Aryl amido iridium complexes supported by a tridentatephosphanyl–carbene–phosphanyl pincer ligand [(PCsp2P)Ir–N(H)Ar, Ar = C6H5, 1a; Ar = 2,6-iPr2C6H3, 1b] react with ace-tonitrile to afford the dimeric complex 2, in which two(PCsp2P)Ir fragments are bridged by a diiminato ligand de-rived from two molecules of CH3CN. Metrical parameters ob-tained from X-ray structural determination of 2 confirm thatthe bridging ligand is a diiminato species rather than anenediamido moiety, which indicates that the reductive cou-
Introduction
Tridentate “pincer” ligands are a versatile class of donorsfor transition metals from across the periodic table. In ad-dition to the stability provided by the multidentate natureof the ligand, a prime feature of pincer ligands is the nearlyendless possible combinations of the donor atoms.[1] Thus,the formal ligand charge can range from 0 to –3, and ma-nipulation of the three donors (one central, two flanking)is available as a tool for fine-tuning the donor properties ofthe ligand array. Furthermore, the potential for metal–ligand cooperation in these tridentate donors can open upnew reaction pathways.[2]
Strongly donating, electron-rich pincer ligands are ofparticular interest in the activation of small molecules be-cause of their ability to labilize ligands trans to the centralatom donor in addition to their ability to stabilize high-oxidation-state intermediates.[3] PCsp2P pincer framework Ifirst published by Shaw et al. some years ago[4] exemplifiesthese attributes, but limitations of this ligand arising fromβ-elimination side reactions[5] involving the saturated brid-
[a] Department of Chemistry, University of Calgary,2500 University Dr. NW, Calgary, Alberta, T2N 1N4, CanadaPhone: +1-403-220-5746E-mail: [email protected]: http://www.ucalgary.ca/wpiers/Supporting information for this article is available on theWWW under http://dx.doi.org/10.1002/ejic.201300152.
pling is mediated by one electron per iridium center. Experi-ments suggest that the reaction proceeds by heterolyticcleavage of the Ir–N(H)Ar bond followed by one-electron re-duction of the resulting IrI–NCCH3 cation by the amidoanion. The resulting anilino radical rapidly abstracts a hydro-gen atom from the solvent. The reductive coupling of aceto-nitrile at a late transition metal center is unusual and in thisinstance occurs as a result of the highly σ-donating, electron-rich nature of the PCsp2P ligand.
ges prompted us to modify the ligand to incorporate arylgroups as linkers.[6] PCsp2P ligand II is thus strongly elec-tron donating and capable of labilizing ligands trans to thecentral carbene, while sculpting stable tridentate pincer en-vironments about the metal.
Results and Discussion
This assertion is manifested in the observed chemistry ofiridium anilido derivatives 1a and 1b incorporating the iPr-substituted version of II. These compounds are preparedstraightforwardly from the chloro precursor[6] and the anil-ido lithium reagents as shown in Scheme 1. Compound 1bwas prepared previously,[6] but unsubstituted 1a can besimilarly synthesized from LiN(H)C6H5. Upon mixing thetwo reagents, the deep-green solution, which is representa-tive of the chloride, becomes purple, which is characteristicof the anilido complexes. In the 1H NMR spectrum of 1a,the N–H resonance is observed as a broad singlet at δ =6.44 ppm (δ = 5.91 ppm for 1b[6]), and the 31P{1H} NMRspectrum shows a single peak at δ = 44.0 ppm (δ =43.7 ppm for 1b[6]).
In attempts to crystallize a sample of 1a or 1b from adegassed solution of acetonitrile, a color change from ma-genta to turquoise over the course of approximately 12 hwas observed, and upon standing, X-ray-quality blue crys-tals deposited from solution. Decanting of the motherliquor followed by successive washings of the product crys-tals with acetonitrile allowed isolation of the product in 55–60% yield. The same product was obtained regardless ofwhether 1a or 1b was used, and this was the major iridium-containing product in the reaction. The 31P{1H} NMRspectrum exhibits a singlet at δ = 46.8 ppm, whereas the
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Scheme 1. Reductive coupling of acetonitrile at iridium.
13C NMR spectrum indicates that the anchoring carbene ispreserved with a chemical shift of δ = 192.13 ppm. C2 sym-metry within the ligand remains, as the ortho-phenylene lin-kers of the backbone show only four signals in the aromaticregion. A singlet integrating to three hydrogen atoms (rela-tive to the ligand hydrogen atoms) appears at δ = 2.46 ppmin the 1H NMR spectrum; this resonance disappears when[D3]acetonitrile is used in the reaction. It correlates to asignal at δ = 21.77 ppm in the 13C NMR spectrum, whichalso displays a weak signal at δ = 147.22 ppm. Finally, incomparison to the IR spectra of 1a and 1b, a strong newband at 1556 cm–1 is present in the IR spectrum of theseturquoise crystals. Taken together, these data are completelyconsistent with the formulation of the product as dimer 2shown in Scheme 1, in which two molecules of acetonitrilewere coupled to form a bridging diiminato ligand. This as-signment was confirmed by X-ray structural analysis(vide infra).
Reductive coupling of nitriles by transition metals is rela-tively well known,[7] but it is usually mediated by highlyreducing, low-valent early transition metals. Typically, thecoupling produces a bridging “enediamido” fragment (i.e.,III) through formal reduction by two electrons per metalcenter; examples for TiII,[8] Nb/TaIII,[9] and WIV [10] are welldocumented. Alternatively, and somewhat more rarely, thecoupling proceeds to give bridging “diiminato” ligand IV,which is formed by overall two-electron reduction. Thistype of nitrile coupling occurs when only one reducingequivalent from each metal is available, and the process hasbeen observed for TiIII,[11] MoIII,[12] and WI [13] derivatives.The character of the coupling is principally determined byexamination of the metrical parameters associated with thebridging N2C2 unit, and in particular the C–N and C–Cbond lengths.
To that end, X-ray analysis of 2 revealed the structureand metrical data given in Figure 1. The structure exhibitspositional disorder with respect to the bridging ligandatoms, and two site occupancy factors are evident in an
approximately 75:25 ratio; the disorder is depicted in Fig-ure S2 (Supporting Information). The metrical parametersassociated with each structure are statistically identical, soonly the major structure is shown in Figure 1 and discussedbelow.
Figure 1. ORTEP illustration of 2 with thermal ellipsoids drawn atthe 50% probability level (hydrogen atoms are omitted for clarity).The structure is a 75:25 ratio of two structures differing only inthe placement of the four central carbon atoms. The 25% carbonplacement is omitted (see the Supporting Information). Select bondlengths [Å]: Ir(1)–C(1) 1.932(6), Ir(1)–P(1) 2.2819(17), Ir(1)–P(2)2.2714(18), Ir(1)–N(1) 1.920(12), N(1)–C(26) 1.257(15), C(26)–C(26*) 1.525(18), C(26)–C(27) 1.531(10). Selected bond angles [°]:P(2)–Ir(1)–P(1) 165.87(6), N(1)–Ir(1)–C(1) 175.8(4), N(1)–Ir(1)–P(1) 98.7(14), N(1)–C(26)–C(27) 122.1(12), N(1)–C(26)–C(26*)122.7(10).
The dimer is C2 symmetric, and each iridium center is ofdistorted square-planar geometry [P(2)–Ir(1)–P(1) =165.87(6)°]. The planes defined by the (PCsp2P)Ir atoms areessentially perpendicular to the plane defined by the atomsof the bridging NC(CH3)C(CH3)N unit. The ruffled confor-mation of the aryl linkers in the backbone of the PCsp2Pligand is consistent with observations for other structureswith this ligand.[6] The Ir(1)–C(1) distance of 1.932(6) Å iscomparable to the distance of 1.899(7) Å seen in the chlor-ide starting material, and the bond angles about C(1) showthat the ligand retains its central carbene donor. The bondlengths within the bridging unit clearly point to diiminatofragment IV as opposed to more-common enediamido moi-ety III. The C(26)–C(26*) bond length of 1.525(18) Å iscommensurate with a single bond, and the N(1)–C(26)bond length of 1.257(15) Å is indicative of a C=N bond.Finally, the Ir(1)–N(1) distance of 1.920(12) Å is consistentwith an Ir–N single bond, and it is significantly longer thanthe distances of 1.712(7)–1.750(3) Å observed in iridiumimido compounds of general formula Cp*Ir=NR, in whichthe Ir–N bond order is at least 2.[14]
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The reductive coupling of acetonitrile, although relativelycommon for low-valent early transition metals, is somewhatsurprising for iridium(I) complexes and speaks to the elec-tron-rich nature of the PCsp2P ligand framework in com-pounds 1a and 1b. The mechanism of this coupling and itsimplications in catalysis by using this novel metal–ligandplatform is still under investigation, but two possible path-ways are outlined in Scheme 2. Both invoke Ir–N bondcleavage in formally IrI amides 1, which is a consequence ofthe high trans influence of the strongly σ-donating centralcarbene ligand. Path A corresponds to homolytic cleavageand involves formal oxidation of the anilido ligand by theIr center to give anilino radical complex V,[15] which rapidlyundergoes substitution with the acetonitrile solvent to pro-duce species VII; this species leads to 2 upon dimerization.The anilino radical that is produced in this process is shortlived[16] and picks up a hydrogen atom (likely from the sol-vent) to form the corresponding aniline. This product wasobserved and quantified in the mass spectrum of the reac-tion mother liquor; it is formed in an amount consistentwith expectations based on the observed yield of 2 in thisprocess. Although species such as V have been charac-terized,[17] invariably they are formed by oxidation of anamido ligand by an external oxidant; given the electronrichness of the Ir centers in 1a and 1b, it seems unlikely that
Scheme 2. Proposed mechanism for the formation of 2.
this intramolecular oxidation would be favorable. Alternatepath B involves heterolytic cleavage of the Ir–N bonds in 1aand 1b to give cationic IrI species V, which is stabilized bya coordinated molecule of acetonitrile (Scheme 2). In thisscenario, the anilido counterion then reduces the cation togenerate VII and the anilino radical, which leads to 2 andthe observed aniline product.
We currently favor this latter proposal as the most likelypath on the basis of the observations depicted in Scheme 3.Treatment of (PCsp2P)Ir–Cl with NaB[3,5-(CF3)2C6H3]4 andNaBArF
4 {BArF4 = tetrakis[3,5-bis(trifluoromethyl)phen-
yl]borate} in C6D6 in the presence of at least one equivalentof acetonitrile yields isolable cation 3, in which the chlorideligand is replaced by a coordinated acetonitrile ligand. Thiscompound is stable in solution and is isolable as an analyti-cally pure green powder; it features upfield-shifted reso-nances for the ligand phosphorus atoms (δ = 51.9 vs.46 ppm) and the carbene carbon atom (δ = 230.6 vs.199.2 ppm) in the 31P{1H} NMR and 13C{1H} NMR spec-tra, respectively, relative to the neutral chloride starting ma-terial. Integration of the resonance for the coordinated ace-tonitrile methyl hydrogen atoms in the 1H NMR spectrumof an isolated sample shows that only one molecule of ace-tonitrile ligates the iridium in this formally IrI complex.Treatment of a solution of 3 with the one-electron reduct-
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ant[18] decamethylcobaltocene (Cp*2Co) resulted in an im-mediate color change from turquoise to deep-blue, and theNMR spectra are identical to those determined for coupledproduct 2. Together, these two reactions demonstrate theviability of path B in Scheme 2, especially noting that treat-ment of 1a with Cp*2Co in the presence of one equivalentof acetonitrile does not lead to 2. The actual mechanism ofdimerization of species VII is not known, and as Cumminset al. have elegantly shown,[12] other bonding modes for theacetonitrile ligand may be involved. Nonetheless, we believethe ability of the PCsp2P ligand to engage in resonance delo-calization in VII,[19] as illustrated in Scheme 2, and its elec-tron-rich nature play important roles in mediating this re-ductive coupling.
Scheme 3. One-electron reduction of cationic complex 3 to give 2.
Conclusions
The reductive coupling of acetonitrile at an electron-richiridium center was observed, and the structure of product2 was determined to consist of a bridging diiminato ligand.This suggests a one electron per metal reduction of the co-ordinated acetonitrile. Mechanistic experiments point to apath that involves formation of a cationic iridium–acetoni-trile complex through heterolytic bond cleavage of an Ir–N(H)Ar bond, followed by one-electron reduction of the IrI
cation by the anilido counteranion. The d9 Ir0 acetonitrilespecies that results rapidly dimerizes to the observed prod-uct.
The observation of reductive coupling of a nitrile at alate transition metal center is unusual. Most of the nitrilereductive couplings reported in the literature involve highlyreducing, low-valent early transition metals. Furthermore,most are stoichiometric in nature, as the higher-oxidation-state products of these reactions tend to be thermodynami-cally and kinetically quite stable. The observation of nitrilecoupling at a potentially more reactive late transition metal
center raises the possibility of further chemistry that mightrelease the coupled fragment. Indeed, complex 2 reacts (al-beit slowly) with hydrogen gas to give the known iridiumpolyhydride complex (PCsp3P)IrH4,[6] although the releaseddiimine product is not detectable because of ill-defined tau-tomerization.
Experimental Section
For general experimental methods and the synthesis of 1a, see theSupporting Information.
Synthesis of 2: In a vial, compound 1a (0.020 g, 0.03 mmol) wasdissolved in degassed acetonitrile (0.55 mL, 10.5 mmol). The result-ant purple solution was shaken for 30 s and then allowed to sitovernight at room temperature. After 12 h, needle-shaped bluecrystals were observed. The supernatant was decanted followed bythree successive washings with acetonitrile. The volatiles were re-moved from the crystals in vacuo to give the product in 60% yield(0.011 g). GC–MS analysis of the supernatant showed m/z = 93(aniline). 1H NMR (600 MHz, C6D6): δ = 8.02 (d, J = 8.0 Hz, 4H), 7.28 (t, J = 7.4 Hz, 4 H), 7.15 (m, 4 H), 6.76 (t, J = 7.6 Hz, 4H), 2.65 (dtd, J = 9.2, 7.0, 7.0, 4.8 Hz, 4 H), 2.59 (dtd, J = 10.0,7.7, 7.5, 5.0 Hz, 4 H), 2.49 (s, 6 H), 1.33 (ddd, J = 10.3, 7.3, 7.2 Hz,24 H), 1.26 (dd, J = 7.0, 7.0 Hz, 12 H), 1.21 (dd, J = 7.1, 7.1 Hz,12 H) ppm. 13C{1H} NMR (151 MHz, C6D6): δ = 192.48 (C=Ir),173.61 (C-C-P, Ar), 147.57 (s, N=C-CH3), 137.08 (t, J = 22.2 Hz,C-P, Ar), 133.07 (CH, Ar), 131.80 (CH, Ar), 123.62 (CH, Ar),122.22 (t, J = 7.5 Hz, CH, Ar), 30.55 (CH3, iPr), 25.26 (td, J =13.1, 2.2 Hz, CH, iPr), 21.77 (N=C-CH3), 20.42 (d, J = 19.5 Hz,CH3, iPr), 19.61 (CH3, iPr), 19.26 (CH3, iPr) ppm. 31P{1H} NMR(243 MHz, C6D6): δ = 46.77 ppm. C54H78Ir2N2P4 (1257.51): calcd.C 51.33, H 6.22, N 2.21; found C 50.53, H 5.96, N 2.21.
Synthesis of 3: Benzene (5 mL) was added by vacuum transfer to aflask charged with PCP(iPr)=IrCl (0.057 g, 0.09 mmol) and sodiumtetrakis[3,5-(trifluoromethyl)phenyl]borate (0.083 g, 0.09 mmol).The solution was stirred for 5 min. Following freezing at –78 °Cand the addition of acetonitrile (2 mL) by vacuum transfer, thesolution was warmed to room temperature and stirred for 4 h. Animmediate change in the color of the solution to turquoise/greenwas observed. The solvent was removed in vacuo followed by theaddition of benzene, trituration at room temperature for 15 min,and filtration through a frit. The solvent was removed in vacuofrom the filtrate to afford a green powder in 43% yield (0.058 g).1H NMR (400 MHz, CD2Cl2): δ = 8.37 (t, J = 7.4 Hz, 2 H), 7.89(d, J = 7.9 Hz, 2 H), 7.73 (m, 8 H), 7.57 (s, 4 H), 7.51 (dt, J = 7.5,3.7 Hz, 2 H), 7.00 (t, J = 7.7 Hz, 2 H), 3.05 (ddt, J = 9.9, 7.1,4.3 Hz, 4 H), 1.28 (m, 24 H) ppm. 13C NMR (101 MHz, CD2Cl2):δ = 230.63 (t, J = 2.7 Hz, C=Ir), 172.32 (t, J = 17.3 Hz, C-C-P, Ar),161.73 [q, J = 49.8 Hz, C-B, B(ArF)4, Ar], 136.96 (t, J = 20.8 Hz, C-P, Ar), 134.78 [CH, B(ArF)4, Ar], 134.26 (CH, Ar), 133.33 (CH,Ar), 130.98 (t, J = 3.2 Hz, CH, Ar), 128.84 [m (app q), C-CF3,B(ArF)4, Ar], 125.20 [q, J = 272.3 Hz, B(ArF)4, CF3], 125.60 (t, J
CCDC-920554 (for 2) contains the supplementary crystallo-graphic data for this paper. These data can be obtained free ofcharge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.
Supporting Information (see footnote on the first page of this arti-cle): Further experimental details, calibration curves and GCtraces, structural refinement details, ORTEP illustration of the cou-pled core of 2 with disorder shown, IR spectra, and NMR spectra.
Acknowledgments
W. E. P. acknowledges the Natural Sciences and Engineering Re-search Council (NSERC) of Canada for a Discovery Grant and theCanada Council of the Arts for a Killam Research Fellowship(2012-14). R. J. B. thanks NSERC for a PGS-D Award and theAlberta Innovates–Technology Futures for a graduate studentscholarship.
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