Ruthenium complexes of chelating amido-functionalized N-heterocyclic carbene ligands: Synthesis, structure and DFT studies

Abstract

Synthesis, structure and density functional theory (DFT) studies of a series of new ruthenium complexes, [1-(R)-3-N-(benzylacetamido)imidazol-2-ylidene]RuCl(p-cymene) [R = Me (1c), i-Pr (2c), CH 2 Ph (3c); p-cymene = 4-i-propyltoluene] supported over N /O-functionalized N-heterocyclic carbene (NHC) ligands are reported. In particular, the ruthenium (1-3)c complexes were synthesized from the respective silver complexes, [1-(R)-3-N-(benzylacetamido)imidazol-2-ylidene] 2 Ag + Cl − [R = Me (1b), i-Pr (2b), CH 2 Ph (3b)] by the treatment with [Ru(p-cymene)Cl 2 ] 2 in 65-76% yields. The molecular structures of (1-3)c revealed the chelation of the N-heterocylic carbene ligand through the carbene center and an amido sidearm of the ligand in all of the three complexes. The density functional theory studies on the ruthenium (1-3)c complexes indicated strong binding of the NHC ligand to the metal center as was observed from the deeply buried NHC-Ru σ-bonding molecular orbitals.

Cyanosilylation of Aromatic Aldehydes by Cationic Ruthenium(II) Complexes of Benzimidazole-Derived O-Functionalized N-Heterocyclic Carbenes at Ambient Temperature under Solvent-Free Conditions

“…The molecular structures of all of the three ( 1 – 3 ) c complexes (Table 2 and Figures 5–7) have been determined by single-crystal X-ray diffraction studies, which showed these complexes exhibiting a conventional “piano stool” structure with the ruthenium center being bound to η 6 - p -cymene, η 2 -amido-functionalized N-heterocyclic carbene ligand and chloride atoms. The Ru–C carbene bond distances in 1c [2.0556(15) Å], 2c [2.038(3) Å], and 3c [2.0410(18) Å] were slightly shorter than the sum of individual covalent radii of Ru and C atoms (2.19 Å) 52 but compared well with other reported analogues, namely, [{1-( N -benzylacetamido)-3-(R 4 )-imidazol-2-ylidene}Ru( p -cymene)Cl]Cl {R 4 = Me [2.0172(19) Å], i -Pr [2.033(5) Å], and CH 2 Ph [2.019(3) Å]} 56 and [{1-(4,4-dimethyl-4,5-dihydrooxazol-2-yl)-3-(mesityl)-imidazol-2-ylidene}Ru( p -cymene)Cl]PF 6 [2.038(3) Å]. 57 Likewise, the Ru–Cl bond distances in 1c [2.4041(4) Å], 2c [2.3907(8) Å], and 3c [2.4005(5) Å] were in agreement with the related complexes [{1-( N -benzylacetamido)-3-(R 4 )-imidazol-2-ylidene}Ru( p -cymene)Cl]Cl {R 4 = Me [2.4256(14) Å], i -Pr [2.4325(8) Å], and CH 2 Ph [2.4404(7) Å]}, 56 [{1-(4,4-dimethyl-4,5-dihydrooxazol-2-yl)-3-(mesityl)-imidazol-2-ylidene}Ru( p -cymene)Cl]PF 6 [2.420(1) Å], 57 and the Ru–O amido bond distances in 1c [2.1563(11) Å], 2c [2.148(2) Å], and 3c [2.1276(13) Å] with a related complex [{1-( N -benzylacetamido)-3-(pyridin-2-ylmethyl)-imidazol-2-ylidene}Ru(CH 3 CN) 2 (PPh 3 )](PF 6 ) 2 [2.143(4) Å].…”

One pot tandem dehydrogenative cross-coupling of primary and secondary alcohols by ruthenium amido-functionalized 1,2,4-triazole derived N-heterocyclic carbene complexes

“…The synthesis of the 1,2,4-triazole derived N-heterocyclic carbene complexes of ruthenium (1–3)b were similar to that of the related imidazole based N-heterocyclic carbene analogous reported by us. 30,32 The 1 H NMR spectrum of the (1–3)b complexes showed the conspicuous absence of the amido (CON H̲ ) resonance, unlike that of its starting silver (1–3)a complexes and there by suggesting the deprotonation of the (CON H̲ ) moiety leading to the chelation of the amido sidearm to the ruthenium centre. As expected, the two (C H̲ 2 ) moieties of the ruthenium (1–3)b complexes showed two pairs of diastereotopic resonances in the 1 H NMR.…”

“…11,13 Transition metals spear-headed the development of the homogeneously catalyzed alcohol-alcohol coupling reactions with a variety of rst row transition metals like Mn, 14 Fe, 4,15 Co, 16,17 Cu, [18][19][20] and Ni, 21,22 second row transition metals like Ru 10,[23][24][25][26] and Rh, 27 and third row transition metal namely Ir, 11,25,28,29 been reported. With our recent interest on ruthenium, 30,31 we decided to focus on ruthenium for the alcoholalcohol coupling reactions. 32 The ruthenium catalyzed alcohol-alcohol coupling reactions have been developed on the back of different ancillary ligands that primarily range from a variety of [PCP], 33 [NNN], 10,34 [NPN], 35 [NNP] 26 and [NNC] 26 pincer ligands to tris-chelating tris(pyrazolyl)borate (Tp) ligands 23 to bis-chelating NN-bipyridine 36 and [NC]pyrimidine based NHC ligands 25 to mono-chelating phosphine ligands.…”

Studies of the Electronic Properties of N‐Heterocyclic Carbene Ligands in the Context of Homogeneous Catalysis and Bioorganometallic Chemistry

“…[26,27] The natural bond orbital analysis of the NHC-M bond (M = Ag, Au, Pd, Ni, and Ru) in the corresponding transition metal complexes reveals electron donation occurring from the filled sp 2 -type carbene orbital to the unfilled metal orbital, depending upon the electronic and geometric demands. [26,27] The natural bond orbital analysis of the NHC-M bond (M = Ag, Au, Pd, Ni, and Ru) in the corresponding transition metal complexes reveals electron donation occurring from the filled sp 2 -type carbene orbital to the unfilled metal orbital, depending upon the electronic and geometric demands.…”

“…Extensive studies by density functional theory were undertaken to probe the metal–NHC interaction occurring in a wide variety of transition metal N‐heterocyclic carbene complexes of the types [(NHC) a M b X c ] and [(NHC)MX d (L)], where NHC=N‐heterocyclic carbene; M = Ag I , Au I/III , Pd II , Ni II , and Ru II ; X = halide; L = pyridine and p ‐cymene; a = 1, 2; b = 1, 2; c = 0, 1, 2, 3, and d = 1, 2 26,27. The natural bond orbital analysis of the NHC–M bond (M = Ag, Au, Pd, Ni, and Ru) in the corresponding transition metal complexes reveals electron donation occurring from the filled sp 2 ‐type carbene orbital to the unfilled metal orbital, depending upon the electronic and geometric demands.…”