Cobalt N-Heterocyclic Phosphenium Complexes Stabilized by a Chelating Framework: Synthesis and Redox Properties

Abstract

Two cobalt complexes containing coordinated N-heterocyclic phosphenium (NHP) ligands are synthesized using a bidentate NHP/phosphine chelating ligand, [PP]. Treatment of Na[Co(CO)] with the chlorophosphine precursor [PP]Cl (1) affords [PP]Co(CO) (2), which features a planar geometry at the NHP phosphorus center and a short Co-P distance [1.9922(4) Å] indicative of a Co═P double bond. The more electron-rich complex [PP]Co(PMe) (3), which is synthesized in a one-pot reduction procedure with 1, CoCl, PMe, and KC, has an even shorter Co-P bond [1.9455(6) Å] owing to stronger metal-to-phosphorus back-donation. The redox properties of 2 and 3 were explored using cyclic voltammetry, and oxidation of 3 was achieved to afford [[PP]Co(PMe)] (4). The electron paramagnetic resonance spectrum of complex 4 features hyperfine coupling to both Co andP, suggesting strong delocalization of the unpaired electron density in this complex. Density functional theory calculations are used to further explore the bonding and redox behavior of complexes 2-4, shedding light on the potential for redox noninnocent behavior of NHP ligands.

N‐Heterocyclic Phosphenium Complex of Manganese: Synthesis and Catalytic Activity in Ammonia Borane Dehydrogenation

“…[1,2] Because the transition-metal fragment in the complexes adopts al ow (sometimes negative) formal oxidation state and the PR 2 + moiety often contains p-donating heteroatom substituents and is classified as a s-donor/p-acceptor ligand, the metal-ligand interaction is described as superposition of ligand-to-metal s-donation andm etal-to-ligand p-back-donation contributions with close parallels to the bondingi nF ischer-type carbene complexes. [3] The carbene complex analogy incites am etal-phosphorus double bond character of the neutralp hosphenium complexes, which is not only in accord with the planar phosphorus coordination geometry and the short phosphorusÀmetal bond lengths, but is also reflected in their chemical behaviour.T hus, complexes like I-IIIu ndergo typical reactions of double bond systemsl ike [2+ +1] cycloadditionr eactions of carbeneso ri solobal analogues thereof, or 1,2-addition of protic reagents (Scheme 1). [3] The carbene complex analogy incites am etal-phosphorus double bond character of the neutralp hosphenium complexes, which is not only in accord with the planar phosphorus coordination geometry and the short phosphorusÀmetal bond lengths, but is also reflected in their chemical behaviour.T hus, complexes like I-IIIu ndergo typical reactions of double bond systemsl ike [2+ +1] cycloadditionr eactions of carbeneso ri solobal analogues thereof, or 1,2-addition of protic reagents (Scheme 1).…”

“…[4] The opposite 1,2-addition regioselectivity noted for complexes II and III illustrates ad ichotomy that culminated in ad ebate over whether these species can still be pictureda s phosphenium complexes, or have crossed the borderline to planar phosphido complexes. [2] More recently,s uch considerations led to the elaboration of late-transition-metal complexes in which the geometricd istortions, resulting from the incorporation of aP R 2 unit in aP P-chelating [3]d or PPP-pincerl igand, [6] were considered to reflect af orced shift from phosphenium-to phosphido-types tructures. [2] More recently,s uch considerations led to the elaboration of late-transition-metal complexes in which the geometricd istortions, resulting from the incorporation of aP R 2 unit in aP P-chelating [3]d or PPP-pincerl igand, [6] were considered to reflect af orced shift from phosphenium-to phosphido-types tructures.…”

Addition of H2 Across a Cobalt–Phosphorus 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 + .…”

“…[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. [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. [42] We now turn our attention to the coordination of 1 to CoCl 2 ,followed by reduction to generate areactive Co I NHP À phosphido complex.…”

Oxidative P‐P‐Bindungsaddition an Cobalt(−I): Bildung eines Low‐spin‐Cobalt(III)‐Phosphanidokomplexes

“…Das Cobaltatom ist verzerrt tetraedrisch koordiniert [P‐Co‐P 110.6(1)° und 90.66(1)°]. Ähnliche Co‐P‐Bindungslängen wie in 1 [2.1437(5) Å] finden sich in Cobalt(I)‐, Cobalt(II)‐ und Cobalt(III)‐Komplexen mit verbrückenden Phosphanidoliganden und in einigen Cobaltphosphankomplexen ,,. Die P‐Atome weisen eine trigonal‐pyramidale Umgebung auf [Summe der Bindungswinkel an einem Phosphoratom: 337.2(2)°].…”