Synthesis and Structure of Chromium(VI) Nitrido Cyclopentadienyl

Sep 10, 2015 - A series of rare nitrido cyclopentadienyl complexes have been prepared of chromium(VI). An improved synthesis of NCr(I)(NPri2)2 is prov...
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Synthesis and Structure of Chromium(VI) Nitrido Cyclopentadienyl Complexes Brennan S. Billow, Ross D. Bemowski, Stephen A. DiFranco, Richard J. Staples, and Aaron L. Odom* Department of Chemistry, Michigan State University, 578 S. Shaw Lane, East Lansing, Michigan 48824, United States S Supporting Information *

ABSTRACT: A series of rare nitrido cyclopentadienyl complexes have been prepared of chromium(VI). An improved synthesis of NCr(I)(NPri2)2 is provided, which reacts with NaCp to give NCr(η1-Cp)(NPri2)2 (1) in 82% isolated yield. The η1-indenyl complex 2 can be prepared using a similar procedure. Replacement of one diisopropylamido ligand was accomplished to prepare NCr(NPri2)Cp(O2CPh) (3), NCr(NPri2)Cp(Cl) (4), and [NCr(NPri2)Cp(NCMe)]+[SbF6]− (5). In the series, the Cr−C interactions increase in bond order, 3 < 4 < 5, as the cyclopentadienyl compensates for the decreasing donor ability of the other ligands on chromium. The Ligand Donor Parameters (LDPs) for the η1-cyclopentadienyl and η1-indenyl ligands were measured as 13.73 and 13.76 kcal/ mol, respectively. Compounds 1−5 were characterized by single-crystal X-ray diffraction.

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Cambridge Structural Database.4 The complexes are shown in Chart 1. The molybdenum complex from the Parkin group has

INTRODUCTION The cyclopentadienyl ligand marked the beginning of modern organometallic chemistry, and the ligand and its derivatives remain the state of the art for many applications.1 The ligand is used throughout the periodic table with great versatility in its substituents and in its bonding modes. A terminal nitride is also a very common ligand in the middle of the transition series, but there are surprisingly few reported structures of cyclopentadienyl complexes with terminal nitride coligands. Herein, we report on the synthesis and structure of a group of cyclopentadienyl derivatives of chromium(VI) nitrido complexes. Cyclopentadienyl can coordinate with several hapticities. The most commonly invoked are η1-, η3-, and η5-bonding modes. References to η2- and η4-Cp are rare but can also be found in the literature. The most common method of bonding to a transition metal by far is as η5-Cp, and the ligand can “slip” to the lower coordinating modes generally to avoid going over 18 electrons at the metal center. Ring slipping is more common for indenyl ligands and can lead to tremendous rate enhancements known as the “indenyl effect”. For metal centers bearing Cp-like ligands, understanding the effect has seen tremendous attention since Basolo’s detailed studies describing it.2 In some cases, an η3-Cp ring can fold (“envelope”) somewhat, making its hapticity quite obvious. However, a slipped Cp, where the five-membered ring is still planar, is not necessarily more donating than its folded counterpart, and the folding has been shown to be extraneous to the Cp donor ability overall, a product of either steric constraints or bending to relieve antiaromatic character.3 At the time of this writing there are only four structurally characterized examples of terminal nitrido Cp complexes in the © XXXX American Chemical Society

Chart 1. Previously Structurally Characterized Terminal Nitrido Complexes Containing Cyclopentadienyl Derivatives

an η3-Cp, with Cp-ring slipping necessary to avoid a 20-electron count at the metal center. The other structurally characterized complexes of this class have cyclopentadienyl ligands with electron counts of 18 or less if bearing an η5-Cp, but the ligand is somewhat slipped away from the strongly donating nitrido with three shorter and two longer M−C bonds.5 Simple electron counting can hide some complexity in that the π-donor ability of the Cp is often in competition with other Received: July 31, 2015

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DOI: 10.1021/acs.organomet.5b00661 Organometallics XXXX, XXX, XXX−XXX

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Organometallics π-donors on the metal. In studies here, we investigated some commonly employed donors and their effects on Cp hapticity to evaluate the relative π-donor ability of some of these substituents as part of a larger study on the donor ability of ligands toward high-valent metals. Using the synthetically versatile system NCr(NPri2)2X, we evaluated many of the most common ligands employed in inorganic and organometallic chemistry for their donor abilities (Figure 1).6

Scheme 1. Synthesis of NCr(NPri2)3 and the Improved Synthesis of NCr(NPri2)2(I)

equiv of iodotrimethylsilane to an ethereal solution of NCr(NPri2)3 and 2 equiv of HOBut gives the iodo complex NCr(NPri2)2I in near quantitative yield. Treatment of NCr(NPri2)2I with NaCp in THF provides the cyclopentadienyl nitrido NCr(NPri2)2Cp (1), the structure of which is shown in Figure 2. In previous studies, it was found Figure 1. System used to determine LDP and a few representative examples of LDP values. The errors on the values given are ±0.2 kcal/ mol, except NMe2, where the error bar is ±0.32 kcal/mol. *This value was determined by line shape analysis. All values are previously reported except η1-indenyl and η1-Cp from this work.6,7

The ligand donor abilities for X in NCr(NPri2)2X are evaluated relative to the π-donor ability of the diisopropylamido ligands. The rate constant for Cr−NPri2 bond rotation is easily measured using 1H NMR spin saturation transfer (SST). The rate constant is converted to the free energy barrier for the reaction using the Eyring equation, with its pendant assumptions. Finally, the free energy barrier is converted to the approximately temperature independent enthalpic barrier by assuming the entropic barrier for amido rotation is invariant with X. With all these assumptions in place, the enthalpic barrier has been called the Ligand Donor Parameter (LDP), which is larger for less donating ligands and gives a measure of the total σ- and π-donor ability of the X ligand in question.6 In this study, several compounds of the general formula NCr(NPri2)(X)Cp were prepared and characterized. These new complexes are evaluated in the context of complexes from the literature based on the calculated bond order of the carbons in the Cp and structural parameters from X-ray diffraction. In addition, we report the LDP values for η1-cyclopentadienyl and η1-indenyl.

Figure 2. Synthesis and structure of NCr(NPri2)2(η1-Cp) (1). The ORTEP structure with 50% ellipsoids from X-ray diffraction is shown without hydrogens in calculated positions.

that ligands bound to the NCr(NPri2)2 fragment tend to be κ1. Cooling solutions (−60 °C in CDCl3) of 1 in the 1H NMR spectrometer gave a sharp singlet for the cyclopentadienyl. An X-ray diffraction study on the complex shows the cyclopentadienyl to be η1 in the solid state (vide infra). The singlet in the 1H NMR spectrum for the cyclopentadienyl is thus attributed to fast “ring whizzing”, 1,2-migration, of the metal around the ring on the NMR time scale. The spectroscopy of 1 can be compared with the prototypical η1-Cp complex Fe(η5-Cp)(η1-Cp)(CO)2, which is often called FpCp.9 In the case of FpCp it is possible to “freeze out” the rapid 1,2-migration of the η1-Cp ring.10,11 In most cases, as in ours, it is not possible to slow rotation sufficiently for solution observation of the different hydrogens in the ring.12 Synthesis of the related indenyl (Ind) complex NCr(NPri2)2(η1-Ind) (2) was accomplished by reaction of NCr(NPri2)2I with LiInd in hexane/ether (eq 1). The compound exhibits a broadened peak in the 1H NMR spectrum for the two

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RESULTS The most straightforward route to access the diisopropylamido nitrido chromium(VI) system is through nitrogen atom transfer from NCr(OBut)3 to Cr(NPri2)3, which provides NCr(NPri2)3 in excellent yield (Scheme 1).6 Previously, one of the most common starting materials for NCr(NPri2)2X complexes, NCr(NPri2)2I, was prepared from reaction of NCr(NPri2)3 with lutidinium iodide.8 Here, we report a simplified procedure (Scheme 1). We could use tertbutanol as a proton source since the reaction of tert-butyl alcohol with NCr(NPri2)3 is quite slow and the reaction of the alcohol with ISiMe3 is quite fast to generate HI. Addition of 2 B

DOI: 10.1021/acs.organomet.5b00661 Organometallics XXXX, XXX, XXX−XXX

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However, rapid addition at room temperature of HCl provided more appreciable quantities (59% yield) of 3 from protonolysis of a diisopropylamide. Similar to the benzoate, perhaps the best description of the cyclopentadienyl hapticity is η3 with the longer Cr−C bonds being trans to the nitrido ligand. The free energy barrier, ΔG⧧, to rotation in 4 was found by SST to be 19.2 kcal/mol at 319 K.15 Attempts were made to prepare the cation [NCr(Cp)NPri2]+ by addition of silver salts to chloride 4. Experiments in acetonitrile with silver hexafluoroantimonate resulted in isolation of the acetonitrile adduct of the desired cation (Figure 3), [NCr(NPri2)Cp(NCMe)][SbF6] (5). We were able to grow X-ray quality crystals from the impure mixture that results, and the structure is shown in Figure 3.

protons attached to the α-carbons of the indenyl group due to rapid exchange of the metal center between these two sites and broadened resonances for many of the carbons in the roomtemperature 13C NMR spectrum. Analysis of 2 by VT-NMR shows the indenyl peaks have a coalescence point around −50 °C. However, we were unable to reach the slow exchange limit with our instrumentation. The LDP values for both the η1-Cp and η1-indenyl were measured using SST to have very similar values of 13.73 and 13.76 kcal/mol, respectively. This places them near the middle of the series of ligands we have examined (Figure 1). Complex 1 has, in addition to the η1-Cp and nitrido, two strong π-donating ligands, diisopropylamides. One can argue that these large and strongly donating ligands are preventing the Cp from reaching its more typical η5-conformation. As a result, we investigated the replacement of one diisopropylamide in 1 with less donating (based on LDP) ligands. Would the metal compensate for the lower donor ability of the ligand set on chromium by increasing the hapticity of the cyclopentadienyl? Reaction of 1 with benzoic acid in toluene led to formation of NCr(NPri2)Cp(O2Ph) (3), as shown in eq 2. For the

Figure 3. (Top) Synthesis of [NCr(NPri2)(η3-Cp)(NCMe)][SbF6] (5). (Bottom) Two ORTEP structures with 50% ellipsoids from X-ray diffraction studies on 4 (left) and 5 (right), shown without hydrogens in calculated positions and without the anion for 5. The cyclopentadienyl ligand is flat and slipped from the η5-conformation in both cases, but the slipping is more extensive in 4.

moment, the cyclopentadienyl can be described as η3, with a more detailed discussion to appear later in this article. Again, the cyclopentadienyl hydrogens provide a single, sharp resonance in the 1H NMR spectrum. The benzoate in 3 was analyzed by FT-IR for its denticity. The procedure involves comparison of the symmetric (νs) and asymmetric (νa) stretches in the complex versus the similar vibrations in very ionic NaO2CPh. A difference in these stretching frequencies, ΔCO2Ph = νs − νa, larger than the value in the sodium salt has been ascribed to κ1-carboxylates, and smaller values in the difference are for κ2-carboxylates.13 The result was a ΔCO2Ph value larger than the sodium salt indicating a κ1(O)-benzoate. This is consistent with the solid-state structure from X-ray diffraction, which gives a distance from chromium to the second oxygen of the carboxylate of 3.169 Å. It also suggests that any long-range donation from the second oxygen is likely negligible.14 In addition to benzoate 3, the chloride complex 4 was prepared by addition of HCl to 1 as shown in eq 3. Controlled,

The synthesis of complex 5 was attempted by other methods and on larger scales, but isolation of the product proved difficult, as decomposition readily occurred. For example, protonation of the amide group by HSbF6 in acetonitrile did not provide isolable quantities of 5. Attempts to produce [NCr(NPri2)Cp(NCMe)]+ with other counterions, e.g., BPh4− and B(3,5-C6H3(CF3)2)4−, also were unsuccessful.

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DISCUSSION The hapticities of cyclopentadienyl ligands are generally described as limiting forms: η1, η3, and η5. As in most cases, there are gradations between these limiting forms worthy of description, as they, in this case, tell the deeper story of competition between the cyclopentadienyl ligand and the other ligands on chromium. It is worth discussing the bonding for some well-accepted cases from the literature to compare with complexes 1−5 and their hapticities. The Wilkinson complex FpCp contains both an η5- and η1-Cp; this complex will be used as the model for both of these ligand types. The complex whose structure identified η3-Cp as a possible bonding form was W(η5-Cp)(η3Cp)(CO)2 by Brintzinger and co-workers. This complex is ideal as a comparison in that it clearly contains an η3-Cp. These two

low-temperature addition of HCl in ether to 1 largely gave known NCr(NPri2)2Cl by protolytic cleavage of the Cp ligand. C

DOI: 10.1021/acs.organomet.5b00661 Organometallics XXXX, XXX, XXX−XXX

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Brintzinger and co-workers’ W(η5-Cp)(η3-Cp)(CO)2 is the archetypal complex for η3-Cp bonding. The other cyclopentadienyl pendant to the same metal, to reach an 18-electron configuration, should be η5. Certainly one of the Cp ligands has bond distances and bond orders that are closely packed around the mean, indicative of an η5-bonding mode. While such a ligand with small bond difference between the metal and carbons could be regarded as “slipped”, there are substantial bond orders to all five carbons, and the ligand would generally be regarded as an η5-Cp. Note, the horizontal lines in Figure 4 for the mean M−C calculated bond orders for the η5-Cp ligand in iron and tungsten models are essentially identical at 0.39 and 0.38, respectively. The η3-cyclopentadienyl ligand in W(η5-Cp)(η3-Cp)(CO)2 has three distinctly shorter M−C bonds along with three higher M−C bond orders; it is clearly η3 by these metrics. The mean W−C calculated bond order for the η3-Cp drops slightly (Figure 4 bottom) to 0.34. Another method of analysis is to look at the standard deviation from the mean for the M−C bond orders. For the η5Cp ligands in both complexes there is a quite small deviation from the mean of 0.02 and 0.07 for the Fe and W complexes, respectively. For the η1-Cp a very large deviation of 0.29 is found, while the η3-Cp has an intermediate value of 0.20. The analyses for the prototypical complexes, FpCp and W(η5-Cp)(η3-Cp) (CO)2, containing η1-, η3-, and η5-cyclopentadienyl ligands seem relatively clear. We now carry out the same analysis with chromium complexes 1−5 (Figure 5). It is apparent from the bond distance and order plots that complexes 1 and 2 are η1-cyclopentadienyl and η1-indenyl complexes, respectively. Consistent with this, 1 and 2 have standard deviations from the mean in the bond orders of 0.30 and 0.33 similar to the value for the η1-Cp in FpCp. All three of the other chromium nitrido Cp complexes 3−5 appear to be η3 with weaker interactions to the remaining two carbons and deviations from the mean of 0.14, 0.13, and 0.12, respectively, but are perhaps closer to η5 than the prototype, η3-Cp in W(η5Cp)(η3-Cp)(CO)2. In all three complexes NCr(Cp)(NPri2)X (3−5), where X = O2CPh, Cl, and acetonitrile (cationic), the two longest Cr−C bonds of the cyclopentadienyl ligand are the farthest from the nitrido ligand, i.e., approximately trans to the strongest donor. The changes in the longest two Cr−C bond distances are statistically significant between the cation 5 and neutral 3 and 4.18 Likewise, the mean Cr−C(Cp) bond order increases slightly from O2CPh and Cl to the acetonitrile cation (Figure 5 bottom). This coincides with the anticipated LDP values for the X ligands as well, where O2CPh is a more donating (lower LDP) than chloride. The LDP of the acetonitrile cation is the subject of current study, but preliminary studies suggest an LDP value much higher than chloride for acetonitrile.19 Consequently, it appears that the cyclopentadienyl ligand trends toward η5 to compensate as more electron-deficient ligands are placed on the metal center. This can be seen in the mean values for the calculated Cr−C bond orders for 1, 3, 4, and 5 of 0.19, 0.29, 0.29, and 0.32, respectively (horizontal lines in Figure 5).20

complexes contain all three common limiting hapticities for the cyclopentadienyl. To examine hapticity of the Cp, the complexes were examined by their M−C(Cp) bond distances and Mayer bond order. The calculations were done on the full molecules using DFT with the B3PW91 functional, and the bond order calculation was done using Mayer’s program BORDER.16 The results of both the bond distance and Mayer bond order analysis are shown in Figure 4 for the two test compounds. The

Figure 4. (Top) Metal−carbon bond distances to the cyclopentadienyls in the literature test compounds with Fe(η5-Cp)(η1Cp)(CO)2 = [Fe] and W(η5-Cp)(η3-Cp)(CO)2 = [W].17 (Bottom) Metal−carbon Mayer bond orders to the cyclopentadienyl ligands. In both plots the colored horizontal line is the mean value of the points for that column.

complexes contain an η5-Cp in addition to another cyclopentadienyl of a different hapticity, and analysis for both cyclopentadienyls of both complexes are shown. In FpCp, the η1- and η5-cyclopentadienyls have distinctly these hapticities for the two rings, as judged by the bond lengths and orders. One would expect that a “true” η5-Cp would have all M−C bond lengths and orders the same, and this is certainly true within error for this ligand despite having different ligands around the basal set (two carbonyls and one η1-Cp). The η1-Cp in the same complex has one high bond order (∼0.7) to carbon, and the others are essentially zero.

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CONCLUSIONS The highest symmetry attainable for a four-coordinate chromium(VI) nitrido complex of general formula NCrX3 is C3v. Because of this symmetry, the nitrido ligand along the zaxis interacts with an s−dz2−pz hybrid, and, due to the very D

DOI: 10.1021/acs.organomet.5b00661 Organometallics XXXX, XXX, XXX−XXX

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EXPERIMENTAL SECTION

General Considerations. All reactions and manipulations were carried out in an MBraun glovebox under a nitrogen atmosphere and/ or using standard Schlenk techniques. Ethereal solvents, pentane, and toluene were purchased from Aldrich Chemical Co. and purified through alumina columns to remove water after sparging with dinitrogen to remove oxygen. Silver hexafluoroantimonate was purchased from Aldrich Chemical Co. and used as received. tertButanol was purchased from Jade Chemical Co. and dried over 3 Å molecular sieves to remove water after being sparged with dry nitrogen to remove oxygen. Trimethylsilyl iodide was purchased from Oakwood Chemical and distilled under dry nitrogen. FpCp was prepared using the literature procedure.9 Lithium indenyl (LiInd) was prepared by addition of 2.5 M n-butyllithium in hexanes from Sigma-Aldrich to a near-frozen ether solution of HInd, and the removal of volatiles after 30 min reaction time gave a white solid, which was used without further purification. All NMR solvents were purchased from Cambridge Isotopes Laboratories, Inc. Deuterated chloroform and acetonitrile were dried over 3 Å sieves and freeze−pump−thaw degassed. The NMR solvents were stored in the glovebox in glass containers with a stopcock. Spectra were taken on Varian instruments located in the Max T. Rogers Instrumentation Facility at Michigan State University. These include a UNITYplus 500 spectrometer equipped with a 5 mm pulsedfield-gradient (PFG) switchable broadband probe and operating at 499.955 (1H) and 125.77 (13C) MHz. 1H NMR chemical shifts are reported relative to residual CHCl3 in CDCl3 as 7.26 ppm. 13C NMR chemical shifts are reported relative to 13C in CDCl3 as 77.0 ppm. Single-crystal X-ray diffraction data were collected in the Center for Crystallographic Research at MSU. Improved Synthesis of NCr(NPri2)2(I). To a solution of NCr(NPri2)3 (100 mg, 0.273 mmol) and tert-butanol (52 μL, 0.546 mmol, 2 equiv) in ether (5 mL) was added dropwise a solution of iodotrimethylsilane (109 mg, 0.546 mmol, 2 equiv) in ether (1 mL). During the reaction the beet-colored solution turned orange, and a precipitate formed. This mixture was stirred at RT for 2 h and evaporated to a dark solid. This solid was then extracted with pentane and filtered through Celite. The filtrate was cooled to −30 °C overnight. The solution was then decanted, and the crystals were dried in vacuo. The solution was reduced in volume and cooled to −30 °C to obtain a second crop of crystals (97 mg, 90%). On large scales, cooling the reaction in an ice bath is advisible during the addition of ISiMe3, as the reaction is noticeably exothermic. For example, the reaction has been run on a 5 g scale and was chilled in a liquid nitrogen cold well prior to iodotrimethysilane addition to prevent fuming. Characterization data in all cases were consistent with that previously reported.6a Synthesis of NCr(NPri2)2(Cp) (1). Under an inert atmosphere, a scintillation vial was loaded with NCr(NPri2)2(I) (0.500 g, 1.271 mmol, 1 equiv), 25 mL of THF, and a stirbar. To this was added a 3.5 M THF solution of sodium cyclopentadienide (1.089 mL, 3.813 mmol, 3 equiv), and the solution was rapidly stirred for 20 h. The volatiles were removed in vacuo. The residue was extracted with pentane (3 × 25 mL) and filtered through Celite. The volatiles were removed in vacuo, yielding 1 as a brown powder. Diffraction quality crystals were obtained from a concentrated pentane solution of NCr(NPri2)2(Cp) held at −30 °C (0.280 g, 1.051 mmol, 82% yield). 1 H NMR (CDCl3, −60 °C, 500 MHz): 6.16 (s, 5 H, Cp), 4.88 (sept, JHH = 6.4, 2 H, CH(CH3)2), 3.59 (sept, JHH = 6.3, 2 H, CH(CH3)2), 1.72 (d, JHH = 6.3, 6 H, CH(CH3)2), 1.42 (d, JHH = 6.4, 6 H, CH(CH3)2), 1.10 (d, JHH = 6.2, 6 H, CH(CH3)2), 1.04 (d, JHH = 6.2, 6 H, CH(CH3)2). 13C{H} NMR (CDCl3, 25 °C, 125 MHz): 115.0, 58.3, 55.2, 30.4, 30.3, 23.3, 18.0. Anal. Calcd for C17H33CrN3: C, 61.60; H, 10.03; N, 12.68. Found: C, 61.59; H, 9.97; N, 12.65. Mp: 90−92 °C (sub). Synthesis of NCr(NPri2)2(Ind) (2). To a partially frozen solution of NCr(NPri2)2(I) (100 mg, 0.254 mmol, 1 equiv) in ether (3 mL) was added a suspension of lithiated indene (34.2 mg, 0.280 mmol, 1.1 equiv) in ether (2 mL). This dark mixture was allowed to warm to

Figure 5. (Top) Metal−carbon bond distances to the cyclopentadienyls in 1−5. (Bottom) Metal−carbon Mayer bond orders to the cyclopentadienyl ligands. In both plots the colored horizontal line is the mean value of the points for that column.

strong donor ability of the nitrido, a hybrid that is mostly pzcharacter is raised high in energy. In the NCr(NPri2)2X system, the two diisopropylamide ligands act as good π-donors, as evidenced by their slow rotation and multiple-bond character. If X acts as only a σ-donor, then the complex is 16-electron, which is preferred because it allows the high-energy hybrid discussed above to remain empty. As a result, the X ligand must compete with the diisopropylamide ligands and nitrido for chromium acceptor orbitals; this is true for both σ- and πdonation, as in this low symmetry both are mixed. It is apparent from the structure of the X = Cp system that the π-donor ability and/or the steric constraints of the two diisopropylamido ligands in the basal set of NCr(NPri2)2(η1Cp) (1) allow only σ-donation from the cyclopentadienyl. If one of the NPri2 ligands is replaced with a weaker donor to give NCr(NPri2)(X′)(Cp) (3−5), cyclopentadienyl donation increases commensurate with the donor ability of ligand X′. The hapticity of these new complexes 3−5 appears to be best described as η3; however there are increasing interactions with all five carbons of the Cp as X′ becomes a weaker donor in the series, with average Cr−C bond orders increasing slightly as 3 ≤ 4 < 5.20 Again, this clearly illustrates the amazing versatility of the C5H5− anion to stabilize metal complexes with finely tuned donation. E

DOI: 10.1021/acs.organomet.5b00661 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

129.19, 110.30, 78.55, 67.60, 49.45, 31.81, 30.41, 20.99, 19.06. 19F NMR (CD3CN, 25 °C, 470 MHz): −113.65 to −134.23 (m). General Procedure for FT-IR Carboxylate Denticity Determination. All FT-IR analysis was done on a Mattson Galaxy Series FTIR 3000 spectrometer. Samples were prepared by pressing ∼10 mg of each compound into anhydrous KBr. The symmetric and asymmetric carbonyl stretches were identified by comparison to its isotopologue, 13C labeled at the carbonyl carbon. Differences between the stretches in the sample were compared to the difference in the symmetric (νs = 1415.5 cm−1) and asymmetric (νa = 1594.8 cm−1) stretches in a sample of sodium benzoate in KBr, which had a value for Δ(NaO2CPh) of 179.3 cm−1. Computational Details. All calculations were done at the High Performance Computing Center (HPCC) at Michigan State University. The optimization of structures was done using G09 with DFT and the B3PW91 functional. Due to the size of the structures, only double-ζ basis sets were used in most cases, with 6-31G** used for all the chromium and iron complexes. In the case of molybdenum and tungsten, the SDD basis was used. The Mayer bond order calculations were done on a departmental cluster using BORDER. In the case of the chromium complexes, the basis set dependence of the Mayer bond orders was examined. For example, for compound 1 the calculation was carried out with 3-21G, 6-31G, 6-31G**, and SDD. Bond orders using these different basis sets were generally comparable, and either 6-31G** or SDD was employed for all the complexes. For example, the Cr−N(nitrido) bond orders in 1 with the different basis sets were 2.71, 2.72, 2.68, and 2.89, respectively. The highest bond order between Cr−C(Cp) in 1 was 0.60, 0.71, 0.73, and 0.71, respectively, using the different basis sets.

room temperature and stirred for 18 h. The volatiles were then removed in vacuo, and the residue was extracted with pentane, filtered through Celite, and evaporated to a dark orange-brown solid. The solids were dissolved in a minimal amount of pentane and chilled to −30 °C overnight, providing 2 as dark orange crystals (26 mg, 27% yield). 1H NMR (CDCl3, 25 °C, 500 MHz): 7.59 (dd, JHH = 5.6, 3.2, 2 H, Ar), 7.02−7.07 (m, 3 H, Ar and β-Ind), 5.65 (br, 2 H, Ar), 4.82 (sept, JHH = 6.3, 2H, CH(CH3)2), 3.48 (sept, JHH = 6.3, 2 H, CH(CH3)2), 1.46 (d, 12 H, CH(CH3)2), 1.04 (d, JHH = 6.4, 6 H, CH(CH3)2), 0.90 (d, JHH = 6.2, 6 H, CH(CH3)2). 13C{H} NMR (CDCl3, 25 °C, 125 MHz): (6 of the signals for the indenyl ligand are not observed due to broadening from fluxionality on the 13C NMR time scale) 136.7, 123.1, 122.4, 57.2, 54.8, 30.7, 29.3, 23.4, 19.9. Anal. Calcd for C21H35CrN3: C, 66.11; H, 9.25; N, 11.01. Found: C, 65.68; H, 9.60; N, 10.95. Mp: 131−133 °C (dec) Synthesis of NCr(NPri2)(O2CPh)(Cp) (3). Under an inert atmosphere a scintillation vial was loaded with 1 (0.178 g, 0.537 mmol, 1 equiv), a stir bar, and toluene (4 mL). The vial was moved to a liquid-nitrogen-cooled cold well for 10 min. The solution was stirred vigorously, and benzoic acid (0.066 mg, 0.537 mmol, 1 equiv) in toluene (6 mL) was added dropwise over 5 min. The solution turned dark red and was allowed to stir at room temperature for 2 h. The volatiles were removed in vacuo, and the residue was dissolved in 2 mL of toluene. The solution was filtered, layered with an equal volume of pentane, and held at −35 °C, yielding crystals of 3 (0.117 g, 0.333 mmol, 62%). 1H NMR (500 MHz, CDCl3, 13 °C): 7.95 (dd, JHH = 8.25 Hz, JHH = 1.5 Hz, 2 H, Ph), 7.41 (tt, JHH = 7.0 Hz, JHH = 2.5 Hz, 1 H, Ph), 7.34 (t, JHH = 7.5 Hz, 2 H, Ph), 6.14 (s, 5 H, C5H5), 5.56 (sept, JHH = 6.0 Hz, 1 H, NCH(CH3)2), 4.31 (sept, JHH = 6.0 Hz, 1 H, NCH(CH3)2), 2.11 (d, JHH = 6.0 Hz, 3 H, NCH(CH3)2), 1.75 (d, JHH = 6.0 Hz, 3 H, NCH(CH3)2), 1.29 (d, JHH = 6.0 Hz, 3 H, NCH(CH3)2), 1.11 (d, JHH = 6.0 Hz, 3 H, NCH(CH3)2). 13C{1H} NMR (125 MHz, CDCl3, 13 °C): 170.74, 135.26, 130.77, 129.66, 127.83, 108.22, 73.71, 63.71, 31.06, 29.83, 20.64, 20.15. FT-IR (KBr): 1639.2 cm−1 (νs CO2), 1415.5 cm−1 (νa CO2). Satisfactory elemental analysis was not obtained after several attempts. Synthesis of NCr(NPri2)(Cp)Cl (4). Under an inert atmosphere a Schlenk flask was loaded with 1 (50 mg, 0.151 mmol, 1 equiv) and ether (5 mL). To the solution of 1 was added rapidly 2.0 M HCl (0.226 mL, 0.453 mmol, 3 equiv) in ether. The dark mixture turned reddish, and some precipitate formed during addition. This mixture was stirred at RT for 30 min. The volatiles were then removed in vacuo, and the residue was washed with pentane (2 × 5 mL). The solid was then extracted with ether, filtered through Celite, and concentrated to ∼2 mL. This dark solution was cooled to −30 °C overnight, providing 4 as dark red crystals (24 mg, 59%). 1H NMR (CDCl3, 25 °C, 500 MHz): 6.09 (s, 5 H, Cp), 5.23 (sept, JHH = 6.5, 1 H, CH(CH3)2), 4.36 (sept, JHH = 6.3, 1 H, CH(CH3)2), 2.17 (d, JHH = 6.3, 3 H, CH(CH3)2), 1.80 (d, JHH = 6.4, 3 H, CH(CH3)2), 1.26 (d, JHH = 6.5, 3 H, CH(CH3)2), 1.20 (d, JHH = 6.5, 3 H, CH(CH3)2). 13 C{H} NMR (CDCl3, 25 °C, 125 MHz): 108.9, 74.7, 64.4, 30.8, 29.3, 20.2, 17.8. Anal. Calcd for C11H19ClCrN2: C, 49.53; H, 7.18; N, 10.50. Found: C, 49.50; H, 7.56; N, 10.45. Mp: 121−123 °C (dec). Synthesis of [NCr(NPri2)(Cp)(NCMe)][SbF6] (5). Under an inert atmosphere, a scintillation vial was loaded with 4 (25 mg, 0.124 mmol, 1 equiv) and CD3CN (1 mL). To this was added a solution of AgSbF6 (85 mg, 0.248 mmol, 2 equiv) in CD3CN (1 mL). The reaction was allowed to run and monitored by 1H NMR. After 4 d, it was observed that all starting material peaks had disappeared. The volatiles were removed in vacuo, and the residue was washed with ether (5 mL). The solids were extracted with chloroform (1 mL), filtered through Celite, layered with ether, and chilled to −30 °C for recrystallization. Despite being of quality for single-crystal diffraction, the compound was unstable, and the bulk material was consistently impure. 1H NMR (CD3CN, 25 °C, 500 MHz): 6.24 (s, 5 H, Cp), 5.33 (sept, JHH = 6.3, 1 H, CH(CH3)2), 4.67 (sept, JHH = 6.2, 1 H, CH(CH3)2), 2.15 (d, JHH = 6.2, 3 H, CH(CH3)2), 1.78 (d, JHH = 6.2, 3 H, CH(CH3)2), 1.28 (d, JHH = 6.8, 3 H, CH(CH3)2), 1.22 (d, JHH = 6.4, 3 H, CH(CH3)2). The resonance for the bound acetonitrile protons was not unambiguously identified in the 1H NMR. 13C{H} NMR (CD3CN, 25 °C, 125 MHz):

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00661. Spectra of new complexes (PDF) X-ray diffraction data for FpCp and 1−5 (CIF)

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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 The financial support of the National Science Foundation CHE-1265738 for this work is greatly appreciated. REFERENCES

(1) (a) Cotton, F. A. J. Organomet. Chem. 2001, 637−639, 18−26. (b) Werner, H. Landmarks in Organo-Transition Metal Chemistry; Springer-Verlag: New York, 2009. Also see: Field, L. D.; Lindall, C. M.; Masters, A. F.; Clentsmith, G. K. B. Coord. Chem. Rev. 2011, 255, 1733−1790 and references therein. (2) For a couple of seminal references see: (a) Rerek, M. E.; Ji, L. N.; Basolo, F. J. Chem. Soc., Chem. Commun. 1983, 1208−9. (b) Calhorda, M. J.; Romão, C. C.; Veiros, L. F. Chem. - Eur. J. 2002, 8, 868−875. (3) Veiros, L. F. Organometallics 2000, 19, 5549−5558. (4) Cambridge Structural Database, updated last 2015-04-1 when searched online at http://webcsd.ccdc.cam.ac.uk/. (5) (a) Shin, J. H.; Bridgewater, B. M.; Churchill, D. G.; Baik, M.-H.; Friesner, R. A.; Parkin, G. J. Am. Chem. Soc. 2001, 123, 10111−10112. (b) Miyazaki, T.; Tanaka, H.; Tanabe, Y.; Yuki, M.; Nakajima, K.; Yoshizawa, K.; Nishibayashi, Y. Angew. Chem., Int. Ed. 2014, 53, 11488−11492. (c) Koch, J. L.; Shapley, P. A. Organometallics 1997, 16, 4071−4076. (d) Johnson, C. E.; Kysor, E. A.; Findlater, M.; Jasinski, J. F

DOI: 10.1021/acs.organomet.5b00661 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics P.; Metell, A. S.; Queen, J. W.; Abernethy, C. D. Dalton Trans. 2010, 39, 3482−3488. (6) (a) DiFranco, S. A.; Maciulis, S. A.; Staples, R. J.; Batrice, R. J.; Odom, A. L. Inorg. Chem. 2012, 51, 1187−1200. (b) Bemowski, R. D.; Singh, A. K.; Bajorek, B. J.; DePorre, Y.; Odom, A. L. Dalton Trans. 2014, 43, 12299−12305. (7) The error bars on the values for LDP were previously found using a propagation of error method that was overly complex. We recommend the use of error bars of ±0.2 kcal/mol for all the values we have reported using SST, which is similar to the previous error values. (8) (a) Odom, A. L.; Cummins, C. C. Polyhedron 1998, 17, 675−688. (b) Odom, A. L.; Cummins, C. C. Organometallics 1996, 15, 898−900. (c) Odom, A. L.; Cummins, C. C.; Protasiewicz, J. D. J. Am. Chem. Soc. 1995, 117, 6613−6614. These procedures made anhydrous lutidinium iodide from ISiMe3 and an alcohol, which was then used in the synthesis of NCr(I)(NPri2)2. In the new procedure, we generate HI directly in the presence of NCr(NPri2)3 to form the iodide. (9) Piper, T. S.; Wilkinson, G. J. Inorg. Nucl. Chem. 1956, 3, 104− 124. (10) Bennett, M. J., Jr.; Cotton, F. A.; Davison, A.; Faller, J. W.; Lippard, S. J.; Morehouse, S. M. J. Am. Chem. Soc. 1966, 88, 4371− 4376. The structure of FpCp can be found in this reference. For the sake of discussion it was useful to obtain the structure from X-ray diffraction on our instrumentation, and the structure was reexamined. (11) Ariafard, A.; Tabatabaie, E. S.; Yates, B. F. J. Phys. Chem. A 2009, 113, 2982−2989. (12) Faller, J. W.; Murray, H. H.; Saunders, M. J. Am. Chem. Soc. 1980, 102, 2306−2309. (13) Deacon, G. B.; Phillips, R. J. Coord. Chem. Rev. 1980, 33, 227− 250. (14) A similar study on NCr(NPri2)2(O2CPh) reported previously suggested that this complex also contains a κ1-benzoate. (15) If one assumes that the value for ΔS⧧ = −9 cal/mol·K, as is done in the LDP measurements, then ΔH⧧ = 16.33 kcal/mol. (16) (a) Mayer, I. J. Comput. Chem. 2007, 28, 204−221. (b) Mayer, I. BORDER 1.0; Chemical Research Center, Hungarian Academy of Sciences: Budapest, 2005. (17) If more than one chemically identical but crystallographically distinct molecule is present in the crystal structure, distances for one of the molecules are shown. (18) The longest two Cr−C bond distances in 3 and 4 are the same within three standard deviations. (19) On the basis of preliminary studies that are ongoing, the LDP value for acetonitrile is significantly higher than chloride. Aldrich, K. E.; Billow, B. S.; Odom, A. L. Unpublished results. (20) The Cr−C bond order trend of 3 ≤ 4 < 5 is consistent with the LDP values of 14.45 < 15.05 < ∼15.5 kcal/mol for O2CPh, Cl, and CNMe, respectively.

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DOI: 10.1021/acs.organomet.5b00661 Organometallics XXXX, XXX, XXX−XXX