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. 2018 May 31;3(5):4986-5001.
doi: 10.1021/acsomega.8b00777. Epub 2018 May 8.

Roles of Hydrogen Bonding in Proton Transfer to κP,κN,κP-N(CH2CH2P i Pr2)2-Ligated Nickel Pincer Complexes

Nadeesha P N Wellala  1 , John D Luebking  1 , Jeanette A Krause  1 , Hairong Guan  1
Affiliations

Roles of Hydrogen Bonding in Proton Transfer to κP,κN,κP-N(CH2CH2P i Pr2)2-Ligated Nickel Pincer Complexes

Nadeesha P N Wellala et al. ACS Omega. .

Abstract

The nickel PNP pincer complex ( i PrPNP)NiPh ( i PrPNP = κP,κN,κP-N(CH2CH2P i Pr2)2) was prepared by reacting ( i PrPNP)NiBr with PhMgCl or deprotonating [( i PrPNHP)NiPh]Y ( i PrPNHP = κP,κN,κP-HN(CH2CH2P i Pr2)2; Y = Br, PF6) with KO t Bu. The byproducts of the PhMgCl reaction were identified as [( i PrPNHP)NiPh]Br and ( i PrPNP')NiPh ( i PrPNP' = κP,κN,κP-N(CH=CHP i Pr2)(CH2CH2P i Pr2)). The methyl analog ( i PrPNP)NiMe was synthesized from the reaction of ( i PrPNP)NiBr with MeLi, although it was contaminated with ( i PrPNP')NiMe due to ligand oxidation. Protonation of ( i PrPNP)NiX (X = Br, Ph, Me) with various acids, such as HCl, water, and MeOH, was studied in C6D6. Nitrogen protonation was shown to be the most favorable process, producing a cationic species [( i PrPNHP)NiX]+ with the NH moiety hydrogen-bonded to the conjugate base (i.e., Cl-, HO-, or MeO-). Protonation of the Ni-C bond was observed at room temperature with ( i PrPNP)NiMe, whereas at 70 °C with ( i PrPNP)NiPh, both resulting in [( i PrPNHP)NiCl]Cl as the final product. Protonation of ( i PrPNP)NiBr was complicated by site exchange between Br- and the conjugate base and by the degradation of the pincer complexes. Indene, which lacks hydrogen-bonding capability, was unable to protonate ( i PrPNP)NiPh and ( i PrPNP)NiMe, despite being more acidic than water and MeOH. Neutral and cationic nickel pincer complexes involved in this study, including ( i PrPNP')NiBr, ( i PrPNP)NiPh, ( i PrPNP')NiPh, ( i PrPNP)NiMe, [( i PrPNHP)NiPh]Y (Y = Br, PF6, BPh4), [( i PrPNHP)NiPh]2[NiCl4], [( i PrPNHP)NiMe]Y (Y = Cl, Br, BPh4), [( i PrPNHP)NiBr]Br, and [( i PrPNHP)NiCl]Cl, were characterized by X-ray crystallography.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Scheme 1
Scheme 1. Hydrogen Transfer in (De)Hydrogenation Reactions Catalyzed by PNP Pincer Complexes
Scheme 2
Scheme 2. Independent Synthesis of Complex 2
Scheme 3
Scheme 3. Independent Synthesis of Complex 2′
Figure 1
Figure 1
Oak Ridge thermal ellipsoid plot (ORTEP) drawing of (iPrPNP′)NiPh (2′) at the 50% probability level. Hydrogen atoms except those attached to the pincer backbone are omitted for clarity.
Figure 2
Figure 2
ORTEP drawing of (iPrPNP)NiPh (2) at the 50% probability level. Hydrogen atoms are omitted for clarity.
Figure 3
Figure 3
ORTEP drawing of [(iPrPNHP)NiPh]Br ([2H]Br) at the 50% probability level. Hydrogen atoms except the one attached to the nitrogen are omitted for clarity.
Figure 4
Figure 4
ORTEP drawing of [(iPrPNHP)NiPh]PF6 ([2H]PF6) at the 50% probability level. Hydrogen atoms except the one attached to the nitrogen are omitted for clarity. The PF6 counterion is disordered in the fluorine atoms.
Figure 5
Figure 5
ORTEP drawing of [(iPrPNHP)NiPh]BPh4 ([2H]BPh4) at the 50% probability level. THF molecules (co-crystallized with [2H]BPh4) and all hydrogen atoms except the one attached to the nitrogen are omitted for clarity.
Figure 6
Figure 6
ORTEP drawing of [(iPrPNHP)NiPh]2[NiCl4] ([2H]2[NiCl4]) at the 50% probability level. Co-crystallized C6D6 and H2O molecules and all hydrogen atoms, except the one attached to the nitrogen, are omitted for clarity.
Scheme 4
Scheme 4. Synthesis of Cationic Nickel Methyl Complexes
Figure 7
Figure 7
ORTEP drawings of nickel methyl complexes at the 50% probability level. Hydrogen atoms except the one attached to the nitrogen are omitted for clarity.
Scheme 5
Scheme 5. Protonation of Nickel Halide Complexes
Scheme 6
Scheme 6. Reaction of 1 with H2O
Scheme 7
Scheme 7. Protonation of 2 by HCl
Figure 8
Figure 8
31P{1H} NMR spectra of 2 (in C6D6) with various amounts of MeOH added.
Scheme 8
Scheme 8. Two Parallel Pathways for the Protonation of 3 by HCl
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