Heterobimetallic Complexes with Polar, Unsupported Cu–Fe and Zn

Article pubs.acs.org/Organometallics

Heterobimetallic Complexes with Polar, Unsupported Cu−Fe and Zn−Fe Bonds Stabilized by N‑Heterocyclic Carbenes Upul Jayarathne, Thomas J. Mazzacano, Sharareh Bagherzadeh, and Neal P. Mankad* Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607, United States S Supporting Information *

ABSTRACT: Heterobimetallic complexes of the formulations (NHC)Cu−FeCp(CO)2 (NHC = IPr, IMes, SIMes), (IPr)Cu− MoCp(CO)3, and (IPr)(Cl)Zn−FeCp(CO)2 were synthesized in high yield from readily available starting materials and characterized crystallographically. The solid-state structures of the Cu−Fe systems reveal close, secondary interactions between Cu and one CO ligand from the [FeCp(CO)2] unit that are absent in the Zn−Fe analogue. The heterobimetallic complexes feature short yet polar Cu−Fe, Cu−Mo, and Zn−Fe bonds in which the electrophilic metal (Cu, Zn) is later in the transition series than the nucleophilic metal (Fe, Mo), thereby subverting the more common early−late heterobimetallic paradigm. DFT analyses were used to assess M−M′ bond polarity and examine effects on M−M′ bonding of systematic modifications to both the nucleophilic and electrophilic fragments. Experimental confirmation of Cu−Fe bond polarity was obtained by analysis of product mixtures resulting from the reactions between (NHC)Cu−FeCp(CO)2 complexes and MeI, which produced (NHC)Cu−I and Me−FeCp(CO)2 products.

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INTRODUCTION

N-Heterocyclic carbenes (NHCs) are known to stabilize electrophilic, late-metal monometallic complexes.8 We sought to extend this ability to heterobimetallic complexes.9 One particularly convenient method for the construction of early− late heterobimetallics is the combination of nucleophilic metal fragments such as the Fp− anion (Fp = FeCp(CO)2) with cationic early-transition-metal fragments to form unsupported M−M′ bonds.1c,10 However, this strategy has not been applied extensively to late-metal electrophiles. Here we report that this strategy is competent for constructing polar, unsupported Cu− M (M = Fe, Mo) and Zn−Fe bonds stabilized by NHCs. We further demonstrate that the Cu−M and Zn−Fe bonds are polarized in such a way that places electrophilic character on the later, more electronegative, higher d-electron count metal of the pair using both DFT calculations and preliminary stoichiometric reactivity studies with MeI. We also explore the effects of systematic tuning within both the nucleophilic and electrophilic fragments on the nature of metal−metal bonding. Though similar unsupported metal−metal bond connections have been known for some time,11 the current complexes collectively advance knowledge of structure and bonding properties in heterobimetallic complexes by allowing for systematic study of the effects of such variations on the bimetallic cores.

Heterobimetallic complexes with polar metal−metal bonds are often capable of either activating polar substrates or polarizing apolar substrates in various fascinating stoichiometric transformations.1,2 It has long been a goal of organometallic chemists to harness such metal−metal cooperativity for catalytic applications. However, heterobimetallic cooperativity in homogeneous catalysis remains underdeveloped, especially in comparison to the increasing importance to catalysis of homobimetallic cooperativity3 as well as nucleophile/electrophile cooperativity with main-group frustrated Lewis pairs.4 The vast majority of heterobimetallic, metal−metal-bonded complexes fall into the so-called “early−late” regime, wherein their polar metal−metal bonds are composed of electrophilic, early d-block elements (electropositive, low d-electron count) bound to nucleophilic, late d-block elements (electronegative, high d-electron count).1 These early−late heterobimetallic complexes are well understood with respect to structure, bonding, and reactivity, but they have not been exploited to develop any catalytic reactions that are unavailable to their monometallic counterparts. One possible drawback of the early−late heterobimetallic design strategy is the use of early transition metals as electrophiles, since catalysis may be hampered in many cases by formation of thermodynamically robust metal−heteroatom bonds (M−O, M−N, M−Cl, etc.) in potential intermediates. By comparison, polar metal−metal bonds featuring late-metal electrophiles are less common5,6 and are only beginning to be studied systematically in terms of their bonding and reactivity.7 © XXXX American Chemical Society

Received: May 25, 2013

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dx.doi.org/10.1021/om400471u | Organometallics XXXX, XXX, XXX−XXX

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Scheme 1. Synthesis of NHC-Stabilized Heterobimetallic Complexes of Cu and Zn

Figure 1. Solid-state structures of (IPr)Cu-Fp (1a; left), (SIMes)Cu-Fp (1c; middle), and (IPr)(Cl)Zn-Fp (3; right) as 50% ellipsoids. Hydrogen atoms and cocrystallized solvent molecules have been omitted.

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RESULTS AND DISCUSSION Synthesis. Because NHC ligands are capable of stabilizing two-coordinate structures featuring group 11 metals,8 we were particularly interested in accessing heterobimetallic complexes featuring [(NHC)Cu]+ fragments. Though metal−metal bonds featuring two-coordinate metals are common for heavy d10 elements such as Au(I), Cd(II), and Hg(II), they are rare for 3d metals, including Cu(I),12 and might be expected to exhibit unique properties. Salt-metathesis reactions between KFp and various (NHC)CuCl starting materials in THF solution produced the air-sensitive, yellow complexes (NHC)Cu-Fp in yields of 63−76% (Scheme 1; NHC = IPr (1a), IMes (1b), SIMes (1c)). A previous report indicates that (dmpe)Cu-Fp is unstable at room temperature;11b NHC ligands, therefore, apparently are better able to stabilize Cu-Fp bonding than phosphine ligands. Complexes 1a,b are stable indefinitely at room temperature and displayed stability in our hands for at least 24 h under thermolytic (80 °C in C6D6 solution) conditions. Because of our interest in understanding the nature of polar metal−metal bonding through systematic variation, we also explored the syntheses of related compounds in which either the nucleophile or electrophile had been modified from the baseline (NHC)Cu-Fp cases. For example, the salt-metathesis reaction between NaMp (Mp = MoCp(CO)3) and (IPr)CuCl in THF/DME solution produced (IPr)Cu-Mp (2) in 58% yield (Scheme 1). Complex 2 has been prepared previously by Nolan and co-workers from the condensation reaction between (IPr)CuOH and HMp.13 Similarly, the reaction between KFp and (IPr)ZnCl2(THF) in THF solution produced orange (IPr)(Cl)Zn-Fp (3) in 64% yield (Scheme 1). Complexes 1−3 have been conveniently characterized by 1H NMR spectrosco-

py. In addition, FT-IR spectroscopy is particularly useful for monitoring the positions of the characteristic CO vibrations within the Fp and Mp units. The CO vibrations for the Zn-Fp complex 3 were observed at higher energy by 30−40 cm−1 in comparison to those for its Cu-Fp analogue 1a. Structural Characterization. The solid-state structures of complexes 1a, 1c, and 3 are shown in Figure 1. 1a and 1c both feature modestly bent CNHC−Cu−Fe angles (1a, 170.16(7)°; 1c, 167.00(10)°). The Cu−Fe distances (1a, 2.3462(5) Å; 1c, 2.3514(7) Å) are close to the sum of Pyykkö’s single-bond covalent radii of Cu and Fe (2.28 Å)14 and are among the shortest Cu−Fe distances of any type in the Cambridge Structural Database (CSD). In fact, only (Ph2EtP)3Fe(μH)3Cu(PPh2Et) has a shorter Cu−Fe distance (2.319(1) Å), presumably due to the presence of multiple bridging hydride ligands.15 The median Cu−Fe distance in the CSD, 2.549 Å, is much longer than the Cu−Fe distances in 1a and 1c. Similarly, complex 3 features a very short Zn−Fe distance of 2.3714(4) Å that is similar to previous observations in Zn-Fp complexes.16 The solid-state structures of Cu-Fp complexes 1a and 1c each feature one short Cu···CO distance (2.423(3) and 2.439(4) Å, respectively), indicating a significant secondary interaction between the Cu center and one carbonyl C (Figure 1). In both cases, the Fe−C−O angles (177.2(2) and 176.6(3)°, respectively), Fe−C distances (1.735(3) and 1.727(4) Å, respectively), and C−O distances (1.169(3) and 1.174(4) Å, respectively) associated with these “bridging” CO ligands actually resemble terminal Fe−CO interactions. Furthermore, the CO stretching vibrations for 1a and 1c reside in the terminal CO regions rather than the bridging CO regions of their FT-IR spectra. Thus, we propose that the observed Cu···CO contacts do not resemble classical bridging B

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CO interactions but instead are nonclassical bridging CO interactions in which the CO ligand interacts much more strongly with the Fe center than with the Cu center. The solidstate structure of 2, reported by Nolan and co-workers,13 features close contacts between the Cu center and two of the three CO ligands within the Mp unit. In contrast, the solid-state structure of 3 lacks evidence for any close Zn···CO contacts (Figure 1), with Zn···CCO distances of 2.707(2) and 2.953(2) Å. Computational Analysis. The nature of metal−metal bonding was examined using DFT calculations at the BVP86 level with triple-ζ basis sets for the metal centers and 6311+G(d) for the light atoms. The N,N′-dimethyl-substituted NHC ligand IMe was utilized in all calculations to decrease computational time. In general, good agreement was found between experimentally determined values and calculated values both for bond metrics and for CO vibrational frequencies (Table 1). One notable exception is the optimized Table 1. Comparisons of Calculated and Observed Values for (NHC)M−M′Cp(CO)n Complexesa ν(CO)/cm−1

d(M−M′)/Å system

calcd

b

obsd

c

(NHC)Cu-Fp (NHC)Cu-Mp

2.330 2.588

2.3462(5) 2.5600(8)d

(NHC)(Cl)ZnFp [(NHC)ZnFp]+

2.451 2.356

calcd

b

Figure 2. Calculated frontier molecular orbital diagram for (IMe)CuFp with surfaces shown for orbitals that contribute to Cu−Fe bonding (BVP86/LANL2TZ(f)/6-311+G(d), 0.04 isocontours).

obsdc

2.3714(4)

1875, 1926 1785, 1812, 1926 1894, 1941

1849, 1914 1789, 1824, 1926 1888, 1944

N/A

1956, 1996

N/A

Table 2. Natural Population Analysis Charge Distribution for (IMe)M−M′Cp(CO)n Complexesa model complex (IMe)CuFp (IMe)CuMp (IMe)(Cl) Zn-Fp [(IMe)ZnFp]+

a

The basis sets are BVP86/LANL2TZ(f) for Cu, Fe, and Mo, LANL2TZ+ for Zn, LANL2DZ ECP for Cl, and 6-311+G(d) for C, H, N, and O. bCalculated for NHC = IMe. cObserved for NHC = IPr. d Reference 13.

structure of (IMe)Cu-Fp, which features two short Cu···CO contacts rather than one and therefore has the least accurate vibrational calculation. Summary comparisons are shown in Table 1, and more thorough comparisons between calculated data and observed values can be found in the Supporting Information. Preliminary examinations with the B3LYP functional indicated poorer correlation with experimental vibrational data than for BVP86. Both methods systematically overestimated bond distances involving Zn (see Table 1 and the Supporting Information). As an illustrative example of molecular orbital analysis, the calculated frontier orbital surfaces that contribute strongly to Cu−Fe bonding for (IMe)Cu-Fp are shown in Figure 2. The Cu−Fe single-bond formulation is supported by occupied orbitals of σ(Cu−Fe) and π*(Cu−Fe) character (HOMO-2 and HOMO/HOMO-1, respectively) along with an unoccupied orbital of σ*(Cu−Fe) character (LUMO+3). Also noteworthy is the significant σ(Cu−CCO) overlap evident in the HOMO-2 orbital (Figure 2), consistent with the observed attractive interactions between Cu and one or more carbonyl carbons. Two particularly effective orbital-based analyses that have been used previously to examine metal−metal bond polarity are the natural charge distribution obtained by natural population analysis (NPA) and the Wiberg bond order.1c,17 Summaries of these two analyses for the current systems are shown in Tables 2 and 3, respectively. The NPA of (IMe)Cu-Fp can be taken as a baseline case. The calculated charges for the Cu and Fe

q(M)b,c q(M′)b,d

q(IMeM)b,c q(M′Cp(CO)n)b,d

WBIe

0.39

−1.19

0.63

−0.63

0.39

0.45

−1.19

0.72

−0.72

0.28

1.01

−1.31

1.23

−0.59

0.37

1.05

−1.39

1.34

−0.34

0.55

a

The basis sets are BVP86/LANL2TZ(f) for Cu, Fe, and Mo, LANL2TZ+ for Zn, LANL2DZ ECP for Cl, and 6-311+G(d) for C, H, N, and O. bq = charge. cM = NHC-bound metal. dM′ = Cp-bound metal. eWBI = Wiberg bond index.

Table 3. Calculated Wiberg Bond Indices for (IMe)M− M′Cp(CO)n Complexesa WBIb M−M′c,d M−CNHCc M···COc

M′−COd

C−O

M−Clc

(IMe)CuFp

(IMe)CuMp

(IMe)(Cl)ZnFp

[(IMe)ZnFp]+

0.39 0.58 0.15 0.13 N/A 1.20 1.18 N/A 2.02 1.99 N/A N/A

0.28 0.59 0.24 0.24 0.02 1.33 1.33 1.29 1.92 1.92 2.06 N/A

0.37 0.36 0.08 0.09 N/A 1.20 1.20 N/A 2.02 2.02 N/A 0.50

0.55 0.35 0.08 0.09 N/A 1.13 1.15 N/A 2.12 2.10 N/A N/A

a

The basis sets are BVP86/LANL2TZ(f) for Cu, Fe, and Mo, LANL2TZ+ for Zn, LANL2DZ ECP for Cl, and 6-311+G(d) for C, H, N, and O. bWBI = Wiberg bond index. cM = NHC-bound metal. dM′ = Cp-bound metal. C

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centers (0.39 and −1.19 e, respectively) are clearly consistent with a Cu−Fe bond polarized in such a way that the Cu center has electrophilic character and the Fe center has nucleophilic character. For comparison, NPA of a recent phosphinoamide Cu/Fe heterobimetallic complex that was formulated as having a dative Fe→Cu bond had calculated natural charges of approximately 0.9 e on Fe and −0.1 e on Cu,18 consistent with the relative electronegativities of the two metals. Perhaps more informative than atomic charges are fragment charges; again, a partial positive charge (0.63 e) is localized on the (IMe)Cu fragment while a partial negative charge (−0.63 e) is localized on the Fp fragment. The Wiberg bond order for the Cu−Fe bond (0.39) is significantly less than unity, indicating that even though the Cu−Fe bond distance is very short, the bond does not have significant covalent character and is best viewed as a Fe→Cu dative bond having substantial ionic character. Small but significant bond orders also were found between the Cu center and each of the carbonyl carbon atoms engaging in close contact (Table 3). When the nucleophilic fragment is changed from Fp to Mp, the Cu−M′ bond becomes more polarized according to DFT calculations. The calculated partial positive charge on the (IMe) Cu fragment and partial negative charge on the M′Cp(CO)n fragment both increase in magnitude to 0.72 e, and the Wiberg bond order for the Cu−Mo bond decreases to 0.28. It may seem counterintuitive that the Cu−M′ bond polarization increases in this way when substituting the more electropositive Mo in place of Fe, but clearly the charge distribution is influenced by the increased number of electron-withdrawing CO ligands. An even greater metal−metal bond polarization is seen when the electrophilic fragment in (IMe)Cu-Fp is changed to [(IMe)(Cl)Zn]. Here, the Zn center stabilizes a 1.01 e positive charge. As a result, even though the Fp fragment bears roughly the same degree of anionic character as in (IMe)Cu-Fp and the Wiberg bond order is approximately equal, the (IMe)Zn fragment has a larger partial positive charge of 1.23 e in comparison to the (IMe)Cu fragment. Much of this additional positive charge is balanced by negative charge localization on the chloride ligand (−0.64 e). Therefore, in order to make a more direct comparison to (IMe)Cu-Fp, we also calculated the hypothetical cation [(IMe)Zn-Fp]+, for which we have yet to obtain an experimental analogue. Though the overall molecular charge in this case has increased by one unit, relatively little of this additional positive charge is localized on the Zn center or the (IMe)Zn fragment in [(IMe)Zn-Fp]+. Instead, the Fe center and the Fp unit bear less of the partial negative charge, as much of the additional positive charge is spread through the Cp and CO ligands according to NPA. In other words, hypothetical chloride abstraction from 3 is predicted to attenuate the nucleophilicity of the Fe center while maintaining essentially constant electrophilicity at Zn. Interestingly, the calculated M− M′ Wiberg bond order in [(IMe)Zn-Fp]+ (0.55) is significantly higher than in (IMe)Cu-Fp or (IMe)(Cl)Zn-Fp, possibly due in part to a predicted Zn−Fe contraction caused by the increased molecular charge (2.356 vs 2.451 Å). Reactivity with Methyl Iodide. Experimental confirmation of Cu−Fe bond polarity was provided by preliminary reactivity studies. The Cu−Fe-bonded complexes 1a and 1b were each exposed to equimolar methyl iodide (Scheme 2). In both cases, analyses of the product mixtures indicated quantitative conversion to (NHC)Cu-I and Me-Fp, with no detectable formation of (NHC)Cu-Me or I-Fp products. This

Scheme 2. Methyl Iodide Reactivity of Cu−Fe-Bonded Systems

transformation, which can be viewed as a bimetallic oxidative addition reaction, is much more rapid for 1b (complete upon mixing) than for the more hindered 1a (complete after 48 h) at room temperature. The observed reactivity is consistent with Cu +−Fe − polarity indicated by DFT calculations and furthermore provides early indication that such polar M−M′ bonds featuring late-metal electrophiles indeed can be used in bond activation processes. Further reactivity studies are ongoing with the Cu−Fe, Cu−Mo, and Zn−Fe systems reported herein. In addition, studies toward incorporating these unsupported metal−metal-bonded systems into catalytic cycles involving metal−metal cooperativity are currently underway in our laboratory.

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EXPERIMENTAL SECTION

General Considerations. Unless otherwise specified, all reactions and manipulations were performed under purified N2 in a glovebox or using standard Schlenk line techniques. Glassware was oven-dried prior to use. Reaction solvents (CH2Cl2, n-pentane, Et2O, toluene, THF) were sparged with argon and dried using a Glass Contour Solvent System built by Pure Process Technology, LLC, or freshly distilled according to standard procedures (EtOAc, hexanes).19 Anhydrous 1,2-dimethoxyethane was purchased from a commercial vendor and degassed before use by sparging with N2 for 2 h. Deuterated solvents (C6D6, CD3CN, CD2Cl2) for NMR spectroscopy were degassed by repeated freeze−pump−thaw cycles and dried by prolonged storage over activated 3 Å molecular sieves. 1H NMR spectra were recorded using a Bruker Avance DPX-400 spectrometer, and 13C{1H} NMR spectra were recorded using a Varian Mercury-Vx300 spectrometer. NMR spectra were recorded at ambient temperature, and chemical shifts were referenced to residual solvent peaks. Note: for all Cp-containing complexes, the cyclopentadienyl resonances consistently gave anomalously low integration values in the 1H NMR spectra; elemental analysis data confirmed that the empirical formula assignments were consistent with solid-state structures determined by single-crystal X-ray crystallography. This phenomenon likely is due to slow relaxation, as integration values closer to predicted values are evident when using longer delay times (see the Supporting Information). FT-IR spectra were recorded on solid samples in a glovebox using a Bruker ALPHA spectrometer fitted with a diamond-ATR detection unit. Elemental analyses were performed by the Microanalysis Laboratory at the University of Illinois at Urbana−Champaign in Urbana, IL, and by Midwest Microlab, LLC, in Indianapolis, IN. A series of literature procedures were used to prepare the ligand salts IPr·HCl, IMes·HCl, and SIMes·HCl.20 Literature procedures were used to prepare the complexes (IPr)CuCl,21 (IMes)CuCl,21 (SIMes)CuCl,21 KFp,22 and (IPr)ZnCl2(THF).23 All other chemicals were purchased from commercial sources and used without further purification. Preparation of (IPr)CuFp (1a). A solution of KFp (83.3 mg, 0.385 mmol) in THF (8 mL) was added to a stirred solution of (IPr)CuCl (188.0 mg, 0.385 mmol) in THF (5 mL), and additional THF (3 mL) was used to complete the transfer of residual KFp. The solution immediately became yellow-brown with formation of a precipitate. The reaction mixture was stirred for 12 h and then pipet-filtered through Celite. Upon evaporation of the solvent under reduced pressure, a yellow solid was obtained. Crystallization of 1a was D

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reaction with an additional portion of THF (1 mL). The mixture was warmed to room temperature gradually and stirred for 18 h. The resulting cloudy, light orange solution was pipet-filtered through Celite and evaporated to an orange residue. Et2O (10 mL) was added, and the mixture first was stirred vigorously for 5 min and then pipetfiltered through Celite. The filtrate was concentrated to approximately 7 mL, at which point a small amount of solid precipitate began to form. Evaporation was halted, the supernatant was transferred to a tared vial, and n-pentane (3 mL) was layered on top of the solution. The vial was placed in a −35 °C glovebox freezer for 24 h, resulting in large light orange crystals (yield: 104 mg, 57%) that were pure according to 1H NMR spectroscopy. The mother liquor from this crystallization was concentrated to approximately 3 mL, n-pentane (4 mL) was layered on top of the solution, and this mixture was placed in the 35 °C freezer in order to obtain a second crop of crystals. Combined, two-crop yield: 116 mg, 0.174 mmol, 64%. 1H NMR (400 MHz, C6D6): δ 7.20 (t, J = 7.6 Hz, 2H, p-CH), 7.14 (d, J = 2.0 Hz, 4H, m-CH), 6.55 (s, 2H, NCH), 4.26 (s, Cp), 3.11 (sept, J = 7.0 Hz, 4H, CH(CH3)2), 1.56 (d, J = 6.7 Hz, 12H, CH(CH3)2), 1.02 (d, J = 6.8 Hz, 12H, CH(CH3)2). 13C{1H} NMR (300 MHz, C6D6): δ 219.2 (CO), 181.1 (NCZn), 146.1 (NCH), 134.3 (ipso-C), 131.2 (o-C), 124.6 (m-C), 124.1 (p-C), 79.9 (Cp), 28.9 (CH(CH3)2), 26.3 (CH(CH3)2), 23.2 (CH(CH3)2). IR (solid, cm−1): 2965, 2929, 2868, 1944 (νCO), 1888 (νCO), 1463, 1444, 1403, 1327, 1208, 1101, 1058, 934, 802, 755, 654, 596. Anal. Calcd for C34H41ClFeN2O2Zn: C, 61.28; H, 6.20; N, 4.20. Found: C, 60.90; H, 6.46; N, 4.24. Reaction of 1a with CH3I. (IPr)CuFp (15.5 mg, 0.0246 mmol) was dissolved in C6D6 (1 mL), and this solution was pipet-filtered through Celite. The resulting solution was transferred to a J. Young NMR tube. CH3I (1.5 μL, 0.024 mmol) was syringed into the J. Young NMR tube. The reaction was monitored by 1H NMR for 48 h and resulted in IPrCuI and MeFp. IPrCuI: 1H NMR (400 MHz, C6D6) δ 7.19 (t, J = 7.6 Hz, 2H, p-CH), 7.04 (d, J = 8.0 Hz, 4H, m-CH), 6.25 (s, 2H, NCH), 2.55 (sept, J = 6.9 Hz, 4H, CH(CH3)2), 1.37 (d, J = 6.9 Hz, 12H, CH(CH3)2), 1.05 (d, J = 6.9 Hz, 12H, CH(CH3)2). MeFp: 1 H NMR (400 MHz, C6D6) δ 4.00 (s, Cp), 0.31 (s, 3H, CH3). Independent Synthesis of (IPr)CuI. A literature procedure24 was used for the preparation of this compound. Literature 1H NMR data were recorded in CDCl3 but are presented in C6D6 here for direct comparison. 1H NMR (400 MHz, C6D6): δ 7.18 (t, J = 7.2 Hz, 2H, pCH), 7.03 (d, J = 7.8 Hz, 4H, m-CH), 6.24 (s, 2H, NCH), 2.54 (sept, J = 6.9 Hz, 4H, CH(CH3)2), 1.37 (d, J = 6.9 Hz, 12H, CH(CH3)2), 1.04 (d, J = 6.9 Hz, 12H, CH(CH3)2). Independent Synthesis of MeFp. KFp (51.5 mg, 0. 238 mmol) was dissolved in toluene (5 mL), generating an orange suspension. CH3I (14.7 μL, 0.236 mmol) was quickly syringed into the solution, generating a hazy dark brown solution. The solution was stirred overnight. The solution was then pipet-filtered through Celite into a tared vial and then dried in vacuo. Yield: 21.9 mg (48%). 1H NMR (400 MHz, C6D6): δ 4.00 (s, Cp), 0.31 (s, CH3). Reaction of 1b with CH3I. (IMes)CuFp (14.6 mg, 0.0267 mmol) was dissolved in C6D6 (1 mL) and was passed through Celite with a pipet filter. The resulting solution was transferred to a J. Young NMR tube. CH3I (1.7 μL, 0.027 mmol) was syringed into the J. Young NMR tube. According to 1H NMR spectroscopy, the reaction had reached complete conversion within a few minutes and resulted in IMesCuI and MeFp. Reaction monitoring was continued for 48 h to ensure no further conversions occurred. IMesCuI: 1H NMR (400 MHz, C6D6) δ 6.70 (s, 4H, m-CH), 6.00 (s, 2H, NCH), 2.10 (s, 6H, p-CH3), 2.00 (s, 12H, o-CH3). MeFp: 1H NMR (400 MHz, C6D6) δ 4.00 (s, Cp), 0.31 (s, 3H, CH3). X-ray Crystallography. Single-crystal X-ray diffraction studies were conducted using a Bruker AXS diffractometer with Mo Kα radiation (λ = 0.71073 Å) and an APEX-I area detector. Crystals were evaluated under a microscope, mounted to a goniometer head with a coating of Paratone-N oil, and centered under a cold stream of nitrogen held at a constant temperature of 100 K using a Kryoflex II cryostat. Initial unit cell constants were determined from a matrix of 3 sets of 12 frames, and the final cell constants were calculated using reflections from the actual data collection. A full sphere of data was

accomplished by leaving a concentrated solution of it in toluene at −35 °C overnight. Yield: 154 mg, 0.244 mmol, 63%. 1H NMR (400 MHz, C6D6): δ 7.22 (t, J = 8.0 Hz, 2H, p-CH), 7.11 (d, J = 7.7 Hz, 4H, m-CH), 6.28 (s, 2H, NCH), 4.19 (s, Cp), 2.68 (sept, J = 6.9 Hz, 4H, CH(CH3)2), 1.49 (d, J = 7.0 Hz, 12H, CH(CH3)2), 1.95 (d, J = 6.9 Hz, 12H, CH(CH3)2). 13C{1H} NMR (300 MHz, C6D6): δ 220.3 (CO), 180.7 (NCCu), 145.9 (o-C), 135.3 (ipso-C), 130.6 (p-C), 124.2 (m-C), 121.6 (NCH), 77.54 (Cp), 29.04 (CH(CH3)2), 24.5 (CH(CH3)2), 24.0 (CH(CH3)2). IR (solid, cm−1): 2962, 2868, 1914 (νCO), 1849 (νCO), 1456, 1404, 1327, 1107, 1058, 935, 802, 757, 656, 586. Anal. Calcd for C34H41CuFeN2O2: C, 64.91; H, 6.57; N, 4.45. Found: C, 64.62; H, 6.46; N, 4.33. Preparation of (IMes)CuFp (1b). A round-bottom flask with a stir bar was charged with a sample of KFp (492 mg, 2.27 mmol) and a sample of (IMes)CuCl (917 mg, 2.77 mmol). Upon addition of THF (20 mL) a reddish solution resulted and gradually turned brown over a period of 0.5 h. The stirring was continued for 18 h, and then the mixture was filtered with suction through a bed of Celite to give a yellow-brown solution. After evaporation of the solvent under reduced pressure, a yellow residue was obtained. This residue was washed with pentane (3 × 10 mL) and dried for 12 h to yield a spectroscopically pure powder. An analytically pure, crystalline solid was obtained by layering pentane over a concentrated solution of 1b in toluene and leaving it in the freezer (−35 °C) for 24 h. Yield: 942 mg, 1.72 mmol, 76%. 1H NMR (400 MHz, C6D6): δ 6.78 (s, 4H, m-CH), 5.99 (s, 2H, NCH), 4.26 (s, Cp), 2.08 (s, 18H, overlapping o- and p-CH3). 13 C{1H} NMR (300 MHz, C6D6): δ 220.6 (CO), 179.3 (NCCu), 139.2 (p-C), 135.85 (ipso-C), 135.0 (o-C), 129.5 (m-C), 120.5 (NCH), 77.4 (Cp), 21.0 (p-CH3), 17.9 (o-CH3). IR (solid, cm−1): 2965, 2919, 1905 (νCO), 1842 (νCO), 1486, 1400, 1276, 1234, 1012, 856, 813, 756, 655, 583. Anal. Calcd for C28H29CuFeN2O2: C, 61.71; H, 5.36; N, 5.14. Found: C, 61.57; H, 5.36; N, 4.93. Preparation of (SIMes)CuFp (1c). A solution of KFp (77.2 mg, 0.357 mmol) in THF (10 mL) was added to a stirred solution of (SIMes)CuCl (145.6 mg, 0.385 mmol) in THF (5 mL), and additional THF (3 mL) was used to complete the transfer of residual KFp. The solution immediately became brown with formation of a precipitate. The reaction mixture was stirred for 12 h and then pipet-filtered through Celite. Upon evaporation of the solvent under reduced pressure, a yellow residue was obtained. This residue was washed with pentane (3 × 5 mL) and dried for 6 h. Crystallization of 1c was accomplished by leaving a concentrated solution of it in Et2O at −35 °C overnight. Yield: 133 mg, 0.243 mmol, 68%. 1H NMR (400 MHz, C6D6): δ 6.79 (s, 4H, m-CH), 4.20 (s, Cp), 3.03 (s, 4H, NCH), 2.24 (s, 12H, o-CH3), 2.07 (s, 6H, p-CH3). 13C{1H} NMR (300 MHz, C6D6): δ 220.3 (CO), 200.4 (NCCu), 138.4 (ipso-C), 135.9 (o-C), 135.8 (p-C), 129.8 (m-C), 77.4 (Cp), 50.2 (NCH), 20.9 (p-CH3), 18.0 (o-CH3). IR (solid, cm−1): 2951, 2916, 1903 (νCO), 1839 (νCO), 1485, 1446, 1374, 1267, 1012, 851, 806, 655, 586. Anal. Calcd for C28H21N2O2CuFe: C, 61.49; H, 5.71; N, 5.12. Found: C, 60.96; H, 5.45; N, 5.23. Preparation of (IPr)CuMp (2). A modified literature procedure was employed in the synthesis of the title compound.13 A three-neck 250 mL flask was charged with Mo(CO)6 (0.20 g, 0.75 mmol) and NaCp (2.0 M in THF, 0.375 mL, 0.750 mmol). Dry 1,2dimethoxyethane (20 mL) was added. The resulting bright yellow solution was stirred at 80 °C overnight. The resulting dark yellow solution was cooled to room temperature, and then (IPr)CuCl (0.365 g, 0.75 mmol) was dissolved in THF (20 mL) and added to the reaction flask. The solution was stirred at room temperature overnight. The resulting light brown solution was Schlenk-filtered through a plug of Celite, and the solution was concentrated in vacuo to give 2 as a yellow powder. Yield: 0.306 mg, 0.48 mmol, 58%. 1H NMR and IR characterization data matched the previously reported values.13 Preparation of (IPr)(Cl)ZnFp (3). KFp (59.4 mg, 0.275 mmol) was suspended in THF (2 mL) and cooled to −35 °C in the glovebox freezer. Separately, (IPr)ZnCl2(THF) (163.0 mg, 0.273 mmol) was dissolved in THF (2 mL) and cooled to −35 °C in the glovebox freezer. With stirring, the KFp suspension was added dropwise to the (IPr)ZnCl2(THF) solution, and the KFp residue was washed into the E

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collected using ω scans. Absorption corrections were applied using semiempirical methods (SADABS). Solution and refinement was accomplished with the SHELXTL suite of programs25 using standard techniques.26 Computations. All calculations were performed using Gaussian09, Revision B.01.27 Density functional theory (DFT) calculations were carried out using a hybrid functional, BVP86, consisting of Becke’s 1988 gradient-corrected Slater exchange functional28 combined with the VWN5 local electron correlation functional and Perdew’s 1986 nonlocal electron correlation functional.29 Mixed basis sets were employed: the LANL2TZ(f) triple-ζ basis set30 with effective core potential31 was used for Cu, Fe, and Mo, the LANL2TZ+ triple-ζ basis set32 with effective core potential31 was used for Zn, the LANL2DZ double-ζ basis set with effective core potential was used for Cl,31 and the Gaussian09 internal 6-311+G(d) basis set was used for C, H, N, and O. Crystallographically determined coordinates for (IPr)Cu-Fp and (IPr)Cu-Mp13 were used as starting points to construct input geometries; the 2,6-diisopropylphenyl groups were truncated to methyl groups in order to minimize computational time. Geometries were optimized to a minimum, and then frequency calculations were performed to confirm that no imaginary frequencies were present. Natural population analysis was used to determine atomic and fragment charges, and Wiberg bond indices were used to determine bond orders: both were obtained from NBO v. 3.133 calculations within Gaussian09. Molecular orbital surfaces were viewed using Gaussview 4.1.34

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Spectroscopic characterization data, crystallographic data, computational output data and comparisons. 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 for N.P.M.: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Donald J. Wink for assistance with X-ray diffraction data collection and Prof. Petr Kral for donation of computational time. Funding was provided by the Department of Chemistry and the College of Liberal Arts and Sciences at the University of Illinois at Chicago (UIC), as well as by a grant through the UIC Campus Research Board Pilot Research program. The NSF provided support for the purchase of X-ray diffraction equipment (NSF-EAR-012982).

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ABBREVIATIONS Cp, cyclopentadienyl; CSD, Cambridge Structural Database; DFT, density functio nal theory ; dm pe, 1,2-bis(dimethylphosphino)ethane; Fp, FeCp(CO)2; HOMO, highest occupied molecular orbital; IPr, 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; IMe, 1,3-dimethylimidazol-2-ylidene; IMes, 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene; LUMO, lowest unoccupied molecular orbital; Mp, MoCp(CO)3; NHC, Nheterocyclic carbene; NPA, natural population analysis; SIMes, 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene; WBI, Wiberg bond index F

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