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Coordination or Oxidative Addition? Activation of N-H with [Tp'Rh(PMe 3 )].

Authors :: Yuwen J
Brennessel WW
Jones WD
Source :: Inorganic chemistry [Inorg Chem] 2019 Jan 07; Vol. 58 (1), pp. 557-566. Date of Electronic Publication: 2018 Dec 24.
Publication Year :: 2019
Abstract: A thermal reaction of amines, anilines, and amides with Tp'Rh(PMe <subscript>3</subscript> )(CH <subscript>3</subscript> )H (1, Tp' = tris(3,5-dimethyl-pyrazolyl)borate) is described in this report. No N-H bond cleavage was observed for reactions between ammonia or unsubstituted aliphatic amines with the reactive fragment [Tp'Rh(PMe <subscript>3</subscript> )]. Instead, amine coordination products (κ <superscript>2</superscript> -Tp')Rh(PMe <subscript>3</subscript> )(NHR <superscript>1</superscript> R <superscript>2</superscript> ) (R <superscript>1</superscript> = H, R <superscript>2</superscript> = H, <superscript>n</superscript> Pr, <superscript>i</superscript> Pr, octyl; R <superscript>1</superscript> = R <superscript>2</superscript> = Et; R <superscript>1</superscript> , R <superscript>2</superscript> : pyrrolidine) were observed, and the crystal structure of (κ <superscript>2</superscript> -Tp')Rh(PMe <subscript>3</subscript> )(NH <subscript>2</subscript> <superscript>i</superscript> Pr) is reported. No coordination products were observed when 1 was reacted with 1,1,1,3,3,3-hexafluoropropan-2-amine, anilines, and amides. Instead, the oxidative addition products (κ <superscript>3</superscript> -Tp')Rh(PMe <subscript>3</subscript> )(NHR)H (R = CH(CF <subscript>3</subscript> ) <subscript>2</subscript> , C <subscript>6</subscript> H <subscript>5</subscript> , 3,5-dimethylbenzyl, C <subscript>6</subscript> F <subscript>5</subscript> , C(O)CH <subscript>3</subscript> , C(O)CF <subscript>3</subscript> ) were observed. Both Rh <superscript>I</superscript> -N coordination products (κ <superscript>2</superscript> -Tp')Rh(PMe <subscript>3</subscript> )(NH <subscript>2</subscript> CH <subscript>2</subscript> CF <subscript>3</subscript> ) and Rh <superscript>III</superscript> N-H addition products (κ <superscript>3</superscript> -Tp')Rh(PMe <subscript>3</subscript> )(NHCH <subscript>2</subscript> CF <subscript>3</subscript> )H were generated when 1 was reacted with 2,2,2-trifluoroethylamine. Coordination products dissociate ammonia and amines in benzene much faster than oxidative addition products eliminate anilines and amides. The relative metal-nitrogen bond energies were studied using established kinetic techniques. Analysis of the relationship between the relative M-N bond strengths and N-H bond strengths showed a linear correlation with a slope = R <subscript>M-N/N-H</subscript> of 0.91 (10), indicating that the Rh-N bond strength varies in direct proportion to the N-H bond strength.