Synthesis of a Tris(phosphaalkene)phosphine Ligand and

Article pubs.acs.org/Organometallics

Synthesis of a Tris(phosphaalkene)phosphine Ligand and Fundamental Organometallic Reactions on Its Sterically Shielded Metal Complexes Preston M. Miura-Akagi,† Mika L. Nakashige,† Caitlin K. Maile,† Shelly M. Oshiro,† Joshua R. Gurr,† Wesley Y. Yoshida,† A. Timothy Royappa,⊥ Colleen E. Krause,‡ Arnold L. Rheingold,⊥ Russell P. Hughes,§ and Matthew F. Cain*,† †

Department of Chemistry, University of Hawai‘i at Manoa, 2545 McCarthy Mall, Honolulu, Hawaii 96822, United States ̅ Department of Chemistry, University of Hartford, 200 Bloomfield Avenue, West Hartford, Connecticut 06117, United States § 6128 Burke Laboratory, Department of Chemistry, Dartmouth College, Hanover, New Hampshire 03755, United States ⊥ Department of Chemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093, United States ‡

S Supporting Information *

ABSTRACT: A new tris(phosphaalkene)phosphine ligand (1) was synthesized via phospha-Wittig methodology. Metalation of 1 with [RhCl(C2H4)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·5CHCl3, and 2b·2.5CHCl3 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 NaN3 afforded Rh−N3 complex 3; computational analysis, IR spectroscopy, and 15N{1H} NMR spectroscopy on isotopologue 15N-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.

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INTRODUCTION Tetradentate ligands1 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 N2 unit with a proton source and reducing agent resulted in the first example of homogeneous and catalytic ammonia production.5

The guiding principle of using sterics to dictate metalcentered reactivity6 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 behavior8 are pincer-derived metal complexes, which undergo metal−ligand cooperativity via dearomatization/aromatization processes9 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 XP3supported (X = Si,11 C,12 B,13 and N14) TBP first-row metal complexes designed to catalyze dinitrogen reduction (Scheme 1).15 Fe16 and Co complexes17 ligated by the BP3 ligand facilitated the catalytic conversion of N2 to NH3 (in the presence of HBArF and KC8), while the performance of SiP3 and CP3 analogues was stoichiometric at best; no N2 reduction has been reported with the NP3 ligand set.17 Trivalent boranes (BAr3) are often counted as zero-electron ligands,18 but recent evidence has suggested that the tricoordinate boron unit (BAr3) in BP3 can act as a zero-, one-, or two-electron ligand depending on the oxidation state of the metal complex.19 This

Scheme 1. Two Different Modes of Metal-Catalyzed Dinitrogen Reduction Featuring Tetradentate Ligands

Received: March 29, 2016

© XXXX American Chemical Society

A

DOI: 10.1021/acs.organomet.6b00250 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics embedded ligand “flexibility” is believed to be the source of the enhanced reactivity of the BP3-supported metal complexes.16,17 In designing a new tetradentate ligand capable of enforcing reactivity at a single site, we wanted to combine the advantages associated with steric control, while simultaneously introducing an electronic component capable of enhancing reactivity through redox-active/noninnocent processes. Electronically, phosphaalkenes (R′PCHR) are analogues of the ubiquitous CO ligand,20 possessing low-lying π* orbitals capable of acting as π acceptors21 and recently implicated in several metalmediated bond activation events.22 However, unlike carbon in CO, the phosphorus donor atom of a phosphaalkene exerts an element of significant steric control. In order to prevent oligomerization of the PC fragment, phosphorus must be kinetically stabilized by a sterically substantial group such as Mes* (Mes* = 2,4,6-tri-tert-butylbenzene).23 If the organic scaffolding of the targeted ligand (1) is based on tris(2carboxyaldehyde)triphenylphosphine,24 the installment of three PMes* moieties25 creates a sterically protected binding pocket for a TBP metal complex, which will be incorporated into a potentially redox-active/noninnocent framework. The rigid three-carbon linker between the apical P and the phosphaalkene donor atoms should enforce tetradentate binding, as calculations indicated the apical P should adopt an endo configuration (Chart 1).26 We report here the successful synthesis of a tris(phosphaalkene)phosphine ligand and reactivity of its corresponding Rh and Ir chloride complexes.

Scheme 2. Synthesis of 1

about the P−Mes* bonds on the NMR time scale; both t-Bu signals were sharp and integrated in a 2:1 ratio. Mass spectrometry and X-ray crystallography unequivocally confirmed the assignment of 1 (Figure 1). Although phosphaal-

Chart 1. Design Parameters of Tetradentate Ligand 1

Figure 1. X-ray crystal structure of 1·1.5THF. The solvent and disorder are not shown.

kenes are known to display remarkable stability to oxidation and hydrolysis,29 we were pleased that 1 could also be handled, manipulated, and purified under aerobic conditions. In both the solid state and solution over the course of months, 1 showed no signs of decomposition by 31P{1H} NMR spectroscopy. Electrochemistry on 1. In an effort to gauge potential redox-active behavior of 1, its redox properties were probed electrochemically.31 Cyclic voltammetry studies of 1 in dichloromethane (DCM) revealed a quasi-reversible system32 at −1.56 V, while in tetrahydrofuran (THF), the wave was shifted to −1.81 V (Figure 2). Similar reductions with phosphaalkene-based pincer ligands have been attributed to the accessibility of the low-lying π* orbitals.32a,33 An additional anodic peak was observed at 0.62 V in DCM, tentatively assigned as oxidation of the apical phosphorus. However, bulky triphenylphosphine derivatives can exhibit complicated electrochemical behavior due to an interplay between more stable radical cations and pronounced structural reorganization.34 In THF, a more complicated series of weak anodic features was noted at −0.12, 0.48, and 0.99 V, respectively. The origin of these secondary electrochemical waves is unidentified.

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RESULTS AND DISCUSSION Ligand Synthesis. The desired ligand 1 was prepared in a straightforward manner using well-precedented phospha-Wittig methodology.27 Treatment of a frozen benzene mixture of Mes*PCl228 and Zn dust with a room-temperature (RT) solution of PMe3 in benzene generated the phosphinidene transfer agent Mes*PPMe3 (A).29 Addition of a freshly filtered solution of A to triphenylphosphine-derived tris(aldehyde) B,24 followed by heating to 60 °C for 16 h, afforded 1 in 79% yield (Scheme 2). The expected doublet/quartet pattern of 1 was observed by 31 1 P{ H} NMR spectroscopy (4JPP = 30 Hz).30 1H NMR spectroscopy revealed that the phosphaalkene units were all present in the E conformation (δ 8.57, 2JHP = 25 Hz).27 Restricted rotation about the P−Mes* bonds was indicated by the presence of one broad and one sharp t-Bu signal in the 1H NMR spectrum. Warming to 60 °C resulted in fast rotation B


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Organometallics

Likewise, reaction of 1 with 0.6 equiv of [IrCl(COE)2]2 (COE = cyclooctene) in benzene at 40 °C afforded trigonal bipyramidal Ir(I) complex 2b in 58% yield. The 31P{1H} NMR spectrum displayed a distinctly shifted doublet/quartet pattern with increased 2JPP coupling (34 Hz), indicating the ligand had coordinated in a tetradentate fashion. Complex 2b was further characterized by 1H and 13C{1H} NMR spectroscopy, mass spectrometry, and X-ray crystallography (Figure 4). Figure 2. Cyclic voltammograms of 1 in a 0.1 M solution of [nBu4N][PF6] in DCM (left, in blue) and THF (right, in red). Scan rate = 100 mV/s, referenced to the Fc/Fc+ couple.

Synthesis of Rh(I) and Ir(I) Chloride Complexes. Despite its substantial steric profile, we envisioned 1 acting as a multidentate ligand to a single metal center. Addition of a solution of 1 in benzene to 0.5 equiv of [RhCl(C2H4)2]2 resulted in the generation of trigonal bipyramidal Rh(I) complex 2a (Scheme 3).35 Scheme 3. Synthesis of 2a/b

Figure 4. X-ray crystal structure of 2b·2.5CHCl3. The solvent and disorder are not shown.

A comparison of the solid-state structures of 1 and 2a/b provided an opportunity to analyze the effect of coordination on the new tris(phosphaalkene)phosphine ligand. Selected bond lengths and angles are documented in Table 1. Table 1. Selected Bond Lengths (Å) and Angles (deg) Highlight the Structural Consequences of Coordination of 1 to Rh and Ir Centers

The tetradentate binding mode of 1 in Rh(I) complex 2a was confirmed by 31P{1H} NMR spectroscopy, which displayed the anticipated doublet of doublets pattern (1JRh−P = 169, 2JPP = 44 Hz) for the equivalent phosphaalkene groups and (overlapping) doublet of quartets (1JRh−P = 99, 2JPP = 44 Hz) splitting for the apical phosphorus unit. Recrystallization from a concentrated chloroform solution layered with acetonitrile afforded 2a in 95% yield. The structure of 2a was unambiguously determined by X-ray crystallography (Figure 3).

P1−C

P2C P3C P4C C−P1−C

C−P2C C−P3C C−P4C hybridization of P1 hybridization of P2,3,4 a

1

2a·CHCl3

1.838(2) 1.840(2) 1.838(2) 1.679(2) 1.677(2) 1.677(2) 101.74(9) 102.05(9) 100.54(9) 100.91(10) 100.11(10) 99.93(10) lone pair sp0.92 lone pair sp0.50

1.817(3) 1.820(3) 1.826(3) 1.673(4) 1.673(4) 1.673(4) 103.84(16) 103.91(16) 102.93(16) 111.80(17) 111.91(17) 109.88(17) bound P sp2.25 bound P sp1.26

2b·CHCl3 1.825(2)a

1.679(3)a

103.48(9)a

111.11(12)a

bound P sp2.33 bound P sp1.34

Three-fold axis present.

Undoubtedly, the most striking feature upon coordination of 1 to Rh(I) and Ir(I) centers was the increase in the C−PC angles. The free ligand displayed typical bond angles (C−P C) around 100 degrees, consistent with high p character in the P−C sigma bonds.36 However, the formation of complexes 2a/ b resulted in an enlargement of the angles about all three phosphaalkene phosphorus atoms, indicating a significant increase in the s character of the P−C sigma bonds; DFT

Figure 3. X-ray crystal structure of 2a·5CHCl3. The solvent and disorder are not shown. C


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Organometallics

purified by filtration and fully characterized by NMR spectroscopy, mass spectrometry, and elemental analysis. In agreement with related late metal azide complexes,39 the IR spectrum of 3 showed a strong band at 2048 cm−1 (DFT predicted: 2044 cm−1),37 with the 15N isotopologue (15N-3), synthesized from Na15N3 (98% terminally labeled; CAS: 609374), displaying a stretching vibration at 2037 cm−1. The 31 1 P{ H} NMR spectrum (CDCl3) of 15N-3 exhibited additional splitting of the apical P resonance, indicative of the introduction of another NMR-active nuclei (15N, I = 1/2). At RT, the 15 N{1H} NMR spectrum (CDCl3) showed a single signal (δ −242.2); however, upon cooling to −60 °C, the expected two signals were observed at −246.5 (singlet, Nχ) and −346.2 (dd, 1 JRh−N = 32, 2JP−N = 15 Hz, Nα) ppm, consistent with an intact Rh−15N3 unit (Figure 6).40 Related temperature-dependent

calculations support this finding (Table 1).37 Le Floch and coworkers observed similar structural adaptions with a dibenzofuran-derived phosphaalkene ligand upon generation of group 11 metal complexes.29 The widening of the C−PC angles forces the Mes* groups into closer proximity to the metal center, creating a binding pocket that is more narrow and sterically protected than we expected. The space-filling diagram depicted in Figure 5 highlights this pronounced effect. A side view of 2a appears to show an “all-

Figure 5. Space-filling diagrams of 2a. On the left is a side view; on the right is a top view looking down the Rh−Cl bond. The hydrogens are included in the top view.

carbon cavity”, and only the smallest sections of phosphorus atoms can be observed (gold-colored atoms). However, viewing the space-filling diagram down the Rh−Cl bond reveals a narrow and sterically encumbered access point (chlorine = green-colored atom). In addition, the 13C{1H} and 1H NMR spectra of 2a and 2b show the presence of restricted rotation about the P−Mes* bond, rendering all six carbon atoms on the aryl ring including both protons and the ortho t-Bu signals inequivalent. Variable-temperature 1H NMR experiments reveal this behavior is maintained up to 60 °C. Substitution on Rh(I) Complexes. Despite the congested environment of 2a, we anticipated that small nucleophiles would enter the coordination sphere and displace the chloride, generating Rh-based precursors of interest. Treatment of a solution of 2a in chloroform with 15 equiv of NaN3 resulted in smooth, but slow conversion (∼72 h) to azido complex 3 in near-quantitative yield (Scheme 4). The sluggish reaction is

Figure 6. 15N{1H} NMR spectrum (CDCl3, −60 °C) of 15N-3.

broadening and sharpening of NMR signals have been reported with 15N-labeled phosphazide-based (RNN15NPPh3) tungsten complexes and were attributed to an unidentified dynamic process.41 Alternatively, the NMR observations may be the result of adjacent 14N quadrupole nuclei. DFT calculations established the bent azide isomer was considerably lower in energy than the linear isomer (∼10 kcal/ mol). In fact, the linear azide is not a minimum, but a secondorder saddle point between bent azide forms and, therefore, not a true viable species. Any bent to linear conversion is low energy and would be fast on the NMR time scale.37 A computed structure of 3 is shown in Figure 7.


likely due to solubility problems, as the use of a more soluble azide source such as [PNN][N 3 ] (PPN = bis(triphenylphosphine)iminium; Ph3PNPPh3)38 accelerated the reaction rate considerably. The conversion of 2a to 3 was monitored by 1H NMR spectroscopy, which revealed a pronounced shift in the phosphaalkene (ArCHPMes*) signal (CDCl3: δ 8.16 to 7.86). The crude product (3) was

Figure 7. Computed structure of 3 displaying the bent azide functionality. D


Organometallics

Article

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CONCLUSIONS This Article reported the synthesis and potential applications of new tris(phosphaalkene)phosphine ligand 1. Upon coordination to a Rh(I)/Ir(I) center, the C−PC angles of 1 incorporating the phosphaalkene P atoms increased significantly and created highly shielded late metal chlorides 2a and 2b, respectively. Rh−Cl complex 2a was converted into azido complex 3 by nucleophilic substitution, while Ir−Cl complex 2b underwent halide abstraction to generate cationic Ir−NCMe complex 4. Future investigations using 3 and 4 as precursors will target new bond activation and catalytic processes.

It is well documented that M−N3 complexes will extrude N2 photochemically42 and/or thermally,43 producing highly reactive terminal metal nitrogen species. Recently, Smith and co-workers harnessed this reactivity with a tris(carbene)boratesupported Fe(II)−N3/Fe(IV)N system to generate a synthetic cycle for the aziridination of styrenes.44 We are interested in developing related processes associated with ammonia production45 and other N atom functionalization with Rh46 and hypothesize the implementation of a metal− ligand framework with a sterically protected M−N3 unit will control reactivity and selectivity. Halide Abstraction on Ir(I) Complexes. Similar sterically shielded and potentially reactive sites should also be accessible from 2b. To investigate if a vacant coordinate site could be generated, we treated 2b with AgOTf in the presence of CH3CN at RT to afford the acetonitrile-trapped Ir(I) salt 4 (Scheme 5).47

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EXPERIMENTAL SECTION

All manipulations were conducted under a nitrogen or argon atmosphere in a MBraun drybox or using Schlenk techniques unless otherwise specified. All glassware was oven-dried prior to use. Anhydrous-grade acetonitrile, pentane, toluene, benzene, ether, dichloromethane, and THF were purchased from commercial suppliers (Aldrich or Acros) and pumped directly into the glovebox. All solvents were stored over oven-activated 4 Å molecular sieves (Aldrich). The starting materials and reagents [RhCl(C2H4)2]2, [IrCl(COE)2]2, AgOTf, NaN3 (and Na15N3), and Zn dust were purchased from commercial suppliers. PMe3 was purchased from Strem and dried over molecular sieves prior to use. Mes*PCl228 and tris(aldehyde) B24 were prepared according to literature protocols. NMR spectra were obtained on Varian spectrometers operating at either 300 or 500 MHz; all spectra shown in the Supporting Information are on the 500 MHz spectrometer. NMR chemical shifts are reported as ppm relative to tetramethylsilane and are referenced to the residual proton or 13C signal of the solvent (1H CDCl3, 7.27 ppm; 1 H C6D6, 7.16 ppm; 13C CDCl3, 77.16 ppm; 13C C6D6, 128.06 ppm). Mass spectrometry was conducted at the University of California at Irvine (Dr. J. Greaves) on compound 1. Other mass spectroscopic data were obtained at the University of Hawai‘i at Manoa on an Agilent ̅ 6545 Accurate-Mass Q-TOF LC/MS (NSF CHE-1532310). Analytical data were obtained from the CENTC Elemental Analysis Facility at the University of Rochester, funded by NSF CHE-0650456. All X-ray quality crystals were analyzed at the Small Molecule X-ray Crystallography Facility located at the University of California at San Diego. Synthesis of 1. Mes*PCl2 (100 mg, 0.288 mmol) and Zn dust (94 mg, 1.44 mmol, 5 equiv) were combined in a vial, and 2 mL of benzene was added. The heterogeneous mixture was cooled in the glovebox freezer (−35 °C) for 15 min. Treatment of the frozen benzene mixture with a room-temperature solution of PMe3 (56 mg, 0.736 mmol, 2.6 equiv) in 3 mL of benzene resulted in an olive green reaction mixture. The reaction mixture was stirred for 2−2.5 h at room temperature and filtered through a Celite plug. The bright yelloworange filtrate (Mes*PPMe3, 31P NMR (C6H6): δ 4.2 and 134.3, J = 580 Hz) was added to solid tris(aldehyde) B (20 mg, 0.058 mmol, 1/5 equiv), affording a homogeneous bright yellow solution. The solution was transferred to a J-Young tube, sealed, brought outside of the glovebox, and heated to 60 °C for 16 h. 31P NMR spectroscopy confirmed complete conversion to the desired product. Identifiable byproducts included unreacted Mes*PPMe3, PMe3(O) (δ 31), PMe3 (δ −62), and PH2Mes* (δ −130). The contents of the J-Young tube were then transferred to a vial (and exposed to oxygen) and concentrated under vacuum. The yellow residue was dissolved in 3 mL of pentane and filtered through a Celite plug. The filtrate was concentrated under vacuum and recrystallized from 5 mL of a 50:50 solution of THF and acetonitrile at −20 °C to afford a yellow crystalline solid (50 mg, 0.044 mmol, 76%). Crystals suitable for X-ray diffraction were obtained by dissolving 25 mg of the ligand in 4 mL of a 50:50 solution of THF and acetonitrile at 0 °C. On scale-up to 1 g of Mes*PCl2, 467 mg of 1 was isolated (79% yield); 1/5.5 equiv of B is employed. Elemental analysis data were consistently low (three attempts) in carbon, for example: Anal. Calcd for C75H102P4: C, 79.89; H, 9.12. Found: C, 79.33; H, 9.30. HRMS: m/z calcd for C75H102P4Na


Complex 4 was easily identified by 31P{1H} NMR spectroscopy (C 6 D 6 ) due to its significantly upfield shifted phosphaalkene and apical P signals.48 In addition, the 1H NMR spectrum (C6D6) displayed a new singlet at 2.39 ppm, integrating in a 1:1 ratio (3H:3H) with the phosphaalkene protons. A 13C−1H NMR HSQC experiment confirmed this signal corresponds to the methyl group of the bound acetonitrile unit. Conducting the halide abstraction in CD3CN resulted in the disappearance of the 1H NMR signal at 2.39 ppm, lending further support to our assignment. Complex 4 was further characterized by 13C{1H} and 19F{1H} NMR spectroscopy and mass spectrometry. Generating open coordination sites on metal complexes ligated by multidentate ligands provides the opportunity to introduce typically unreactive substrates into a reactive coordination sphere.4,5 For example, pioneering work by Jensen and co-workers demonstrated that an unsaturated 14electron Ir(PCP)+ pincer fragment can dehydrogenate alkanes to afford alkenes.49,50 Milstein has shown late metal complexes (usually Ru or Ir) featuring vacant coordination sites and dearomatized PNN and PNP pincer ligands can activate other inert and difficult to functionalize substrates such as water, ammonia, and related molecules with strong bonds.9 Upcoming studies will investigate exposing the “naked” Ir center to typically unreactive substrates with the goal of uncovering new modes of reactivity. E


Article

Organometallics (M + Na)+ 1149.6830, found 1149.6853. 31P{1H} NMR (CD2Cl2): δ 266.5 (d, J = 30 Hz, 3P, PC), −32.3 (quartet, J = 30 Hz, 1P, PAr3). 1 H NMR (CD2Cl2): δ 8.57 (dd, J = 25, 5.5 Hz, 3H, Mes*PCHAr), 7.88 (br m, 3H, Ar), 7.30 (t, J = 7.5 Hz, 3H, Ar), 7.25 (6H, Mes*), 7.11(t, J = 7.5 Hz, 3H, Ar), 6.55 (dd, J = 7.5, 4.5 Hz, 3H, Ar), 1.30 and 1.17 (overlapping singlets, 1 sharp and 1 broad, 81H, Mes*). 13C{1H} NMR (CD2Cl2): δ 173.5 (dd, J = 36, 20.5 Hz, PC), 154.2 (Ar), 149.7 (Ar), 145.3 (dd, J = 25, 14 Hz, Ar), 139.8 (d, J = 56 Hz, Ar), 135.1 (Ar), 134.8 (t, J = 11 Hz, Ar), 129.3 (Ar), 128.3 (Ar), 127.1 (d, J = 27 Hz, Ar), 121.9 (Ar), 38.2 (CMe3), 35.2 (t-Bu), 33.7 (CMe3), 31.5 (t-Bu). 13C NMR assignments were aided by DEPT experiments. Synthesis of 2a. A solution of 1 (126 mg, 0.112 mmol) in 5 mL of benzene was added to [RhCl(C2H4)2]2 (22 mg, 0.057 mmol, 0.5 equiv), producing a dark reaction mixture. The reaction mixture was stirred for 24 h at room temperature and then concentrated under vacuum. Subsequently, the crude product was dissolved in 3 mL of chloroform and filtered through a Celite plug. The filtrate was layered with acetonitrile (10 mL) and placed in the freezer at −35 °C. After standing overnight, a dark solid precipitated (134 mg, 0.106 mmol, 95%, 2 crops) from solution. X-ray quality crystals were obtained from an additional recrystallization of 10 mg of 2a in 1 mL of chloroform layered with 3 mL of acetonitrile at −35 °C. Elemental analysis data were consistently low (3 attempts) in carbon, for example: Anal. Calcd for C75H102P4RhCl: C, 71.16; H, 8.12. Found: C, 69.87; H, 8.05. HRMS: m/z calcd for [C75H103P4RhCl (M + H)+]: 1265.5754, found 1265.5740. 31P{1H} NMR (CDCl3): δ 241.1 (dd, JRh−P = 169, JPP = 44 Hz, PC), 39.2 (overlapping doublet of quartets, JRh−P = 99, JPP = 44 Hz, apical P). 1H NMR (CDCl3): δ 8.16 (br, 3H, PC), 7.39 (t, J = 7.5 Hz, 3H, Ar), 7.32 (3H, Mes*), 7.27 (3H, Mes*), 7.15 (br t, J = 6 Hz, 3H, Ar), 7.09 (t, J = 7.5 Hz, 3H, Ar), 6.32 (dd, J = 8, 12 Hz, 3H, Ar), 1.28 (27H, t-Bu), 1.25 (27H, t-Bu), 1.08 (27H, t-Bu). 13C{1H} NMR (CDCl3): δ 155.7 (Ar), 151.9 (Ar), 150.3 (PC), 149.7 (Ar), 144.4 (d, J = 12 Hz, Ar), 133.5 (Ar), 130.7 (br, likely two overlapping Ar signals), 129.0 (Ar), 126.4 (Ar), 122.7 (Ar), 122.3 (Ar), 115.7 (br, Ar), 39.9 (CMe3), 38.8 (CMe3), 34.8 (CMe3), 34.5 (t-Bu), 33.3 (t-Bu), 31.2 (t-Bu). 13C NMR assignments were aided by DEPT experiments. Synthesis of 2b. A solution of 1 (100 mg, 0.087 mmol) in 4 mL of benzene was added to [IrCl(COE)2]2 (48 mg, 0.052 mmol, 0.6 equiv), producing a dark reaction mixture. The reaction mixture was transferred to a screw-top Teflon Schlenk bomb, heated at 40 °C for 24 h with stirring, and then concentrated under vacuum. The crude product was dissolved in pentane and filtered through a Celite plug. The filtrate was concentrated under vacuum, and the residue was washed with cold acetonitrile (−35 °C) to wash away any remaining COE. The crude solid was dissolved in 1 mL of chloroform and layered with 1 mL of acetonitrile, resulting in the precipitation of a dark crystalline solid at room temperature (70 mg, 0.050 mmol, 58%, 2 crops). Dissolving 10 mg of 2b in a 3 mL 1:2 solution of chloroform and acetonitrile at −35 °C produced X-ray quality crystals. Elemental analysis data were consistently off (three attempts), for example: Anal. Calcd for [C75H102P4IrCl]: C, 66.47; H, 7.59. Found: C, 70.90; H, 8.26. HRMS: m/z calcd for [C75H103P4IrCl (M + H)+] 1355.6328, found 1355.6351. 31P{1H} NMR (CDCl3): δ 211.0 (d, J = 34 Hz, P C), −8.4 (quartet, J = 34 Hz, apical P). 1H NMR (C6D6): δ 8.22 (br, 3H, PC), 7.58 (6H, overlapping Mes*), 6.96 (t, J = 7.5 Hz, 3H, Ar), 6.85 (t, J = 6 Hz, 3H, Ar), 6.72 (t, J = 7.5 Hz, 3H, Ar), 6.58 (dd, J = 8, 12.5 Hz, 3H, Ar), 1.57 (27H, t-Bu), 1.39 (27H, t-Bu), 1.31 (27H, tBu). 13C{1H} NMR (CDCl3): δ 155.7 (Ar), 151.9 (Ar), 149.9 (Ar), 144.2 (d, J = 11 Hz, Ar), 140.0 (br, PC), 133.4 (Ar), 130.5 (Ar), 130.0 (br, Ar), 127.5 (Ar), 125.4 (d, J = 10 Hz, Ar), 122.8 (Ar), 122.0 (Ar), 114.0 (br, Ar), 39.9 (CMe3), 39.1 (CMe3), 34.8 (CMe3), 34.3 (tBu), 33.0 (t-Bu), 31.2 (t-Bu). 13C NMR assignments were aided by DEPT experiments. Synthesis of 3. A solution of 2a (70 mg, 0.055 mmol) in 3 mL of chloroform was added to NaN3 (54 mg, 0.83 mmol, 15 equiv) and stirred for 72 h at room temperature. The progress of the reaction can be tracked by 1H NMR spectroscopy (CDCl3, [Rh]−Cl phosphaalkene signal: δ 8.19 to [Rh]−N3 phosphaalkene signal: δ 7.89). After full conversion to the azide complex, the dark reaction mixture was filtered through a Celite plug, and the filtrate was concentrated under

vacuum, affording the product as a dark brown solid (69 mg, 0.54 mmol, 98%). Anal. Calcd for C75H102P4RhN3: C, 70.79; H, 8.08; N, 3.30. Found: C, 70.52; H, 8.37; N, 3.15. HRMS: m/z calcd for [C75H102P4RhN3] 1271.6079, found 1271.6071. 31P{1H} NMR (CDCl3): δ 230.3 (dd, JRh−P = 168, JPP = 48 Hz, PC), 37.0 (overlapping doublet of quartets, JRh−P = 97, JPP = 48 Hz, apical P). 1H NMR (CDCl3): δ 7.86 (br, 3H, PC), 7.39−7.34 (overlapping, 9H, 2 Mes* and Ar), 7.09 (t, J = 7.5 Hz, 3H, Ar), 7.06 (t, J = 7.5 Hz, 3H, Ar), 6.25 (dd, J = 7.5, 12 Hz, 3H, Ar), 1.45 (27H, t-Bu), 1.29 (27H, t-Bu), 1.06 (27H, t-Bu). 13C{1H} NMR (CDCl3): δ 155.4 (Ar), 153.3 (Ar), 152.9 (br, PC), 150.1 (Ar), 144.3 (d, J = 12 Hz, Ar), 133.1 (Ar), 132.6 (br, Ar), 130.9 (two overlapping Ar signals), 126.8 (d, J = 8 Hz, Ar), 123.2 (Ar), 122.9 (Ar), 116.1 (br), 39.5 (CMe3), 38.6 (CMe3), 34.8 (CMe3), 34.0 (t-Bu), 33.9 (t-Bu), 31.2 (t-Bu). 13C NMR assignments were aided by DEPT experiments. The 15N-labeled isotopologue 15N-3 was synthesized in an analogous fashion. 15N{1H} NMR (CDCl3, RT): δ −242.2. 15N{1H} NMR (CDCl3, −60 °C): δ −246.5 (singlet, Nχ), −346.2 (dd, JRh−N = 32, JP−N = 15 Hz, Nα). Synthesis of 4. A solution of 2b (36 mg, 0.027 mmol) in 1 mL of benzene with three drops of acetonitrile was added to solid AgOTf (8 mg, 0.032 mmol, 1.2 equiv) and stirred for 1 h at room temperature. The resulting dark red solution was filtered through a Celite plug. The filtrate was layered with pentane and placed in the freezer at −35 °C, resulting in the precipitation of a dark red solid (22 mg, 0.015 mmol, 55%). HRMS: m/z calcd for [C77H105IrNP4 (M − NCMe)+] 1360.6827, found 1360.6849. 31P{1H} NMR (C6D6): δ 198.2 (d, J = 37 Hz, PC), −15.0 (quartet, J = 37 Hz, apical P). An unidentified impurity is present at 24.9 ppm (singlet). 1H NMR (C6D6): δ 8.23 (br, 3H, PC), 7.75 (3H, Mes*), 7.57 (3H, Mes*), 6.95 (t, J = 7.5 Hz, 3H, Ar), 6.69 (overlapping, 6H, 2 Ar), 6.32 (dd, J = 7.5, 13 Hz, 3H, Ar), 2.39 (3H, NCMe), 1.47 (27H, t-Bu), 1.37 (27H, t-Bu), 1.20 (27H, t-Bu). 13C{1H} NMR (C6D6): δ 154.7 (Ar), 153.5 (Ar), 153.0 (Ar), 144.6 (br, PC), 143.6 (d, J = 11 Hz, Ar), 133.5 (d, J = 7 Hz, Ar), 131.8 (Ar), 130.9 (br, Ar), 124.5 (Ar), 124.0 (Ar), 39.9 (CMe3), 39.3 (CMe3), 35.5 (CMe3), 34.1 (t-Bu), 32.9 (t-Bu), 31.5 (t-Bu), 4.4 (NCMe). 19F{1H} NMR (C6D6): δ −77.8 (OTf). Note: Some of the Ar peaks are obscured by the strong C6D6/C6H6 signal. The 13C{1H} and 1H NMR are complicated by the fact that complex 4 is somewhat unstable; prolonged exposure to vacuum, the presence of CHCl3, and even standing in solution resulted in unidentified decomposition pathways. DFT Computational Details. Full-molecule calculations were carried out using the hybrid M06 functional51 and the LACV3P** basis,52 as implemented in the Jaguar53 suite of programs. Natural bond orbital calculations54 were performed on the optimized structure using NBO 6.0,55 as implemented in Jaguar. Computed structures were confirmed as minima by calculating the vibrational frequencies using second-derivative analytic methods and confirming the absence of imaginary frequencies for minima. Thermodynamic quantities were calculated assuming an ideal gas and zero point energy corrected. Graphical representations of structures were made using the CYLView program.56 Electrochemistry Experimental Details. All electrochemical measurements were conducted on a CHI 1040 electrochemical analyzer with a three-electrode system. A glassy carbon electrode, a platinum wire, and a Ag/Ag+ electrode were employed as the working, auxiliary, and reference electrodes, respectively. The Fc/Fc+ couple was employed as the internal standard. Cyclic voltammetry (CV) was performed in a 0.1 M solution of [Bu4N][PF6] in anhydrous dichloromethane or tetrahydrofuran at room temperature under a nitrogen atmosphere. Individual CV plots are shown in the Supporting Information.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00250. F


Article

Organometallics

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Complete experimental details with NMR spectra included, expanded details on electrochemistry and individual CV plots, and full details on DFT computational methodology (PDF) X-ray structures (CIF) (CIF) (CIF) Optimized Cartesian coordinates (XYZ)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.F.C. thanks the University of Hawai‘i at Manoa for generous ̅ start-up funds and laboratory space. M.F.C. also acknowledges the UHM faculty for support and access to their chemical inventories and the Strem Chemical Co., Sigma-Aldrich, Chemglass Life Sciences, Fisher Scientific, and WilmadLabglass for their generous discounts on chemicals and/or laboratory equipment and supplies. In addition, Dr. Christian Ehm provided excellent feedback on an early draft of the manuscript, and William Brennessel (University of Rochester) conducted elemental analyses on the new compounds. Discussions on 14N/15N NMR with Tom Apple (UHM) were also extremely valuable.

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