Robert S. P. Turbervill

Transition Metal Complexes of Anionic N-Heterocyclic Dicarbene Ligands†

The development of N-heterocylic carbene (NHC) chemistry has advanced enormously since the pioneering work of Wanzlick and Schçnherr, and that of fele. Arguably, one of the most significant breakthroughs in this area was the isolation of the first stable N-heterocyclic carbene (NHC), 1,3-bis(adamantyl)-imidazol-2-ylidene, by Arduengo and coworkers. Since this species was first reported, NHCs have gone from chemical curiosities to ubiquitous ligands in the span of just twenty years. Their use as supporting ligands for the formation of transition metal complexes with varied chemical properties has been extensively documented. Similarly, the exploitation of their strong sigma-donor ability has been used in main-group chemistry for the stabilization of numerous low-coordinate complexes containing elements in low oxidation states, including the remarkable group 14 and 15 diatomic species E2 (E = Si, Ge, P, As) [5–8] and P2 . The chemistry of NHCs is dominated by ligand coordination through the C2 atom in the so-called classical mode (A, Figure 1). However, following the discovery of the first transition metal complex of an abnormal NHC (aNHC, B, Figure 1) by Crabtree and co-workers, numerous additional examples of such abnormally bonded species have been isolated, including the first example of an isolated metalfree abnormal carbene. A closely related, metal-free mesoionic carbene was also isolated by Bertrand and coworkers. More recently, Robinson and co-workers reported the first example of an anionic N-heterocyclic dicarbene, that is, an NHC in which one of the sites of the imidazol-2-ylidene backbone has been deprotonated, giving rise to an NHC capable of coordinating through the C2 and C4 positions simultaneously (NHDC, C, Figure 1). A related anionic NHC which can be rationalized as an adduct of a dicarbene and B(C6F5)3 was also recently reported. [15] Whereas lanthanide complexes of an N-heterocyclic dicarbene have previously been isolated, to our knowledge no examples of transition metal complexes of N-heterocyclic dicarbenes have been reported to date. Related neutral dicarbenes based on 1,2,4-triazole-3,5-diylidenes were first identified in coordination polymers of silver(I) salts. Herein, we report the first example of a transition metal complex containing anionic N-heterocyclic dicarbene ligands K[{DC[N(2,6-iPr2C6H3)]2(CH)C}2Mn(mes)(thf)]·THF (2). Reaction of 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene (IPr) with Mn3(mes)6 in diethylether afforded the complex [Mn(IPr)(mes)2] (1; Figure 2) as an insoluble offwhite solid in near quantitative yields. Crystals suitable for single crystal X-ray diffraction could be obtained by slow diffusion of diethylether into a pyridine solution of 1 at 4 8C. Analogous reactions giving rise to related three

‘Classical’ and ‘Abnormal’ Bonding in Tin (II) N-Heterocyclic Carbene Complexes

Reaction of Sn(OTf)2 (OTf– = OSO2CF3–) with one and two equivalents of the N-heterocyclic carbene (NHC) 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene (IPr) yielded the complexes [Sn(IPr)(OTf)2] (1) and [Sn(IPr)(aIPr)(OTf)][OTf] (2), respectively. Both species were characterised by single crystal X-ray diffraction, multi-element NMR spectroscopy, and elemental analysis. Both compounds display an NHC ligand bonded to the tin(ii) metal centre via the C2 carbon in a ‘classical’ mode, while 2 also contains an ‘abnormal’ C4/C5-bonded carbene (aIPr). These observations highlight the subtle steric and electronic effects affecting the coordination modes of these ligands. Solution phase NMR experiments on 1 and 2 reveal complex behaviour resulting in the protonation of the IPr ligands to yield the 1,3-bis(2,6-diisopropylphenyl)-imidazolium cation via an unidentified reaction mechanism.