A Well-Defined Isocyano Analogue of HCo(CO)₄. 2: Relative Brønsted Acidity as a Function of Isocyanide Ligation

Abstract

The m-terphenyl isocyanide complex, HCo­(CNArMes2)4 (ArMes2 = 2,6-(2,4,6-Me3C6H2)2C6H3), serves as a unique example of a well-defined isocyano analogue to HCo­(CO)4. Given the well documented Brønsted acidity of HCo­(CO)4 in both protic and nonprotic media, the Brønsted acidity of HCo­(CNArMes2)4 was assessed for a quantitative comparison. Acid bracketing experiments in THF solution revealed that HCo­(CNArMes2)4 has a Morris relative pK α THF value of 38.5–40.7, which is considerably higher than that of HCo­(CO)4 (pK α THF (calc) = 11.4) and thereby indicates insignificant Brønsted acidity. Furthermore, the relative acidity of HCo­(CNArMes2)4 rivals that of tetra-phosphine cobalt hydrides (i.e., HCo­(PR3)4; pK α THF (calc) ≥ 48), despite the good π-acidity properties of the isocyano unit. To systematically determine the effect of substituting an isocyanide for a CO ligand on the acidity of the Co–H unit in HCoL4 complexes, the full series of HCo­(CO) n (CNArMes2)4–n monohydrides and [Co­(CO) n (CNArMes2)4–n ]− (n = 1–4) metalates were prepared and characterized. Acid bracketing studies on the [Co­(CO) n (CNArMes2)4–n ]− metalates in THF solution revealed a regular progression of increasing pK α THF values as isocyanides are added to the Co center. However, the monoisocyanide tricarbonyl hydride, HCo­(CO)3(CNArMes2), possesses a pK α THF value of 28.6–32.5, which is also significantly higher than that of HCo­(CO)4 and the monophosphine complex HCo­(CO)3(PPh3). Accordingly, the unconventional ability of isocyanide ligands to function as stronger σ-donors than organophosphines is discussed within the context of both the Brønsted acidity and spectroscopic features of the HCo­(CO) n (CNArMes2)4–n monohydrides.

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.…”

“…[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. 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.…”

“…Accordingly,h erein we report the use of m-terphenyl isocyanides [19][20][21][22] for the preparation of the heteroleptic cluster Na[Fe 4 (m 4 -N)(CO) 8 (CNAr Mes2 ) 4 ]( Na [1], Ar Mes2 = 2,6-(2,4,6-Me 3 C 6 H 2 ) 2 C 6 H 3 ), where multiple isocyanide-for-carbonyl substitutions successfully activate the nucleophilic character of the interstitial nitride.T his enhanced reactivity profile, coupled with the encumbering nature of the CNAr Mes2 ligands, allows for controlled cluster growth with avariety of electrophiles.S uch cluster expansion reactions can be used for the systematic incorporation of coordinatively and electronically unsaturated transition-metal and main-group centers into the multimetallic framework, thereby providing reactive sites in am olecular system that mimic some of the characteristics known for heterogeneous surfaces. [1] In our initial attempts to prepare cluster 1 À ,wefound that it cannot be directly obtained by simple thermolytic ligand exchange on [Fe 4 (m 4 -N)(CO) 12 ] À .P rolonged heating of [Na-(diglyme) 2 ][Fe 4 (m 4 -N)(CO) 12 ]i nt he presence of excess CNAr Mes2 led to intractable mixtures of products as assessed by 1 HNMR spectroscopy.However,heating abenzene slurry of [Na(diglyme) 2 ][Fe 4 (m 4 -N)(CO) 12 ]a tr eflux in the presence of 1.0 equiv of benzoic acid and 4.2 equiv of CNAr Mes2 for 21 h generated the monohydride HFe 4 (m 4 -N)(CO) 8 (CNAr Mes2 ) 4 (1-H;S cheme 1), as indicated by the consumption of free isocyanide and the appearance of an upfield singlet at À30.87 ppm in the 1 HNMR spectrum of the reaction mixture.…”

“…Thehydride cluster 1-H can subsequently be deprotonated with NaO t Bu in THF solution to yield Na[Fe 4 (m 4 -N)-(CO) 8 (CNAr Mes2 ) 4 ]( Na [1]), as determined by X-ray diffraction ( Figure 1). Thes olid-state structure of Na [1]r eveals ac ontact-ion pair where the Na + counterion engages in several cation p(CN)/p(arene) interactions [20][21][22] and makes along,but significant, contact with the interstitial nitride unit (2.580 (avg)). In addition, the uneven distribution of isocyanide ligands found in the solid state for 1-H is retained in Na [1].…”

Dianionic Mononuclear Cyclo‐P4 Complexes of Zero‐Valent Molybdenum: Coordination of the Cyclo‐P4 Dianion in the Absence of Intramolecular Charge Transfer

“…Theo ther K + ion is engaged in contact-ion interactions with the CNa nd arene portions of the CNAr Dipp2 ligands as suggested from the IR spectroscopic data. [38][39][40] Tr eatment of [K 2 (db18-c-6)] [3]w ith up to five additional equivalents of dibenzo[db18-crown-6] did not succeed in the encapsulation of the second K + ion, thereby indicating that contact-ion pairing in this complex is substantial.…”