Borane-Bridged Ruthenium Complex Bearing a PNP Ligand: Synthesis and Structural Characterization

Influence of Multisite Metal-Ligand Cooperativity on the Redox Activity of Noninnocent N2S2 Ligands

Metal-ligand cooperativity (MLC) relies on chemically reactive ligands to assist metals with small-molecule binding and activation, and it has facilitated unprecedented examples of catalysis with metal complexes. Despite growing interest in combining ligand-centered chemical and redox reactions for chemical transformations, there are few studies demonstrating how chemically engaging redox active ligands in MLC affects their electrochemical properties when bound to metals. Here we report stepwise changes in the redox activity of model Ru complexes as zero, one, and two BH₃ molecules undergo MLC binding with a triaryl noninnocent N₂S₂ ligand derived from o-phenylenediamine (L1). A similar series of Ru complexes with a diaryl N₂S₂ ligand with ethylene substituted in place of phenylene (L2) is also described to evaluate the influence of the o-phenylenediamine subunit on redox activity and MLC. Cyclic voltammetry (CV) studies and density functional theory (DFT) calculations show that MLC attenuates ligand-centered redox activity in both series of complexes, but electron transfer is still achieved when only one of the two redox-active sites on the ligands is chemically engaged. The results demonstrate how incorporating more than one multifunctional reactive site could be an effective strategy for maintaining redox noninnocence in ligands that are also chemically reactive and competent for MLC.

Metal–ligand cooperativity across two sites of a square planar iron(ii) complex ligated by a tetradentate PNNP ligand

A square planar (PNNP)FeII complex is shown to readily activate two B–H bonds across the Fe–amide linkages in an overall four-electron process facilitated by metal–ligand cooperativity.

Synthesis of Pyrazines and Quinoxalines via Acceptorless Dehydrogenative Coupling Routes Catalyzed by Manganese Pincer Complexes

Base-metal catalyzed dehydrogenative self-coupling of 2-amino alcohols to selectively form functionalized 2,5-substituted pyrazine derivatives is presented. Also, 2-substituted quinoxaline derivatives are synthesized by dehydrogenative coupling of 1,2-diaminobenzene and 1,2-diols. In both cases, water and hydrogen gas are formed as the sole byproducts. The reactions are catalyzed by acridine-based pincer complexes of earth-abundant manganese.

Benzylene-linked [PNP] scaffolds and their cyclometalated zirconium and hafnium complexes

The benzylene-linked [PNP] scaffolds HN(CH₂-o-C₆H₄PPh₂)₂ ([A]H) and HN(C₆H₄-o-CH₂PPh₂)₂ ([B]H) have been used for the synthesis of zirconium and hafnium complexes. For both ligands, the dimethylamides [A]M(NMe₂)₃ ([A]1-M) and [B]M(NMe₂)₃ ([B]1-M) were prepared and converted to the iodides [A]MI₃ ([A]2-M) and [B]MI₃ ([B]2-M) (M = Zr, Hf). Starting from these iodides, the corresponding benzyl derivatives [A]MBn₃ ([A]3-M) and [B]MBn₃ ([B]3-M) (M = Zr, Hf) were obtained via reaction with Bn₂Mg(OEt₂)₂. For zirconium, the benzylic ligand positions in [A]3-Zr and [B]3-Zr were found to cyclometalate readily, which led to the corresponding κ⁴-[PCNP]ZrBn₂ complexes [A]4-Zr and [B]4-Zr. As these complexes failed to hydrogenate cleanly, cyclometalated derivatives with only one alkyl substituent were targeted and the mixed benzyl chlorides κ⁴-[PCNP]MBnCl ([B]5-M, M = Zr, Hf) were obtained in the case of ligand [B]. Upon hydrogenation of [B]5-Zr, the η⁶-tolyl complex [B]Zr(η⁶-C₇H₈)Cl ([B]6-Zr) was generated cleanly, but the corresponding hafnium complex [B]5-Hf was found to decompose unselectively in the presence of H₂. Using a closely related carbazole-based [PNP] ligand, Gade and co-workers have shown recently that zirconium η⁶-arene complexes similar to [B]6-Zr may serve as zirconium(ii) synthons, namely when reacted with 2,6-Dipp-NC (L) or pyridine (py). Both these substrates were shown to react cleanly with [B]6-Zr, which led to the formation of the bis-isocyanide complex [B]ZrCl(L)₂ ([B]7-Zr) and the 2,2'-bipyridine derivative [B]ZrCl(bipy) ([B]8-Zr), respectively. Upon reaction of [B]Zr(η⁶-C₇H₈)Cl ([B]6-Zr) with NaBEt₃H, the cyclometalated derivative κ⁴-[PCNP]Zr(η⁶-C₇H₈) ([B]9-Zr) was isolated. In an attempt to synthesise terminal hydrides, complexes [A]MI₃ ([A]2-M) were treated with KBEt₃H, which led to the isolation of the cyclometalated hydrido complexes κ⁴-[PCNP]M(H)(κ³-Et₃BH) ([A]10-M; M = Zr, Hf) featuring a κ³-bound triethyl borohydride moiety.

(BB)-Carboryne Complex of Ruthenium: Synthesis by Double B-H Activation at a Single Metal Center

The first example of a transition metal (BB)-carboryne complex containing two boron atoms of the icosahedral cage connected to a single exohedral metal center (POBBOP)Ru(CO)2 (POBBOP = 1,7-OP(i-Pr)2-2,6-dehydro-m-carborane) was synthesized by double B-H activation within the strained m-carboranyl pincer framework. Theoretical calculations revealed that the unique three-membered (BB)>Ru metalacycle is formed by two bent B-Ru σ-bonds with the concomitant increase of the bond order between the two metalated boron atoms. The reactivity of the highly strained electron-rich (BB)-carboryne fragment with small molecules was probed by reactions with electrophiles. The carboryne-carboranyl transformations reported herein represent a new mode of cooperative metal-ligand reactivity of boron-based complexes.

Diamidophosphines with six-membered chelates and their coordination chemistry with group 4 metals: development of a trimethylene-methane-tethered [PN2]-type "molecular claw"

The coordination chemistry of the phosphine-tethered diamidophosphine ligands PhP(CH2CH2CH2NHPh)2 (pr[NPN]H2) and PhP(1,2-CH2-C6H4-NHSiMe3)2 (bn[NPN]H2) featuring six-membered N–C3–P chelates was explored with group 4 metals, which allowed for the consecutive development of a new trimethylene-methane-tethered [PN2] scaffold. In the case of the propylene-linked system pr[NPN]H2, access to the sparingly soluble dibenzyl derivative pr[NPN]ZrBn2 (3-Zr) was gained, while thermally sensitive zirconium and hafnium diiodo complexes bn[NPN]MI2 (5-M, M = Zr, Hf) were isolated in the case of the benzylene-linked derivative bn[NPN]H2. Despite the related phosphine-tethered backbone architectures of both of these ligands, their group 4 complexes were found to exhibit either C1-symmetric (bn[NPN]MX2) or averaged CS-symmetric (pr[NPN]MX2) structures in solution. To restrain the overall flexibility of these systems and thereby control the properties of the resulting complexes without disrupting the six-membered chelates, the new trimethylene-methane-tethered N,N′-di-(tert-butyl)-substituted [PN2]H2 protioligand was designed. This tripodal ligand system was prepared on a gram scale and its CS-symmetric dichloro complexes [PN2]MCl2 (6-M, M = Ti, Zr, Hf) were isolated subsequently. The benzene-soluble dibenzyl derivative [PN2]ZrBn2 (7-Zr) was synthesised as well and characterised by X-ray diffraction. These results are discussed not only in conjunction with the known [NPN]-coordinated group 4 complexes incorporating five-membered chelates, but also in the context of “molecular claws” that are related to the new [PN2] tripod.