Coordination of an N-Heterocyclic Phosphenium Containing Pincer Ligand to a Co(CO)₂ Fragment Allows Oxidation To Form an Unusual N-Heterocyclic Phosphinito Species

Abstract

A tridentate pincer ligand featuring a central Nheterocyclic phosphenium (NHP + ) donor has been coordinated to a Co(CO) 2 fragment to generate the Co NHP complex [PPP]Co(CO) 2 (2). The NHP unit adopts an unusual pyramidal geometry with a relatively long Co−P distance, suggesting a stereochemically active nonbonding phosphorus lone pair. Interestingly, treatment of 2 with trimethylamine N-oxide affords [P(PO)P]Co(CO) 2 (3), in which the Co-bound central phosphorus donor has been oxidized to an unprecedented N-heterocyclic phosphinito species. The bonding and electronic properties of these complexes are discussed in the context of DFT and NBO computational data. W hile N-heterocyclic carbene ligands (NHCs) have become ubiquitous in the field of transition metal and organo-catalysis, 1−5 far less focus has been placed on the potential applications of their isovalent group 15 analogues, Nheterocyclic phosphenium cations (NHPs). 6−10 First reported in 1972, 11,12 theoretical investigations of the electronic structure of NHPs have shown that they are poor σ donors but excellent π acceptors, in contrast to NHCs, which are very good σ-donors and poor π-acceptors. Recent advancements in the chemistry of NHPs include new preparative methods 13−16 and new reactivity patterns, 13,17−19 but the coordination chemistry and catalytic capabilities of NHPs are relatively unexplored compared to their NHC counterparts.One of the more interesting aspects of NHP ligands is their ability to adopt different binding modes toward transition metals depending on the electronic nature of the transition metal center. Most commonly, when binding to an electronpoor fragment, NHPs act as both σ-donors and π-acceptors, leading to a planar geometry at the phosphorus atom (NHP + description). However, when binding to an electron-rich fragment, NHPs adopt a pyramidal geometry indicative of a nonbonding phosphorus lone pair. Two limiting descriptions for the bonding in such pyramidal NHPs have been suggested:(1) the Lewis acidic NHP phosphorus acts solely as an electron pair acceptor (NHP + /M n ) and (2) the bonding between the metal and the NHP is covalent and the NHP is best described as a phosphido ligand (NHP − /M n+2 ). In this sense, a convincing analogy between NHPs and nitrosyls, which can adopt either linear or bent geometries, can be made, 17 highlighting the potential noninnocent character of these ligands and bringing about potential ambiguities in the formal metal oxidation state. Key advantages of phosphenium ligands over their noninnocent nitrosyl counterparts are (1) the ability to modify their steric and electronic properties via derivatization and (2) the ability to incorporate these ligands into chelating frameworks. These strategies may also be effective at imparting stability in transition-metal NHP complexes and protecting them from nucleophilic attack. Transition-metal complexes of multidentate ligands featuring NHPs are noticeably absent from the literature, particularly in comparison to the growing number of NHC-containing...

Addition of H2 Across a Cobalt–Phosphorus Bond

“…[28][29][30][31][32] Extension of the metal-ligand cooperativity observed for amide/amine systems to phosphide/phosphines poses as ignificant challenge,b ut may result in more reactive systems since P À H bonds are generally weaker and more acidic than N À Hbonds, and because metal phosphides are more nucleophilic than their metal amide congeners. [40,42,43] By using experimental and computational data, it was determined that the pyramidal geometry about the central phosphorus is the result of atwo-electron transfer from the metal to the ligand to generate an NHP À phosphido, and the preference for the pyramidal phosphido resonance form in the tridentate motif is thought to be the result of delocalization of the nitrogen lone pairs into the p system of the coplanar aryl side arms. [38][39][40] In this work, we used atridentate PPP pincer ligand with ac entral N-heterocyclic phosphenium/phosphido (NHP + / NHP À )donor,developed in our laboratory, [41] to demonstrate the first example of H 2 activation across af irst-row MÀP bond.…”

“…[34][35][36][37] In addition, the ability of disubstituted phosphorus ligands to adopt either PR 2 À or PR 2 + forms imparts unique ambiphilic behavior to these ligands and, by extension both protic and hydridic qualities to metal-bound HPR 2 functionalities. [40,[42][43][44] Thepotential for NHPs to convert between ap hosphenium (NHP + )a nd phosphido (NHP À )form poses the possibility for non-innocent behavior from ar edox standpoint, [40,45] and both NHP + and NHP À forms have the respective electrophilic and nucleophilic characteristics to potentially participate in bifunctional substrate-activation processes.P revious work in the Thomas group has explored the coordination and subsequent reactivity of a(PPP)Co(CO) 2 complex synthesized through anion metathesis from ac hlorophosphine precursor (PP Cl P, 1)a nd Na[Co(CO) 4 ]; however, this complex is unreactive towards small-molecule substrates,and removal of the CO ligands was unsuccessful. When NHP + ligands,which are isolobal analogues of Nheterocyclic carbenes,a re incorporated into at ridentate ligand framework, the central phosphorus tends to adopt ap yramidal geometry rather than the planar geometry normally observed in metal-bound untethered NHPs + .…”

The Mechanism of Alkane Selective Oxidation by the M1 Phase of Mo–V–Nb–Te Mixed Metal Oxides: Suggestions for Improved Catalysts

“…A metal-carbon bond is formed in the reaction by binding directly with thewhich the metal binds direction to the P=O. Several such an organometallic structures have been synthesized recently, [42][43][44][45][46][47][48][49] but we find that none leads to a particularly strong bond of H to the P=O. For such a motif, after hydrogen binds with O=P, P-M σ-bond becomes a donor-acceptor bond and metal is reduced by one electron (Scheme 4).…”

Computational Design of a Pincer Phosphinito Vanadium ((OPO)V) Propane Monoxygenation Homogeneous Catalyst Based on the Reduction-Coupled Oxo Activation (ROA) Mechanism

“…Indeed a few such complexes have been reported. 9,12 Our prior investigation led to the proposal that the ligand bis(2-phenoxyl)phosphinite κ-P coordinated on vanadium, abbreviated (OPO)V, is a candidate catalyst for C-H activation (Scheme 1). We expect that this complex should be robust based on factors such as the chelation effect, the stability of O-V bonds, and the rigidity of the ligand, which guards against insertion of small activating molecules into the V-P bond.…”