Square Planar Nucleophilic and Radical Pt(II) Carbenes

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Cite This: Organometallics XXXX, XXX, XXX−XXX

Square Planar Nucleophilic and Radical Pt(II) Carbenes Anthony P. Deziel, Melissa R. Hoffbauer, and Vlad M. Iluc* Department of Chemistry and Biochemistry, University of Notre Dame, 251 Nieuwland Science Hall, Notre Dame, Indiana 46556, United States

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ABSTRACT: A square planar platinum(II) carbene complex [{PC(sp2)P}HPt(PMe3)] ([PC(sp2)P]H = (bis[2-(di-iso-propylphosphino)phenyl]methylene) was synthesized through the dehydrohalogenation of [{PC(sp3)HP}HPtCl] in a microwave reactor. The tert-butyl substituted analogue, [{PC(sp2)P}tBuPt(PMe3)] ([PC(sp2)P]tBu = bis[2-(di-iso-propylphosphino)4-tert-butylphenyl]methylene), was synthesized via an analogous route. The nucleophilic nature of the carbene carbon was confirmed through DFT calculations and reactivity with HCl. Additionally, [{PC(sp2)P}HPt(PMe3)] was treated with 0.5 equiv of I2 to generate a paramagnetic product, [{PC(sp2)P}HPtI]. The Evans method and EPR spectroscopy revealed that a one-electron oxidation occurred at the carbene carbon, thus generating a persistent radical carbene.

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NN double bonds.22 Additionally, Piers et al. synthesized a non-heteroatom stabilized carbene on a Ni metal center. The isolation of such a complex was achieved through the utilization of a PCP pincer ligand scaffold. This [PC(sp2)P] ([PC(sp2)P] = (bis[2-(di-iso-propylphosphino)phenyl]methylene) complex of Ni proved to be competent in a variety of small molecule activations including NH3, H2O, and H2.4 Furthermore, [PC(sp2)P] was valuable in the generation of carbenes on a variety of metal centers, with examples reported for Ir, Rh, Pd, and Co.5,23−28 Interestingly, the synthetic route by which the carbene is formed varies by the group of the metal. For example, iridium and rhodium complexes, [{PC(sp2)P}IrCl] and [{PC(sp2)P}RhCl],23,26 were synthesized through a double C−H activation process, whereas [{PC(sp2)P}Pd(PMe3)], reported by our group,5 was synthesized via a double dehydrohalogenation pathway. Comprehensive reactivity studies of [{PC(sp2)P}Pd(PMe3)] revealed the nucleophilic nature of the carbene carbon, and it was further corroborated by DFT calculations. Platinum carbenes lacking heteroatom stabilization are rare and not well researched. Templeton and co-workers observed a platinum(IV) carbene that could not be fully characterized due to its thermal instability and increased reactivity.29 Similarly, Carmona and co-workers observed a cationic platinum(II) alkylidene by NMR spectroscopy.30,31 Yet again, structural characterization was not possible due to its instability.30,31 Rourke and co-workers reported a non-heteroatom stabilized platinum carbene complex, with the metal attached to a 1,4dihydropyridinylene, but a resonance structure could be redrawn as protonated pyridinyl.32 Similarly, Urriolabeitia and co-workers reported a structurally characterized platinum carbene complex, cis-[Pt(C6F5)2{C(CO2Me)-C(PPh3)(CO2Me)}(PPh3)], but the ylide carbene exists in a delocalized

INTRODUCTION Transition metal carbenes have been defined as either Fischer or Schrock type species and are categorized based on their electronic structure and reactivity.1−3 In recent years, carbenelike complexes exhibiting features that do not align with Fischer type or Schrock type classifications have surfaced. The reactivity and properties of these complexes are not as straightforward as previously considered, and their characterization has made these classifications incomplete. In particular, late transition metal carbenes with nucleophilic character have been synthesized, and the study of their reactivity prompted further classification and investigations.4,5 One of the main reasons that late transition metal carbenes have garnered substantial interest over the years is due to their applications in catalysis.6−11 Specifically, a variety of palladium coupling reactions are postulated to proceed through a process involving a migratory insertion of a carbene functionality into a palladium−carbon bond.12−17 However, the transient nature of these species resulted in difficulties surrounding their synthesis and characterization. Thus, the isolation and characterization of such compounds would allow unique insight into the mechanism of complex reactions. The majority of isolated late transition metal carbenes feature heteroatom stabilization or N-heterocyclic carbenes (NHCs).18,19 Compounds without heteroatom stabilization tend to be highly reactive and unstable; as such, only a few of these metal complexes have been isolated, structurally characterized, and had their reactivity studied.20 The first non-heteroatom stabilized carbene of a group 10 metal center was reported by Hillhouse and co-workers in 2002.21 Their report contained a novel trigonal planar diphenylcarbene, (dtbpe)NiCPh2 (dtbpe = 1,2-bis(di-tertbutylphosphino)ethane. The robust nature of this molecule allowed in-depth reactivity studies that revealed the ability of the Ni(II) carbene complex to generate new CN, CO, and © XXXX American Chemical Society

Received: November 29, 2018

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DOI: 10.1021/acs.organomet.8b00864 Organometallics XXXX, XXX, XXX−XXX

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expected, with the bidentate phosphine ligand coordinating in a cis fashion observed in the analogous palladium complex.37 Efforts to dehydrochlorinate 2, by using methods analogous to those employed in the case of the nickel and palladium analogues, were unsuccessful. In contrast, the Ni and Pd dichloride species were successfully dehydrohalogenated by increasing the temperature of the reaction, resulting in C−H bond activation accompanied by the loss of the corresponding acid, HBr or HCl. Even when [{PC(sp3)H2P}HPtCl2] was treated with KN(TMS)2 at elevated temperatures up to 100 °C for 7 days, no reaction occurred. Despite the manipulation of reaction conditions and thermal parameters, only the unreacted starting material was recovered. Interestingly, [{PC(sp3)HP}HPtCl] (3) was successfully formed through a microwave assisted dehydrohalogenation of 2 in the presence of KN(TMS)2 (Scheme 1), which allowed for its full characterization by multinuclei NMR spectroscopy and X-ray diffraction crystallography. The activated backbone C-H resonance at 5.85 ppm in the 1H NMR spectrum of [{PC(sp3)HP}HPtCl] contains platinum satellite peaks (2JHPt = 145.8 Hz), supporting the formation of a new Pt−C single bond. This was further corroborated by the appearance of platinum satellites on the Pt-Cbackbone (JCPt = 681.6 Hz) carbon in the 13C{1H} NMR spectrum. Two similar cationic platinum(II) complexes with coordinating CO or CNXyl (Xyl = 2,6-Me2C6H3) on a PC-CP ligand (PC-CP = 2,2′ bis(di-iso-propylphosphino)-3,3′ methyl-diphenyl-1,2-ethane) were reported by Carmona and co-workers.31 [{PC-CP}Pt(CO)] and [{PC-CP}Pt(CNXyl)] displayed Pt−C coupling constants of 500 and 495 Hz, respectively, slightly lower than our findings for 3, likely due to electronic differences between these systems. Additionally, Wendt and co-workers reported a Pt monochloride complex, trans-[PtCl{cis-1,3-bis(di-tert-butylphosphino)}cyclohexane], which has a Pt−C coupling constant of 730 Hz.38 Thus, the observed Pt−C coupling for 3 is in line with previously reported values for similar Pt(II) complexes. A single resonance appears at 50.21 ppm in the 31P{1H} NMR spectrum, shifted downfield from the dichloride species, with a smaller JPPt coupling constant of 2976.5 Hz than the JPPt of 3574.1 Hz for 2. The smaller coupling constant is expected as the phosphines are no longer in a cis coordination mode, as they shifted to trans due to cycloplatination. Although a direct comparison cannot be made since 2 and 3 are not isomers, cis phosphine platinum splitting coupling constants are generally larger than trans phosphine splitting coupling constants.35 The solid state molecular structure of 3 shows a square planar complex (∑angles = 333.55°), with a Pt−C distance of 2.074 Å, similar to previously reported Pt−C distances.31,39,40 For example, Carmona and co-workers reported Pt−C distances of 2.080 and 2.073 Å for [{PC-CP}Pt(CO)] and [{PC-CP}Pt(CNXyl)], respectively.31 Successful formation of [{PC(sp2)P}HPt(PMe3)] (4) was achieved through a dehydrohalogenation process induced by the treatment of [{PC(sp3)HP}HPtCl] with KN(TMS)2 in the presence of PMe3 (eq 1), analogous to the method reported for the Ni and Pd [PC(sp2)P]H congeners.4,5,41 The reaction occurs immediately with a stark color change from pale yellow to black upon the addition of 1.5 equiv of PMe3 to a solution of KN(TMS) 2 and [{PC(sp 3)HP} HPtCl]. As the reaction proceeds, 4 readily precipitates from the reaction mixture. The formation of 4 was confirmed by 1H NMR spectroscopy through the absence of the Pt-CbackboneH proton resonance at 5.85 ppm. Furthermore, the 31P{1H} NMR spectrum contained two

system where further stabilization can be achieved through resonance.33 Recently, our group reported the first structurally characterized, non-heteroatom stabilized platinum carbene, in which stabilization was made possible through the use of a [PterP] ([PterP] = 1,2-bis(2-(di-iso-propylphosphino)phenyl)benzene) ligand.34 The ability of [PterP] to coordinate to platinum with a widened bite angle in a cis orientation resulted in an enhanced stability of the carbene complex. The robust nature of [(PterP)PtC(p-tol)2] permitted studies that indicated electrophilic reactivity at the carbene carbon. Herein, we report a [PC(sp2)P] complex of platinum that completes the group 10 metal series of carbene complexes bearing this ligand framework and adds to the family of structurally characterized platinum carbene complexes. In addition, [{PC(sp2)P}tBuPt(PMe3)] was synthesized because of its increased solubility in common organic solvents in order to obtain full characterization via NMR spectroscopy. Lastly, we report a one-electron oxidation of [{PC(sp2)P}HPt(PMe3)] to a persistent radical carbene complex.

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RESULTS AND DISCUSSION The reaction of [PC(sp3)H2P]H ([PC(sp3)H2P]H = (bis[2-(diiso-propylphosphino)phenyl]methane, 1) with an equivalent of [Pt(COD)Cl2] in THF for 30 min at ambient temperature (Scheme 1) yielded [{PC(sp3)H2P}HPtCl2] (2). Compound 2 Scheme 1. Synthesis of 2 and 3

was fully characterized by 1H, 13C{1H}, 31P{1H}, 195Pt{1H} NMR spectroscopy, and X-ray crystallography. At room temperature, the 1H and 31P{1H} NMR spectra appeared broad, whereas no resonance could be found in the 195Pt{1H} NMR spectra. Increasing the temperature resulted in the sharpening of the spectra, indicating a dynamic process. At 65 °C, the symmetrical nature of the compound was evidenced through a single resonance at 16.41 ppm (JPPt = 3574.1 Hz) in the 31P{1H} NMR spectrum and a triplet at −4175.86 ppm (JPtP = 3595.3 Hz) in the 195Pt{1H} NMR spectrum. Furthermore, these Pt satellites are consistent with cis coupling phosphines.35,36 These observations were also reflected in the solid state molecular structure (Figure 1), where the P(2)−Pt−P(1) and Cl(2)−Pt−Cl(1) angles were found to be 100.72(3)° and 85.11(3)°, respectively. Additionally, the solid state molecular structure of 2 revealed a square planar geometry at platinum (∑angles = 338.74°). The observed coordination mode was as B

DOI: 10.1021/acs.organomet.8b00864 Organometallics XXXX, XXX, XXX−XXX

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Figure 1. Thermal ellipsoid (50% probability level) representation of [{PC(sp3)H2P}HPtCl2] (2, left) and [{PC(sp3)HP}HPtCl] (3, right). Most hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (deg): For 2: Pt−P(2) 2.2637(9); Pt−Cl(2) 2.3556(8); Pt−P(1) 2.2931(9); Pt−Cl(1) 2.3718(8); P(2)−Pt−P(1) 100.72(3); P(1)−Pt−Cl(2) 170.84(3); P(1)−Pt−Cl(1) 86.47(3); P(2)−Pt−Cl(2) 88.20(3); P(2)−Pt−Cl(1) 167.90(3); Cl(2)−Pt−Cl(1) 85.11(3). For 3: Pt−C 2.074(5); Pt−P(2) 2.2914(17); Pt−P(1) 2.2579(15); Pt−Cl 2.3965(15); C−Pt−P(1) 85.30(13); P(1)−Pt−P(2) 160.28(4); P(1)−Pt−Cl 93.49(5); C−Pt−P(2) 82.25(13); C−Pt−Cl 173.27(13); P(2)−Pt−Cl 100.58(5).

distinct resonances at 54.72 ppm and −35.16 ppm, indicative of the two phosphines of the supporting ligand and PMe3, respectively. The doublet resonance at 54.72 ppm shows coupling to the PMe3 phosphorus (JPP = 20.45 Hz) and to 195 Pt (JPPt = 2870.3 Hz), while the triplet resonance at −35.16 ppm shows coupling to the phosphines of the supporting ligand and 195Pt, resulting in coupling constants of 20.3 and 1691.8 Hz, respectively. Interestingly, [{PC(sp2)P}HPt(PMe3)] is a robust molecule that can withstand temperatures up to 120 °C before any decomposition is observed and can be stored as a crystalline solid at −35 °C for a long period of time. The solid state molecular structure revealed a Pt−C distance of 2.087(5) Å, nearly identical to the Pt−C distance of 3 and indicative of a Pt− C single bond (Figure 2). Our group recently reported the only other crystallographically characterized non-heteroatom stabilized PtC complex, [(PterP)PtC(p-tol)2], which, interestingly, contained a significantly shorter platinum−carbon distance of 1.942 Å. Furthermore, DFT calculations confirmed the formation of a double bond between platinum and carbon.34 The lengthy platinum−carbon distance found in 4 can also be understood using DFT calculations that show that the HOMO (highest occupied molecular orbital) has an antibonding character, suggesting a platinum−carbon single bond with a negative charge on the carbene moiety and a positive charge on the Pt metal center (Figure 3). These findings are similar to those reported for the palladium analogue5 and were first observed in methanediide complexes.42,43 The 4-coordination in the square planar environment of the metal center in 4 has a major impact on the Pt−C bond order. For the previously reported trigonal planar, 3-coordinate carbene complex [(PterP)-

Figure 2. Thermal ellipsoid (50% probability level) representation of [{PC(sp2)P}HPt(PMe3)] (4). Hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (deg): Pt−C 2.087(5); Pt−P(2) 2.3007(14); Pt−P(1) 2.2993(14); Pt−P(3) 2.3202(14); C−Pt−P(1) 81.86(14); P(1)−Pt−P(2) 164.50(5); P(1)−Pt−P(3) 95.75(5); C− Pt−P(2) 82.86(14); C−Pt−P(3) 176.17(14); P(2)−Pt−P(3) 99.64(5).

Figure 3. HOMO (highest occupied molecular orbital) for 3.

PtC(p-tol)2],34 the π* orbital is not populated and the Pt−C interaction was described as a double bond. Issues with the insolubility of 4 in common organic solvents prohibited the preparation of an NMR sample concentrated enough to locate the carbene resonance in the 13C{1H} NMR C

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Figure 4. Thermal ellipsoid (50% probability level) representation of [{PC(sp3)H2P}tBuPtCl2] (6, left) and [{PC(sp3)HP}tBuPtCl] (7, right). Most hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (deg): For 6: Pt(1)−P(12) 2.2434(8); Pt(1)−Cl(12) 2.3568 (8); Pt(1)− P(11) 2.2879(8); Pt(1)−Cl(11) 2.3524(8); P(12)−Pt(1)−P(11) 101.69(3); P(11)−Pt(1)−Cl(11) 84.58(3); P(11)−Pt(1)−Cl(12) 168.80(3); P(12)−Pt(1)−Cl(11) 165.22(3); P(12)−Pt(1)−Cl(12) 88.71(3); Cl(11)−Pt(1)−Cl(12) 86.30(3). For 7: Pt−C 2.077(3); Pt−P(2) 2.2561(8); Pt−P(1) 2.2894(7); Pt−Cl 2.3823(8); C−Pt−P(1) 85.85(8); P(2)−Pt−P(1) 164.62(3); P(2)−Pt−Cl 93.22(3); C−Pt−P(1) 83.07(8); C−Pt−Cl 178.73(8); P(1)−Pt−Cl 98.00(3).

spectrum. Therefore, the synthesis of a platinum carbene complex supported by [PC(sp2)P]tBu ([PC(sp2)P]tBu = bis[2(di-iso-propylphosphino)-4-tert-butylphenyl]methylene) was carried out. This ligand was previously used by our group to enhance the solubility of the analogous Pd carbene complex and allowed the determination of the carbene resonance by 13C{1H} NMR spectroscopy.44 The treatment of [PC(sp3)H2P]tBu ([PC(sp3)H2P]tBu = (bis[2-(di-iso-propylphosphino)-4-tert-butylphenyl]methane) (5) with [Pt(COD)Cl2] at room temperature for 2 h yielded [{PC(sp3)H2P}tBuPtCl2] (6). Compound 6 was isolated in good yield and showed an enhanced solubility in common organic solvents. Similar to 2, the 1H and 31P{1H} spectra of [{PC(sp3)H2P}tBuPtCl2] appeared rather broad at room temperature. Sharpening of the spectra was observed through variable temperature experiments. All spectroscopic parameters are similar to those reported for [{PC(sp3)H2P}HPtCl2]. For instance, the 31P{1H} NMR spectrum displays a single resonance that experiences coupling to the 195Pt nucleus (JPPt = 3577.09 Hz), a coupling constant representative of cis coordinated phosphines. Additionally, the 13C{1H} backbone resonance appears as a singlet with no platinum satellites, thus indicating that no C−H activation occurred and the ligand is only coordinated through the phosphine atoms. The solid state molecular structure revealed a square planar geometry at the metal center with P(11)−Pt−Cl(12) and P(12)−Pt−Cl(11) angles of 168.80(3)° and 165.22(3)°, respectively (∑angles = 334.02°, Figure 4). A successful dehydrohalogenation occurred through a microwave-assisted reaction between [{PC(sp3)H2P}tBuPtCl2] and KN(TMS)2 to give [{PC(sp3)HP}tBuPtCl] (7, Scheme 2). The solid state molecular structure of 7 revealed a square planar geometry at platinum (∑angles = 343.35°) and a Pt−C distance of 2.077 Å. Additionally, the Pt-CbackboneH carbon appeared as a singlet at 38.29 ppm showing Pt satellites (JCPt = 678.8 Hz) that indicate the formation of a bond between platinum and the backbone carbon of the supporting ligand. Analogous to 4, the new platinum carbene [{PC(sp2)P}tBuPt(PMe3)] (8) was synthesized in good yield through the reaction of 7 with PMe3

Scheme 2. Synthesis of 7 and 8

and KN(TMS)2. Unsurprisingly, the 31P{1H} NMR spectrum of 8 was nearly identical to that of 4, with a doublet at 54.625 ppm (JPPt = 2885.79 Hz, 2JPP = 20.55 Hz) and a triplet at −34.99 ppm (JPPt = 1698.59 Hz, 2JPP = 20.55 Hz). The 195Pt{1H} NMR spectrum features a triplet of doublets at −4999.8 ppm with phosphorus coupling of JPtP = 2888.35 Hz and JPtP = 1698.86 Hz. A resonance for a Cbackbone-H proton was absent in the 1H NMR spectrum, showing the deprotonation of the coordinating backbone carbon. The increased solubility of this carbene from the tBu groups on the ligand allowed for the carbene carbon resonance to be located in the corresponding 13C{1H} NMR spectrum as a doublet of triplets at 131.48 ppm, with JCP values of 99.13 and 3.52 Hz for the trans coupling and cis coupling, respectively (Figure 5). The chemical shift of the carbene carbon is consistent with the reported value for that of the palladium analogue that appears at 136.06 ppm at room D

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carbene [{PC(sp2)P}HPtI] (9). Compound 9 was found to be paramagnetic, resulting in silent 1H and 31P{1H} NMR spectra. The Evans method was used to determine the effective magnetic moment, μeff = 1.27 μΒ. These results confirmed the paramagnetic nature of 9 and indicate that the carbene carbon undergoes a one-electron oxidation when treated with I2. EPR spectroscopy corroborated the existence of a ligand centered radical with the appearance of a resonance at g = 2.0105. Additionally, the coupling to the active 195Pt nucleus was found to be 92 G, similar to a previously reported platinum complex that contains an unpaired electron on the ligand, rather than a platinum centered radical (Figure 7).51 Moreover, the spin

Figure 5. Carbene region of the 13C{1H} NMR (126 MHz, 20 °C) spectrum for 8.

temperature,45 and is significantly shifted upfield compared to that of the previously reported trigonal planar, 3-coordinate carbene complex [(PterP)PtC(p-tol)2] (262.8 ppm, JCP = 66 Hz).34 The solid state molecular structure of 8 (Figure 6)

Figure 6. Thermal ellipsoid (50% probability level) representation of [{PC(sp2)P}tBuPt(PMe3)] (8). Hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (deg): Pt−C 2.070(4); Pt−P(1) 2.2912(9); Pt−P(2) 2.2822(9); Pt−P(3) 2.3107(10); C−Pt−P(2) 81.87(11); P(2)−Pt−P(1) 163.74(3); P(2)−Pt−P(3) 95.56(4); C− Pt−P(1) 82.52(10); C−Pt−P(3) 177.37(11); P(1)−Pt−P(3) 100.08(3).

indicated a Pt−C distance of 2.070(4) Å, and a square planar geometry at platinum (∑angles = 341.1°). The Pt−C distance is slightly longer than previously reported values of Pt−C distances in NHC carbenes (∼2.0 Å)46−48 and in heteroatom-stabilized platinum carbenes (1.95−2.0 Å),29,49,50 suggesting a single bond character (Figure 6).

Figure 7. Top: EPR spectrum (298 K, 100 mM solution in toluene, Xband). The blue line represents the experimental data and the red line the simulated spectrum. Bottom: calculated spin density for 9.

In order to compare the reactivity of the palladium and platinum carbene complexes, 4 was oxidized with 0.5 equiv of I2 in toluene at room temperature (eq 2), yielding the radical

density calculated using DFT methods indicates that the unpaired electron is ligand-based, localized mostly on the carbene carbon atom and slightly delocalized over the two phenyl rings (Figure 7). The bond order between this carbon atom and platinum should increase; during the oxidation process, one electron was removed from the π C-Pt antibonding orbital. This is corroborated by the distance between the two atoms observed through X-ray spectroscopy (vide infra). Interestingly, when treated with I2, [{PC(sp2)P}HPd(PMe3)] is not isolated as a monomeric radical species but, rather, E

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Organometallics undergoes a Gomberg type dimerization,52−54 which couples the radical in the para position of the phenyl ring with the radical in the ligand backbone of another molecule. This dimerization could be circumvented through substitution in the para position of the ligand. However, 9 does not dimerize under ambient conditions neither in solution nor in the solid state, as supported by the paramagnetic nature of its NMR spectra and its solid state molecular structure (Figure 8). The latter revealed a Pt−C

and are interesting additions to the small family of nonheteroatom stabilized transition metal carbenes. In particular, this is the second non-heteroatom stabilized platinum carbene to be structurally characterized. A one-electron oxidation of the platinum carbene resulted in a persistent radical, which did not undergo dimerization as observed in the palladium analogue. Metric and spectroscopic data (Table 1) reveal the effect of geometry and coordination number on the nature of the Pt−C π bonding in these platinum carbene complexes.

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EXPERIMENTAL SECTION

General Remarks. Unless otherwise noted, experiments were performed under a N2 atmosphere using glovebox techniques. Solvents were dried by passing through a column of activated alumina and were stored under N2. All commercial chemicals were used as received. [Pt(COD)Cl2] was purchased from Sigma-Aldrich. Deuterated solvents were obtained from Cambridge Isotope Laboratories. THFd8 was dried over sodium, while C6D6 was dried by refluxing over dry CaH2 and filtered prior to use. For microwave reactions, a Discover/ Explorer12 CEM microwave reactor was used with the pressure set to the maximum (300 psi), the power set to the maximum (300 W), and the temperature set to 80 °C. NMR spectra were recorded on a Bruker 400 or 500. Chemical shifts are reported in ppm relative to residual internal deuterated solvent for 1H and 13C{1H} NMR spectra, and to external H3PO4 for 31P{H} spectra. For 195Pt{H} NMR spectra, shifts are reported relative to Na2[PtCl4] (δ = −1616 ppm);56 J values are given in Hz. All assignments are based on one-dimensional 1H and 13 C{1H}, 31P{1H}, and 195Pt{H} experiments unless otherwise noted. Compounds 2 and 5 were synthesized according to literature procedures.23,44 Magnetic moments were determined by the Evans method using capillaries containing hexamethylsiloxane in C6D6 as a reference and hexamethylsiloxane in the sample solution.57−59 EPR spectra were recorded on a Bruker EMXplus EPR spectrometer with a standard X-band EMXplus resonator and an EMX premiumX microwave bridge. CHN analyses were performed on a CE-440 elemental analyzer or by Midwest Microlab, LLC. Gaussian 03 (revision D.02) was used for all reported calculations.60 The B3LYP (DFT) method was used to carry out the geometry optimizations on model compounds specified in the text using the LANL2DZ basis set. The validity of the true minima was checked by the absence of negative frequencies in the energy Hessian. CIFs were deposited with the CCDC, identifiers 1879691 (2· 2CH2Cl2), 1879692 (3), 1879693 (4), 1879694 (6), 1879695 (7), 1879696 (8·Et2O), and 1879697 (9). Synthesis of [{PC(sp3)H2P}HPtCl2] (2). In a 20 mL scintillation vial, a solution of [PC(sp3)H2P]H (102.7 mg, 0.2564 mmol) in 2 mL of THF was added to a slurry of [Pt(COD)Cl2] (92.6 mg, 0.247 mmol) in 2 mL of THF at room temperature. The reaction mixture was stirred for 30 min. The volatiles were removed under a reduced pressure and the residue was washed with n-pentane to give a solid as a white powder. Yield: 163.7 mg, 0.246 mmol, 99%. Colorless single crystals were obtained from diethyl ether at room temperature. 1H NMR (400 MHz, CDCl3): δ 7.51−7.46 (m, 4H, ArH), 7.39 (t, J = 7.5 Hz, 2H, ArH, 7.30

Figure 8. Thermal ellipsoid (50% probability level) representation of [{PC(sp2)P}HPtI] (9). Hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (deg): Pt(1)−C(1) 2.000(5); Pt(1)−P(1) 2.2805(9); Pt(1)−P(1)# 2.2804(9); Pt(1)−I(1) 2.6783(4); C(1)−Pt(1)−P(1)# 83.84(2); P(1)#−Pt(1)−P(1) 167.68(5); P(1)#1−Pt(1)−I(1) 96.16(2); C(1)−Pt(1)−P(1) 83.84(2); C(1)−Pt(1)−I(1) 180.0; P(1)−Pt(1)−I(1) 96.16(2).

distance of 2.000(5) Å and a square planar geometry at platinum (∑angles = 347.68°). For the analogous Pd species, the dimerization was observed for the iodo species but not for the corresponding chloro and bromo radicals.55 This was explained by the small difference in energy between the monomeric and the dimeric species. A Gomberg type dimerization is unfavorable in the case of the iodo species for the platinum radical carbene even without blocking the para position.

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CONCLUSIONS The synthesis of [{PC(sp2)P}HPt(PMe3)] (4) completes the series of group 10 metal carbenes supported by the [PC(sp2)P]H ligand. Though thermally stable to 120 °C, its insolubility required the synthesis of the analogous [{PC(sp2)P}tBuPt(PMe3)] (8) carbene for characterization purposes. Both diamagnetic platinum carbene complexes, 4 and 8, were structurally characterized by X-ray diffraction crystallography

Table 1. Comparison between Relevant Data for Complexes 7−9 and Previously Reported Compounds compound

13

195

C NMR shift (ppm)

Pt NMR shift (ppm)

M−Ccarbene distance (Å)

orbital population (MC π and π*)

−4226.3 (t, JPtP = 2990.7 Hz) −4999.8 (td, JPtP = 2888.3 Hz, JPtP = 1698.8 Hz) N/A

2.077(3) 2.070(4)

N/A π: 2e−, π*: 2e−, bond order: 1

9

38.29 (s) 131.48 (td, JCPtrans = 99.13, JCPcis = 3.50 Hz) N/A

2.000(5)

[{PC(sp2)P}tBuPd(PMe3)]44 [{PC(sp2)P}tBuPdI]45

136.06 (d, JCPtrans = 103.7 Hz) N/A

N/A N/A

2.076(3) 2.022(3)

[(PterP)PtC(p-tol)2]34

262.8 ppm (t, JCP = 66 Hz)

−3702.9 (t, JPtP = 2234.0 Hz)

1.942(3)

π: 2e−, π*: 1e−, bond order: 1.5 π: 2e−, π*: 2e−, bond order: 1 π: 2e−, π*: 1e−, bond order: 1.5 π: 2e−, π*: 0e−, bond order: 2

7 8

F

DOI: 10.1021/acs.organomet.8b00864 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics (t, J = 7.55 Hz, 2H, ArH), 7.27−7.24 (d, J = 14.6 Hz, 1H, CH2), 3.94 (d, J = 14.6 Hz, 1H, CH2), 3.86−3.77 (br m, 2H, CH(CH3)2), 1.85 (br s, 2H, CH(CH3)2), 1.49 (app. q, J = 7.0 Hz, 6H, CH(CH3)2), 1.36 (br s, 6H, CH(CH3)2), 1.24 (dd, J = 16.8 Hz, 6.7 Hz, 6H, CH(CH3)2), 0.82 (br s, 6H, CH(CH3)2). 13C{1H} NMR (101 MHz, CDCl3): δ 144.16 (br s, ArC), 133.26 (d, JCP = 8.4 Hz, ArC), 131.40 (d, JCPt = 25.8 Hz, JCP = 2.3 Hz, ArC), 131.10 (d, JCP = 1.8 Hz, ArC), 126.64 (d, JCP = 7.3 Hz, ArC), 43.77 (t, JCP = 8.1 Hz, CH(CH3)2), 26.72 (d, JCP = 38.8 Hz, CH(CH3)2), 24.71 (d, JCP = 38.8 Hz, CH(CH3)2), 21.18 (br s, CH(CH3)2), 20.92 (d, JCP = 2.6 Hz, CH(CH3)2), 20.48 (br s, CH(CH3)2). 31P{1H} NMR (162 MHz, CDCl3, 25 °C): δ 16.20 (s, JPPt = 3647.78 Hz). 31P{1H} NMR (162 MHz, CDCl3, 65 °C): δ 16.41 (s, JPPt = 3574.1 Hz). 195Pt{1H} NMR (86 MHz, CDCl3, 65 °C): δ −4175.86 (s, JPtP = 3595.3 Hz). Anal. Calcd for C25H38P2Cl2Pt: C, 45.05; H, 5.75. Found: C, 45.12; H, 5.84. Synthesis of [{PC(sp3)HP}HPtCl] (3). In a 10 mL microwave vial, a solution of KN(TMS)2 (22.0 mg, 0.1103 mmol) in 2 mL of THF was added to a slurry of [{PC(sp3)H2P}HPtCl2] (73.5 mg, 0.1103 mmol) in 2 mL of THF. The reaction mixture was heated at 80 °C for 60 min in a microwave reactor. The final mixture was filtered through a plug of Celite to remove impurities. The volatiles were removed under a reduced pressure and the residue was washed with n-pentane to give the solid as a light-yellow product. Yield: 56.2 mg, 0.0892 mmol, 80.9%. Colorless single crystals were obtained through recrystallization from npentane at −35 °C. 1H NMR (400 MHz, C6D6): δ 7.40 (d, J = 7.8 Hz, 2H, ArH), 7.1−7.06 (m, 2H, ArH), 7.02 (t, J = 7.4 Hz, 2H, ArH), 6.88 (t, J = 7.4 Hz, 2H, ArH), 5.85 (d, J = 145.8 Hz, 1H, CbackboneH), 2.78− 2.70 (m, 2H, CH(CH3)2), 2.49−2.40 (m, 2H, CH(CH3)2), 1.42 (app. q, J = 7.8 Hz, 6H, CH(CH3)2), 1.36 (app. q, J = 7.9 Hz, 6H, CH(CH3)2), 1.06 (dd, J = 15.0, 7.4 Hz, 2H, CH(CH3)2), 0.99 (dd, J = 14.9, 7.3 Hz, 2H, CH(CH3)2). 13C{1H} NMR (101 MHz, C6D6): δ 159.59 (t, JCP = 14.3 Hz, ArC), 134.54 (t, JCP = 22.0 Hz, ArC), 131.87 (t, JCP = 16.9 Hz, ArC), 130.01 (s, ArC), 127.11 (t, JCP = 7.2 Hz, ArC), 125.32 (t, J = 3.6 Hz, ArC), 39.51 (s, JCPt = 681.6 Hz, Pt-Cbackbone), 26.13 (t, JCP = 13.6 Hz, CH(CH3)2), 24.99 (t, JCP = 15.6 Hz, CH(CH3)2), 18.74 (t, JCP = 2.1 Hz, CH(CH3)2), 18.21 (s, CH(CH3)2), 18.10 (s, CH(CH3)2), 17.92 (t, JCP = 10.5 Hz, CH(CH3)2). 31P{1H} NMR (162 MHz, C6D6): δ 50.21 (s, JPPt = 2976.5 Hz). 195Pt{1H} NMR (86 MHz, C6D6): δ −4221.88 (t, JPtP = 2981.2 Hz). Anal. Calcd for C25H37P2ClPt: C, 47.66; H, 5.92. Found: C, 47.93; H, 5.71. Synthesis of [{PC(sp2)P}HPt(PMe3)] (4). In a 20 mL scintillation vial, a solution of KN(TMS)2 (17.8 mg, 0.0892 mmol) in 2 mL of toluene was added to a solution of [{PC(sp3)HP}HPtCl] (56.2 mg, 0.0892 mmol) in 2 mL of toluene. 1.5 equiv of PMe3 in toluene (1.01 M, 134 μL, 0.134 mmol) was added to the reaction vial. An immediate color change to black was observed. The reaction mixture was stirred for 5 min. The volatiles were removed under a reduced pressure. The crude product was dissolved in toluene and filtered through a plug of Celite to remove impurities. The solid was either recrystallized in toluene layered with pentane or through a trituration of n-pentane with toluene. Yield: 38.1 mg, 0.0569 mmol, 63.8%. 1H NMR (400 MHz, Toluene-d8): δ 8.05 (s, 2H, ArH), 6.84 (s, 2H, ArH), 6.76 (s, 2H, ArH), 6.20 (s, 2H, ArH), 2.31−2.23 (m, 4H, CH(CH3)2), 1.20 (d, J = 7.3 Hz, 9H, P(CH)3), 1.16 (app. q, J = 7.08 Hz, 12H, CH(CH3)2), 1.08 (app. q, J = 8.29 Hz, 12H, CH(CH3)2). 31P{1H} NMR (162 MHz, Toluene-d8): δ 54.72 (d, JPPt = 2870.3 Hz, JPP = 20.45 Hz), −35.16 (t, JPPt = 1691.83 Hz, JPP = 20.56 Hz). Anal. Calcd for C28H45P3Pt: C, 50.22; H, 6.77. Found: C, 50.28; H, 6.93. Reaction of [{PC(sp2)P}HPt(PMe3)] (4) with HCl. In a 20 mL scintillation vial, HCl (1.0 M, 220 μL, 0.220 mmol) in diethyl ether was added to [{PC(sp2)P}HPt(PMe3)] (76 mg, 0.1135 mmol) in 2 mL of toluene. The reaction was stirred for 30 min at room temperature. The volatiles were removed under a reduced pressure and the residue was washed with n-pentane to give [{PC(sp3)HP}HPtCl] (3) as a lightyellow product. Yield: 38.2 mg, 0.0606 mmol, 53.4%. Synthesis of [{PC(sp3)H2P}tBuPtCl2] (6). In a 20 mL scintillation vial, a solution of [PC(sp3)H2P]tBu (116.6 mg, 0.227 mmol) in 2 mL of THF was added to a slurry of [Pt(COD)Cl2] (77 mg, 0.206 mmol) in 2 mL of THF at room temperature. The reaction mixture was stirred for 2 h. The volatiles were removed under a reduced pressure and the residue

was washed with n-pentane to give the solid as a white powder. Yield: 145.6 mg, 0.187 mmol, 90%. 1H NMR (400 MHz, CDCl3, 65 °C): δ 7.5 (d, J = 8.7 Hz, 2H, ArH), 7.37 (s, 4H, ArH), 7.18 (d, J = 14.3 Hz, 1H, CH2), 3.87 (d, J = 14.5 Hz, 2H, CH2), 3.83−3.70 (br m, 2H, CH(CH3)2), 1.82 (br s, 2H, CH(CH3)2), 1.55−1.50 (m, 6H, CH(CH3)2), 1.40−1.34 (m, 6H, CH(CH3)2), 1.30 (s, 18H, C(CH3)3), 1.27−1.25 (m, 6H, CH(CH3)2), 0.83 (br s, 6H, CH(CH3)2). 13C{1H} NMR (101 MHz, CDCl3): δ 149.29 (d, JCP = 6.2 Hz, ArC), 141.52 (d, JCP = 9.6 Hz, ArC), 132.84 (d, JCP = 8.9 Hz, ArC), 128.53 (s, JCPt = 26.5 Hz, ArC), 127.80 (s, ArC), 42.58 (t, JCP = 7.9 Hz, CH(CH3)2), 34.75 (s, CH2), 31.33 (s, CH(CH3)3), 31.10 (s, CH(CH3)3), 26.58 (d, JCP = 38.9 Hz, CH(CH3)2), 24.90 (d, JCP = 31.6 Hz, CH(CH3)2), 21.12 (d, JCP = 23.5 Hz, CH(CH3)2), 20.48 (s, CH(CH3)2). 31P{1H} NMR (162 MHz, CDCl3, 25 °C): δ 16.36 (br s, JPPt = 3671.62 Hz). 31P{1H} NMR (162 MHz, CDCl3, 65 °C): δ 16.82 (s, JPPt = 3577.09 Hz). 195Pt{1H} NMR (86 MHz, CDCl3, 65 °C): δ −4174.23 (t, JPtP = 3569.48 Hz). Anal. Calcd for C33H54P2Cl2Pt: C, 50.90; H, 6.99. Found: C, 50.21; H, 6.58. Synthesis of [{PC(sp3)HP}tBuPtCl] (7). In a 10 mL microwave vial, a solution of KN(TMS)2 (18.9 mg, 0.0947 mmol) in 2 mL of THF was added to a slurry of [{PC(sp3)H2P}PtCl2] (73.9 mg, 0.0949 mmol) in 2 mL of THF. The reaction mixture was heated to 80 °C for 60 min in a microwave reactor. The reaction solution was filtered through a plug of Celite to remove impurities. The volatiles were removed under a reduced pressure and the residue was washed with n-pentane to give the solid as a light yellow-orange product. Yield: 50.9 mg, 0.0685 mmol, 72.3%. 1H NMR (400 MHz, C6D6): δ 7.50 (d, J = 8.13 Hz, 2H, ArH), 7.41−7.38 (m, 2H, ArH), 7.18 (d, J = 8.22 Hz, 2H, ArH), 5.87 (s, JHPt = 146.01 Hz, 1H, CbackboneH), 2.93−2.86 (m, 2H, CH(CH3)2), 2.59− 2.51 (m, 2H, CH(CH3)2), 1.47 (app. q, J = 8.05 Hz, 6H, CH(CH3)2), 1.41 (app. q, J = 8.11, 7.73 Hz, 6H, CH(CH3)2), 1.21 (s, 18H, C(CH3)3), 1.16 (app. q, J = 7.57 Hz, 6H, CH(CH3)2), 1.08 (app. q, J = 7.55 Hz, 6H, CH(CH3)2). 13C{1H} NMR (101 MHz, C6D6): δ 157.00 (t, JCP = 14.3 Hz, ArC), 147.98 (t, JCP = 3.3 Hz, ArC), 134.23 (t, JCP = 21.8 Hz, ArC), 128.28 (t, JCP = 16.9 Hz, ArC), 127.19 (s, ArC), 126.70 (t, JCP = 7.5 Hz, ArC), 38.29 (s, JCPt = 678.8 Hz, Pt-Cbackbone), 31.36 (s, C(CH3)3) 26.21 (t, JCP = 13.5 Hz, CH(CH3)2), 24.98 (t, JCP = 15.5 Hz, CH(CH3)2), 22.57 (s, C(CH3)3), 18.80 (s, JCP = 2.1 Hz, CH(CH3)2), 18.38 (s, CH(CH3)2), 18.23 (s, CH(CH3)2), 17.98 (t, JCP = 11.5 Hz, CH(CH3)2). 31P{1H} NMR (162 MHz, C6D6): δ 50.52 (s, JPPt = 2990.26 Hz). 195Pt{1H} NMR (86 MHz, C6D6): δ −4226.31 (t, JPtP = 2990.7 Hz). Anal. Calcd for C33H53P2ClPt: C, 53.40; H, 7.20. Found: C, 53.62; H, 7.25. Synthesis of [{PC(sp2)P}tBuPtP(Me3)] (8). In a 20 mL scintillation vial, a solution of [{PC(sp3)HP}tBuPtCl] (56.4 mg, 0.076 mmol) in 2 mL of toluene was added to a solution of KN(TMS)2 (18.4 mg, 0.091 mmol) in 2 mL of toluene. 1.5 equiv of PMe3 (1.01 M, 113 μL, 0.114 mmol) in toluene was added to the mixture. The solution was stirred for 30 min. The volatiles were removed under a reduced pressure and the product was redissolved in toluene and filtered through a plug of Celite to remove impurities. The product was either recrystallized with npentane layered on toluene or through a trituration with n-pentane/ toluene. Yield: 42.7 mg, 0.054 mmol, 71.6%. 1H NMR (500 MHz, toluene-d8): δ 7.93 (d, J = 8.83 Hz, 2H, ArH), 6.89−6.85 (m, 4H, ArH), 2.43−2.34 (m, 4H, CH(CH3)2), 1.39 (s, 18H, C(CH3)3), 1.26 (app. q, J = 6.78 Hz, 12H, CH(CH3)2), 1.25 (d, J = 7.24 Hz, 9H, P(CH3)3), 1.12 (app. q, J = 8.76 Hz, 12H, CH(CH3)2). 13C{1H} NMR (126 MHz, toluene-d8): δ 164.16 (td, JCP = 17.44, 4.35 Hz, JCP = 82.49 Hz, ArC), 133.08 (t, JCP = 3.97 Hz, ArC), 131.48 (dt, JCP = 99.13, 3.50 Hz, PtCcarbene), 128.94 (s, ArC), 128.45 (s, ArC), 116.92 (td, JCP = 10.09, 5.57 Hz, ArC), 116.75 (td, JCP = 27.32, 11.32 Hz, ArC), 34.36 (s, CH(CH3)2), 33.47 (s, C(CH3)3), 27.51 (t, J = 16.22 Hz, CH(CH3)2), 22.63 (s, C(CH3)3), 21.30 (t, J = 1.88 Hz,, CH(CH3)2), 21.09 (t, J = 1.85 Hz, CH(CH3)2), 20.61 (s, CH(CH3)2), 18.77 (t, J = 11.5 Hz, CH(CH3)2). 31P{1H} NMR (162 MHz, Toluene-d8): δ 54.625 (d, JPPt = 2885.79 Hz, JPP = 20.55 Hz), −34.99 (t, JPPt = 1698.59 Hz, JPP = 20.55 Hz). 195Pt{1H} NMR (86 MHz, Toluene-d8): δ −4999.8 (td, JPtP = 2888.35 Hz, JPtP = 1698.86 Hz). Anal. Calcd for C36H61P3Pt: C, 55.30; H, 7.86. Found: C, 55.19; H, 8.38. Synthesis of [{PC(sp2)P}HPtI] (9). In a 20 mL scintillation vial, half an equivalent of I2 (12 mg, 0.047 mmol) in 2 mL of toluene was added G

DOI: 10.1021/acs.organomet.8b00864 Organometallics XXXX, XXX, XXX−XXX

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Organometallics to [{PC(sp2)P}HPt(PMe3)] (60 mg, 0.0896 mmol) in 2 mL of toluene. The reaction was stirred for 30 min. A solid dark red product of [{PC(sp2)P}HPtI] (9) was isolated after a n-pentane wash and recrystallization in ether at −35 °C. Yield: 30.2 mg, 0.419 mmol, 46.7%. Data for 9 are as follows. Anal. Calcd for C25H36IP2Pt: C, 41.68; H, 5.04. Found: C, 41.38; H, 5.12. Evans: μeff = 1.27. EPR (X-band, 9.4858 GHz): g = 2.0105, 195Pt splitting = 92 G.

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(8) Schrock, R. R. Multiple Metal−Carbon Bonds for Catalytic Metathesis Reactions (Nobel Lecture). Angew. Chem., Int. Ed. 2006, 45 (23), 3748−3759. (9) Silbestri, G. F.; Flores, J. C.; de Jesús, E. Water-Soluble NHeterocyclic Carbene Platinum(0) Complexes: Recyclable Catalysts for the Hydrosilylation of Alkynes in Water at Room Temperature. Organometallics 2012, 31 (8), 3355−3360. (10) Fürstner, A.; Davies, P. W. Catalytic Carbophilic Activation: Catalysis by Platinum and Gold π Acids. Angew. Chem., Int. Ed. 2007, 46 (19), 3410−3449. (11) Fü rstner, A.; Stelzer, F.; Szillat, H. Platinum-Catalyzed Cycloisomerization Reactions of Enynes. J. Am. Chem. Soc. 2001, 123 (48), 11863−11869. (12) Miyaura, N.; Suzuki, A. Palladium-Catalyzed Cross-Coupling Reactions of Organoboron Compounds. Chem. Rev. 1995, 95 (7), 2457−2483. (13) Zhang, C.; Huang, J.; Trudell, M. L.; Nolan, S. P. Palladium− Imidazol-2-ylidene Complexes as Catalysts for Facile and Efficient Suzuki Cross-Coupling Reactions of Aryl Chlorides with Arylboronic Acids. J. Org. Chem. 1999, 64 (11), 3804−3805. (14) Xia, Y.; Xia, Y.; Zhang, Y.; Wang, J. Palladium-catalyzed coupling of N-tosylhydrazones and β-bromostyrene derivatives: new approach to 2H-chromenes. Org. Biomol. Chem. 2014, 12 (46), 9333−9336. (15) Liu, Z.; Wang, J. Cross-Coupling Reactions Involving Metal Carbene: From C=C/C−C Bond Formation to C−H Bond Functionalization. J. Org. Chem. 2013, 78 (20), 10024−10030. (16) Zhang, Y.; Wang, J. Recent Developments in Pd-Catalyzed Reactions of Diazo Compounds. Eur. J. Org. Chem. 2011, 2011 (6), 1015−1026. (17) Xiao, Q.; Zhang, Y.; Wang, J. Diazo Compounds and NTosylhydrazones: Novel Cross-Coupling Partners in Transition-MetalCatalyzed Reactions. Acc. Chem. Res. 2013, 46 (2), 236−247. (18) Schuster, O.; Yang, L.; Raubenheimer, H. G.; Albrecht, M. Beyond Conventional N-Heterocyclic Carbenes: Abnormal, Remote, and Other Classes of NHC Ligands with Reduced Heteroatom Stabilization. Chem. Rev. 2009, 109 (8), 3445−3478. (19) Hahn, F. E.; Jahnke, M. C. Heterocyclic Carbenes: Synthesis and Coordination Chemistry. Angew. Chem., Int. Ed. 2008, 47 (17), 3122− 3172. (20) Peloso, R.; Carmona, E. Non-heteroatom-substituted alkylidene complexes of groups 10 and 11. Coord. Chem. Rev. 2018, 355, 116−132. (21) Mindiola, D. J.; Hillhouse, G. L. Synthesis, Structure, and Reactions of a Three-Coordinate Nickel-Carbene Complex, {1,2Bis(di-tert-butylphosphino)ethane}NiCPh2. J. Am. Chem. Soc. 2002, 124 (34), 9976−9977. (22) Harrold, N. D.; Waterman, R.; Hillhouse, G. L.; Cundari, T. R. Group-Transfer Reactions of Nickel−Carbene and −Nitrene Complexes with Organoazides and Nitrous Oxide that Form New C=N, C=O, and N=N Bonds. J. Am. Chem. Soc. 2009, 131 (36), 12872− 12873. (23) Burford, R. J.; Piers, W. E.; Parvez, M. beta-Elimination-Immune PCcarbeneP Iridium Complexes via Double C-H Activation: LigandMetal Cooperation in Hydrogen Activation. Organometallics 2012, 31 (8), 2949−2952. (24) Burford, R. J.; Piers, W. E.; Parvez, M. Acetonitrile Coupling at an Electron-Rich Iridium Center Supported by a PCsp2P Pincer Ligand. Eur. J. Inorg. Chem. 2013, 2013 (22−23), 3826−3830. (25) Doyle, L. E.; Piers, W. E.; Borau-Garcia, J. Ligand Cooperation in the Formal Hydrogenation of N2O Using a PCsp2P Iridium Pincer Complex. J. Am. Chem. Soc. 2015, 137 (6), 2187−2190. (26) Logan, J. R.; Piers, W. E.; Borau-Garcia, J.; Spasyuk, D. M. Ligand Attachment Chemistry in the Preparation of PCsp3P and PCsp2P Complexes of Rhodium. Organometallics 2016, 35 (9), 1279−1286. (27) Sung, S.; Wang, Q.; Krämer, T.; Young, R. D. Synthesis and reactivity of a PCcarbeneP cobalt(i) complex: the missing link in the cobalt PXP pincer series (X = B, C, N). Chem. Sci. 2018, 9 (43), 8234− 8241.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00864. NMR spectra for compounds 2−4, 6−8, X-ray crystallographic data, and computational data (PDF) Cartesian coordinates (XYZ) Accession Codes

CCDC 1879691−1879697 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Vlad M. Iluc: 0000-0001-6880-2470 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported through the 2018 Vincent P. Slatt Fellowship for Undergraduate Research in Energy Systems and Processes and the 2018 College of Science Summer Undergraduate Research Fellowship to A.P.D. This work was also supported by the National Science Foundation (NSF), CAREER grant CHE-1552397 to V.M.I. We are grateful to Dr. Allen Oliver for crystallographic assistance.

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REFERENCES

(1) de Frémont, P.; Marion, N.; Nolan, S. P. Carbenes: Synthesis, properties, and organometallic chemistry. Coord. Chem. Rev. 2009, 253 (7), 862−892. (2) Dötz, K. H.; Stendel, J. Fischer Carbene Complexes in Organic Synthesis: Metal-Assisted and Metal-Templated Reactions. Chem. Rev. 2009, 109 (8), 3227−3274. (3) Montgomery, C. D. Fischer and Schrock Carbene Complexes: A Molecular Modeling Exercise. J. Chem. Educ. 2015, 92 (10), 1653− 1660. (4) Gutsulyak, D. V.; Piers, W. E.; Borau-Garcia, J.; Parvez, M. Activation of Water, Ammonia, and Other Small Molecules by PCcarbeneP Nickel Pincer Complexes. J. Am. Chem. Soc. 2013, 135 (32), 11776−11779. (5) Comanescu, C. C.; Iluc, V. M. Synthesis and Reactivity of a Nucleophilic Palladium(II) Carbene. Organometallics 2014, 33 (21), 6059−6064. (6) Grubbs, R. H. Olefin-Metathesis Catalysts for the Preparation of Molecules and Materials (Nobel Lecture). Angew. Chem., Int. Ed. 2006, 45 (23), 3760−3765. (7) Chauvin, Y. Olefin Metathesis: The Early Days (Nobel Lecture). Angew. Chem., Int. Ed. 2006, 45 (23), 3740−3747. H

DOI: 10.1021/acs.organomet.8b00864 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.8b00864 Organometallics XXXX, XXX, XXX−XXX