A Rigid Cyclic (Alkyl)(amino)carbene Ligand Leads to Isolation of Low‐Coordinate Transition‐Metal Complexes

In the last decade the use of diamino N-heterocyclic carbenes (NHCs) A as ancillary ligands for transition-metal catalysts and as organic catalysts on their own has proven very fruitful.[1] The efficiency of NHCs is attributed to their strong σ-donor properties and sterically demanding structure.[1,2] Not only have NHCs yielded improved transition metal catalysts, but they have also lead to the isolation of unusual low-coordinate metal complexes,[2] which often play a key role in catalytic processes. Recently, we reported the synthesis of stable cyclic (alkyl)(amino)carbenes (CAACs) B[3] and demonstrated that CAACs can be even stronger σ donors than NHCs, and lead to highly efficient palladium catalysts for the α-arylation of ketones and aldehydes. Here we report that the peculiar steric and electronic properties of rigid CAAC ligands allow the preparation of low-coordinate metal complexes, hitherto not isolable with any other ligands. To further evaluate the electronic properties of rigid CAAC B1, we attempted to prepare the [RhCl(CO)2(B1)] complex, since the CO stretching frequencies of this type of rhodium complexes are recognized as an excellent measure of the σ-donor ability of the ligand L, and a large amount of data is available for comparison.[4] Reaction of carbene B1 with half an equivalent of [{RhCl(cod)2}2] (cod = 1,5-cyclooctadiene) afforded complex 1, which after purification by column chromatography on silica gel was obtained as orange crystals in 79% yield (Scheme 1). The structure of 1 was unambiguously determined by single-crystal X-ray diffraction.[5] Following the classical procedure, a solution of 1 in chloroform was treated with an excess of CO. Surprisingly, the 13 C NMR spectrum of the product (95% yield) showed only one CO signal, with a large coupling constant (JRh,C = 134 Hz), and not two signals with smaller J values (JRh,C = 33–82 Hz), as observed for [RhCl(CO)2(L)] complexes.[4] It is known that when bulky ligands L are present [RhCl(CO)2L] complexes can readily lose one CO ligand to give a chloro-bridged dimer.[6] However, single-crystal X-ray diffraction[5] revealed that even in the solid state complex 2 is formally a 14-electron [RhCl(CO)L] monomer (Figure 1). Figure 1 Molecular structure of 2 in the solid state. Selected bond lengths [A] and angles [°]: N–C1 1.3174(14), C1–C2 1.5368(15), C1–Rh 1.9399(10) Rh–Cl 2.3740 (3), Rh–C3 1.7955(11), C3–O 1.1433(14), ... Scheme 1 Synthesis of [RhCl(CO)(B1)] complex 2. Interestingly, 2 can also be synthesized in 80% yield by reaction of one equivalent of carbene B1 with half an equivalent of [{RhCl(CO)2}2]. The extreme hindrance provided by the menthyl ring of CAAC B1, which is locked in the most sterically demanding conformation with respect to the metal center, explains in part why dimerization does not occur. Moreover, the short Rh–Ha (2.183 A) and Rh–Hb (2.231 A) distances suggest the existence of agostic interactions, which bring additional stabilization to the metal center. In solution, an agostic interaction is also apparent in the 1H NMR spectrum, which shows a broad multiplet (1H) at 0.08 ppm. Complex 2 is indefinitely stable at room temperature in air. Related [RhClL2] complexes, exemplified by the active species of Wilkinson3s catalyst [RhCl(PPh3)2], are known only as transient species.[7] They have only been generated in situ by ligand dissociation[8] or by hapticity changes;[9] otherwise they readily form chloro-bridged dimers, even when two very bulky ligands L are present.[6] Although other neutral T-shaped formally 14-electron RhI complexes have been isolated, they are still very rare, and none of them have a bridging-capable halide ligand like complex 2.[10,11] The isolation of this low-coordinate RhI complex 2 made us confident that other low-ligated transition-metal complexes could be stabilized by rigid CAAC B1. After close scrutiny of the literature, it came to our attention that other ligands, particularly NHCs, have consistently failed to allow the isolation of cationic 14-electron [Pd(All)(L)]+ complexes (Scheme 2, All =allyl).[12] In 2003, Nolan et al.[12a] were able to abstract chloride from complex 3, but the cationic complex generated was either unstable and decomposed upon workup or isolated and structurally characterized as an acetonitrile adduct. In 2005, Porschke et al.[12b] reported that chloride abstraction with thallium salts from NHC complexes 3 led to ionic binuclear compounds, which result from combination of the transiently formed desired cationic species with the starting neutral complex. Most recently, Glorius et al.[12c] generated cationic complex 4, which, despite intramolecular stabilization by double-bond complexation, appeared to be rather unstable and was only characterized by 1H NMR spectroscopy. Scheme 2 Previous attempts to prepare cationic 14-electron [Pd(All)(L)]+ complexes. In marked contrast, chloride abstraction from [PdCl-(All)(B1)] (5)[3] with a stoichiometric amount of AgBF4 produced the elusive cationic palladium complex 6 as yellow crystals (88% yield, Scheme 3). The structure of 6 was unambiguously established by single-crystal X-ray diffraction (Figure 2).[5] As expected, 6 has a T-shaped geometry with no interaction between the metal atom and tetrafluoroborate anion (shortest Pd–F distance 3.88 A). Similar to the above-mentioned RhI complex 2, at least one of the axial H atoms of the menthyl ring provides a stabilizing agostic interaction (Pd–Ha 2.048, Pd–Hb 2.509 A). This interaction is also apparent from the 1H NMR spectrum, which shows a broad multiplet (1H) at −0.17 ppm. This is the first example of a stable, formally 14-electron, PdII cation.[13] Figure 2 Structure of the cation of 6 in the solid state. Selected bond lengths [A] and angles [°]: N–C1 1.293(3), C1–C2 1.519(3), C1–Pd 2.038(3), Pd–Ha 2.048, Pd–Hb 2.509, Pd–C3 2.272(13), Pd–C4 ... Scheme 3 Synthesis of [Pd(All)(B1)]+ complex 6. To further demonstrate the increased stabilization of metal centers provided by the rigid menthyl moiety of carbene B1, we attempted to prepare complexes analogous to 2 and 6, using the CAAC B2, featuring the unlocked and therefore flexible cyclohexyl ring.[3,14] Treatment of [{RhCl(CO)2}2] with two equivalents of carbene B2 afforded exclusively the classical dicarbonyl complex 7, while treatment of 8[3] with AgBF4 led to rapid formation of palladium black (Scheme 4). Clearly, in contrast to B1, the cyclohexane ring of CAAC B2 can flip to the less sterically demanding conformation, in which no H atoms are available for strong agostic interactions with the metal center. Scheme 4 Synthesis of [RhCl(CO)2(B2)] complex 7, and attempted chloride abstraction from [PdCl(All)(B2)] complex 8. The isolation of complexes 2 and 6 clearly demonstrates the advantages of rigid CAACs over classical ligands. By manipulating the quaternary carbon atom adjacent to the carbene center it is possible to design ligands with an electronically active wall of protection for the metal center. We are currently exploring the possibility that CAAC ligands will stabilize even lower coordinate metal complexes.