NH-NHC complex

Scheme 10 Imidazole-bipy C-C coupling (A) or transformation of N-RIm to NH-NHC complexes (B).

Following this pnblication, the anthors tested a series of Pd-NHC complexes (33-36) for the oxidative carbonylation of amino compounds (Scheme 9.8) [44,45]. These complexes catalysed the oxidative carbonylation of amino compounds selectively to the nreas with good conversion and very high TOFs. Unlike the Cu-NHC catalyst 38-X, the palladium complexes catalysed the oxidative carbonylation of a variety of aromatic amines. For example, 35 converted d-Me-C H -NH, d-Cl-C H -NH, 2,4-Me3-C H3-NH3, 2,6-Me3-C H3-NH3, and 4-Ac-C H3-NH3 to the corresponding nreas with very high TOFs (>6000) in 1 h at 150°C, in 99%, 87%, 85%, 72%, and 60% isolated yields, respectively (Pco,o2 = 3.2/0.8 MPa). 

Phenylbiguanide, C6H6NHC(NH)NHC(NH)NH2, combines with trivalent cobalt, as does biguanide itself, to form complex tris(phenylbiguanido) cobalt (III), its hydroxide, and tris(phenylbiguanide)cobalt salts.1 All of these resem-... 

COMPLEXES WITH PROTIC N-HETEROCYCLIC CARBENE (NR,NH-NHC) LIGANDS... 

Figure 9.1 Complexes bearing NR,NR- (A), NR,NH- (B), and NH,NH-substituted (C) A-heterocyclic carbenes (NHC)s and hydrogen bonding between a coordinated NR,NH-NHC and DMPU (D).

In this contribution, a review on methods for the preparation of complexes bearing protic NRJNH- and NH,NH-NHCs is presented. In addition, potential applications for these complexes are discussed. 

COMPLEXES WITH NR,NH- AND NH.NH-NHCS FROM CYCLIC LIGAND PRECURSORS... 

The option to alkylate the coordinated NH,NH-NHC ligand offers the possibility to generate macrocyclic poly-NHC ligands in a template-controlled approach. For example, four NH,NH-NHC ligands coordinated to Pt in a square-planar complex have been linked to a macrocyclic tetra-NHC ligand with crown-ether topology [57]. 

Attempts to generate three NH,NH-NHC ligands at the CpFe " -template demonstrated again the important role of the electronic situation at the metal center. Reduction and cyclization of the 2-azidoethyl isocyanide ligands in complex 47 creates NH,NH-NHC ligands that are much better a-donors than their a-donor/jr-acceptor predecessor isocyanide ligands 41. Thus, every cyclization of an isocyanide ligand to an NH,NH-NHC leads to a more electron-rich metal center. After the... 

Scheme 9.13 Template syntheses of a complex bearing a saturated NH,NH-NHC ligand.

While the template-stabilized 2-aminoethyl isocyanide ligand in 48 was observed more or less by accident, the directed stabilization of /3-amino functionalized isocyanides is possible at electron-rich metal templates. Isocyanides 35, 41 and 2-nitrophenyl isocyanide form the electron-rich complexes of types 50 and 51. Reduction of the 2-azido or 2-nitro functions leads to the 2-amino-substituted isocyanides in complexes 52 and 53 that are stabilized by M CsNR backbonding and thus undergo no cyclization to the NH,NH-NHC ligands (Scheme 9.16) [64]. 

COMPLEXES WITH NR,NH-NHCS BY OXIDATIVE ADDITION OF AZOLES... 

Complexes of type G are rather rare and have so far only been obtained by deprotonation of coordinated NR,NH-NHCs [67, 68]. The direct access to complexes featuring NR,N-NHC ligands by the oxidative addition of the C2—Cl bond of a 2-halogenato-Al-alkylbenzimidazole is much more facile and allows a rich follow-up chemistry like the N-protonation or Al-alkylation of the NR,N-NHC ligand. Particularly, complexes bearing protic NHC ligands can be obtained easily by this way. 

Complexes bearing protic NHC ligands are accessible by various synthetic routes such as the deprotonation of azoles followed by reaction with a transition metal complex, the template-controlled cyclization of functionalized isocyanides, and the oxidative addition of different azoles to transition metal complexes. The complexes with simple monodentate NR,NH-NHCs often tend to tautomerize to give the N-bound azoles. This type of tautomerization is prevented in complexes with donor-functionalized NR,NH-NHCs. Recent smdies demonstrate that complexes with protic NHCs obtained from C2-H azoles are formed by an oxidative addition/reductive elimination reaction sequence. The N—H group in complexes with protic NR,NH-NHCs can serve as a hydrogen bond donor and thus as a molecular recognition unit and may enable various types of bifunctional catalysis. Recent smdies indicate that even biomolecules such as caffeine can be C8-metallated. It... 

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