Element Misidentification in X-ray Crystallography: A Reassessment of the [MCl₂(diazadiene)] (M = Cr, Mo, W) Series

Abstract

A series of reports describing the syntheses and structures of [MCl2(diazadiene)] (M = Cr, Mo, W) complexes is reassessed in the context of known chemistry of low-valent Group VI metal complexes, crystallographic trends such as M-Cl bond lengths and unit cell volumes, and calculated metal-ligand bond lengths. Crystallographic data and computational results are inconsistent with any of these species being second or third row transition metal complexes. A review of the crystallographic information files accompanying the [MCl2(diazadiene)] (M = Mo, W) published structures reveals that the metal atoms were inappropriately treated with partial site occupancy factors (0.775 for Mo; 0.4005 and 0.417 for W), the effect of which was to manifest lighter-element behavior and better refinement in accord with the metal atoms' correct identity. A deliberate synthesis and characterization by X-ray diffraction of [ZnCl2((Mes)dad(Me))] ((Mes)dad(Me) = 1,4-bis(2,4,6-trimethylphenyl)-2,3-dimethyl-1,4-diaza-1,3-butadiene) are reported. Refinement of this structure with the same combination of second or third row metal and offsetting partial site occupancy is shown to provide final refinement statistics essentially the same as with the correct model employing M = Zn at site occupancy 1.00. Use of the published method for synthesis of [WCl2(diazadiene)] with (Mes)dad(Me) and [WBr4(MeCN)2] in lieu of [WCl4(MeCN)2] is shown to produce [ZnBr2((Mes)dad(Me))], which has also been characterized by X-ray diffraction. It is concluded that the unusual putative 12-electron [MCl2(diazadiene)] (M = Cr, Mo, W) complexes are in all cases the corresponding [ZnCl2(diazadiene)] complexes, Zn having been commonly employed as reducing agent in their synthesis.

Crystal structure of unsymmetrical α-diimine palladium(II) complex cis-[{ArN=C(Me)–(Et)C=NAr}PdCl2] [Ar = 2,6-(iPr)2C6H3]

“…-Diimines (or) 1,4-diaza-1,3-butadienes (DAD) are one of the most versatile classes of chelating nitrogen-donor ligands, and are well known to stabilize several transition metal complexes at various oxidation levels (Bart et al, 2005;Greene et al, 2014). Nickel and palladium complexes of -diimines are reported to be effective catalysts for various olefin polymerization and co-polymerization reactions (Ittel et al, 2000).…”

Resolving Space-Group-Choice Dilemma in Small-Molecule Crystallography for Chemistry Students Using Case-Based Learning Modules

“…However, it is exactly this level of comfort toward crystallography that also has caused several corrections and retractions on high-profile publications by some researchers due to their lack of appropriate training and understanding of basic but important concepts. Among the common reported mistakes such as mis-assigned atom identities and bond connections, , a more fundamental misunderstanding often overlooked yet having caused problematic results is the space-group-choice dilemma. ,,− …”

Electronic and Geometric Structures of Paramagnetic Diazadiene Complexes of Lithium and Sodium

“…1,4-Diaza-1,3-dienes( DADs) are well established as highly versatile ligandsf or nearly every element in the periodic table. [1][2][3] The list of stable DAD complexese ncompasses main-group metals, [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] early [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] and late [36][37][38][39][40][41][42][43][44][45][46][47] transition metals, as well as the lanthanides anda ctinides. [48][49][50]…”

“…1,4‐Diaza‐1,3‐dienes (DADs) are well established as highly versatile ligands for nearly every element in the periodic table . The list of stable DAD complexes encompasses main‐group metals, early and late transition metals, as well as the lanthanides and actinides . Important practical applications of metal diazadiene complexes include homogeneous catalysis, C−H bond activation, materials science, and the synthesis of single‐molecule magnets .…”

“…This electronic flexibility in addition to the possibility of tuning the steric properties of DADs by using various substituents at both the C and N atoms account for the high versatility of DAD ligands in coordination chemistry. Scheme shows the different coordination modes of DAD ligands that have been reported in the literature …”