Triazolylidene‐Iridium Complexes with a Pendant Pyridyl Group for Cooperative Metal–Ligand Induced Catalytic Dehydrogenation of Amines

Abstract

Two iridium(III) complexes containing a C,N-bidentate pyridyl-triazolylidene ligand were prepared that are structurally very similar but differ in their pendant substituent. Whereas complex 1 contains a non-coordinating pyridyl unit, complex 2 has a phenyl group on the triazolylidene substituent. The presence of the basic pyridyl unit has distinct effects on the catalytic activity of the complex in the oxidative dehydrogenation of benzylic amines, inducing generally higher rates, higher selectivity towards formation of imines versus secondary amines, and notable quantities of tertiary amines when compared to the phenyl-functionalized analogue. The role of the pyridyl functionality has been elucidated from a set of stoichiometric experiments, which demonstrate hydrogen bonding between the pendant pyridyl unit and the amine protons of the substrate. Such N ⋅⋅⋅H-N interactions are demonstrated by X-ray diffraction analysis, H NMR, and IR spectroscopy, and suggest a pathway of substrate bond-activation that involves concerted substrate binding through the Lewis acidic iridium center and the Lewis basic pyridyl site appended to the triazolylidene ligand, in agreement with ligand-metal cooperative substrate activation.

Aerobic dehydrogenation of amines to nitriles catalyzed by triazolylidene ruthenium complexes with O2 as terminal oxidant

“…The five-membered metallacycle features a typically acute C trz -Ru-N py bite angle of 77 (±1)° (Table 1). 41,42 The Ru-C trz bond length is 2.04(2) Å, and is thus in the expected range when compared to related triazolylidene complexes. 44,45 All four complexes show similar bond distances suggesting that variation of the ligand substitution pattern has only minor influences on the ligand bonding.…”

“…40,41 The absence of the triazolylidene proton as well as the splitting of the aromatic p-cymene protons into four distinct doublets indicate chelation and therefore N,Cbidentate coordination of the pyridyl-triazolylidene ligand in complexes 4-6. Furthermore, the pyridyl α proton is considerably deshielded, e.g., a doublet at δ H = 8.81 for complex 6b, which is characteristic [41][42][43] for N-coordination of the pyridyl unit to the ruthenium center (cf. δ H = 8.22 ppm in the ligand precursor 3b).…”

Mesoionic and Related Less Heteroatom-Stabilized N-Heterocyclic Carbene Complexes: Synthesis, Catalysis, and Other Applications

“…A similar bonding mode is accessible with a tris(triazolylidene) borate ligand, as demonstrated with the nickel complex 67 and the manganese analogue (68; see Scheme 15). 153,185 Triazolium salts with pyridyl substituents give C trz ,Nbidentate chelating complexes after metalation, irrespective of whether the pyridyl substituent is positioned either at N1 (R; M = Ir, 13,74,86,171 Ru, 74,93,95,99,100 Os, 74 Ni 173 ) or at C4 (S; M = Ir, 65,69,71,77,88,196−198 Re, 177 Ru, 95,99,100,107 Ni, 173 Pd, 163 and Co 130 ). Interestingly, when both C4 and N1 contain a pyridyl substituent, only the N-bound pyridyl site forms a chelate while the other pyridyl unit is idle (coordination type R).…”

“…Interestingly, when both C4 and N1 contain a pyridyl substituent, only the N-bound pyridyl site forms a chelate while the other pyridyl unit is idle (coordination type R). 86 This selective bonding is attributed to the higher basicity of the N-bound pyridyl ring due to the electron-donating effect of the triazole nitrogen into that pyridine ring. The bite angle of the triazole−pyridyl chelate is acute, about 77°, which positions the metal closer to the chelating pyridyl and too remote to the second pyridyl for coordination.…”

“…For example, complexes 176a and 186 containing either an inert phenyl group or a basic pyridyl substituent catalyze the oxidative coupling of amines with concomitant production of H 2 gas (Scheme 59). 86 Under standard conditions, that is, 5 mol % iridium loading and 150 °C reaction temperature, a mixture of products is obtained that consists of benzylidene benzylimine, dibenzylamine, and traces of the tertiary amine. Complex 186 with a pendant pyridyl unit favors the formation of imine products.…”