Comparative Measure of the Electronic Influence of Highly Substituted Aryl Isocyanides

Abstract

To assess the relative electronic influence of highly substituted aryl isocyanides on transition metal centers, a series of C4v-symmetric Cr(CNR)(CO)5 complexes featuring various alkyl, aryl, and m-terphenyl substituents have been prepared. A correlation between carbonyl-ligand (13)C{(1)H} NMR chemical shift (δCO) and calculated Cotton-Kraihanzel (C-K) force constant (kCO) is presented for these complexes to determine the relative changes in isocyanide σ-donor/π-acid ratio as a function of substituent identity and pattern. For nonfluorinated aryl isocyanides possessing alkyl or aryl substitution, minimal variation in effective σ-donor/π-acid ratio is observed over the series. In addition, aryl isocyanides featuring strongly electron-releasing substituents display an electronic influence that nearly matches that of nonfluorinated alkyl isocyanides. Lower σ-donor/π-acid ratios are displayed by polyfluorinated aryl isocyanide ligands. However, the degree of this attenuation relative to nonfluorinated aryl isocyanides is not substantial and significantly higher σ-donor/π-acid ratios than CO are observed in all cases. Substituent patterns for polyfluorinated aryl isocyanides are identified that give rise to low relative σ-donor/π-acid ratios but offer synthetic convenience for coordination chemistry applications. In order to expand the range of available substitution patterns for comparison, the syntheses of the new m-terphenyl isocyanides CNAr(Tripp2), CNp-MeAr(Mes2), CNp-MeAr(DArF2), and CNp-FAr(DArF2) are also reported (Ar(Tripp2) = 2,6-(2,4,6-(i-Pr)3C6H2)2C6H3); p-MeAr(Mes2) = 2,6-(2,4,6-Me3C6H2)2-4-Me-C6H2); p-MeAr(DArF2) = 2,6-(3,5-(CF3)2C6H3)2-4-Me-C6H2); p-FAr(DArF2) = 2,6-(3,5-(CF3)2C6H3)2-4-F-C6H2).

Kinetic Destabilization of Metal–Metal Single Bonds: Isolation of a Pentacoordinate Manganese(0) Monoradical

“…Additionally, investigations into the one-electron activation chemistry of [Mn(CO) 5Àn L n ]s pecies with small molecule substrates are limited. [22,23] Thesterically encumbering nature of these isocyanidel igands effectively prevents the formation of MnÀMn single bonds,thus rendering this 17e À radical both isolable in the solid state and persistent in solution. [19][20][21] Accordingly,successful strategies leading to the kinetic stabilization and isolation of such [MnL 5 ] monoradicals,while explicitly preventing the formation of the common Mn À Mn single-bond motif,a re of importance in these processes.F urthermore,a ccess to long-lived [MnL 5 ] monoradicals offers ap otential avenue for the development of selective,well-defined metal-mediated one-electron transformations.H erein we describe the isolation, structure,a nd spectroscopic features of the neutral monoradical [Mn-(CO) 3 (CNAr Dipp2 ) 2 ]( Ar Dipp2 = 2,6-(2,6-(iPr) 2 C 6 H 3 ) 2 C 6 H 3 )f eaturing two m-terphenyl isocyanide ligands.…”

“…[16,17] Given the challenges faced in exploring the properties of [Mn(CO) 5 ]a nd related zerovalent [MnL 5 ]c omplexes,i ti s notable that they continue to be of significant interest as transient species.F or example,[ MnL 5 ]m onoradicals of the type [Mn(CO) 3 (bipy)] (bipy = 2,2'-bipyridine) are now recognized as the key intermediates on the pathway to catalyst deactivation (by MnÀMn bond formation) in electrocatalytic CO 2 reduction. Additionally,asaresult of the isolobal analogy between carbon monoxide (CO) and organoisocyanides (C NR), [22,24] Figure 1A). [22,23] Thesterically encumbering nature of these isocyanidel igands effectively prevents the formation of MnÀMn single bonds,thus rendering this 17e À radical both isolable in the solid state and persistent in solution.…”

Controlled Expansion of a Strong‐Field Iron Nitride Cluster: Multi‐Site Ligand Substitution as a Strategy for Activating Interstitial Nitride Nucleophilicity

“…[15][16][17][18] Thegreater nucleophilicity of [Fe 4 (m 4 -C)(CO) 12 ] 2À relative to [Fe 4 (m 4 -N)(CO) 12 ] À has been rationalized qualitatively by two main factors,namely 1) the greater charge of the carbide cluster (À2v s. À1) and 2) increased orbital energies,w hich are due to the more electropositive character of carbon compared to nitrogen. Given this rationale,w ep ostulated that organoisocyanides, which are strong donors,y et provide stabilizing p-acidity properties, [19,20] could yield the multiple substitutions necessary (n > 2) to meet the electronic threshold for activation of the interstitial nitride. [11] Additionally, [Fe 4 (m 4 -N)(CO) 12Àn (L) n ] À clusters with increasing levels of phosphine or other strong-donor substitution have remained elusive.T hese observations suggest that two ligand substitutions inhibit subsequent CO labilization prior to the onset of increased nucleophilicity at the interstitial nitrogen atom.…”

Kinetic Destabilization of Metal–Metal Single Bonds: Isolation of a Pentacoordinate Manganese(0) Monoradical

“…[16,17] Given the challenges faced in exploring the properties of [Mn (CO) ) 2 ]( Ar Dipp2 = 2,6-(2,6-(iPr) 2 C 6 H 3 ) 2 C 6 H 3 )f eaturing two m-terphenyl isocyanide ligands. [22,23] Thesterically encumbering nature of these isocyanidel igands effectively prevents the formation of MnÀMn single bonds,thus rendering this 17eÀ radical both isolable in the solid state and persistent in solution. Additionally,asaresult of the isolobal analogy between carbon monoxide (CO) and organoisocyanides (C NR), [22,24] ) 2 ]delivers 1 most cleanly and in the highest yield after isolation (92 %; Figure 1A).…”

“…[22,23] Thesterically encumbering nature of these isocyanidel igands effectively prevents the formation of MnÀMn single bonds,thus rendering this 17e…”