Unsymmetrical PNP-Pincer Type Phosphaalkene Ligands Protected by a Fused-Ring Bulky Eind Group: Synthesis and Applications to Rh(I) and Ir(I) Complexes

We recently reported that 2-(phospholanylmethyl)-6-(2phosphaethenyl)pyridine (PPEP) with a 2,4,6-tri-tert-butylphenyl group (Mes*) as steric protection of the PC bond serves as a noninnocent ligand on Ir(I), leading to extremely high reactivity toward metal−ligand cooperative activation of ammonia and acetonitrile. The high reactivity is largely due to the strong π-accepting properties of the PC bond. However, PPEP had a stability problem that provokes the loss of the PC bond on other transition metals, including Rh(I), and restricts its utilization. This paper describes the synthesis of Eind-PPEP protected by an octaethyl-s-hydrindacen-4-yl group (Eind) instead of Mes*. The fusedring bulky Eind group successfully prevents the loss of the PC bond and enables us to compare the reactivity of Rh(I) and Ir(I) complexes toward ammonia. The complex K[RhCl(Eind-PPEP*)], bearing a dearomatized Eind-PPEP* ligand, undergoes simple ligand displacement to give [Rh(NH3)(Eind-PPEP*)], whereas the iridium analogue K[IrCl(Eind-PPEP*)] causes N−H bond cleavage to form [Ir(NH2)(Eind-PPEP)]. DFT calculations indicate a thermodynamic cause of the metal-dependent product change. ■ INTRODUCTION Phosphaalkenes with a PC double bond are an interesting class of phosphorus compounds, which possess an extremely low lying π* orbital and thus serve as strong π acceptors toward transition metals. We are currently interested in the application of this particular ligand property in organometallic chemistry. This paper describes the synthesis of a novel pyridine-based PNP-pincer type phosphaalkene ligand that exhibits noninnocent behavior on group 9 metals. Metal−ligand cooperative activation of chemical bonds aided by noninnocent behavior of pyridine-based pincer ligands has found wide application in catalytic transformations. PNPand PNN-pincer complexes with a phosphanylmethyl group on the pyridine core undergo deprotonation at the benzylic position to cause dearomatization of the pyridine ring. The dearomatized complex cleaves chemical bonds in a heterolytic manner, along with regeneration of the aromatic pyridine ring, where the metal center and the dearomatized ligand serve as a Lewis acid and a Brønsted base, respectively. We recently found that the reactivity of dearomatized PNPpincer complexes of Ir(I) toward metal−ligand cooperation could be remarkably enhanced by incorporating a phosphaalkene unit into the PNP-pincer scaffold. For example, complex K[3b] in Scheme 1 bearing a dearomatized PNPpincer type phosphaalkene ligand (PPEP*) causes instant cleavage of the N−H bond of ammonia (1 atm) at room temperature. DFT calculations have revealed that the reactivity is effectively enhanced by strong π back-donation from the metal to the phosphaalkene unit. Complex K[3b] is prepared from 1b bearing a 2,6bis(phosphaethenyl)pyridine ligand (BPEP), as illustrated in Scheme 1. While the Mes* substituent is commonly used for steric protection of PC bonds, the Mes*-PCH group has been known to undergo C−H addition/cyclization on transition metals to form a phospholanylmethyl group in some instances. The conversion of BPEP into the unsymmetrical PNP-pincer ligand PPEP in Scheme 1 (1b → 2b) proceeds via this process. A crucial requirement is that BPEP undergoes C−H addition/cyclization exclusively at one of the Mes*-PCH groups. The reaction of Ir(I) complex 1b proceeded in perfect selectivity, and the resulting 2b was Special Issue: Organometallics in Asia Received: February 10, 2016 Published: May 2, 2016 Article pubs.acs.org/Organometallics © 2016 American Chemical Society 1526 DOI: 10.1021/acs.organomet.6b00113 Organometallics 2016, 35, 1526−1533 totally stable to further C−H addition/cyclization. On the other hand, attempts at other transition-metal complexes, including Rh(I) complex 1a, were unsuccessful due to the occurrence of successive C−H addition/cyclization at both Mes*-PCH groups, giving rise to complete loss of PC bonds. We therefore attempted to synthesize a novel BPEP ligand that can be selectively converted to PPEP on various metals. In this case, the most reliable way is the introduction of a steric protecting group that is stable to C−H addition/cyclization into one of the phosphaalkene units. We found that the 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl group (Eind), having a fused-ring bulky structure, effectively prevents undesirable loss of the PC bond. Using the newly synthesized phosphaalkene ligand Eind-PPEP, we could observe a distinct change in the reaction products of K[M(Cl)(Eind-PPEP*)] (M = Rh, Ir) and ammonia depending on the metals. ■ RESULTS AND DISCUSSION Synthesis of Eind-BPEP (7). Compound 7, having Mes*PCH and Eind-PCH groups at the 2,6-positions of pyridine, was synthesized by a two-step procedure starting from 2,6-pyridinedicarboxaldehyde (5) (Scheme 2). While phosphaalkene units may be constructed either by a phosphaPeterson reaction or by a phospha-Wittig reaction, we chose the phospha-Wittig reaction due to the difficulty in preparing the phospha-Peterson reagent Eind-P(SiMe3)Li. The first step is the treatment of 5 with Eind-PPMe3, generated in situ from Eind-PCl2, 10 PMe3, and zinc dust in THF. The phosphaWittig reaction took place selectively at one of the formyl groups, and the desired compound 6 was isolated as a pale yellow powder in 74% yield by column chromatography. Compound 6 was then treated with Mes*-PPMe3 in THF, and Eind-BPEP (7) was obtained in 97% yield. This product was contaminated with a small amount of Mes*-PH2, which could not be removed by repeated chromatographic purifications. Accordingly, compound 7 was identified by NMR spectroscopy and mass spectrometry and used for complex formation without further purification. The P{H} NMR spectrum displayed two singlet signals at δP 285.8 and 275.2, consistent with the unsymmetrical structure of 7 having different substituents on phosphorus atoms. Synthesis of Eind-BPEP Complexes 8 and 9. Eind-BPEP (7) was coordinated with rhodium and iridium chlorides by the reactions with [Rh(μ-Cl)(η-C2H4)2]2 and [Ir(μ-Cl)(η coe)2]2, respectively (Scheme 3). Both reactions took place instantly at room temperature to give [RhCl(Eind-BPEP)] (8) and [IrCl(Eind-BPEP)] (9) quantitatively, as confirmed by