Synthesis of a Tris(phosphaalkene)phosphine Ligand and Fundamental Organometallic Reactions on Its Sterically Shielded Metal Complexes

Abstract

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

Osmium(II) Complexes Containing a Dianionic CCCC-Donor Tetradentate Ligand

“…The chain of the acyclic tetradentate ligands with a branched chain can be bifurcated at a donor atom, at an atom other than a donor atom, or with a bridging group linking two bidentate moieties . The first type is the most common and best known among the ligands of this category, being tetraphosphines and tetraarsines classical molecules of this class …”

Transition Metal-Induced Cyclization of 1-Phosphabutadienes: Selective Formation of 1-Phosphet-2-ene or P═C-Substituted Phosphacyclohexene Ligands

“…Despite the established π-accepting character of PC ligands, the PC bond length of the E - 1a ligand does not change upon coordination to Au I . This has been attributed to a rehybridization of the P-center whereupon the P–C σ-bonds have increased s -character in the complex, thereby giving a shorter σ-bond which cancels out the lengthening due to π-acceptance. − This rehybridization appears to be supported by the observed increase in the Mes*PC angle in [Au­( E - 1a )­Cl] [108.23(14)°] compared to E - 1a [103.4(2)°] and a shortening of the Mes*–P bond {[Au­( E - 1a )­Cl]: 1.822(3); E - 1a 1.860(4) Å}. The P(1)–Au(1) bond length [2.2228(8) Å] is in the range expected for related phosphaalkene-Au I complexes [2.20(5) to 2.242(11) Å] …”

Overview of the Synthesis and Catalytic Reactivity of Transition Metal Complexes Based on C═P Bond Systems

“…27,28 These P�C compounds can take various forms. 29,30 Phosphaalkene complexes are rather poor δ-donors and strong π-acceptors which is particularly important in the selection of these complexes in catalytic reactions. The donation of electrons to the metal occurs from the nonbonding lone pair of phosphorus, while the back bonding from the metal center occurs in the antibonding empty orbit π* P�C .…”