Exposure of 10π-electron benzazaphosphole 1 to HCl, followed by nucleophilic substitution with the Grignard reagent BrMgCCPh afforded alkynyl functionalized 3 featuring an exocyclic –CC–Ph group with an elongated P–C bond (1.7932(19) Å).

A new tris(phosphaalkene)phosphine ligand (1) was synthesized via phospha-Wittig methodology. Metalation of 1 with [RhCl(C 2 H 4 ) 2 ] 2 and [IrCl(COE) 2 ] 2 (COE = cyclooctene) produced trigonal bipyramidal metal chlorides 2a (M = Rh) and 2b (M = Ir) in which the ligand coordinates in a tetradentate fashion. X-ray crystallographic studies on 1·1.5THF, 2a·5CHCl 3 , and 2b·2.5CHCl 3 combined with DFT calculations revealed a pronounced change in hybridization of the phosphaalkene phosphorus atoms upon coordination to the Rh/Ir centers, resulting in highly sterically congested metal complexes. Nucleophilic substitution on 2a with NaN 3 afforded Rh−N 3 complex 3; computational analysis, IR spectroscopy, and 15 N{ 1 H} NMR spectroscopy on isotopologue 15 N-3 provided additional structural insights. Halide abstraction of the chloride in 2b with AgOTf in the presence of acetonitrile afforded cationic Ir−NCMe complex 4. Evidence of the bound acetonitrile unit was obtained by 2D NMR spectroscopy and deuterium labeling studies. ■ INTRODUCTIONTetradentate ligands 1 featuring sterically demanding groups are becoming increasingly popular, 2 as the resulting metal center contains a highly crowded, but well-defined binding pocket for the activation of small molecules. 3 Schrock and co-workers successfully exploited bulky TREN ligand derivatives (TREN = 2,2′,2′′-triaminotriethylamine; HIPT = hexaisopropylterphenyl, shown in Scheme 1) to direct dinitrogen coordination into a sterically protected trigonal bipyramidal (TBP) Mo pocket. 4 Subsequent treatment of the bound N 2 unit with a proton source and reducing agent resulted in the first example of homogeneous and catalytic ammonia production. 5The guiding principle of using sterics to dictate metalcentered reactivity 6 is evolving as ligands capable of electronically influencing reactivity have experienced a surge of interest. 7 The most common examples of ligands engaging in this redoxactive/noninnocent behavior 8 are pincer-derived metal complexes, which undergo metal−ligand cooperativity via dearomatization/aromatization processes 9 or by ligand-centered reduction through easily accessible, low-lying, and extended π networks. 10 Recently though, the Peters lab has directly observed this electronic influence with tetradentate XP 3 -supported (X = Si, 11 C, 12 B, 13 and N 14 ) TBP first-row metal complexes designed to catalyze dinitrogen reduction (Scheme 1). 15 Fe 16 and Co complexes 17 ligated by the BP 3 ligand facilitated the catalytic conversion of N 2 to NH 3 (in the presence of HBAr F and KC 8 ), while the performance of SiP 3 and CP 3 analogues was stoichiometric at best; no N 2 reduction has been reported with the NP 3 ligand set. 17 Trivalent boranes (BAr 3 ) are often counted as zero-electron ligands, 18 but recent evidence has suggested that the tricoordinate boron unit (BAr 3 ) in BP 3 can act as a zero-, one-, or two-electron ligand depending on the oxidation state of the metal complex. 19 This

Dearomatization of the five-membered ring of N-Dipp (Dipp = 2,6-diisopropylphenyl)-substituted benzazaphosphole 2 with HCl generated 1−Cl, which undergoes substitution with halide sources to provide the remaining members of the Phalogenated series (1−F, 1−Br, and 1−I). These P-heterocycles (1) were characterized by multinuclear [ 31 P{ 1 H}, 1 H, 13 C{ 1 H}, and 19 F (if applicable)] NMR spectroscopy, elemental analysis, and X-ray crystallography. 1 H and 13 C{ 1 H} NMR spectroscopy revealed that derivatives 1−F, 1−Cl, and 1−Br have C 1 symmetry in solution. In contrast, 1−I has effective C s symmetry in solution due to a rapid, concentration-dependent, inversion at phosphorus, shown by density functional theory (DFT) calculations (B3LYP-D3/6-311G**++) to involve a dimeric iodine-bridged transition structure. In the solid state, 1−F through 1−I all exhibited C 1 symmetry with varying degrees of elongation of their P−X bonds. Elongation of the P−X bonds is shown by DFT/natural bond orbital studies to involve N LP → σ*(P−X) negative hyperconjugation, which increases down the halogen series but is less pronounced than that for the closely related NHP−X counterparts (3−F through 3−I). Treatment of 1−Br/I with [Pt(P(t-Bu) 3 ) 2 ] afforded Pt(I)−Pt(I) dimers 4−Br/I, which were characterized by 31 P{ 1 H}, 1 H, and 13 C{ 1 H} NMR spectroscopy, elemental analysis, and X-ray crystallography. Addition of 1−F to Pt(PPh 3 ) 4 gave Pt(1−F)(PPh 3 ) 2 (6), a coordination compound in which the P-heterocyclic ligand is bound through its lone pair without P−F bond cleavage. Recrystallization attempts resulted in ligand exchange, furnishing Pt(0) complex 7, which features 1−F and PPh 3 donors in a 2:1 ratio. Reactions of 1−Cl with all tested starting materials produced unidentifiable product mixtures by 31 P{ 1 H} NMR spectroscopy, but the combination of Pt(PPh 3 ) 4 or Pt(PPh 3 ) 2 (C 2 H 4 ) and 1−Cl in acetone generated an unisolable Pt complex containing tentatively assigned PtP 2 metallacyclopropane structural unit 8.