Dissecting Transmetalation Reactions at the Molecular Level: Phenyl Transfer in Metal Borate Complexes

Abstract

Despite the wide use of transmetalation reactions in organic and organometallic synthesis, little is known about the involved intermediates and mechanisms. Here, we use a combination of electrospray multistage-mass spectrometry experiments and quantum chemical calculations to examine the unimolecular gas-phase reactivity of [M n (BPh 4 ) n+1 ] − and [M n (BPh 4 ) n−1 ] + ions (M = Li and Ag) as model for transmetalation. This approach excludes any bimolecular equilibrium processes occurring in solution and thus ensures exact control over the composition of the probed species. Our findings show how the nature of the metal M, the charge of the complex, and the oligomeric state of the complex influence the occurrence of transmetalation. Efficient transmetalation reactions only proceed for the silver-containing complexes but not for their lithium-containing counterparts. This behavior reflects the better stabilization of the soft Lewis base Ph − by the softer Lewis acid Ag + than by the harder Lewis acid Li + , in accordance with the predictions of HSAB theory. The participation of a second silver ion in [Ag 2 (BPh 4 )] + further facilitates the abstraction of the phenyl anion from boron. The resulting product complex closely resembles a transmetalation product isolated from solution and thus directly demonstrates the implications of our gas-phase model for condensed-phase chemistry.

Identifying reactive intermediates by mass spectrometry

“…27 The work of Auth et al shows how to use ESI-MS 2 for investigation of the transformation from the reactive complex to the product. 28 The authors studied transmetallation reaction within phenylborate complexes. They mixed Li(BPh 4 ) salt with AgOTf and studied possible transfer of the phenyl group from boron to either lithium or silver.…”

Phenyl argentate aggregates [AgnPhn+1]− (n = 2–8): Models for the self-assembly of atom-precise polynuclear organometallics

“…While CID of silver tetraphenylborate aggregates generates related phenyl silver aggregates, many of these retain tetraphenyl borate ligands, adding structural complexity. 37 Given that ESI-MS of organocuprate solutions generates aggregates, 38,39 we were encouraged to explore such an approach to generate anionic phenyl silver aggregates.…”

Lability of Ta–NHC adducts as a synthetic route towards heterobimetallic Ta/Rh complexes

“…Transmetallation is a key elementary reaction in organometallic chemistry which is frequently used for the preparation of organometallic derivatives [1][2][3] and utilized with success in homogeneous catalysis, especially in cross-coupling reactions. [4][5][6] Transmetallation approaches are well-established and popular for the synthesis of N-heterocyclic carbene (NHC) complexes because the direct metalation of NHC ligands often fails, due to the harsh basic conditions required for the deprotonation of imidazolium proligands which can lead to degradation.…”

“…COD), 3.51 (m, 1H, COD), 3.93 (s, 1H, Ta=CH t Bu), 4.65 (d, 2 JHH = 14.3 Hz, 1H, NCH2), 5.31 (m, 1H, COD), 5.41 (m, 1H, COD), 5.78 (d, 2 JHH = 14.3 Hz, 1H, NCH2),6.00 (d,3 JHH = 1.9 Hz, 1H, CHimid), 6.68 (s, 1H, m-CHMes), 6.89 (s, 1H, m-CHMes), 7.29 (d,3 JHH = 1.9 Hz, 1H, CHimid) 13. C{ 1 H} NMR (125.7 MHz, C6D6, 296K) δ = 17.8 (p-CH3Mes), 20.3 (o-CH3Mes), 28.4 (o-CH3Mes), 28.9 (CH2COD), 29.2 (CH2COD), 30.1 (CH3), 30.9 (CH3), 32.5 (CH2COD), 34.1 ( t Bu), 34.3 (CH2COD), 34.8 ( t Bu), 34.9 ( t Bu), 35.0 ( t Bu), 46.2 ( t Bu), 62.5 (NCH2), 68.0 (d, 1 JRh-C = 14 Hz, CHCOD), 85.3 (OC), 94.8 (Ta-CH2 t Bu), 95.1 (Ta-CH2 t Bu), 97.1 (d, 1 JRh-C = 14 Hz, CHCOD), 122.3 (CHimid), 122.9 (CHimid), 128.6 (CHAr), 130.1 (CHAr), 134.5 (CAr), 137.0 (CAr), 138.0 (CAr), 138.8 (CAr), 184.5 (d, 1 JRh-C = 52 Hz, Rh-Ccarbene), 238.2 (s, Ta=C).Synthesis of 5.A 4 mL colorless toluene solution of tris(tertbutoxy) silanol (54 mg, 0.21 mmol, 1.0 equiv) was added dropwise to an orange stirring solution of complex Ta(L)(CH t Bu)(CH2 t Bu) 1 (135 mg, 0.21 mmol, 1.0 equiv) in 6 mL of toluene.…”