Potassium and Lithium Complexes with Monodeprotonated, Dearomatized PNP and PNC^NHC Pincer-Type Ligands

The reaction of 2,6-bis(di-tert-butylphosphinomethyl)pyridine (^tBuPN^tBuP) with 1 molar equiv of KCH₂C₆H₅ or LiCH₂SiMe₃ gave M(^tBuP∗N_a^tBuP) (M = K, Li; P∗ = vinylic P donor, ^tBuP = P^tBu₂, N_a = anionic amido N donor) after monodeprotonation of the α-lutidinyl-CH₂ and concomitant dearomatization of the heterocycle. Evidence is provided that the anion ^tBuPz.ast;N_a^tBuP may exist as Z- and E-isomers, interconvertible by rotation about the C_α-N-C_α-P exocyclic formal double bond. Thus, the two isomers of K(^tBuP∗N_a^tBuP), i.e., [K{(Z)-(^tBuP∗N_a^tBuP-κP∗,κN_a,κP)}(THF)], 1-Z·(THF), and [K{(E)-(^tBuP∗N_a^tBuP-κN_a,κP)}(THF)], 1-E·(THF), cocrystallized from THF in a 4:1 ratio. However, in the presence of DME, the isomerically pure [K{(E)-(^tBuP∗N_a^tBuP-κN_a,κP)}(DME)₂], 1-E·2(DME), was crystallized. The α-picolinyl-CH₂ moiety in ^RPNC^NHC (N = substituted 2-picoline, ^RP = PCy₂, R = Cy; ^RP = P^tBu₂, R = ^tBu; C^NHC = N-heterocyclic carbene) was similarly deprotonated with concomitant dearomatization using LiN(SiMe₃)₂. This afforded the complexes Li(^RP∗N_aC^NHC), which were crystallized as the Z- or E-isomers, [Li{(E)-(^tBuP∗N_aC^NHC-κN_a,κC^NHC)}(Et₂O)₂], 3^tBu-E·2(Et₂O), [Li{(Z)-(^CyP∗N_aC^NHC-κP∗,κN_a,κC^NHC)}(Et₂O)], 3^Cy-Z·(Et₂O), and [Li{(Z)-(^tBuP∗N_aC^NHC-κP∗,κN_a,κC^NHC)}(Et₂O)], 3^tBu-Z·(Et₂O). The Z- and E-isomers reversibly interconvert in solution as shown by ³¹P{¹H} and ⁷Li NMR spectroscopy. The acidity of the α-picolinyl-CH₂ in ^RPNC^NHC is higher than in the known ^tBuPN^tBuP (ca. by 6 pK_a units).