Stabilization of an Electron-Unsaturated Pd(I)–Pd(I) Unit by Double Hemichelation

Abstract

The competition between conventional coordination and hemichelation of a Pd(II) center of three different palladacycles was probed by reacting the tricarbonyl(η 6 -3-phenylprop-1-enyl)chromium anion with μ-chloro-bridged palladacycles. Structural X-ray diffraction analysis indicates that the main products of the reaction are conventional (η 3 -allyl)Pd(II) complexes. The latter display significant dynamic behavior in solution, as suggested by 1 H NMR spectroscopy. According to DFT calculations this dynamic behavior can be related to conformational equilibria and to possible chemical exchanges leading to Pd(II) hemichelates. The (η 3 -allyl)Pd(II) complexes convert readily to homoleptic bischelates of Pd(II) and to a bis-hemichelate of a Pd(I)−Pd(I) unit via a reductive disproportionation reaction. The latter Pd(I)−Pd(I) complex bears structural features very similar to those of electron-saturated bis(μ-allyl)-bridged Pd(I) complexes in the literature: the Pd−Pd interaction is only weakly covalent and is dominated by noncovalent attractive interactions, as revealed by NCI analyses. The incipient covalent interaction of the Pd(I) centers with CO ligands of the Cr(CO) 3 moieties is too weak to significantly hinder the motion of the metalcarbonyl rotor in solution, which leaves each Pd center formally unsaturated with a 14-valence-electron count. DFT investigations sustained by QTAIM, NCI, ELF, and ETS-NOCV analyses suggest the predominance of noncovalent attractive forces in the stabilization of the bis(μ-allyl)-bridged Pd(I)−Pd(I) complex. ■ INTRODUCTIONThe quest for a comprehensive account of the forces that contribute to chemical bonding in transition-metal complexes containing metal−metal interactions 1 is a challenging domain of research that underwent renewed interest 2 with the recent development of theoretical methods 3 capable of accounting for nonlocal attractive interactions such as the London force 4 in large complexes (up to 120 atoms) at moderate computational cost. 5 Such new tools pave the way for a more comprehensive description of the forces that are responsible for molecular cohesion 1b,2b,c,6 and potentially allow the rational elaboration of new paradigms for coordination chemistry that can now include noncovalent attractive forces as a significant source of molecular cohesion and stability. The term hemichelation 7 is one of these new concepts that was recently proposed to define a nonclassical chelation mode of d-block transition metals by a heteroditopic ligand with which both covalent and noncovalent interactions between the ligand and a metal are central to the stabilization of the associated transition-metal complex. 8

Evidence of a Donor–Acceptor (Ir–H)→SiR3Interaction in a Trapped Ir(III) Silane Catalytic Intermediate

“…To establish the bonding relationship within the critical Ir–Si–H motif of [ 3 ] + , DFT investigations were carried out using DFT-D functionals that reproduce the effect of nonlocal dispersion interactions. , Computations carried out with generalized gradient approximation (GGA), meta-GGA, and hybrid functionals (ZORA PBE-D3­(BJ), TPSS-D3­(BJ), and PBE0-dDsC, respectively) associated with a Slater-type basis set (TZP) produced geometries displaying a good match with the X-ray diffraction based structure (Figure b). Geometry optimizations using GGA and hybrid functionals (PBE-D3 and PBE0-D3, respectively) associated with an effective core potential Gaussian-type basis set (ECP SDD) performed satisfactorily as well.…”

Effect of Enhanced Electron Withdrawal on the Cohesion of Cr‐Pd Hemichelates

“…Figure displays the NCI plots of models of 4a and 4b characterized by the red‐colored isosurfaces that materialize some attractive interaction between the Cr(CO) 3 moiety and the arene ligands, and the attractive noncovalent interaction existing between the Pd centre and the proximal Cr(CO) 3 moiety. The rather spread out intermetallic attractive NCI isosurface is similar to all hemichelates synthesized to date, where the attractive NCI component of the intermetallic interaction is only weakly covalent and largely dominated by electrostatics and electron correlation , , . The blue isosurfaces materialize either repulsive (steric) interactions or non‐bonded so‐called van der Waals contacts.…”

“…The rather spread out intermetallic attractive NCI [13] isosurface is similar to all hemichelates synthesized to date, where the attractive NCI component of the intermetallic interaction is only weakly covalent and largely dominated by electrostatics and electron correlation. [3,4,[5][6][7]14] The blue isosurfaces materialize either repulsive (steric) interactions or non-bonded so-called van der Waals contacts. The QTAIM-based Interacting Quantum Atoms energy partitioning method developed by Pendas and co-workers [16] is a useful tool for investigating noncovalent interactions that has rarely been used to date in the study of transition metal complexes.…”

“…[1b] Thus it opens new perspectives in the design of new architectures of interest in catalysis and allows the study of otherwise difficult processes such as the rarely observed trans‐cis isomerization of T‐shaped complexes . A number of subsequent reports have shown that although the dominating non‐covalent character of the interaction is directly responsible for the dynamic behavior of complexes in solution, hemichelates of Mn(I), Re(I), Pd(II[3a], [3b] and I), Pt(II)[3a] and Rh(I)[3c] could be generally isolated as moderately air sensitive compounds and characterized in the solid state. A recent independent investigation of the molecular dynamics of a Pd(II) hemichelate further confirms the evanescent and mostly non‐covalent nature of the interaction of the Pd(II) centre with the Cr(CO) 3 fragment, which exhibits fluxionality, the Pd centre binding alternatively the two proximal Cr‐bound CO ligands and the rotation of the Cr(CO) 3 moiety being largely slower than this rapid coordinative exchange equilibrium.…”

Designing M-bond (X-M···Y, M = transition metal): σ-hole and radial density distribution

“…38 In the same line, r-lump 39 interactions in noble metal nanoparticles and coinage metal dimers are also proposed. 40 Other types of non-covalent interactions involving transition metal complexes are hemichelation and double hemichelation, [41][42][43] donoracceptor interactions, 44,45 stabilizing heterodox bonding, [46][47][48] etc. However, our detailed analysis of each of them revealed that none of them falls under the definition of the Metal-bond.…”