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Title: 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(CNAr Mes2) 4 (Ar Mes2 = 2,6-(2,4,6-Me 3C 6H 2) 2C 6H 3), 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(CNAr Mes2) 4 was assessed for a quantitative comparison. Acid bracketing experiments in THF solution revealed that HCo(CNAr Mes2) 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(CNAr Mes2) 4 rivals that of tetra-phosphine cobalt hydrides (i.e., HCo(PR 3) 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 HCoL 4 complexes, the full series of HCo(CO) n(CNAr Mes2) 4-n monohydrides and [Co(CO) n(CNAr Mes2) 4-n] - (n = 1-4) metalates were prepared and characterized. Acid bracketing studies on the [Co(CO) n(CNAr Mes2) 4-n] - metalates in THF solution revealed a regular progression of increasingmore » pK α THF values as isocyanides are added to the Co center. However, the monoisocyanide tricarbonyl hydride, HCo(CO) 3(CNAr Mes2), 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(PPh 3). 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(CNAr Mes2) 4-n monohydrides.« less

Authors:
 [1];  [1];  [1];  [1]
  1. Univ. of California, San Diego, CA (United States). Dept. of Chemistry and Biochemistry
Publication Date:
Research Org.:
Univ. of California, San Diego, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22); National Science Foundation (NSF)
OSTI Identifier:
1260851
Grant/Contract Number:
SC0008058; CHE-1464978
Resource Type:
Journal Article: Published Article
Journal Name:
Organometallics
Additional Journal Information:
Journal Volume: 35; Journal Issue: 14; Journal ID: ISSN 0276-7333
Publisher:
American Chemical Society
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY

Citation Formats

Carpenter, Alex E., Chan, Chinglin, Rheingold, Arnold L., and Figueroa, Joshua S. A Well-Defined Isocyano Analogue of HCo(CO)4. 2: Relative Brønsted Acidity as a Function of Isocyanide Ligation. United States: N. p., 2016. Web. doi:10.1021/acs.organomet.6b00375.
Carpenter, Alex E., Chan, Chinglin, Rheingold, Arnold L., & Figueroa, Joshua S. A Well-Defined Isocyano Analogue of HCo(CO)4. 2: Relative Brønsted Acidity as a Function of Isocyanide Ligation. United States. doi:10.1021/acs.organomet.6b00375.
Carpenter, Alex E., Chan, Chinglin, Rheingold, Arnold L., and Figueroa, Joshua S. 2016. "A Well-Defined Isocyano Analogue of HCo(CO)4. 2: Relative Brønsted Acidity as a Function of Isocyanide Ligation". United States. doi:10.1021/acs.organomet.6b00375.
@article{osti_1260851,
title = {A Well-Defined Isocyano Analogue of HCo(CO)4. 2: Relative Brønsted Acidity as a Function of Isocyanide Ligation},
author = {Carpenter, Alex E. and Chan, Chinglin and Rheingold, Arnold L. and Figueroa, Joshua S.},
abstractNote = {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 pKα THF value of 38.5-40.7, which is considerably higher than that of HCo(CO)4 (pKα 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; pKα 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 pKα THF values as isocyanides are added to the Co center. However, the monoisocyanide tricarbonyl hydride, HCo(CO)3(CNArMes2), possesses a pKα 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.},
doi = {10.1021/acs.organomet.6b00375},
journal = {Organometallics},
number = 14,
volume = 35,
place = {United States},
year = 2016,
month = 7
}

Journal Article:
Free Publicly Available Full Text
Publisher's Version of Record at 10.1021/acs.organomet.6b00375

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  • The m-terphenyl isocyanide complex, HCo(CNAr Mes2) 4 (Ar Mes2 = 2,6-(2,4,6-Me 3C 6H 2) 2C 6H 3), 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(CNAr Mes2) 4 was assessed for a quantitative comparison. Acid bracketing experiments in THF solution revealed that HCo(CNAr Mes2) 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.more » Furthermore, the relative acidity of HCo(CNAr Mes2) 4 rivals that of tetra-phosphine cobalt hydrides (i.e., HCo(PR 3) 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 HCoL 4 complexes, the full series of HCo(CO) n(CNAr Mes2) 4-n monohydrides and [Co(CO) n(CNAr Mes2) 4-n] - (n = 1-4) metalates were prepared and characterized. Acid bracketing studies on the [Co(CO) n(CNAr Mes2) 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(CNAr Mes2), 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(PPh 3). 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(CNAr Mes2) 4-n monohydrides.« less
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