Synthesis and characterization of novel Pd(ii) complexes with chelating and non-chelating heterocyclic iminocarbene ligands

Abstract

The imidazolium salts [3-R1-1-{2-Ar-imino)-2-R2-ethyl}imidazolium] chloride (C-N; Ar = 2,6-iPr2C6H3; R1/R2 = Me/Me (a), Me/Ph (b), Ph/Me (c), 2,4,6-Me3C6H2 (d), 2,6-iPr2C6H3 (e)) react with Ag(2)O to give Ag(I) iminocarbene complexes (C-N)AgCl (4a-e) in which the iminocarbene ligand is bonded to Ag via the imidazoline-2-ylidene carbon atom. The solid-state structures of 4b and 4d were determined by X-ray crystallography and revealed the presence of monomeric (carbene)AgCl units with Z and E configurations at the imine C=N bonds, respectively. Carbene transfer to Pd occurs when compounds 4b-e are treated with (COD)PdCl2 to yield bis(carbene) complexes (C-N)2PdCl2 (6b-e) containing two kappa1-C bonded iminocarbene moieties. NMR spectroscopic data indicated a trans coordination geometry at Pd. This conclusion was supported by an X-ray structure determination of 6b which clearly demonstrated the non-chelating nature of the iminocarbene ligand system. EXSY 1H NMR spectroscopy suggests that the non-chelating structures undergo E/Z isomerization at the imine C[double bond, length as m-dash]N double bonds in solution. The preparative results contrast our earlier report that the reaction between 4a and (COD)PdCl2 results in a chelating kappa2-C,N bonded iminocarbene complex (C-N)PdCl2. The coordination mode and dynamic behavior of the iminocarbene ligand systems have been found to be dramatically affected by changes in the substitution pattern of the ligand system. Sterically unencumbered systems (a) favor the formation of kappa2-C,N chelate structures containing one iminocarbene moiety per metal upon coordination at Pd(II); these complexes were demonstrated to engage in reversible, solvent-mediated chelate ring-opening reactions. Sterically encumbered systems (b-e) form non-chelating kappa1-C iminocarbene Pd(II) complexes containing two iminocarbene ligands per metal. Transannular repulsions across the chelate ring are believed to be the origin of these structural differences.

Synthesis and characterization of novel Pd(II) and Pt(II) complexes with 5-ring chelating iminoylcarbene ligands

“…The separation between the two doublets was 0.43 ppm. This separation agrees with a previously observed empirical trend, in which the separation of the two i Pr doublets is consistently larger in 6-membered chelate j 2 -(C,N) iminocarbene structures than in non-chelating j 1 -(C) structures [7,8]. The same trend appears to be seen in the 5-membered chelate iminoylcarbene complexes reported in this work.…”

“…The iminocarbene ligand systems in Scheme 1 are conveniently prepared by the reaction between an a-chloroimine and a monosubstituted imidazole, which provides the iminoimidazolium salt precursor of the carbene [7,8]. We reasoned that an analogous reaction between an iminoyl chloride and a monosubstituted imidazole should provide the imidazolium salt precursor for the desired 5-membered chelates.…”

“…The chelate complexes exhibited complex dynamic behavior in solution, involving conformational ''ring flip'' of the boat conformation of the 6-membered chelate ring and solvent-dependent interconversion of chelating j 2 -(C,N) and non-chelating j 1 -(C) solvento species. Subsequently, it was demonstrated that sterically more demanding substituents (Scheme 1, right; R 1 /R 2 = Me/Ph, Ph/Me, 2,4,6-Me 3 C 6 H 2 /Me, 2,6-i Pr 2 C 6 H 3 /Me) changed the structural preferences from chelating mono(j 2 -(C,N)) to nonchelating bis(j 1 -(C)) iminocarbene Pd complexes that also exhibited dynamic conformational changes in solution [8].…”

“…The synthetic strategy in our previous contributions [7,8] involved the preparation of imidazolium salt precursors of the carbene moiety. Deprotonation of these with concomitant carbene formation could not be effected with conventional bases (KH, KO t Bu) due to competing deprotonation at the activated methylene linker between the imine and the imidazole ring.…”

Synthesis, characterization and catalytic properties of cis-dibromo{1,1′-di[3,4,5-trimethoxybenzyl]-3,3′-butylenedibenzimidazol-2,2′-diylidene}palladium (II)

“…In addition, the Pd-C carben distances are also slightly shorter than the other Pd-C carben single bonds; 1-(o-methoxybenzyl)-3-tert-butylimidazol-2-ylidene) 2 PdCl 2 (2.036(5) Å) [27], trans-{3-(2-((2,6-diisopropylphenyl)-imino)propyl)-1-methylimidazol-2-ylidene}PdCl 2 , (2.037(1) Å) [28], [(κ 1−nPr CN CHPh ) 2 PdCl 2 ], (2.067(10) Å) [29], bis(1-(benzoxazolin-2-yl)-3-mesitylimidazol-2-ylidene) diiodo palladium(II) (2.016(5) Å) [30], and bis(1,3-bis-(2,4,6-trimethylbenzyl)benzimidazol-2-ylidene)palladium(II) chloride (2.035 (2) Å) [12]. The Pd-C carben bond lenght values (Pd1-C12, 1.985 (9) and Pd1-C1, 1.986(9) Å) differ, but are only slightly greater than those in the similar platinum complex [1.963(10) and 1.950(10) Å] [30].…”