Building Square Planar Cobalt (II) Complexes via Sodium Mediated Cobaltation of Fluoroarenes

Abstract

While cobalt complexes have already shown their potential for CÀ H and CÀ F bond activation of fluoroarenes, their reactivity as metalating agents via CoÀ H exchange towards these substrates has not been explored. Herein, we report a Co(HMDS) 2 [HMDS = N(SiMe 3 ) 2 ] system which, when synergistically enhanced via sodium amide Na(HMDS) mediation, can render chemo-and regioselective cobaltation of a series of fluoroarenes to produce a new class of homoleptic square planar [Na 2 CoAr 4 ] complexes. Density functional theory calculations elucidate the key roles of the Na/Co counterparts in a stepwise sodiation/cobalt transmetalation process, leading to this novel CÀ H metalation. Depending on the reaction stoichiometry, this process can occur inter-or intramolecularly, furnishing transient [NaCo(HMDS) 2 Ar] intermediates which can undergo ligand rearrangement to afford [Na 2 CoAr 4 ] with concomitant formation of Co(HMDS) 2 and [NaCo-(HMDS) 3 ].

Applying Na/Co(ii) bimetallic partnerships to promote multiple Co–H exchanges in polyfluoroarenes

“…Mechanistic studies revealed that the reaction occurs with initial formation of [NaCo(HMDS) 2 (C 6 F 5 )] (II) which undergoes ligand redistribution to form I ( Scheme 1 ). 7 This work concluded that while formally these reactions are cobaltations, sodium plays a key role not only on mediating the metalation but also on the stabilisation of I via formation of Na–F interactions. The mild reaction conditions employed and the thermal stability of I contrasts with previous reports on the metalation of fluoroarenes using organolithium reagents which inevitably require use of cryogenic temperatures in order to avoid unwanted side reactions (autometalation, benzyne formation, multimetalation, etc.…”

“… 5 Interestingly, contrasting with other established C–H bond activation processes, 6 these reactions operate without a change in the oxidation state of the 3d-transition metal. 7 …”

“…Demonstrating the importance on the choice of base to promote these metalations, while 3 and 4 are formed when using 2 molar excess of 1, if 1,3,5-trifluorobenzene or 1,2,4,5-tetrafluorobenzene are treated with one equivalent of 1 in combination with another equivalent of NaHMDS, which potentially could also promote dimetalation, selective monocobaltation is preferred yielding [Na 2 CoAr 4 ] species like I ( Scheme 1 ). 7 Furthermore, treating these tetra(aryl) cobaltates with another four equivalents of 1 failed to promote a second Co–H exchange even under forcing conditions.…”

“…If the reaction mixture is allowed to reach room temperature, 5 undergoes ligand redistribution to afford the tetra-arene [Na 2 CoAr 4 ] (6) (Ar = C 6 F 4 H) with the same structure as I ( Scheme 2 ). 7 However, when low temperature was maintained and a second equivalent of base 1 was added, an immediate colour change from light turquoise to dark green was observed together with the disappearance of the H ortho resonance of 5 and concomitant appearance of a new broad signal at −4.17ppm characteristic of the HMDS groups of the dimetalated product 4 ( Scheme 2 and Fig. S26, ESI † ).…”

“…5 Interestingly, contrasting with other established C-H bond activation processes, 6 these reactions operate without a change in the oxidation state of the 3d-transition metal. 7 Within fluoroarene studies sodium cobaltate NaCo(HMDS) 3 [HMDS = N(SiMe 3 ) 2 ] (1), in combination with homometallic NaHMDS, has shown particular promise in the regioselective cobaltation of a range of fluoroarenes at room temperature including pentafluorobenzene with excellent atom efficiency to give disodium tetra(aryl) Co(II) as the THF complex [(THF) 4 Na 2 -Co(C 6 F 5 ) 4 ] (I). In this molecule Co exhibits an unusual square planar geometry (Scheme 1).…”