Pyrazole-Based PCN Pincer Complexes of Palladium(II): Mono- and Dinuclear Hydroxide Complexes and Ligand Rollover C–H Activation

Abstract

Palladium complexes of the novel unsymmetrical phosphine pyrazole-containing pincer ligands PCN H (PCN H = 1-[3-[(di-tert-butylphosphino)methyl]phenyl]-1H-pyrazole) and PCN Me (PCN Me = 1-[3-[(di-tert-butylphosphino)methyl]phenyl]-5-methyl-1H-pyrazole) have been prepared and characterized through single-crystal X-ray diffraction and multinuclear 1 H, 13 C{ 1 H}, and 31 P{ 1 H} NMR spectroscopy. In preparations of the monomeric hydroxide species (PCN H )Pd(OH), an unexpected N detachment followed by C−H activation on the heterocycle 5-position took place resulting in conversion of the monoanionic {P,C − ,N} framework into a dianionic {P,C − ,C − } ligand set. The dinuclear hydroxide-bridged species (PCN H )Pd(μ-OH)Pd(PCC) was the final product obtained under ambient conditions. The "rollover" activation was followed via 31 P{ 1 H} NMR spectroscopy, and dinuclear cationic μ-OH and monomeric Pd II hydroxide intermediates were identified. DFT computational analysis of the process (M06//6-31G*, THF) showed that the energy barriers for the pyrazolyl rollover and for C−H activation through a σ-bond metathesis reaction are low enough to be overcome under ambient-temperature conditions, in line with the experimental findings. In contrast to the PCN H system, no "rollover" reactivity was observed in the PCN Me system, and the terminal hydroxide complex (PCN Me )Pd(OH) could be readily isolated and fully characterized. ■ INTRODUCTIONThe use of pincer-ligated transition-metal complexes as catalysts for organic transformations has grown dramatically in recent years. 1 The robust tridentate binding motif coupled with the tunability of the steric and electronic parameters of pincer ligands has proven highly effective in stabilizing and allowing isolation of a variety of uncommon types of metal complexes. For example, pincer ligands have been very useful in the preparation of mononuclear late-metal complexes bearing M−OR and M−NR 2 bonds. Notably, there are significantly fewer mononuclear late-transition-metal hydroxide, alkoxide, and amide complexes relative to their metal alkyl (M−C) analogues. 2 However, such M−OR and M−NR 2 linkages are pertinent to catalysis, 3,4 and thus isolation and study of model metal hydroxide, alkoxide and amide complexes are of great value.The variety of available pincer ligands has increased in recent years. While early pincer ligands were symmetric with respect to ligand "arms" (e.g., PCP, PNP, POCOP, etc.), 5 pincer-type complexes bearing unsymmetrical arms have begun to appear in greater numbers (NCC, PNN, PCO, PCS, etc.). 6 PCN-type systems in particular are intriguing, because in (PCN)M(L) n complexes (M = transition metal; L = ancillary ligand) the tridentate hybrid ligand contains both hard (N) and soft (P) donor functions, thus leading to novel and unprecedented chemical properties. 7 In such species, there is a marked difference in the trans effect between the two different donor arms. This difference results in the group with the weaker trans effect (N) being more likely to dissociate fro...

Hydrogenolysis of Dinuclear PCNR Ligated PdII μ‐Hydroxides and Their Mononuclear PdII Hydroxide Analogues

“…However,i fd issociation of ap incera rm was relativelyf acile (hemilabile ligand arm), the reactioni nstead proceeded through formation of as quare-planar intermediate wherein dihydrogen has replaced the hemilabile arm. [14] (1-OH)w ithH 2 (1-7 atm), as low transformation was observed by 1 Ha nd 31 PNMR spectroscopy over days at room temperature,w ith the loss of the signal associated with the hydroxide ligand,a nd the growth of an ew hydridic species, {[(PCN H )Pd] 2 (m-H)}(OTf) (1-H)a sc onfirmed below.I nt he 1 HNMR spectrum of 1-H,t he bridgingh ydride signal at d H = À8.68 ppm is split into at riplet by two equivalent phosphorous nuclei of the PCN H ligands( 2 J PH = 12.6 Hz). Further reactionl eads to release of water,C ÀHr eductive eliminationo ft he pincerl igand, and formation of aP d 0 complex.…”

“…Exposure of as olution of the mononuclear Pd hydroxide 3-OH in [D 8 ]THF at room temperature to H 2 (7 atm) resulted in the appearance of an ew hydride species 7-H within hours. [14] After 24 ho fr eaction, the same product 5,o bserved in the case of 1-H,i sf ound in the reaction mixture by the growth of as inglet at d P = 61.1 ppm in the 31 P{ 1 H} NMR spectrum, accompanied by Pd 0 precipitation. This was confirmed in a 1 H{ 31 P} NMRe xperiment where the signal at d H = À8.65 ppm appearsc learly as as inglet.T he 31 P{ 1 H} NMR spectrum displays two singlets at d P = 85.9 ppm and d P = 103.8 ppm.…”

“…[14] The reaction of 2-OH with H 2 Scheme1.Hydrogenolysis of dinuclear 1-OH and 2-OH and reduction of the resultingd inuclear hydride complexes. Complex 2-OH was chosen as ar epresentativee xample owing to its increased stabilityc ompared with 1-OH.…”

“…[14] Therefore, hydrogenolysis of 3-OH was also performed in a[ D 8 ]THF/D 2 Om ixed-solvent system. [14] Therefore, hydrogenolysis of 3-OH was also performed in a[ D 8 ]THF/D 2 Om ixed-solvent system.…”

“…[11] With firmly bound tridentatel igands, the reactions proceed similarly to the PCP analogue. [14] Both dinuclear m-OH and mononuclear terminal hydroxidesw ere reported.W e now report the hydroxide complexes's usceptibility towards hydrogenolysis, and the contrasting reactivity between the mono-a nd dinucleara nalogues. In this case, the PdÀOH bond is not cleaved upon the H 2 addition and the water remains bound to the metal center along with the new hydride ligand.…”