 500); δ_H(CDCl₃) 9.17 [1 H, s, CH=N], 8.26 [3 H, m, C₆H₃], 6.50–7.81 [15 H, m Ph₃P], 6.23 [1 H, m, NNH) and 5.20 [5 H, s, C₅H₅]; δ_C 118–144.4 [m, C₆H₃(NO₂)₂], 133.8 [d, J(CP) 41 Hz, o-Ph), 133.1 [d, J(CP) 50 Hz, i-Ph], 130.5 [d, J(CP) 2 Hz, p-Ph], 128.2 [d, J(CP) 11 Hz, m-Ph], 117.0 [s, CH=N], 110.9 [s, NiC≡C], 108.3 [d, J(CP) 48 Hz, NiC≡C] and 93.5 [s, η⁵-C₅H₅].

The reaction of [Ni(η⁵-C₅H₅)(PPh₃)C≡CCHO] with malononitrile and 3-phenyl-5-isoxazolone

A solution of [Ni(η⁵-C₅H₅)(PPh₃)C≡CCHO] 2 (0.1 g, 0.22 mmol) and CH₂(CN)₂ (0.015 g, 0.22 mmol) in CH₂Cl₂ (10 cm³) was stirred for 10 min. Et₃N (3 drops) was added. A rapid colour change from green to deep red took place. The mixture was stirred for 2 h, and then the solvent removed at reduced pressure. The residue was crystallised from chloroform–hexane mixtures at −20 °C overnight and then filtered to give a red powder, [Ni(η⁵-C₅H₅)(PPh₃)C≡CCH=C(CN)₂] 5 (0.098 g, 90%), decomp. > 150 °C (Found: C, 71.4; H, 4.4; N, 5.8%. C₂₉H₂₁N₂NiP requires C, 71.4; H, 4.3; N, 5.7%); ν_max/cm⁻¹ (C≡C) 2050(10), (CN) 2218(1), 2226(1), (C=C) 1534(2.1) (CH₂Cl₂); (C≡C) 2049(10), (CN) 2216(1), 2225(1), (C=C) 1532(2.4) (KBr); λ_max/nm (CH₂Cl₂) 469 (ε/dm³ mol⁻¹ cm⁻¹ 13 000); λ_max/nm (CH₃CN) 463 (ε/dm³ mol⁻¹ cm⁻¹ 11 200); δ_H(CDCl₃) 7.33–7.62 [15 H, m, Ph₃P], 6.43 [1 H, s, CH], 5.25 [5H, s, C₅H₅]; δ_C(CDCl₃) 152.8 [d, J(CP) 48 Hz, NiC≡C], 143.7 [s, HC=C], 134.1 [d, J(CP) 11 Hz, o-Ph], 133.2 (d, J(CP) 50 Hz, i-Ph], 131.4 (d, J(CP) 3 Hz, p-Ph], 129.1 [d, J(CP) 11 Hz, m-Ph], 121.5 [s, NiC≡C], 114.5 [s, C≡N], 114.9 [s, C≡N], 94.2 [s, η⁵-C₅H₅] and 86.3 [d, J(CP) 2.1 Hz, C(CN)₂].

Using the same procedure 3-phenyl-5-isoxazolone, C₉H₇NO₂ and [Ni(η⁵-C₅H₅)(PPh₃)C≡CCHO] 2 gave purple [Ni(η⁵-C₅H₅)(PPh₃)C≡CCH=C₉H₅NO₂] 6 (0.11g, 85%), decomp. > 50 °C (Found: C, 72.1; H, 4.5; N, 2.3%. C₃₂H₂₅NiN₄O₄P requires C, 72.2; H, 4.5; N, 2.4%); ν_max/cm⁻¹ (C≡C) 2050(6), (CO) 1690(10) (CH₂Cl₂); (C≡C) 2051(6), (CO) 1692(10), (KBr); λ_max/nm (CH₂Cl₂) 511 (ε/dm³ mol⁻¹ cm⁻¹ 27 800 ); λ_max/nm (CH₃CN) 507 (ε/dm³ mol⁻¹ cm⁻¹ 26 600); δ_H(CDCl₃) 6.95–7.59 [21 H, m, Ph₃P, Ph, CH=] and 5.22 [5H, s, C₅H₅]; δ_C(CDCl₃) 140.3 [d, J(CP) 48 Hz, NiC≡C], 133.9 [d, J(CP) 12 Hz, o-Ph], 133.0 (d, J(CP) 50 Hz, i-Ph], 128.7–131.4 [m, Ph, isoxazolone], 130.7 [d, J(CP) 2 Hz, p-Ph], 128.6 [d, J(CP) 11 Hz, m-Ph], 120.7 [s, NiC≡C] and 96.7 [s, η⁵-C₅H₅].

The reaction of [Ni(η⁵-C₅H₅)(PPh₃)C≡CX] {X = CH(OEt)₂, CHO and CHC(CN)₂} with [Co₂(CO)₈]

[Co₂(CO)₈] (0.07 g, 0.22 mmol) was added to a solution of [Ni(η⁵-C₅H₅)(PPh₃)C≡CX] {0.22 mmol; X = CH(OEt)₂ and CHO} in tetrahydrofuran (20 cm³). After 2 h the solution was filtered, the solvent removed at reduced pressure and the residue chromatographed (alumina/petroleum ether, bp 40–60 °C). The product was recrystallised from ether–hexane mixtures to give red crystals.

There was no reaction between [Co₂(CO)₈] and 5 where X = CHC(CN)₂.

[{μ-η¹:η¹-Ni(η⁵-C₅H₅)(PPh₃)C≡CCH(OEt)₂}{Co₂(CO)₆}] 7 (0.14 g, 80%), decomp. > 200 °C (Found: C, 63.1; H, 4.5%. C₃₆H₃₁O₈PCo₂Ni requires C, 63.5, H, 4.6%); ν_max/cm⁻¹ terminal (CO) 2058(8), 2031(6) and 1997(10) (CH₂Cl₂); terminal (CO) 2056(8), 2031(6) and 1998(10) (KBr); λ_max/nm (CH₂Cl₂) 400 (ε/dm³ mol⁻¹ cm⁻¹ 12 900); λ_max/nm (CH₃CN) 401 (ε/dm³ mol⁻¹ cm⁻¹ 13 300); δ_H(CDCl₃) 7.24–7.75 [15 H, m, Ph₃P], 5.17 [5 H, s, C₅H₅], 4.90 [1 H, s, CH(OEt)₂], 3.05 [4 H, q, J(HH) 7.3 Hz, CH₂] and 0.9 [6 H, t, J(HH), CH₃]; δ_C(CDCl₃) 206.4 [s, terminal-CO], 134.2 [d, J(CP) 12 Hz, o-Ph], 133.6 [d, J(CP) 50 Hz, i-Ph], 130.5 [d, J(CP) 2 Hz, p-Ph], 128.0 [d, J(CP) 11 Hz, m-Ph], 106.5 [s, NiC≡C], 96.0 [s, C₅H₅], 75.2 [d, J(CP) 48 Hz, NiC≡C], 70.3 [s, CH(OEt)₂], 60.1 [s, CH₂CH₃] and 15.1 [s, CH₂CH₃].

[{μ-η¹:η¹-Ni(η⁵-C₅H₅)(PPh₃)C≡CCHO}{Co₂(CO)₆}] 8 (0.14 g, 85%), decomp. 150 °C (Found: C, 53.0; H, 3.0%. C₃₂H₂₁O₇PCo₂Ni requires C, 53.0, H, 2.9%); ν_max/cm⁻¹ terminal (CO) 2072(6), 2034(6) and 2007(10), (CHO) 1654(8) (CH₂Cl₂); terminal (CO) 2071(6), 2032(6) and 2006(10), (CHO) 1654(1) (KBr); λ_max/nm (CH₂Cl₂) 406 (ε/dm³ mol⁻¹ cm⁻¹ 12 000); λ_max/nm (CH₃CN) 402 (ε/dm³ mol⁻¹ cm⁻¹ 14 300); δ_H(CDCl₃) 8.43 [1 H, s, CHO], 7.42–7.70 [15 H, m, Ph₃P] and 5.18 [5 H, s, C₅H₅]; δ_C(CDCl₃) 201.8 [s, terminal CO], 185.7 [s, CHO], 133.8 [d, J(CP) 11 Hz, o-Ph], 132.5 [d, J(CP) 50 Hz, i-Ph], 131.2 [d, J(CP) 2 Hz, p-Ph], 128.8 [d, J(CP) 10 Hz, m-Ph], 118.0 [s, NiC≡C], 107.1 [d, J(CP) 48 Hz, NiC≡C] and 96.6 [s, C₅H₅].

Structures of [Ni(η⁵-C₅H₅)(PPh₃)C≡CCHO] 2, [Ni(η⁵-C₅H₅)(PPh₃)C≡CCH=C(CN)₂] 5, and [{μ-η¹:η¹-Ni(η⁵-C₅H₅)(PPh₃)C≡CCHO}{Co₂(CO)₆}] 8

The crystal data for 2, 5 and 8 are summarised in Table 1. The structures were solved using the Patterson function of SHELXS97,¹⁴ and refined by full-matrix least squares using SHELXL97.¹⁴ The diagrams were obtained using the PLATON¹⁵ and ORTEX¹⁶ programs. The molecular structures and atom labelling for 2, 5 and 8 are illustrated in Figs. 1, 2 and 3, respectively. Selected bond lengths and angles are listed in Table 2 together with those previously reported for 1.¹⁰

Table 1 Crystal data and structure refinement for [Ni(η⁵-C₅H₅)(PPh₃)C≡CCHO] 2, [Ni(η⁵-C₅H₅)(PPh₃)C≡CCHC(CN)₂] 5, and [{μ-η¹:η¹-Ni(η⁵-C₅H₅)(PPh₃)C≡CCHO}{Co₂(CO)₆}] 8

	2	5	8
Empirical formula	C26H21NiOP	C29H21N2NiP	C32H21Co2NiO7P
Formula weight	439.11	487.16	725.03
Temperature/K	297(2)	290(1)	294(2)
Wavelength/Å	0.71073	0.71073	0.71073
Crystal system, space group	Orthorhombic, Pbca	Triclinic, P1̄	Monoclinic, I2/a
a/Å	10.2026(14)	10.6684(9)	18.698(3)
b/Å	16.4365(18)	11.0455(10)	16.8087(11)
c/Å	26.152(2)	11.7757(8)	21.090(3)
α/°	90	95.774(7)	90
β/°	90	113.486(7)	109.150(14)
γ/°	90	106.520(8)	90
Volume/Å^3	4385.5(8)	1183.46(17)	6261.3(14)
Z, Calculated density/Mg m^−3	8, 1.330	2, 1.367	8, 1.538
Absorption coefficient/mm^−1	0.971	0.907	1.739
Reflections collected/unique	6439/5249	4396/3658	7152/7152
Final R indices {I > 2σ(I)]	R 1 = 0.062, wR2 = 0.099	R 1 = 0.037, wR2 = 0.046	R 1 = 0.047, wR2 = 0.078
R indices (all data)	R 1 = 0.168, wR2 = 0.129	R 1 = 0.093, wR2 = 0.096	R 1 = 0.143, wR2 = 0.092

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Table 2 Selected molecular dimensions, bond lengths in Å, bond angles in °, for [Ni(η⁵-C₅H₅)(PPh₃)C≡CCH(OEt)₂] 1, [Ni(η⁵-C₅H₅)(PPh₃)C≡CCHO] 2, [Ni(η⁵-C₅H₅)(PPh₃)C≡CCH=C(CN)₂] 5, and [{μ-η¹:η¹-Ni(η⁵-C₅H₅)(PPh₃)C≡CCHO}{Co₂(CO)₆}] 8

	1	2	5	8 ^b
a Cg1 is the centroid of the η^5–C5H5 ligand. b The range of bond lengths for Co–CO is 1.765(6) to 1.815(6) Å and for C≡O is 1.126(6) to 1.144(5) Å and the Co–C≡O bond angles are in the range 176.2(6)–178.7(6)° (cf. refs. 20, 24–26). Co1–C1–C2 64.6(2), Co2–C1–C2 67.2(3), Co1–C2–C1 76.8(2), Co2–C2–C1 74.1(3)°.
Ni1–P1	2.1376(10)	2.1418(11)	2.1451(6)	2.1430(11)
Ni1–C1	1.840(5)	1.830(5)	1.833(2)	1.867(4)
C1–C2	1.193(5)	1.205(6)	1.214(3)	1.333(5)
C2–C3	1.474(5)	1.432(8)	1.402(3)	1.460(5)
C11–C12	1.391(5)	1.364(7)	1.346(5)	1.386(6)
C12–C13	1.389(5)	1.418(7)	1.385(5)	1.376(6)
C13–C14	1.406(6)	1.370(7)	1.391(5)	1.400(6)
C14–C15	1.369(6)	1.398(7)	1.356(5)	1.373(6)
C11–C15	1.396(6)	1.426(7)	1.394(5)	1.405(6)
Ni1–C11	2.059(4)	2.127(5)	2.111(3)	2.099(4)
Ni1–C12	2.128(4)	2.099(6)	2.123(3)	2.152(4)
Ni1–C13	2.094(4)	2.097(5)	2.062(30)	2.084(4)
Ni1–C14	2.133(4)	2.121(5)	2.134(30)	2.119(4)
Ni1–C15	2.128(4)	2.066(4)	2.101(3)	2.144(4)
Ni1–Cg1^a	1.7458(4)	1.7354(6)	1.7523(4)	1.7612(6)
C3–O1/C3–C4*		1.147(6)	1.345(4)*	1.179(5)
C3–H3		0.93(3)	0.96(3)	0.93
Others			C4–C5 1.434(4)	Co1–Co2 2.4854(11)
			C4–C7 1.419(5)	Co1–C1 2.084(3)
			C7–N8 1.134(5)	Co2–C1 2.053(4)
				Co1–C2 1.934(4)
				Co2–C2 1.969(5)
P1–Ni1–C1	90.90(10)	95.65(14)	89.93(7)	93.34(11)
Ni1–C1–C2	178.7(3)	176.4(4)	176.6(2)	154.2(3)
Cg1–Ni1–P1	136.6	134.24(4)	137.27(3)	131.19(4)
Cg1–Ni1–C1	132.6	130.10(13)	131.92(9)	135.46(10)
C1–C2–C3	173.9(4)	174.6(6)	177.8(3)	145.0(4)
C2–C3–O1 (C4)*		130.8(6)	125.0(3)*	126.3(5)
Ni1–C1–Co1				128.5(2)
Ni1–C1–Co2				134.3(2)

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Fig. 1 Molecular structure and atom labeling for [Ni(η⁵-C₅H₅)(PPh₃)C≡CCHO] 2. Displacement ellipsoids are drawn at the 30% probability level.

Fig. 2 Molecular structure and atom labeling for [Ni(η⁵-C₅H₅)(PPh₃)C≡CCHC(CN)₂] 5. Displacement ellipsoids are drawn at the 30% probability level.

Fig. 3 Molecular structure and atom labeling for [{μ-η¹:η¹-Ni(η⁵-C₅H₅)(PPh₃)C≡CCHO}{Co₂(CO)₆}] 8. Displacement ellipsoids are drawn at the 10% probability level.

CCDC reference numbers 165064–165066.

See http://www.rsc.org/suppdata/dt/b1/b104442g/ for crystallographic data in CIF or other electronic format.

Results and discussion

Synthesis and reactivity studies

Transition metal acetylides of the general type MC≡CX provide an attractive starting point for the synthesis of organometallic donor–π-acceptor systems. They may often be easily prepared from a CuI–catalysed reaction of metal halides with terminal alkynes HC≡CX,⁷ and when X contains a suitable functional group, it may be modified by conventional organic reactions. In the present work, the reaction of [Ni(η⁵-C₅H₅)(PPh₃)Br] with the alkyne acetal HC≡CCH(OEt)₂ gives [Ni(η⁵-C₅H₅)(PPh₃)C≡CCH(OEt)₂] 1, which is hydrolysed in the presence of oxalic acid to the aldehyde [Ni(η⁵-C₅H₅)(PPh₃)C≡CCHO] 2. This undergoes condensation reactions with hydrazines in the presence of acetic acid to give the hydrazones [Ni(η⁵-C₅H₅)(PPh₃)C≡CCH=NNHAr] {Ar = C₆H₅3, or C₆H₃(NO₂)₂-2,4 4}, and with active methylene compounds such as malononitrile or phenylisoxazolone in the presence of triethylamine to give [Ni(η⁵-C₅H₅)(PPh₃)C≡CCH=C(CN)₂] 5, and [Ni(η⁵-C₅H₅)(PPh₃)C≡CCH=C₉H₅NO₂] 6, respectively (Scheme 1).

Compounds 1–6 are alkynes and would be expected to react with [Co₂(CO)₈] to give the well-known alkyne complexes [(μ²-alkyne)Co₂(CO)₆]. Thus 1 and 2, respectively, gave the derivatives [{μ-η¹:η¹-Ni(η⁵-C₅H₅)(PPh₃)C≡CCH(OEt)₂}{Co₂(CO)₆}] 7, and [{μ-η¹:η¹-Ni(η⁵-C₅H₅)(PPh₃)C≡CCHO}{Co₂(CO)₆}] 8, within two hours. However 5, [Ni(η⁵-C₅H₅)(PPh₃)C≡CCHC(CN)₂], does not undergo this reaction even after 24 h at room temperature, so clearly the ability of the alkynes [Ni(η⁵-C₅H₅)(PPh₃)C≡CX] to react with [Co₂(CO)₈] depends on X and is inhibited by the strongly electron-withdrawing C(CN)₂ group. Furthermore, some characteristic reactions of X appear to be affected by complexation of C≡C to the {Co₂(CO)₆} moiety so that the acetal group in 7 is not hydrolysed to the aldehyde in the presence of oxalic acid, and the aldehyde group in 8 does not undergo a Knoevenagel condensation with CH₂(CN)₂ or react with the Wittig reagent prepared from [Ph₃PCH₂Br]Br and ⁿBuLi. These reactions are facile for the free [Ni(η⁵-C₅H₅)(PPh₃)C≡CX].

A rationalization of these observations will be discussed below.

Spectra and bonding

The IR spectra of complexes 1–8 show many absorption bands, but the ones which yield useful structural information are those due to the ν(C≡C), aldehyde ν(CO) and ν(CN) modes of the C≡CX moieties and the ν(CO) vibrations of the Co₂(CO)₆ groups.

The bands due to the ν(C≡C) vibrations of the [Ni(η⁵-C₅H₅)(PPh₃)C≡CX] complexes are readily identified. Their frequencies (Table 3) decrease along the series X = CH(OEt)₂ > CH=NNHC₆H₅ > CH=NNHC₆H₃(NO₂)₂ > CH=C(CN)₂ ≈ CH=C₉H₅NO₂. This may be correlated with the increasing electron-withdrawing power of X and rationalized by assuming that the ground state electronic structure of these complexes may be described as a resonance hybrid of three mesomers: the acetylenic form A and charge-separated forms B, C and D (Fig. 4).

Table 3 IR, ¹³C NMR and UV/visible spectroscopic data for [Ni(η⁵-C₅H₅)(PPh₃)C≡CX] complexes

	IR spectra^a		^13C spectra^b			UV/visible spectra^c
X	ν(C≡C)^d	ν(C≡C)^e	C1	C2	C5H5	λ max (ε)^e	λ max (ε)^f
a Peak positions in cm^−1 with relative peak heights in parentheses for 2. b Chemical shifts in ppm downfield from Me4Si as an internal standard. In parentheses are multiplicity (d = doublet) and coupling constants JPC in Hz. Other resonances are singlets. Spectra recorded in CDCl3 solution. c Absorption band maxima in nm with band intensities in parentheses in 10^3 dm^3 mol^−1 cm^−1. d Spectra measured as KBr discs. e Spectra measured in CH2Cl2 solution. f Spectra measured in CH3CN solution.
CH(OEt)2, 1	2109	2112	82.0 (d, 40)	114.2	92.7	305 (15.2)	303 (14.9)
CHO, 2	2081 (2), 2034 (3)	2079 (2), 2034 (3)	122.8 (d, 43)	124.6	93.3	364 (16.1)	359 (16.0)
CHNNHC6H5, 3	2091	2090	91.4 (d, 48)	93.1	92.4	350 (12.0)	354 (10.1)
CHNNHC6H3(NO2)2-2,4, 4	2082	2082	108.3 (d, 48)	110.9	93.5	394 (19.6)	394 (18.5)
CHC(CN)2, 5	2049	2050	152.8 (d, 48)	121.5	94.2	469 (13.0)	463 (11.2)
CHC9H5NO2, 6	2051	2050	140.3 (d, 48)	120.7	96.7	511 (27.8)	507 (26.6)

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Fig. 4 Resonance structures of the [Ni(η⁵-C₅H₅)(PPh₃)C≡CX] complexes.

A is always the most important, but there will be an increasing contribution from B, C and D as the electron-withdrawing ability of X increases. The decreasing C≡C bond order results in a lower ν(C≡C) frequency as has been observed by Gladysz and co-workers in the metalloalkynes Re(η⁵-C₅Me₅)(NO)(PPh₃)C≡CC≡CC(OMe)(fluorenyl)¹⁷ which show ν(C≡C) bands at 2018 and 2172 cm⁻¹ whereas in [Re(η⁵-C₅Me₅)(NO)(PPh₃)C≡CC≡CAr₂]⁺ they are found at 1993 and 1902 cm⁻¹ (H₂CAr₂ = fluorene).¹⁷

The IR spectrum of 2 (X = CHO) (Table 4) is different from those of 1, and 3–6. It displays two ν(C≡C) absorption bands in the solid state at 2034 and 2081 cm⁻¹ (intensity ratio 1 ∶ 3). In solution, the relative intensities vary with the ca. 2080 cm⁻¹ band increasing in importance along the series MeCN ≈ CH₂Cl₂ (1 ∶ 3) < toluene ≈ tetrahydrofuran (1 ∶ 5) < hexane (0 ∶ 1) so that for the latter solvent the ca. 2030 cm⁻¹ band is absent. These changes are reversible. The aldehydic ν(CO) band of 2 has two equal components in acetonitrile or dichloromethane solutions ca. 1621 and 1626 cm⁻¹, but not in toluene, tetrahydrofuran, hexane, or in the solid state. Its frequencies are very low even when compared with that of benzaldehyde (1714 and 1702 cm⁻¹ in hexane and CH₂Cl₂, respectively) and PhC≡CCHO (1661 and 1672 cm⁻¹). This frequency variation may be attributed to (a) coupling of the ν(C≡C) and ν(C=O) vibrations which would raise the frequency of the former and lower that of the latter but which is likely to be limited, and (b) the contribution made to the ground state electronic structure of 2 by the cumulenic mesomers B and C (Fig. 4) with their CO⁻ moieties. The latter also accounts for the marked solvent-dependence of this absorption band. Hydrogen-bonding of the negatively charged oxygen atom to solvents such as dichloromethane and acetonitrile (but not hexane) would be expected to lower the frequency of the ν(CO) vibration still further. However, we are unable to account for the presence of two ν(C≡C) bands in the spectrum of 2. This feature is commonly observed for MC≡CX complexes and is usually attributed to Fermi resonance [e.g.ref. 18]. It may be due to the presence of two species, but there is no evidence for this in the case of 2 either in the solid state (X-ray crystallography) or in solution (NMR spectra).

Table 4 The effect of solvent on the ν(CC) and ν(CO) absorption bands in the IR spectrum of [Ni(η⁵-C₅H₅)(PPh₃)C≡CCHO]

Solvent	ν(C≡C) absorption bands^a	ν(CO) absorption bands^a
a Frequency (cm^−1) with relative peak heights in parentheses. b Values for PhC≡CCHO.
CH3CN	2078, 2031 (3 ∶ 1)	1626, 1621 (1 ∶ 1)
CH2Cl2	2079, 2034 (3 ∶ 1)	1625, 1622 (1 ∶ 1)
C6H5CH3	2082, 2026 (5 ∶ 1)	1635
THF	2082, 2026 (5 ∶ 1)	1635
Hexane	2086	1645
KBr	2081, 2034 (3 ∶ 1)	1627
CH2Cl2^b	2192	1661
CH3C6H5^b	2190	1664
Hexane^b	2193	1672

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The IR spectra of 7 and 8 show absorption bands due to the ν(CO) ligands of the Co₂(CO)₆ moiety (Table 5). Their frequencies are a function of X and are ca. 10 cm⁻¹ higher when X is the electron-withdrawing CHO group as compared with CH(OEt)₂. This is attributed to the contribution that mesomers such as G (Fig. 5) make to the electronic structure of these complexes. The same frequency relationship is observed for the ν(CO) bands of [{μ-PhC≡CX}{Co₂(CO)₆}] (X = CH(OEt)₂9, and CHO 10),¹⁹ but whereas the frequencies for 9 and 10 are similar to those of other [(μ-alkyne)Co₂(CO)₆] derivatives, they are ca. 30 cm⁻¹ higher on average than are those of 7 and 8. This is attributed to the electron-donating ability of the (η⁵-C₅H₅)(Ph₃P)Ni moiety and the contribution made by mesomers such as H to an overall description of the bonding in 7 and 8 (Fig. 5). Similar mesomers clearly do not contribute to the bonding of 9 and 10. Similar effects have been observed for [{μ-η¹:η¹-(η⁵-C₅H₅)(OC)₂FeC≡CH}{Co₂(CO)₆}] {ν(CO) = 2060, 2011, 1986, 1961 cm⁻¹}.²⁰ The spectra of 8 and 10 also show an additional weaker band at 1654 and 1663 cm⁻¹ respectively due to their aldehydic ν(CO) vibration. This is much higher than that of the uncomplexed aldehyde 2 but very little changed from PhC≡CCHO. The relative frequencies reflect the greater electron-richness of the Co₂C₂ cluster in 8 as compared with 10. It points to some Ni⋯CHO electronic interaction in 8 that is greatly reduced as compared with that in the parent alkyne 2.

Table 5 C–O stretching frequencies in the IR spectra of [{μ-η¹:η¹-Ni(η⁵-C₅H₅)(PPh₃)C≡CX}{Co₂(CO)₆}] {X = CH(OEt)₂7 and CHO 8} and [{μ-η¹:η¹-PhC≡CX}{Co₂(CO)₆}] {X = CH(OEt)₂9 and CHO 10}

Compound	Absorption bands
a Peak positions (cm^−1) with relative peak heights in parentheses. The bands at ca. 1660 cm^−1 are due to the aldehyde group.
7 {X = CH(OEt)2}	2058 (8)	2031 (6)	1997 (10, br)
8 {X = CHO}	2072 (6)	2034 (6)	2007 (10, br)		1654 (1)
9 {X = CH(OEt)2}	2093 (3)	2056 (10)	2030 (8)	2024 (10)
10 {X = CHO}	2102 (4)	2067 (10)	2041 (10)	2035 (sh)	1663 (1)

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Fig. 5 Resonance structures of [{μ-η¹-η¹-Ni(η⁵-C₅H₅)(PPh₃)C≡CCHO}{Co₂(CO)₆}].

The ¹H NMR spectra of 1–8 show the resonances characteristic of the η⁵-C₅H₅ and PPh₃ ligands, and the group X. They require no further discussion.

The ¹³C NMR spectra show the anticipated resonances. The most interesting are two due to the alkynyl C atoms (Table 3). One is a doublet. It is assigned to the Ni–C₁ atom coupled to the coordinated P atom. The other is a singlet and is assigned to the remote Ni–C≡C₂ atom as there is no coupling to ³¹P. The chemical shifts of both are a function of X so that when X is electron-donating such as CH(OEt)₂, the C₁ resonance is found upfield (δ 82.0) of that due to C₂ (δ 114.2). Both carbon atoms are deshielded when X is electron-withdrawing, but C₁ is affected more than C₂ so that when X is, for example, CH=C(CN)₂, the C₁ resonance is found downfield (δ 152.8) of that due to C₂ (δ 121.5). C₁ is increasingly deshielded along the series X = CH(OEt)₂, CH=NNHC₆H₅, CH=NNHC₆H₃(NO₂)₂-2,4, CHO, CH=C₉H₅NO₂, CH=C(CN)₂. These effects are consistent with the suggestion made above that the charge separated mesomers B, C and D (Fig. 4) make an increasing contribution to the ground state structure of these complexes as the electron-withdrawing ability of X increases. The series for C₁ arises, in part, from its increasing carbene character/decreasing alkyne character as a consequence of rehybridisation of the NiCCX chain, and, in part, from its increasing positive charge as a consequence of charge separation. The series is very similar to that observed for the effect of X on the ν(C≡C) frequency except for the anomalous IR spectrum of 2. C₂ becomes increasingly deshielded for X = CH=NNHC₆H₅, CH=NNHC₆H₃(NO₂)₂-2,4, CH(OEt)₂, CH=C₉H₅NO₂, CH=C(CN)₂, CHO. This is a somewhat different series from that for C₁, but similarly it has the electron-withdrawing X giving rise to the most deshielded carbon atoms. The differences between the two series are probably a reflection of the differing contributions made by mesomers A–D.

The UV-visible spectrum of 1 (Table 3) shows a relatively intense absorption band at 305 nm. It is present in the spectra of 2–6 but its wavelength increases with the increasing electron-withdrawing ability of X to 469 nm when X = CHC(CN)₂ and 511 nm when X = CHC₉H₅NO₂. It is assigned to an electronic transition from the ground state of these complexes to an excited state. As discussed above, the electronic structure of the ground state may be described as a resonance hybrid in which the alkynyl mesomer A predominates over the charge separated cumulenic mesomers. In the excited state, the same mesomers probably contribute to an overall description of the bonding but B, C and D are now the more important. Consequently, the electron-withdrawing groups X preferentially stabilise the excited state, reduce the energy of the electronic transition and increase the wavelength of the observed absorption band. The increasing conjugation present in 5 and 6 may also contribute to lowering the energy of this electronic transition as may the presence of the NHC₆H₃(NO₂)₂-2,4 moiety in 4.

The NMR and UV-visible spectra of 7 and 8 are similar to those of other [μ-(alkyne)Co₂(CO)₆] species and will not be discussed further.

Crystal structures of [Ni(η⁵-C₅H₅)(PPh₃)C≡CCHO] 2, [Ni(η⁵-C₅H₅)(PPh₃)C≡CCHC(CN)₂] 5, and [{μ-η¹:η¹-Ni(η⁵-C₅H₅)(PPh₃)C≡CCHO}{Co₂(CO)₆}] 8

The structures of 2, 5 and 8 are illustrated in Figs. 1, 2 and 3, respectively, and selected molecular dimensions are listed in Table 2 together with those previously reported for 1 (isomer B). All four compounds have the expected half-sandwich structures in which the C₅ and P–Ni–C planes are virtually orthogonal. The P–Ni–C plane may be coincident with a σ_v plane of the C₅ ring (α isomers in Fig. 6) or perpendicular to it (β isomer in Fig. 6). There are two possible α isomer (Fig. 6) and intermediate situations are possible. Complex 2 adopts the α_a structure, 5 the α_b structure, and 1B and 8 the β structure but not completely so. The α isomers show a distortion of the C₅ ring towards a diene with two short non-adjacent C–C bonds (C₁₁–C₁₂/C₁₃–C₁₄ in 2, and C₁₁–C₁₂/C₁₄–C₁₅ in 5) and one particularly short Ni–C_ring bond (to C₁₅ in 2 and C₁₃ in 5). The β isomers show distortions of the ring towards an ene-allyl with a short ene C–C bond (C₁₄–C₁₅ in both 1B and 8) and two longer bonds (C₁₁–C₁₅/C₁₃–C₁₄) and two Ni–C_ring ring bonds (to C₁₁ and C₁₃) which are shorter than the other three; differences which are less marked for 1B than for 8. These distortions are a reflection of the loss of degeneracy of the cyclopentadienyl E₁ and E₂ π orbitals, their different energies and their differential occupancy in these complexes. This has been discussed in ref. 21 and many other papers cited therein.

Fig. 6 Distortions of the η⁵-C₅H₅ ligand in the [Ni(η⁵-C₅H₅)(PPh₃C≡CX] complexes.

The P–Ni–C angles lie in the range 89.9–95.7° and are comparable with values of 93.47(5)° in [Ni(η⁵-C₅H₅)(PPh₃)C≡CC≡CH],⁹ 93.30(10)° in [Ni(1-Me-indenyl)(PPh₃)C≡CC₆H₅],¹¹ and 95.4(7)° in the benzonitrile derivative [Ni(η⁵-C₅H₅)(PPh₃)NCC₆H₄NH₂][PF₆].¹² The Ni–P distances lie in the range 2.1376(10)–2.1451(6) Å. They are identical within experimental error and similar to those of related compounds.^8–11

The Ni–C–C–X systems are almost linear for 1, 2 and 5 with Ni–C₁–C₂ and C₁–C₂–C₃ angles lying between 173.9(4)° and 178.7(3)°. Their Ni–C₁ distances are, within experimental error, independent of X, and although their C₁–C₂ bond lengths increase along the series 1 < 2 < 5, the changes are small and scarcely outside of experimental error, but they do follow the trend expected if the mesomeric form B makes an increasing contribution to the overall bonding with increasing electron-withdrawing power of X. However, our observations are essentially in agreement with those of Humphrey and co-workers who showed that Ni–C_alkynyl and C≡C bond lengths do not vary significantly in a series of [Ni(η⁵-C₅H₅)(PPh₃)(L)C≡CAr] complexes with NO₂ substituents on the aromatic groups Ar.⁸

In 5 the CHC(CN)₂ moiety is planar and oriented almost orthogonal {81.53(10)°} to the P–Ni–C plane. The same is observed for the CHO group in 2, which contrasts with the situation in [Ni(η⁵-C₅H₅)(PPh₃)C≡CPh] derivatives where the P–Ni–C plane and the C₆ ring tend towards coplanarity.²² The C₂–C₃ distance in 2 {1.432(8) Å} is longer than that in 5 {1.402(3) Å}, which is consistent with the increasing contribution that mesomer B makes toward the bonding in 5 as compared with 2 (Fig. 4).

There are no intramolecular interactions of note in 5 and this is primarily due to the orientation of the rigid dicyanovinyl ligand with respect to the Ni(η⁵-C₅H₅)(PPh₃) group; this contrasts with the acetal derivative [Ni(η⁵-C₅H₅)(PPh₃)C≡CCH(OEt)₂]¹⁰ where the flexible CH(OEt)₂ group interacts with the PPh₃ ligand. An intermolecular interaction C₂₃–H₂₃⋯π(C≡C)ⁱ (i = 1 + x, y, z) involving the alkynyl C₁≡C₂–C₃, with C₂₃–H₂₃⋯C₂ 146°, H₂₃⋯C₂ 2.63 Å and C₂₃⋯C₂ 3.438(5) Å, is comparable to similar C–H⋯π(C≡C) interactions reported in Ni(η⁵-C₅H₅)(PPh₃)C≡CC≡CH,⁹ and by Desiraju and Steiner.²³ There are two other intermolecular interactions of note C₄₃–H₄₃⋯{C₃₁,⋯C₃₆}ⁱⁱ (ii = x, y, 1 + z) and C₄₅–H₄₅⋯{C₂₁,⋯C₂₆}ⁱⁱⁱ (iii = 1 − x, 1 − y, 1 − z).

The overall structure of [{μ-η¹:η¹-Ni(η⁵-C₅H₅)(PPh₃)C≡CCHO}{Co₂(CO)₆}], 8, (Fig. 3) is similar to that of other [(μ-η¹:η¹-alkyne)Co₂(CO)₆] complexes^20,24–26 with a Co₂C₂ tetrahedral core. However, inspection of bond angles and bond lengths within this core reveals some interesting distortions. The Co–C distances to C₁ (average 2.068 Å) are considerably longer than those to C₂ (average 1.952 Å). A similar distortion is observed in [{μ-η¹:η¹-(η⁵-C₅H₅)(OC)₂FeC≡CH}{Co₂(CO)₆}]²⁰ whereas in [{μ-η¹:η¹-Bu^tC≡CBu^t}{Co₂(CO)₆}] Co–C are comparable at 1.999 and 1.985 Å.²⁴ It is a reflection of the electron-donating character of (η⁵-C₅H₅)(Ph₃P)Ni and the electron-withdrawing character of CHO. It is consistent with the supposition that the alkyne–Co₂ bonding consists of donation of the alkyne π-electrons to the metal atoms and back-donation of electrons from the metal atoms into the two vacant alkyne π* orbitals in which the latter is the more important and would be preferentially directed to the C atom bearing the most electronegative substituent. There is a second but minor distortion of the Co₂C₂ core as each C is bonded unequally to the two Co atoms with C₁–Co₁ > C₁–Co₂ and C₂–Co₁ < C₂–Co₂ so that the Co–Co and C–C axes are not orthogonal. These solid-state differences involving the (μ-η¹:η¹-C₂)/Co₂(CO)₆ system in 8 may support evidence for predisposed substitution sites in unsymmetrical alkynyl systems in Pauson–Khand reactions.^27–30

As would be expected, the C₁–C₂ bond length in 8 {1.333(5) Å} is much longer than in 2 {1.205(6) Å} but well short of a normal C–C single bond of 1.53 Å.³¹ It is comparable to that found in most complexes of this class of compound²⁴ but it is longer than that in [{μ-η¹:η¹-(η⁵-C₅H₅)(OC)₂FeC≡CH}{Co₂(CO)₆}]{1.305(5) Å}.²⁰ The Ni–C≡C and C≡C–C angles of 154.2(3)° and 145.0(4)° in 8 differ greatly as is usually the case where the substituents on the coordinated C≡C differ greatly in their electron-withdrawing capabilities, (e.g.ref. 25). They are different from the near linear 176.4(4) and 174.6(6)° angles in 2. The Ni–C and aldehyde C=O bond lengths of 1.867(4) Å and 1.179(5) Å, however, are longer in 8 than in 2 as a direct consequence of the μ-η¹ ∶ ¹-(C₂) moiety bonding with the Co₂ group. The orientation of the η⁵-C₅H₅ ring and the P–Ni–C fragment is of the β type with ene-allyl distortions within the cyclopentadienyl ring (see above). The Ni–C_cp bond lengths in 8 {2.084(4)–2.152(4) Å} are lengthened when compared with those in 2 {2.066(4)–2.127(5) Å} and so is the nickel to cyclopentadienyl ring centroid distance, Ni1⋯Cg1 is 1.7612(6) Å in 8 and 1.7354(6) Å in 2.

Conclusions

In [Ni(η⁵-C₅H₅)(PPh₃)C₁≡C₂X] complexes the (η⁵-C₅H₅)(Ph₃P)Ni acts as an electron donor. When X is a suitable electron acceptor, the electronic interaction between the two results in a lowering of the ν(C≡C) frequency, a deshielding of the C₁ and C₂ resonances (particularly the former), and an increase in the wavelength of the lowest energy charge transfer transition of the NiC≡CX system. In the broadest terms, these effects increase in importance as the electron-withdrawing ability of X increases. Although the ordering of X is not the same for each spectroscopic parameter, it is clear that the more strongly electron-withdrawing X [CHO, CH=C(CN)₂ and CH=C₉H₅NO₂] have a greater effect than the less strongly electron-withdrawing [CH(OEt)₂, CH=NNHPh and CH=NNHC₆H₃(NO₂)₂-2,4]. The two least ambiguous of the parameters are the ν(C≡C) frequencies which provide a direct measure of the C≡C bond order, and the chemical shift of C₁ which depends on the charge separation in the molecule as well as hybridisation at C₁. Both suggest that the electron withdrawing ability of X increases along the series CH(OEt)₂ < CH=NNHPh < CH=NNHC₆H₃-(NO₂)₂-2,4 < CHO < CH=C₉H₅NO₂ ≈ CH=C(CN)₂. The spectroscopic changes are attributed to the increasing contribution that charge-separated mesomers make to the overall electronic structure of these compounds, but the crystal structures of [Ni(η⁵-C₅H₅)(PPh₃)C≡CCH(OEt)₂], [Ni(η⁵-C₅H₅)(PPh₃)C≡CCHO] and [Ni(η⁵-C₅H₅)(PPh₃)C≡CCHC(CN)₂] show that this is not reflected in any significant structural changes in the Ni–C≡C–C system.

Although 1–8 are alkynes, not all of them form [{μ-η¹:η¹-Ni(η⁵-C₅H₅)(PPh₃)C₁≡C₂X}{Co₂(CO)₆}] derivatives on reaction with [Co₂(CO)₈]. Such compounds may be isolated when X = CH(OEt)₂7, or CHO 8, but not when X is the strongly electron-withdrawing CH(CN)₂. This is attributed to the relatively low C≡C bond order in 6 as evidenced by its low ν(C≡C) frequency. The donor capability of the (η⁵-C₅H₅)(PPh₃)Ni substituents greatly affects the ν(CO) frequencies of 7 and 8 which are lowered by ca. 35 cm⁻¹ as compared with their [(μ-PhC≡CX){Co₂(CO)₆}] counterparts, and in 8 where one cluster C atom is substituted by the electron donating (η⁵-C₅H₅)(PPh₃)Ni and the other by the electron withdrawing CHO groups, the C₂Co₂ core is greatly distorted with significantly different C–Co distances. Furthermore, complexation of [Ni(η⁵-C₅H₅)(PPh₃)C≡CX] to a Co₂(CO)₆ moiety appears to affect the reactivity of X, but the electronic communication between Ni and X is still apparent and has spectroscopic implications.

Acknowledgements

P.B. thanks Enterprise Ireland and Schering-Plough (Avondale) for financial support. J.F.G. thanks Enterprise Ireland (International collaboration grants) for funding several research visits to the University of Guelph, Canada and especially Professor George Ferguson for use of his Enraf–Nonius CAD4 diffractometer to collect data sets of 5 and 8. The purchase of a Bruker-AXS P4 diffractometer by Dublin City University in 1999 is genuinely appreciated by J.F.G.

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