Elementary Mechanism and Scope of Net Heterolytic B–H and B–B Bond Cleavage Across the Pd–N Bond in a Cationic (PNP)Pd Fragment

The cationic, three-coordinate pincer fragment $[(\mathrm{PNP})\mathrm{Pd}]^+$ (PNP = bis(2-diisopropylphosphino-4-methylphenyl)amido) cleaves the B–H bond of catecholborane and the B–B bond of bis(catecholato)diboron by net heterolytic 1,2-addition across the Pd–N(amido) bond, delivering boron to the amido nitrogen and hydride (or the second boryl group) to palladium. The 2011 source study established the products and their regiochemistry but not the pathway. No kinetic, isotopic-labeling, or computational study of this system has been reported, the proton source behind the trace protolysis products remains unidentified, the non-observed $^{11}\mathrm{B}$ NMR resonances of the diboryl product remain unexplained, and the reaction has not been extended to other boranes, PNP ligand variants, or catalytic conditions on this platform. Determining the elementary mechanism and scope of this cooperative B–H/B–B activation therefore remains an open problem.

Background

Pincer ligands are tridentate ligands that bind a transition metal through three donor atoms in a plane, giving a robust, well-defined coordination environment. Here PNP denotes the monoanionic diarylamido/bis(phosphine) pincer bis(2-diisopropylphosphino-4-methylphenyl)amido: two neutral phosphine arms flank a central amido nitrogen (a deprotonated amine and a strong σ-donor) bound to palladium(II). Removing the fourth ligand gives the cationic, three-coordinate, coordinatively unsaturated fragment $[(\mathrm{PNP})\mathrm{Pd}]^+$, which is accessed with weakly coordinating anions — tetrakis(3,5-bis(trifluoromethyl)phenyl)borate ($\mathrm{BAr^F_4}^-$) or monocarba-closo-dodecaborate ($\mathrm{CB_{11}H_{12}}^-$) — that stay off the open metal site.

'Metal–ligand cooperation' denotes bond activation in which the ligand participates directly: an X–Y σ-bond is cleaved by net heterolytic (ionic) 1,2-addition across a metal–ligand bond, one fragment going to the metal and the other to a basic site on the ligand, with no change in the metal's formal oxidation state. This contrasts with oxidative addition, in which both fragments bind the metal and its oxidation state rises by two. A σ-complex is an adduct in which an intact X–Y σ-bond donates into a vacant metal orbital without full cleavage; σ-borane complexes of this type are the classical entry point to B–H activation at many metals.

The reagents are catecholborane, $\mathrm{HBcat}$ (cat = catecholato, the 1,2-dioxobenzene dianion), and bis(catecholato)diboron, $\mathrm{CatB{-}BCat}$, a B–B-bonded diboron(4) compound. Zhu et al. (2011) showed that $[(\mathrm{PNP})\mathrm{Pd}]^+$ reacts cleanly with $\mathrm{HBcat}$ to give $[(\mathrm{PN(BCat)P})\mathrm{PdH}]^+$ (2) — the boryl fragment on the amido nitrogen, hydride on Pd — and with $\mathrm{CatB{-}BCat}$ to give $[(\mathrm{PN(BCat)P})\mathrm{Pd(BCat)}]^+$ (3), bearing boryl groups on both N and Pd (Pd–B = 1.9879(16) Å): net heterolytic cleavage of B–H and B–B across the Pd–N bond. That study also pinned down constraints any mechanism must satisfy: the two possible regioisomers from $\mathrm{HBcat}$ addition (Pd–H/N–B versus Pd–B/N–H) are both isolable but do not thermally interconvert at 90 °C, whereas heating the Pd–H/N–B compound with excess catecholborane at 95 °C converts it to the Pd–B/N–B compound with evolution of $\mathrm{H_2}$; and the N–B bond hydrolyzes far faster than the Pd–B bond, giving $[(\mathrm{PN(H)P})\mathrm{PdH}]^+$ (1) and $[(\mathrm{PN(H)P})\mathrm{Pd(BCat)}]^+$ (4). Heterolytic splitting of H–X bonds at this same cationic (PNP)Pd center had been demonstrated by Gregor et al. (2010), and borylpalladium complexes are classically reached by oxidative addition of B–X bonds to low-valent Pd or, as shown by Kirai et al. (2013), by σ-bond metathesis at phosphine-ligated Pd(II) — the mechanistic alternatives a pathway proposal must be weighed against.

Problem Statement

Determine the elementary mechanism by which the B–H bond of catecholborane and the B–B bond of bis(catecholato)diboron undergo net heterolytic cleavage across the Pd–N(amido) bond of the cationic three-coordinate fragment $[(\mathrm{PNP})\mathrm{Pd}]^+$ (PNP = bis(2-diisopropylphosphino-4-methylphenyl)amido): specifically, whether the reactions proceed (i) through a discrete σ-B–H (or σ-B–B) complex at the open palladium site followed by intramolecular transfer of H (or boryl) between boron and the amido nitrogen, (ii) by concerted 1,2-addition across the Pd–N bond in a single transition state, or (iii) by oxidative addition at palladium followed by ligand rearrangement, or by another route such as σ-bond metathesis at Pd(II) (Kirai et al. 2013). The list is non-exhaustive: if more than one route is viable, the determination must state which is preferred for each substrate and why. The determination must account for the established observations: exclusive B-to-N / H-to-Pd regiochemistry from $\mathrm{HBcat}$; non-interconversion of the isomers $[(\mathrm{PN(BCat)P})\mathrm{PdH}]^+$ and $[(\mathrm{PN(H)P})\mathrm{Pd(BCat)}]^+$ at 90 °C; conversion of $[(\mathrm{PN(BCat)P})\mathrm{PdH}]^+$ to $[(\mathrm{PN(BCat)P})\mathrm{Pd(BCat)}]^+$ with excess $\mathrm{HBcat}$ at 95 °C with evolution of $\mathrm{H_2}$; and faster hydrolysis of the N–B than the Pd–B bond. The determination must also address the scope of this addition mode on the same $[(\mathrm{PNP})\mathrm{Pd}]^{+}$ platform: whether the net heterolytic cleavage extends to other boranes and diborons, to PNP ligand variants, and to catalytic conditions. Addressing the scope requires assessing the expected generality by analysis of at least one other borane and one other diboron, one PNP ligand variant, and one catalytic turnover cycle, stating the predicted outcome for each. The identity of the proton source behind the trace protolysis products and the non-observed $^{11}\mathrm{B}$ NMR resonances of the diboryl product remain open observations, provided as context rather than as required deliverables.

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