Spontaneous Bicyclometalation of a Single Cobalt(I) Complex Stabilized by a δ‐C–H Agostic Interaction

Abstract

CoCH3(PMe3)4 is known for a long time for its potential to activate less reactive C–H Bonds under extremely mild conditions by methane elimination. A spontaneous second C–H bond activation at the same center of cobalt complex is not expected. In contrast to previous investigations with certain aryl‐iminophosphane chelating ligands, in this work two consecutive metalation steps at the same complex center lead to a rare example of a cobalt metallabicycle. In the first step a C–H activation results in a five‐membered chelate ring, whereas the second ring is formed by an incomplete C–H activation which stops at a stable, rarely observed δ‐C–H agostic interaction. The results are supported by NMR spectroscopy, X ray structure analysis and density functional theory (DFT) calculations. Knowledge of such agostic interactions are essential in order to understand catalytic reaction paths and studies of such molecular structures of their ring sizes, bite angles and electronic properties have shown that these ligands allow sensitive control within the metal coordination sphere.

Steric and Electronic Manipulation of the Agostic and π‐Syndetic Donations in a Known Iminophosphane Ni(II) Complex Containing a Rotatable In‐Plane Aromatic Ring

“…The presence of two donations is similar to that found for other agostic interactions we have carried out calculations on [6] but the total E (2) value (12.0 kcal mol −1 ) is much smaller in magnitude than agostic intermediates [ E (2) values ca 29 [6e] to 70 [6b] kcal mol −1 ]. This total is also smaller than for the Co(I) iminophosphane metallacycle complex [11] H‐relaxed structure we have recently calculated [9] [ E (2) value, 27.7 kcal mol −1 ] but in this case, the Co⋅⋅⋅C 2 separation of 2.162 Å is much smaller than the present complex (2.355 Å), as is the Co⋅⋅⋅H separation (1.690 and 2.064 Å respectively). Also, in the Co complex, the C−H bond length is much longer (1.165 and 1.117 Å respectively) and the C−H bond deformation angle is much larger (28.2 and 12.1° respectively) which is the amount the C−H bond deviates from the plane of the benzene ring.…”

“…A further property of these ligands is that the aromatic ring is rotatable unlike pincer ligand complexes [10] where there is a constrained coordination mode that prevents the aromatic ring from much movement. The reported complexes include a Co(I) example containing an imino phosphane ligand that coordinates through phosphine P ( A ) and imine carbon ( B ) atoms, [11] and Rh(I) and Ni(II) complexes containing phoshine P ( A ) and pyridine ring N ( B ) atoms [12,13] . For the present study, we required a close M⋅⋅⋅C distance and the aromatic ring to be well inclined to the coordination plane which are the required features for π‐syndetic donation to be well developed [9] .…”

“…For the present study, we required a close M⋅⋅⋅C distance and the aromatic ring to be well inclined to the coordination plane which are the required features for π‐syndetic donation to be well developed [9] . Of the 3 complexes quoted above, in the X‐ray structures, the Co(I) complex [11] has the shortest M⋅⋅⋅C distance [2.161(2) Å] but the aromatic ring inclination angle was only 16° which appeared to be the reason for the poorly developed π‐syndetic donation [9] . For the other complexes, the X‐ray structure of the Rh complex [12] showed that M⋅⋅⋅C distance is quite long at 2.3750(15) Å and the aromatic ring inclination angle is 32° whereas for the Ni complex [13] the M⋅⋅⋅C distance is a little shorter at 2.355(8) Å and the aromatic ring inclination angle is not much different at 29.3°.…”

Theoretical Investigations on the Agostic Interactions of the Molybdenum and Manganese Complexes

“…The significance of the agostic interactions [1][2] in organometallic chemistry and homogeneous catalysis is well-established by its critical role in the polymerization, [3][4] CÀ H bond activation, [5][6] βhydride elimination, [7] and cyclometallation. [8][9] A number of α/ β/γ/σ/ɛ-agostic interactions and their isomerization have been reported. [10][11] The energetically similar agostic isomers were defined as agostomer by Bullock and coworkers while studying the Mo β/ γagostic isomers [CpMo(CO) 2 (PiPr 3 )] + [B(C 6 F 5 ) 4 ] À (Mo, 1 and Mo, 2, Scheme 1).…”

“…The significance of the agostic interactions [1–2] in organometallic chemistry and homogeneous catalysis is well‐established by its critical role in the polymerization, [3–4] C−H bond activation, [5–6] β‐hydride elimination, [7] and cyclometallation [8–9] . A number of α/β/γ/σ/ϵ‐agostic interactions and their isomerization have been reported [10–11] …”