4-Phosphino-Substituted N-Heterocyclic Carbenes (NHCs) from the

May 31, 2012 - ... in addition, contain a phosphine moiety. The preparation and spectroscopic characterization of the complexes [(1a)M(cod)Cl] (4) and...
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4-Phosphino-Substituted N-Heterocyclic Carbenes (NHCs) from the Abnormal Reaction of NHCs with Phosphaalkenes Joshua I. Bates and Derek P. Gates* Department of Chemistry, The University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, Canada V6T 1Z1 S Supporting Information *

ABSTRACT: The activation of the PC bond of phosphaalkenes with N-heterocyclic carbenes (NHCs) offers a convenient means to introduce new functionality at the 4position of an NHC. Treatment of MesPCRR′ (2a: R = R′ = Ph; 2b: R = Ph, R′ = 2-C5H4N; 2c: R = R′ = 4-C6H4F) with 1,3-dimesitylimidazol-2-ylidene [:C(NMesCH)2, IMes, Mes = 2,4,6-Me3C6H2] affords 4-phosphino-2-carbenes :C(NMes)2CHC(PMesCHRR′)NMes [1a: R = R′ = Ph; 1b: R = Ph, R′ = 2C5H4N; 1c: R = R′ = 4-C6H4F]. Significantly, these functional NHCs retain the parent carbene functionality and, in addition, contain a phosphine moiety. The preparation and spectroscopic characterization of the complexes [(1a)M(cod)Cl] (4) and cis[(1a)M(CO)2Cl] (5) (M = Ir, Rh) are reported. The average CO stretching frequencies (νav C ̅ O) for 5M=Ir, 5M=Rh, and authentic samples of cis-[(IMes)M(CO)2Cl] (M = Ir, Rh) are presented as a means to evaluate the donor properties of 4-phosphino-2−1 carbene 1a. The average CO stretching frequency is lower energy for 5M=Ir than for cis-[(IMes)Ir(CO)2Cl] (Δν̅av CO = −2 cm ), av −1 whereas the opposite is observed between 5M=Rh and cis-[(IMes)Ir(CO)2Cl] (ΔνC̅ O = 2 cm ). The molecular structures are reported for 1a, 1b, 1c, 4M=Ir, and 5M=Ir.

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Chart 1. First aNHC Metal Complex (A) and the First Isolable Metal-Free aNHC (B)

INTRODUCTION The development of isolable neutral divalent carbon species (carbenes) is an area of considerable current interest.1 Perhaps the most widely studied are the N-heterocyclic carbenes (NHCs) in which the divalent carbon moieties are flanked by one or two π-donor nitrogen atoms within a five-membered heterocycle. Since their first discovery,2 there has been rapid growth in NHC chemistry due to their fundamental importance and their attractive properties as ligands for metal-catalyzed organic synthesis. Recently, NHCs have been having considerable impact in the areas of organocatalysis3,4 and in the field of p-block chemistry, particularly to stabilize unusual low-coordinate species.5,6

facilitated by employing blocking phenyl moieties in the 2- and 5-positions.10 In 2009, we discovered an unusual and unexpected abnormal reaction of an NHC. Specifically, the NHC 1,3-dimesitylimidazol-2-ylidene (IMes) reacts with the phosphaalkene MesP CPh2 to afford the bifunctional 4-phosphino-2-carbene (IMesP, 1a).11 To our knowledge, this compound was the first example of a 4-phosphino-substituted NHC.

One general feature of NHCs is their tendency to bind electrophiles at the most reactive site, namely, the 2-position of the NHC ring.7 Therefore, it was particularly striking when the structure of an Ir complex in which the NHC ligand was bound the “wrong way” was reported (A in Chart 1).8 Since that remarkable discovery, the coordination chemistry of these socalled abnormal NHCs (aNHCs), where the carbene moiety binds at the 4- or 5-position, has been reasonably well established for d- and f-block elements.9 It should be noted that, in many instances, the 2-position must be blocked or bound to a metal to induce abnormal reactivity for NHCs. For example, the isolation of the first metal-free aNHC (B) was © 2012 American Chemical Society

The direct functionalization of an NHC with retention of the carbene moiety, although rarely observed, represents a powerful and convenient methodology to functionalize the 4-position of an NHC. Examples of 4-functionalized NHCs derived from the Received: April 18, 2012 Published: May 31, 2012 4529

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longer present and was replaced by a singlet resonance at −37.3 ppm. Remarkably, analysis of recrystallized product in C6D6 using 13C{1H} NMR spectroscopy suggested that the carbene moiety was still present in the product [δ = 220.3 (d), 3JPC = 5 Hz, cf. IMes: δ = 220.3].2 The identity of the product was confirmed by X-ray crystallography as the remarkable 4phosphino-substituted NHC 1a (Figure 1).

direct reaction of a free NHC are shown in Chart 2 and include the halogenation of NHCs (C),12−15 the deuteration of ItBu Chart 2. Examples of NHCs Bearing Functional Groups at the C4 and/or C5 Positions That Are Derived from the Abnormal Reaction of an Unprotected NHC

Figure 1. Molecular structure of 1a·1/2C6H6 (thermal ellipsoids set at 50% probability). The solvate and all hydrogen atoms except H(1) and H(2) are omitted for clarity.

(D),16 the silylation of ItBu (E),17 the generation of F and G from IDipp,18,19 the silylation of IDipp (H) (Dipp = 2,6-di-isopropylphenyl),20 the lithium-functionalized NHC (I),21 and the synthesis of the anionic NHC (J).22 Notably, F and G represent further examples of 4-phosphino-NHCs. These reactions are of particular importance as they open the door to optimization studies of catalytic activity by fine-tuning the 4- and 5-positions of the NHC backbone. Herein, we report the preparation, characterization, and donor−acceptor properties of several novel 4-phosphinoNHCs using the abnormal reaction of an NHC and a phosphaalkene. The donor properties of these substituted NHCs were investigated by preparing rhodium(I)- and iridium(I)−carbonyl complexes.

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The scope of this fascinating reaction of IMes was explored by varying the phosphaalkene employed. In particular, IMes was treated with the pyridyl-substituted phosphaalkene 2b in THF at 70 °C. After several hours, the 31P NMR spectrum of the reaction mixture revealed that the signal assigned to 2b [δ = 260 (E), 242 (Z)] had been replaced by two new signals assigned to the diastereomers of 1b (δ = −33.7, −35.3 in 1:1 ratio). The analogous reaction of IMes with phosphaalkene 2c similarly afforded 4-phosphino NHC 1c (δ = −37.4). In stark contrast to the reaction of IMes with phosphaalkenes 2a or 2b, which require elevated temperatures (70 °C), the reaction with the electrophilic phosphaalkene 2c occurs at ambient temperature. Both 1b and 1c were characterized by X-ray crystallography, and the molecular structures are shown in Figures 2 and 3. Attempts to detect the 13C{1H} NMR signal assigned to the carbene moiety in 1b were unsuccessful due to the low solubility of 1b combined with the long relaxation time of the carbene moiety. The 13C{1H} NMR spectrum of 1c in C6D6 revealed a broad resonance at 220.4 ppm from which the 3 JPC coupling constant could not be ascertained. The transformation of one NHC into another NHC is surprising because the carbene moiety is by far the most reactive site in the molecule. Because of this high reactivity, the

RESULTS AND DISCUSSION Abnormal Reactions of NHCs with Phosphaalkenes. When we embarked on the present studies, phosphaalkenes had been observed to react with fleeting electrophilic carbenes to afford phosphiranes, neutral PC2 heterocycles.23 The reaction of a phosphaalkene with an NHC had not been described.24 It was known that NHCs can add to multiply bonded phosphorus compounds, such as phosphaalkynes and iminophosphines, to afford zwitterionic species.25 In the case of phosphaalkynes, subsequent cyclizations are often observed. Our interest in the reactions of PC bonds with initiators for polymerization26 provided the initial inspiration for the present investigations. We imagined that the strongly donating NHCs might lead to novel zwitterionic polymerizations or to novel heterocycles analogous to those observed in our study of the reactions of PC bonds with in situ-generated phosphenium (R2P+) ions, which are isovalent to carbenes.27 With this hypothesis in mind, a THF solution of IMes was treated with phosphaalkene 2a (1 equiv) in THF. Although no reaction was observed at room temperature, when the reaction mixture was heated to 70 °C for several hours, its 31P NMR spectrum revealed that the signal for 2a (δ = 234) was no 4530

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Scheme 1. Steric Interaction between the Bulky IMes Donor and the Bulky 2a Acceptor May Be Imagined as a Frustrated Lewis Pair Leading to Donor−Acceptor Formation through the Less Hindered Abnormal IMes

for 3 weeks and periodically monitoring the reaction progress using 31P NMR spectroscopy. We postulate that the frustrated NHC → 2a Lewis pair is alleviated by invoking the abnormal NHC (aIMes or aItBu) as a key intermediate in the formation of 1a−1c and 3a, as depicted in Scheme 1. In addition to being a stronger nucleophile, aIMes is significantly less hindered than IMes. Thus, assuming that traces of aIMes may be present in solutions of IMes (ΔEIMes→aIMes = 51.7 kJ/mol),11 it is plausible that aIMes nucleophilically adds to the PC bond of 2a. If indeed this hypothesis were correct, the subsequent proton transfer from C2 to the −CPh2 moiety would afford 1a. Electronic Properties of IMesP as a Ligand. It is well established that the σ-donating properties of IMesX (X = Cl, Br) bearing halides at the C4 and C5 positions are considerably lower than those of IMes itself.12 Consequently, it would be desirable to ascertain the impact of the 4-phosphino functionality in 1a−1c on the donor−acceptor properties of this species. The most widely used method to assess the donor strength of carbene ligands involves comparing the stretching frequencies of the carbonyl ligands (ν̅CO) in their respective metal carbonyl complexes.34 Although numerous metal systems have been utilized, and consequently different donor-strength scales established, the most common are those based on rhodium(I) and iridium(I) complexes of the type cis-[LM(CO)2Cl] (M = Rh, Ir).34−37 Therefore, 1a was treated with [(cod)MCl]2 to afford complexes 4M=Rh and 4M=Ir, which, upon treatment with CO, were converted to metal carbonyls 5M=Rh and 5M=Ir. Complexes 4 and 5 were fully characterized spectroscopically.

Figure 2. Molecular structure of 1b·1/2C6H6 (thermal ellipsoids set at 50% probability). The solvate and all hydrogen atoms except H(1) and H(2) are omitted for clarity.

Figure 3. Molecular structure of 1c·1/2THF (thermal ellipsoids set at 50% probability). The solvate and all hydrogen atoms except H(1) and H(2) are omitted for clarity.

first step in analogous reactions is believed to involve temporarily blocking the C2 site of IMes through the formation of a donor−acceptor adduct with one of the reagents. This binding at C2 leads to higher acidity for the C4-H proton and thus favors its subsequent deprotonation. This pathway has been proposed for the selective chlorination of both C4 and C5 (C)12 and, more recently, for the selective substitution of either C4 or C5 to afford F, G,10,18 or H.20 In contrast, our preliminary calculations suggested that forming a zwitterionic IMes → 2a donor−acceptor adduct is unfavorable due to the steric influence of the two N substituents of IMes and the bulky phosphaalkene.11 Essentially, the interaction of IMes and 2a can be envisaged as a frustrated Lewis pair28 between the NHC nucleophile and the phosphaalkene electrophile (Scheme 1). The concept of frustrated Lewis pairs as a plausible rationale for the observed abnormal reactions of NHCs with electron pair acceptors has been considered recently for reactions involving bulky NHCs and B(C6F5)3.22,29−32 Moreover, reaction of ItBu with hindered phosphinidene complexes was proposed to proceed through aItBu.33 Interestingly, treating ItBu with phosphaalkene 2a in THF resulted in ca. 50% conversion to 3a (δ31P = −32.3) despite heating the reaction mixture to 100 °C

The 31P and 13C{1H} NMR spectroscopic data confirm that 1a binds though the carbene rather than the phosphine moiety. Specifically, the 31P chemical shifts for the complexes are very similar to those of the free NHC 1a. Furthermore, the 13C{1H} NMR signals assigned to the carbene moiety are significantly upfield for the complexes compared to those for free 1a. A mixture of isomers (∼2:1 ratio) is observed in solution for each of the complexes 4M=Rh and 4M=Ir, analogous to that observed for [(IMes)Rh(cod)Cl].35 This is attributed to restricted rotation about the C−M and C−P bonds. For complexes 4531

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4M=Ir and 5M=Ir, the proposed carbene binding to the iridium(I) center was confirmed by X-ray crystallographic analysis (Figures 4 and 5) (vide infra).

Table 1. Summary of CO Stretching Frequencies for Selected Group 9 Carbonyl Complexes of Selected NHCs complex 5M=Rh IMesRh(CO)2Cl IMesRh(CO)2Cl 5M=Ir IMesIr(CO)2Cl IMesIr(CO)2Cl

ATR ATR CH2Cl2 ATR ATR CH2Cl2

ν̅CO (cm−1) 2071, 1983 2066, 1984 2081, 1996 2056, 1967 2055, 1971 2066, 1980

ν̅COav (cm−1) 2027 2025 2039 2011 2013 2023

ref this this 35 this this 36

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asymmetric CO stretching (νasym C ̅ O ), whereas the higher-energy frequencies (ca. 2070 cm−1) are assigned to the symmetric CO 36 stretching (νsym the average of both asymmetric C ̅ O ). Typically, and symmetric values (νav C ̅ O) are used to evaluate the σ-donating properties of a ligand (L) in cis-[LIr(CO)2Cl], although factors, 34,37,38 such as size of the ligand, also affect ν̅av CO The average CO stretching frequency for 5M=Ir (ν̅av CO = 2011 cm−1) is weaker than that for [(IMes)Ir(CO)2Cl] (ν̅av CO = 2013 cm−1) and suggests that IMesP 1a is a slightly stronger donor than IMes. In contrast, comparison of the average values for av −1 5M=Rh (νav C O = 2027 cm ) and cis-[(IMes)Rh(CO)2Cl] (νC ̅ −1 ̅ O= 2025 cm ) suggests the opposite σ-donating properties. Further illustrating the subtle differences in donor properties between IMesP and IMes, the ν̅Casym for cis-[(IMesP)MO (CO)2Cl] (5M=Rh and 5M=Ir) is lower energy than that for cis−1 [(IMes)M(CO)2Cl] [Δνasym cm−1 (M C ̅ O : −1 cm (M = Rh), −4 −1 sym = Ir)], whereas ν̅sym shows the opposite trend [Δν CO C ̅ O : 5 cm −1 asym (M = Rh), 1 cm (M = Ir)]. For comparison, the ΔνC̅ O , av Δν̅sym CO , and ΔνC ̅ O values for complexes of backbone functionalized 4,5-dichlorocarbenes are ca. 4 cm−1 higher in energy when compared with their respective nonchlorinated analogues, reflecting the poorer donor properties of the former.12 To place these differences in IR stretching frequencies into context, it must be noted that NHC complexes tend to exhibit smaller differences in their CO stretching frequencies compared with their phosphine counterparts (e.g., cis-[(IMes)Ir(CO)2Cl] vs cis-[(ItBu)Ir(CO)2Cl]: ΔνC̅ O = 1 cm−1, whereas cis-[(PPh3)Ir(CO)2Cl] vs cis-[(PCy3)Ir(CO)2Cl]: ΔνC̅ O = 15 cm−1).36 In conclusion, the observed trends in CO stretching frequencies for complexes IMesP suggest that the good σ-donating properties of IMes are retained upon functionalization with a phosphine moiety in the C4 position; however, the subtle differences likely reflect the steric influence of IMesP upon the metal carbonyl systems. Molecular Structures and Metrical Parameters of IMesP. Details of the crystal structure determinations undertaken in the present study are given in Table 2, and important metrical parameters for the new NHCs and their complexes are given in Table 3. Only subtle perturbations in the metrical parameters are observed between the 4-phosphino-substituted NHCs (1a−1c) and IMes itself. For example, the endocyclic angles at C(3) in 1a−1c [C(4)−C(3)−C(2): avg = 104.3(4)°] are smaller than those in IMes (106.5(3)°).2 Furthermore, the bond lengths to C(3) are slightly elongated with respect to those in IMes. Presumably, these observations reflect the steric pressure induced by the bulky P substituent at C(3). For the most part, the bond lengths and angles within the C3N2 ring do not change considerably upon coordination to iridium(I). The C(2)−Ir bond length in 4M=Ir is similar to that in [IMesIr(cod)Cl]36 [2.055(8) Å vs 2.052(7) Å, respectively]. Likewise, similar C−Ir bond lengths are observed in 5M=Ir and [IMesIr(CO)2Cl]36 [2.071(2) Å vs 2.08(2) Å, respectively].

Figure 4. Molecular structure of 4M=Ir·3/2CH2Cl2 (thermal ellipsoids set at 50% probability). The solvate atoms and all hydrogen atoms are omitted for clarity.

Figure 5. Molecular structure of 5M=Ir·C7H8 (thermal ellipsoids set at 50% probability). The solvate atoms and all hydrogen atoms are omitted for clarity.

The ATR-IR (attenuated total reflectance) spectra of carbonyl complexes 5M=Rh and 5M=Ir were collected and compared to those of authentic samples of cis-[(IMes)Rh(CO)2Cl]35 and cis-[(IMes)Ir(CO)2Cl], respectively.36 The results are summarized in Table 1. In such complexes, the lower-energy frequencies (ca. 2000 cm−1) are assigned to the 4532

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Table 2. Crystallographic Data for Compounds 1a, 1b, 1c, 4M=Ir, and 5M=Ir formula fw cryst syst space group Z a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) T (K) μ(Mo Kα) (cm−1) cryst size (mm3) dcalcd (Mg m−3) 2θ(max) (deg) no. reflns no. unique reflns R(int) refln/param ratio R1a [I > 2σ(I)] wR2 (all data)b GOF CCDC No. a

1a·0.5C6H6

1b·0.5C6H6

1c·0.5C4H8O

4M=Ir·1.5CH2Cl2

5M=Ir·C7H8

C46H48N2P 659.83 monoclinic C2/c 8 30.592(2) 15.388(1) 18.693(1) 90 117.843(2) 90 7781.0(7) 173 1.04 0.5 × 0.35 × 0.2 1.127 55.8 43 060 9259 0.0492 20.12 0.0506 0.1401 1.016 748235

C45H47N3P 660.83 monoclinic C2/c 8 30.427(4) 14.980(2) 18.825(2) 90 117.373(5) 90 7619.3(16) 173 1.07 0.7 × 0.4 × 0.1 1.152 56 34 727 9162 0.1151 19.91 0.0673 0.1673 0.972 869364

C45H47F2O0.5N2P 692.82 monoclinic C2/c 8 28.942(2) 15.805(1) 19.737(1) 90 118.977(2) 90 7898.3(9) 173 1.13 0.5 × 0.4 × 0.1 1.165 55.8 44 723 9340 0.1000 19.75 0.0644 0.1561 0.972 869365

C105H120Cl8Ir2N4P2 2167.99 triclinic P1̅ 2 15.941(1) 16.121(1) 20.855(2) 89.941(4) 68.341(5) 76.550(4) 4823.0(6) 100 30.61 0.25 × 0.2 × 0.1 1.493 56.4 116 130 36 890 0.0482 33.26 0.0468 0.1329 1.076 869368

C52H53ClIrN2O2P 996.58 orthorhombic Pbcn 8 29.159(1) 18.337(1) 17.068(1) 90 90 90 9126.2(5) 100 30.61 0.25 × 0.15 × 0.1 1.451 55.8 67 757 10 883 0.029 19.72 0.0234 0.0511 1.011 869369

R1 = ∑||Fo| − |Fc||/∑|Fo|. bwR2 (F2 [all data]) = {∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]}1/2.

Table 3. Selected Metrical Parameters for IMesa and the New Compounds Prepared in This Study IMesa bond lengths (Å) C(2)−N(1) C(2)−N(2) N(2)−C(3) C(3)−C(4) C(4)−N(1) M(1)−C(2) M(1)−Cl(1) bond angles (deg) C2−N1−C4 N1−C2−N2 C2−N2−C3 N2−C3−C4 N1−C4−C3 N1−C2−M1 N2−C2−M1 C2−M1−Cl1 a

1.365(4) 1.371(4) 1.378(4) 1.331(5) 1.381(4)

112.8(3) 101.4(2) 112.8(3) 106.5(3) 106.5(3)

1a 1.363(2) 1.366(3) 1.406(2) 1.349(3) 1.388(2)

113.1(2) 101.2(2) 114.1(2) 104.4(2) 107.2(2)

1b 1.367(3) 1.365(3) 1.403(3) 1.352(4) 1.390(3)

113.1(2) 101.2(2) 114.2(2) 104.2(3) 107.4(3)

1c 1.359(4) 1.368(3) 1.402(4) 1.352(4) 1.390(3)

112.8(2) 101.3(2) 114.2(2) 104.3(2) 107.4(3)

4M=Ir

5M=Ir

1.36(1) 1.36(1) 1.41(1) 1.34(1) 1.39(1) 2.055(8) 2.371(3)

1.356(3) 1.361(3) 1.411(3) 1.354(3) 1.386(3) 2.071(2) 2.351(1)

111.2(7) 103.7(7) 112.0(7) 105.1(7) 108.1(7) 125.2(6) 131.1(6) 93.3(3)

111.2(2) 104.3(2) 111.7(2) 104.9(2) 107.9(2) 126.6(2) 129.0(2) 90.9(2)

See ref 2.

the unusual trends in CO stretching frequencies described above.

For the complexes, the N−C−N angles increase slightly and the C−N−C angles contract slightly when compared with those in 1a. Interestingly, a common feature of all the complexes is the asymmetric binding of 1a to Ir(I). This is evident upon examining the N−C−Ir bond angles, which are larger on the side of the NHC closest to the 4-phosphino substituent [ΔN−C−M: 5.9(9)° (4M=Ir), 2.4(3)° (5M=Ir)]. This may not solely be a consequence of the presence of the bulky 4phosphino moiety since [(IMes)Ir(cod)Cl] displays a similar, albeit smaller, asymmetrical distortion [ΔN−C−Ir: 2.9(8)°].36 Nevertheless, it is possible that these distortions are factors in

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SUMMARY We have demonstrated that hindered phosphaalkenes will react with bulky N-heterocyclic carbenes, such as IMes and ItBu, to afford the corresponding NHC with a phosphine substituent in the 4-position. We postulate that the reaction proceeds via the coordination of an abnormal NHC intermediate to the phosphaalkene, which is favored for these bulky substrates since the donor−acceptor adduct of the normal NHC and 4533

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Organometallics

Article

3H), 1.50 (s, 3H). MS (EI, 70 eV): 622, 621 [3, 7; M+]; 454, 453 [36, 100; M+ − CHPh(C5H4N)]; 169, 168 [12, 60; CHPh(C5H4N)+]. Synthesis of 1c. To a solution of IMes (304 mg, 1 mmol) in THF (2 mL) was added a solution of 2c (352 mg, 1 mmol) in THF (2 mL). The mixture was stirred at room temperature overnight. An aliquot was removed, and 31P NMR spectroscopy indicated quantitative conversion of the phosphaalkene (δ = 234) to a new compound (δ = −36.7). Slow evaporation of the solvent afforded crystals suitable for X-ray crystallography. Yield (1c·1/2THF): 560 mg (81%). 31 P NMR (162 MHz, C6D6): δ = −37.4. 19F NMR (282 MHz, C6D6): δ = −116.0, −116.1. 1H NMR (400 MHz, C6D6): δ = 7.28− 6.40 (m, 15H, 14 aromatic + 1 vinyl), 5.05 (d, JHP = 3.3 Hz, 1H, CAr2H), 2.57 (s, 3H), 2.32 (s, 3H), 2.28 (s, 3H), 2.15 (s, 6H), 1.93 (s, 3H), 1.78 (d, JHP = 2.8 Hz, 3H), 1.77 (s, 3H), 1.49 (s, 3H). 13C{1H} NMR (100 MHz, C6D6): δ = 220.4 (br s), 161.9 (dd, 5JCP = 3 Hz, 1JCF = 246 Hz), 160.8 (dd, 5JCP = 3 Hz, 1JCF = 245 Hz), 147.7, 147.4, 144.3, 140.1, 139.0, 138.1 (d, JCP = 3 Hz), 137.9 (d, JCP = 3 Hz), 137.7, 137.6, 137.4, 137.3 (JCP = 3 Hz), 137.1, 136.5, 135.2, 130.6 (d, J = 8 Hz), 130.5, 130.4 (d, J = 7 Hz), 129.6 (d, J = 7 Hz), 129.5 (d, J = 8 Hz), 129.4 (d, J = 8 Hz), 129.3, 129.2, 129.1, 127.9, 125.2 (d, 1JCP = 20 Hz), 115.7 (d, 2JCF = 21 Hz), 115.1 (d, 2JCF = 21 Hz), 46.3 (d, 1JCP = 10 Hz), 23.5 (d, JCP = 35 Hz), 21.5, 21.0 (2 × C coincident), 20.9, 18.8 (d, JCP = 7 Hz), 18.4, 17.3, 17.0. MS (EI, 70 eV): 657, 656 [3, 6; M+]; 454, 453 [36, 100; M+ − CH(C6H4F)2]; 204, 203 [5, 13; CH(C6H4F)2+]. Synthesis of 3a. To a solution of ItBu (180 mg, 1 mmol) in THF (2 mL) was added a solution of 2a (316 mg, 1 mmol) in THF (2 mL). The mixture was heated at 100 °C in a closed vessel. Over the course of 3 weeks, the mixture was cooled periodically to remove an aliquot for 31P NMR spectroscopic analysis. The spectra revealed the growth of a new signal (δ = −32.3), attributed to compound 3a, at the expense of the signal attributed to the phosphaalkene (δ = 233). The approximate ratio of signals was ∼1:1 after 1 week and remained unchanged over the remainder of the experiment (ca. 3 weeks). The separation of 3a from ItBu and 2a was not attempted. Synthesis of 4M=Rh. To a solution of [(cod)RhCl]2 (100 mg, 0.20 mmol) in THF (2 mL) was added a solution of 1a (266 mg, 0.41 mmol) in THF (2 mL). The mixture was stirred for 1 h, at which point the volatiles were removed in vacuo. The crude product was recrystallized from a mixture of THF and pentane. Yield: 310 mg (88%). 31 P NMR (162 MHz, CD2Cl2): δ = −38.4 (64%), −39.1 (36%). 1H NMR (400 MHz, CD2Cl2) (major isomer): δ = 7.66−6.49 (m, 17H, 16 aromatic + 1 vinyl), 5.26 (d, JHP = 4 Hz, 1H, CPh2H), 4.33 (m, 2H, CHcod), 3.24 (m, 2H, CHcod), 2.60 (s, 3H, CH3), 2.42 (s, 3H, CH3), 2.39 (s, 3H, CH3), 2.37 (s, 3H, CH3), 2.17 (s, 3H, CH3), 2.13 (s, 3H, CH3), 1.74 (m, 4H, CH2cod), 1.58 (s, 3H, CH3), 1.51 (s, 3H, CH3), 1.50 (s, 4H, CH2cod), 1.41 (s, 3H, CH3). 1H NMR (400 MHz, CD2Cl2) (minor isomer): δ = 7.64−6.52 (m, 17H, 16 aromatic + 1 vinyl), 5.24 (d, JHP = 4 Hz, 1H, CPh2H), 4.33 (m, 2H, CHcod), 3.33 (m, 2H, CHcod), 2.62 (s, 3H, CH3), 2.41 (s, 3H, CH3), 2.39 (s, 3H, CH3), 2.37 (s, 3H, CH3), 2.17 (s, 3H, CH3), 2.13 (s, 3H, CH3), 1.86 (s, 3H, CH3), 1.74 (m, 4H, CH2cod), 1.59 (s, 3H, CH3), 1.50 (s, 4H, CH2cod), 1.13 (s, 3H, CH3). 13C{1H} NMR (100 MHz, CD2Cl2): δ = 186.2 (d, JCRh = 51 Hz, major N−C−N), 185.5 (d, JCRh = 50 Hz, minor N−C−N); other signals were not assigned. MS (EI, 70 eV): 869, 868, 867, 866 [10, 24, 28, 50; M+]; 831, 830 [4, 6; M+ − HCl]; 723, 722 [5, 12; M+ − HCl − C8H12]; 623, 622, 621 [12, 50, 100; 1a·H+]; 455, 454 [22, 44; 1a·H+ − CHPh2]; 306, 305 [5, 22; IMesH+]; 304, 303 [14, 53; IMes+ − H]; 168, 167 [12, 52; CHPh2+]. Anal. Calcd for C51H57ClN2PRh: C, 70.62; H, 6.62; N, 3.23. Found: C, 70.33; H, 6.57; N, 3.30. Synthesis of 4M=Ir. To a solution of [(cod)IrCl]2 (50 mg, 0.07 mmol) in THF (2 mL) was added a solution of 1a (94 mg, 0.15 mmol) in THF (2 mL). The mixture was stirred for 1 h, at which point the volatiles were removed in vacuo. The crude product was recrystallized by slow diffusion of pentane into a dichloromethane solution of the product. Yield: 113 mg (79%). 31 P NMR (162 MHz, CD2Cl2): δ = −38.5 (62%), −39.3 (38%). 1H NMR (400 MHz, CD2Cl2) (major isomer): δ = 7.67−6.49 (m, 17H,

phosphaalkene results in a frustrated Lewis pair. Significantly, this unexpected reaction opens up a convenient route to highly functionalized NHCs. The 4-phosphino-NHCs reported herein have been utilized as ligands for rhodium(I) and iridium(I). Noteworthy, the carbonyl stretching frequencies for the rhodium(I)- and iridium(I)−carbonyl complexes of the 4phosphino-NHC suggests that its σ-donor ability is roughly equal to that of unsubstituted IMes. A key difference between IMes and the 4-phosphino-NHCs reported herein is the potential application of the latter as bidentate bridging ligands, which will form the basis for future investigations.

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EXPERIMENTAL SECTION

General Considerations. Unless otherwise noted, all manipulations were performed under an atmosphere of nitrogen. Hexanes, dichloromethane, and toluene were deoxygenated with nitrogen and dried by passing through a column containing activated alumina. Tetrahydrofuran was distilled from Na/benzophenone. C6D6 was stored over molecular sieves (4 Å) before use, and ampules of CD2Cl2 were used as received from CIL. IMes2 and ItBu39 and phosphaalkenes 2a−2c40 were prepared following literature procedures. The [M(cod)Cl]2 (M = Rh, Ir) were used as obtained from Strem. The CO was used as received from Praxair. 31P, 1H, and 13C{1H} NMR spectra were recorded at room temperature on Bruker Avance 300 or 400 MHz spectrometers with chemical shifts (δ) in parts per million (ppm). Chemical shifts are referenced and reported relative to 85% H3PO4 as an external standard (δ = 0.0 for 31P) or referenced to TMS and measured relative to the residual solvent peak (C6HD5 or CHDCl2: δ = 7.16 or 5.32 for 1H, respectively; C6D6 or CD2Cl2: δ = 128 or 53.8 for 13C, respectively). Mass spectra were recorded on a Kratos MS 50 instrument in EI mode (70 eV). IR spectra were recorded on a Thermo IR spectrometer with an attenuated total reflection (ATR) attachment. Synthesis of 1a. To a solution of IMes (500 mg, 1.64 mmol) in THF (2 mL) was added a solution of 2a (520 mg, 1.64 mmol) in THF (2 mL). The mixture was heated at 70 °C overnight in a closed vessel. Upon cooling, an aliquot was removed, and 31P NMR spectroscopy indicated quantitative conversion of the phosphaalkene (δ = 233) to a new compound (δ = −37.3). Volatiles were removed in vacuo, and the product was recrystallized from benzene. Yield: 800 mg (78%). 31 P NMR (162 MHz, C6D6): δ = −37.9. 1H NMR (400 MHz, C6D6): δ = 7.56−6.39 (m, 16H, aromatic), 6.64 (d, JHP = 2.0 Hz, 1H, vinyl), 5.23 (d, JHP = 4.4 Hz, 1H, CPh2H), 2.64 (s, 3H), 2.35 (s, 3H), 2.34 (s, 3H), 2.14 (s, 6H), 1.92 (s, 3H), 1.86 (d, JHP = 2.4 Hz, 3H), 1.78 (s, 3H), 1.52 (s, 3H). 13C{1H} NMR (100 MHz, C6D6): δ = 220.3 (d, JCP = 5 Hz), 147.8, 147.4, 144.6 (d, JCP = 5 Hz), 142.6, 142.4, 141.9, 141.8, 139.7 (d, JCP = 2 Hz), 139.2, 137.9, 137.4, 137.16, 137.15, 136.7, 135.4, 130.4, 129.2, 129.1, 128.8, 128.7 (d, JCP = 2 Hz), 128.3, 128.2, 128.1, 128.0, 127.9, 126.8 (d, JCP = 2 Hz), 126.6 (d, JCP = 2 Hz), 125.8, 125.6, 48.2 (d, JCP = 10 Hz), 23.6 (d, JCP = 36 Hz), 21.6, 21.0 (2 × C coincident), 20.9, 18.8 (d, JCP = 8 Hz), 18.4, 17.6, 17.0. MS (EI, 70 eV): 621, 620 [2, 5; M+]; 455, 454, 453 [10, 50, 100; M+ − CHPh2]; 168, 167 [18, 85; CHPh2+]. Anal. Calcd for C43H45N2P: C, 83.19; H, 7.31; N, 4.51. Found: C, 83.10; H, 7.44; N, 4.61. Synthesis of 1b. To a solution of IMes (104 mg, 0.34 mmol) in C6D6 (2 mL) was added a solution of phosphaalkene E/Z-2b (106 mg, 0.33 mmol) in C6D6 (2 mL). The mixture was heated at 100 °C overnight in a closed vessel. Upon cooling, an aliquot was removed, and 31P NMR spectroscopy indicated quantitative conversion of the E/Z-2b (δ = 261, 242) to new compounds (δ = −33.7, −35.3). Slow evaporation of the solvent afforded crystals suitable for X-ray crystallography. Yield (1b·1/2C6D6): 160 mg (73%). 31 P NMR (121 MHz, C6D6): δ = −33.7, −35.3. 1H NMR (300 MHz, C6D6): δ = 8.34−6.18 (m, 32H, 30 aromatic + 2 vinyl), 5.67 (d, 2 JHP = 5 Hz, 1H, CAr2H), 5.52 (d, 2JHP = 5 Hz, 1H, CPh2H), 2.74 (s, 3H), 2.65 (s, 3H), 2.37 (s, 3H), 2.35 (br s, 3H), 2.29 (br s, 3H), 2.26 (s, 3H), 2.16 (s, 6H), 2.15 (s, 6H), 2.12 (d, JHP = 4 Hz, 3H), 1.93 (d, JHP = 3 Hz, 3H), 1.91 (s, 3H), 1.89 (s, 3H), 1.81 (br s, 6H), 1.51 (s, 4534

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Organometallics

Article

16 aromatic + 1 vinyl), 5.27 (d, JHP = 4 Hz, 1H, CPh2H), 3.94 (m, 2H, CHcod), 2.96 (br m, 2H, CHcod), 2.63 (s, 3H, CH3), 2.37 (s, 3H, CH3), 2.35 (s, 3H, CH3), 2.34 (s, 3H, CH3), 2.20 (s, 3H, CH3), 2.16 (s, 3H, CH3), 1.57 (s, 3H, CH3), 1.56 (br m, 4H, CH2cod), 1.55 (s, 3H, CH3), 1.36 (s, 3H, CH3), 1.24 (s, 3H, CH2cod). 1H NMR (400 MHz, CD2Cl2) (minor isomer): δ = 7.65−6.52 (m, 17H, 16 aromatic + 1 vinyl), 5.25 (d, JHP = 4 Hz, 1H, CPh2H), 3.91 (m, 2H, CHcod), 2.96 (br m, 2H, CHcod), 2.63 (s, 3H, CH3), 2.37 (s, 3H, CH3), 2.35 (s, 3H, CH3), 2.34 (s, 3H, CH3), 2.17 (s, 6H, 2 × CH3), 1.83 (s, 3H, CH3), 1.57 (s, 3H, CH3), 1.56 (br m, 4H, CH2cod), 1.24 (s, 3H, CH2cod), 1.18 (s, 3H, CH3). 13C{1H} NMR (100 MHz, CD2Cl2): δ = 183.7 (s, major N−C−N), 182.8 (s, minor N−C−N); other signals were not assigned. MS (EI, 70 eV): 960, 959, 958, 957, 956, 955, 954 [5, 18, 42, 55, 100, 29, 50; M+]; 920, 919, 918 [2, 3, 6; M+ − HCl]; 792, 791, 790, 789, 788, 787 [2, 5, 6, 14, 4, 6; M+ − CHPh2]; 455, 454, 453 [12, 23, 30; 1a+ − CHPh2]; 168, 167 [25, 99; CHPh2+]. Anal. Calcd for C51H57ClN2PIr: C, 64.03; H, 6.01; N, 2.39. Found: C, 64.04; H, 6.00; N, 3.00. Synthesis of 5M=Rh. A stream of CO gas was bubbled through a stirred solution of 4M=Rh (100 mg, 0.12 mmol) in dichloromethane (5 mL) for 15 min. The solvent was removed in vacuo, and the product was washed with hexanes. Yield: 85 mg (90%). 31 P NMR (162 MHz, CD2Cl2): δ = −38.6. 1H NMR (400 MHz, CD2Cl2): δ = 7.67−6.51 (m, 17H, 16 aromatic + 1 vinyl), 5.29 (d, JHP = 4 Hz, 1H, CPh2H), 2.63 (s, 3H, CH3), 2.36 (s, 3H, CH3), 2.34 (s, 3H, CH3), 2.26 (s, 3H, CH3), 2.20 (s, 3H, CH3), 2.17 (s, 3H, CH3), 1.68 (s, 3H, CH3), 1.56 (d, JHP = 3 Hz, 3H, CH3), 1.23 (s, 3H, CH3). 13 C{1H} NMR (100 MHz, CD2Cl2): δ = 185.7 (d, JCRh = 54 Hz, CO), 183.4 (d, JCRh = 74 Hz, N−C−N), 178.6 (d, JCRh = 47 Hz, CO); other signals were not assigned. IR ν̅CO (cm−1): 2070.5, 1983.2. Anal. Calcd for C45H45ClO2N2PRh: C, 66.30; H, 5.56; N, 3.44. Found: C, 66.34; H, 5.82; N, 3.17. Synthesis of 5M=Ir. A stream of CO gas was bubbled through a stirred solution of 4M=Ir (50 mg, 0.05 mmol) in dichloromethane (5 mL) for 15 min. The solvent was removed in vacuo, and the crude product was recrystallized from slow evaporation of a toluene solution. Yield (5M=Ir·cod): 43 mg (81%). 31 P NMR (162 MHz, CD2Cl2): δ = −38.8. 1H NMR (400 MHz, CD2Cl2): δ = 7.66−6.51 (m, 17H, 16 aromatic + 1 vinyl), 5.29 (d, JHP = 5 Hz, 1H, CPh2H), 2.63 (s, 3H, CH3), 2.35 (s, 3H, CH3), 2.33 (s, 3H, CH3), 2.25 (s, 3H, CH3), 2.21 (s, 3H, CH3), 2.17 (s, 3H, CH3), 1.67 (s, 3H, CH3), 1.55 (d, JHP = 3 Hz, 3H, CH3), 1.22 (s, 3H, CH3). 13 C{1H} NMR (100 MHz, CD2Cl2): δ = 180.8 (CO), 177.1 (N−C− N), 169.1 (CO), 148.3, 148.0, 145.0 (d, JCP = 5 Hz), 141.8 (d, JCP = 13 Hz), 141.4 (d, JCP = 19 Hz), 141.3, 141.1, 140.2, 140.0, 137.3, 136.7, 136.0, 135.6, 135.3, 133.9, 133.3, 133.0, 130.9, 130.4, 129.6, 129.5, 129.4, 129.0, 128.8, 128.4, 128.3, 127.8, 127.2, 123.6 (d, JCP = 21 Hz), 47.8 (d, JCP = 8 Hz), 23.3 (d, JCP = 35 Hz), 21.8, 21.5, 21.4, 21.3, 19.5 (d, JCP = 17 Hz), 18.9, 18.2, 17.5. MS (EI, 70 eV): 907, 906, 905, 904, 903, 902 [1, 2, 2, 4, 1, 2; M+]; 879, 878, 877, 876, 875, 874 [3, 6, 8, 16, 4, 8; M+ − CO]; 792, 791, 790, 789, 788, 787 [2, 5, 6, 14, 4, 6; M+ − CHPh2]; 684, 683, 682, 681, 680, 679 [1, 3, 4, 9, 3, 8; 1a·IrCl+ − CHPh2]; 168, 167 [28, 100; CHPh2+]. IR ν̅CO (cm−1): 2056.2, 1966.6. Anal. Calcd for C45H45ClN2O2PIr·C8H12: C, 62.67; H, 5.36; N, 2.81. Found: C, 63.04; H, 5.42; N, 2.81. X-ray Crystallography. All single crystals were immersed in oil and mounted on a glass fiber. Data were collected on a Bruker X8 APEX 2 diffractomer with graphite-monochromated Mo Kα radiation. Data were collected and integrated using the Bruker SAINT41 software package and corrected for absorption effect using TWINABS42 (for 4M=Ir) and SADABS43 (for all others). All data sets were corrected for Lorentz and polarization effects. All structures were solved by direct methods44 and subsequent Fourier difference techniques and refined anisotropically for all non-hydrogen atoms using the SHELXTL45 crystallographic software package from Bruker-AXS. All data sets were corrected for Lorentz and polarization effects. Additional crystal data and details of the data collection and structure refinement are given in Table 3. The crystals of 1a and 1b presented no crystallographic complications. The structures of 1c and 5M=Ir each exhibited disorder.

For 1c, it appears that the oxygen atom of the THF solvate is disordered to every position of the ring; however, since the solvate simply seems to be filling a void space in the crystala partial benzene molecule in both 1a and 1b occupies the same sitethe partial THF molecule was modeled as a single species. In 5M=Ir, the Cl and CO ligands cis to the carbene ligand are disordered over both positions, with the major isomer (83%) being shown in Figure 5. The crystal of 4M=Ir was a two-component twin, present in a 3:2 ratio, where the minor component is related to the major component by a 180° rotation about the (1 0 0) reciprocal axis.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic data as CIF files. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Natural Sciences and Engineering Research Council (NSERC) of Canada for support of this work. J.I.B. thanks NSERC for PGS M and D scholarships. We thank Dr. Brian O. Patrick for helpful discussions regarding the X-ray crystallography and Dr. Eamonn Conrad for useful discussions.

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REFERENCES

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Organometallics

Article

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dx.doi.org/10.1021/om3003174 | Organometallics 2012, 31, 4529−4536