Bipyridine Complexes Bearing Bis(N-heterocyclic carbene) Ligands

Nov 24, 2017 - California 92093-0358, United States. •S Supporting Information. ABSTRACT: The synthesis and characterization of four new Ni(II) bis(...
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Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Synthesis and Characterization of Heteroleptic Ni(II) Bipyridine Complexes Bearing Bis(N-heterocyclic carbene) Ligands Mark H. Reineke, Tyler M. Porter, Andrew L. Ostericher, and Clifford P. Kubiak* Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, Mail Code 0358, La Jolla, California 92093-0358, United States S Supporting Information *

ABSTRACT: The synthesis and characterization of four new Ni(II) bis(NHC) bipy complexes (bipy = 4, 4′-bipyridine, NHC = N-heterocyclic carbene) are reported. These represent the first examples of the NiII(NHC2) L2 class of compounds, where L is a two-electron-donor ligand. The introduction of a double propyl bridged bis-NHC ligand (CProp2C) imparts reversibility to the electrochemistry of these complexes. The electrochemical response includes two reductions, the first believed to be bipy ligand based, bipy0/−, and the second metal based. The unusual ligand arrangements in these complexes result in electronic structures in which both the HOMOs and LUMOs are ligand-based, as determined by DFT analysis of all four compounds.

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INTRODUCTION Ni(II) diphosphine complexes have been studied for a variety of catalytic transformations, including proton reduction,1,2 hydrogen oxidation,1 oxygen evolution (OER),3 and formate oxidation.4,5 Their frequent choice for these applications is due to several factors. They are highly tunable due to a wide variety of available diphosphine ligands. They are also stable with respect to reduction, with homoleptic diphosphine complexes of nickel characterized in the Ni(II),6−8 Ni(I),6 and Ni(0)7,8 oxidation states. A variety of mixed-ligand Ni(II) diphosphine complexes are also known, including several diphosphine dihalide complexes,9,10 as well as select mixed-ligand Ni(II) bis-diphosphines.6 N-heterocyclic carbenes (NHCs) have been successfully used in place of phosphines in a variety of systems.11−13 In the case of Ni(II) bis-carbene complexes, both mono(NHC) ligands and bis(NHC) ligands have been shown to produce a variety of stable square-planar complexes of the NiII(NHC2)X2 type, where X is a halide or pseudohalide.14−21 These have been formed by direct reaction of NHC ligands or precursors with Ni(II) halides15,21 and by the oxidative addition of aryl halides,22 alkyl halides,23 and organonitriles24 to Ni(0) bis(carbenes). In all cases, the NHC ligands orient trans to each other whenever possible. While there are reported examples of homoleptic Ni(II) bis(NHC) complexes,15,25 there are no examples of complexes of the NiII(NHC2)L2 © XXXX American Chemical Society

type, where L is a two-electron donor other than an NHC ligand. It should be noted that there are extensive examples of tetradentate NiIIbis(carbene−L) systems26,27 (where L = phosphine,28 pyridine,27,28 amido)29−31 applied in numerous catalytic transformations. Despite their highly modular nature, there exist no examples of tetradentate NiIIbis(carbene−L) that incorporate redox-noninnocent L-type ligands. Herein, we present the synthesis and characterization of the first examples of mixed-ligand NiII(NHC2) L2 complexes. Four Ni(II) bis(NHC) bipyridine complexes have been prepared: ( Me C Prop C Me )(bipy)nickel(II) hexafluorophosphate (3a, Me Prop Me C C = 1,3-bis(3-methylbenzimidazolin-2-ylidine)propane, bipy = 2,2′-bipyridine), (CProp2C)(bipy)nickel(II) hexafluorophosphate (3b, CProp2C = doubly bridged analogue of MeCPropCMe), (MeCPropCMe)(tBu-bipy)nickel(II) hexafluorophosphate (4a, tBu-bpy = 4,4′-tert-butyl-2,2′-bipyridine), and Ni(CProp2C)(tBu-bipy)nickel(II) hexafluorophosphate (4b). These compounds represent the first heteroleptic NiII(NHC2)L2 complexes and show a structural motif previously unknown. In addition to their structural characterization, we report the electrochemical properties of these compounds and utilize density functional theory (DFT) analysis to study their electronic structure and redox behavior. Received: November 24, 2017

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DOI: 10.1021/acs.organomet.7b00847 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

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RESULTS AND DISCUSSION Synthesis and Structural Characterization of Ni(II) Precursors. Ligand precursors 1a,b were synthesized according to previously reported procedures.16,32 Ni(II) dibromide precursors (MeCPropCMe)NiBr2 (2a) and (CProp2C)NiBr2 (2b) were synthesized according to the methods for the synthesis of 2a reported by Bouwman16 and Jothibasu15 (Scheme 1).

Ni(II) core. The same bonding motif is also seen when the CProp2C ligand 1b is bound to Rh(I)33 and is seen in related d8 cyclophane-based metal bis(NHC) compounds.17,37,38 In 2a, this arrangement allows the C−Ni−C bite angle to approach the ideal square-planar value of 90°. This angle is narrowed from 85.8(2)° in 2a to 79.92(18)° in 2b by the closure of the second eight-membered nickelacycle. In both cases, these geometries are rigid in solution, as evidenced by complex splitting patterns for the six protons of the three-carbon linker in the 1H NMR spectra.13,14 As noted by Vinh Huynh15 and co-workers, bis(NHC) ligands bridged by a single carbon exclusively form homoleptic tetrakis(NHC) compounds with nickel when synthesized by this method. In these homoleptic compounds, the NHC rings and the nickelacycles formed by coordination approach coplanarity. Longer linkers allow for the arrangement of ligands seen in 2a,b and enable access to stable Ni(II) bis(NHC) dihalide compounds.15,16 Synthesis and Crystallography of Ni(II) Bis(NHC) Bipyridine Compounds. While 2a has been previously synthesized and evaluated as a catalyst for the Kumada coupling reaction,15,16 the coordination chemistry of the bis(NHC) Ni(II) fragment contained within it has not been explored. Because of the preferred cis-chelating nature of these bis(NHC) ligands, particularly in complex 2b, the coordination of well-known cis-chelating bipyridine ligands to the bis(NHC) Ni(II) fragment is straightforward (Scheme 2).

Scheme 1. Synthesis of Ni(II) Bis(NHC) Dibromide Compounds 2a,b

Crystals of 2b suitable for X-ray diffraction were grown by vapor diffusion of diethyl ether into a concentrated dichloromethane solution of the complex. The solid-state structure of 2b is shown in Figure 1, while metrical parameters for 2a,b are shown in Table 1.

Scheme 2. Synthesis of Ni(II) Bis(NHC) Bipyridine Compounds 3a,b and 4a,b

The bromide ligands in 2a,b are sufficiently labile that, at least in the case of bipyridine coordination, strong halide abstraction agents such as Ag+ and Tl+ are not required. Instead, the reaction shown in Scheme 2 is driven by precipitation of the dicationic Ni(II) product in the presence of the hexafluorophosphate anion in methanol. Complex 3a is a pale yellow powder, while the three other bis-carbene bipyridine complexes are nearly white. Crystals of these complexes suitable for X-ray diffraction were grown by vapor diffusion of diethyl ether into a concentrated acetonitrile solution of the complex in question. Solid-state structures of Ni(II) bis(NHC) bipyridine complexes 3a,b and 4a,b are presented in Figure 2, while metrical parameters are presented in Table 2. The nickel coordination environments in 3a,b and 4a,b are nearly square planar. The dihedral twists are negligible, and all four complexes have τδ values close to the ideal value of 0 for square-planar geometries.25 The bis(NHC) ligands retain geometries similar to those found in dibromide precursors 2a,b. Electrochemistry of Ni(II) Bis(NHC) Bipyridine Compounds. The electrochemical behavior of the Ni(II) bipyridine complexes in solution was investigated by cyclic voltammetry (CV) in order to explore the effects of the strongly donating bis(NHC) ligand on the electronic structure of the nickel center and to assess any involvement of the bipyridine ligand in the redox chemistry of these complexes. CV scans were taken in

Figure 1. Solid-state structure of 2b. Hydrogen atoms and solvent molecules are omitted for clarity. Thermal ellipsoids are set to the 50% probability level.

Table 1. Structural Parameters of Compounds 2a,b Ni1−C1 (Å) Ni1−C2 (Å) Ni1−Br1 (Å) Ni1−Br2 (Å) C1−Ni1−C2 (deg) τδa

2ab

2b

1.859(6) 1.859(6) 2.3545(10) 2.3568(10) 85.8(2) 0.09

1.842(4) 1.848(5) 2.3563(7) 2.3552(7) 79.92(18) 0.16

See ref 25. τδ values between 0.0 and 0.2 are consistent with distorted-square-planar geometries. bValues taken from crystallographic data associated with ref 16.

a

The structure of 2b is generally similar to that previously reported for 2a, with the planes of the benzimidazole moieties arranged orthogonally to the plane of the square-planar d8 B

DOI: 10.1021/acs.organomet.7b00847 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 3. Electrochemical Data for 3a,b and 4a,b complex

E1/2 (V vs Fc+/Fc)

3a 3b 4a 4b

−1.42 −1.50 −1.54a −1.59

ΔEPb

a

E1/2 (V vs Fc+/Fc) −1.79 −2.00 −1.85a −2.06

ΔEPb

a

69 76

70 75

a

Pseudoreversible redox features. bE1/2 = 0.5(Epa + Epc), where Epa and Epc are anodic and cathodic peak potentials, respectively.

the carbene ligand inhibits geometry changes about the metal center upon reduction and gives rise to reversible redox features. It is believed that using only one bridging propyl group provides sufficient flexibility in the ligand to allow for deviation from square-planar geometry, leading to a more irreversible response. Support for this effect can further be seen in the scan rate dependence studies for 3a and 4a (Figures S1 and S3 in the Supporting Information), where both the first and second redox features gain reversibility when faster scan rates are used. In all cases, the reductions appear to be singleelectron-redox processes. The peak to peak separations in 3b are 69 and 70 mV for the first and second reductions, respectively. Internal ferrocene in the same scan shows a peak to peak separation of 71 mV. In 4b the peak to peak separations are 76 and 75 mV, in comparison with 79 mV for internal ferrocene. These splittings allow confident assignment of the reductions of 3b and 4b as single-electron-redox processes. We tentatively assign the reductions of 3a and 4a as single-electron processes as well, by analogy to 3b and 4b given the similarity of their calculated ground state electronic structures (vide infra). Scan rate experiments show a linear dependence between peak current and the square root of the scan rate, indicating that all species are freely diffusing (Figures S1−S8 in the Supporting Information). Bipyridine ligands have been shown to act as reversible electron acceptors in a variety of organometallic complexes, including recently studied group 6 and 7 mono(bipyridine) polycarbonyl systems.34−36 The first reduction of these polycarbonyl systems is typically observed in the range of −1.5 to −1.8 V vs Fc+/Fc and is generally assigned as being bipyridine-based. The first reductions of the bis(NHC) bipyridine systems presented here are near this range, suggesting that the site of first reduction at these Ni(II) complexes may be the bipyridine ligand. Further support for this assignment is shown upon substitution of tert-butyl groups at the 4- and 4′-positions. Complex 4a, which contains the tertbutylbipyridine ligand, displays a first reduction that is shifted negative by 120 mV relative to the unsubstituted version 3a. Similarly, the first reduction potential of the tert-butyl derivative 4b is shifted 90 mV negative in comparison to that of 4a. The direction of this shift is consistent with the increased donating ability of the tert-butyl-substituted ligand. Furthermore, the magnitude of this potential shift is similar to that observed for the first reduction of the bipyridine versus tert-butylbipyridine versions of Re(bipy)(CO)3Cl (105 mV),35 Mo(bipy)(CO)4 (80 mV),34 and W(bipy)(CO)4 (90 mV).34 These results are consistent with the assignment of the first reduction as bipyridine-based. In comparison, increased donation on the bipyridine by tertbutyl substitution has less of an effect on the second reduction, as the second potential for both systems is shifted negatively by only 60 mV for the tert-butyl derivatives. Instead, we tentatively assign the second reduction to be primarily metal based. For

Figure 2. Solid-state structures of 3a,b and 4a,b. Hydrogen atoms, solvent molecules, and hexafluorophosphate counterions are omitted for clarity. Thermal ellipsoids are set to the 50% probability level.

Table 2. Structural Parameters of Compounds 3a,b and 4a,b Ni1−C1 (Å) Ni1−C2 (Å) Ni1−N1 (Å) Ni1−N2(Å) C1−Ni1−C2 (deg) N1−Ni1−N2 (deg) dihedral twist (deg) τδa

3a

3b

4a

4b

1.869(5) 1.876(5) 1.922(3) 1.924(4) 84.04(18) 84.24(15) 8.54 0.08

1.874(6) 1.874(6) 1.920(5) 1.920(5) 79.0(3) 83.8(3) 6.73 0.07

1.877(5) 1.877(5) 1.925(4) 1.925(4) 86.8(3) 83.6(2) 0.00 0.03

1.876(3) 1.864(3) 1.909(2) 1.916(2) 79.00(12) 83.71(10) 7.17 0.08

See ref 25. τδ values between 0.0 and 0.2 are consistent with distorted-square-planar geometries.

a

acetonitrile with 0.1 M tetra-n-butylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte. Cyclic voltammograms of 3a,b and 4a,b are shown in Figure 3, while reduction potentials are presented in Table 3.

Figure 3. Cyclic voltammograms (CVs) of 3a and 4a (left) and 3b and 4b (right). Experiments were performed at concentrations of 1 mM in MeCN with 0.1 M TBAPF6 as the supporting electrolyte under a N2 atmosphere with a 3 mm diameter glassy-carbon working electrode, platinum-wire counter electrode, and nonaqueous Ag+/Ag reference electrode. Scans were recorded at 100 mV/s and referenced to an internal ferrocene (Fc) standard.

Complexes 3a and 4a, which are bound by the MeCPropCMe ligand 1a, show two pseudoreversible reductions. For complexes 3b and 4b, which are bound by the doubly bridged CProp2C ligand 1b, these reductions sharpen into fully reversible features. These differences in reversibility between 3a/4a and 3b/4b are attributed to geometric changes upon reduction. In complexes 3b and 4b the addition of a second propyl bridge to C

DOI: 10.1021/acs.organomet.7b00847 Organometallics XXXX, XXX, XXX−XXX

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CONCLUSIONS A series of new Ni(II) bipyridine complexes containing the bis(NHC) ligands 1a,b were synthesized from the dibromide precursors (MeCPropCMe)NiIIBr2 (2a) and (CProp2C)NiIIBr2 (2b). These complexes represent the first examples of heteroleptic NiII(NHC2)L2 complexes and display a structural motif that is unknown in analogous Ni(II) diphosphine complexes. Each complex shows two reductions, although only those ligated by the doubly bridged CProp2C ligand display reversible electrochemistry. In all cases, the first reduction is assigned as bipyridine-based, while the second reduction is assigned as metal-based. In addition to (CProp2C)NiIIBr2 acting as a precursor for these new molecules, we anticipate that it will be able to provide a route to a variety of new structural motifs, as the reversibility of the electrochemistry in 3b and 4b suggests that this ligand may be able to stabilize transition-metal complexes in environments beyond the d8 square-planar examples shown here.

both the bipyridine and tert-butylbipyridine complexes, the doubly bridged CProp2C ligand 1b leads to a 210 mV negative shift of the second reduction in comparison with the singly bridged counterpart. The first reduction is shifted negatively by only 80 and 50 mV for the bipyridine and tert-butylbipyridine complexes, respectively. This suggests that the second reduction is metal-based. The large magnitude of the second reduction shift may be due to the restricted bite angle of the cyclic ligand, which would be expected to destabilize the presumably tetrahedral Ni(I) state. DFT Analysis. In order to further support the redox assignments made on the basis of the electrochemical characterization of these complexes, density functional theory (DFT) analysis of 3b was performed with the COSMO solvation model.37 Agreement between the calculated structure and the solid-state structure was excellent (Table S6 in the Supporting Information). The LUMO of 3b is localized primarily on the bipyridine ligand. Upon reduction of 3b by one electron to give the monocationic complex, the coordination geometry at nickel is computed to remain square planar. The singly occupied molecular orbital (SOMO) of the monocation and the calculated spin density are both localized in the bipyridine ligand (see Figure 4 and Figure S16 in the

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EXPERIMENTAL SECTION

General Considerations. All reagents were obtained from commercial suppliers and used without purification unless otherwise noted. All reactions were carried out under a N2 atmosphere using standard Schlenk and glovebox techniques. Acetonitrile (MeCN) and dichloromethane (DCM) were dried over activated 3 Å molecular sieves and alumina and degassed prior to use. Nickel(II) acetate hexahydrate was dried under vacuum at 60 °C overnight. Complete dehydration was verified gravimetrically. Tetra-n-butylammonium hexafluorophosphate (TBAPF6) was twice recrystallized from methanol (MeOH) and dried under vacuum. Ligand precursors 1a,b and and complex 2a were synthesized as previously reported.15,16,32 The purity of all complexes has been assed by elemental analysis (EA) through NuMega Resonance Laboratories. Synthesis of (CProp2C)NiBr2 (2b). In a procedure analogous to the literature preparation of compound 2a,15 ligand precursor 1b (1.01 g, 2.11 mmol), anhydrous Ni(OAc)2 (460 mg, 2.60 mmol), and tetra-nbutylammonium bromide (10.0 g, 31.02 mmol) were combined in a round-bottom flask with a simple distillation head and heated under vacuum. The temperature rose to 140 °C over the course of 2.5 h and was held there for a further 3.5 h. After it was cooled to room temperature, the crude solid was triturated with 150 mL of water. The resulting yellow precipitate was collected by vacuum filtration and dried overnight under vacuum at 60 °C (1.11 g, 2.07 mmol, 98% yield). Further purification, if necessary, can be accomplished by washing with hot methanol or liquid−liquid extraction with DCM and a saturated aqueous NaBr solution. 1H NMR (500 MHz, d6-DMSO): δ 7.57 (m, 4H, CH), 7.18 (m, 4H, CH), 6.04, (t, br, 4H, CH2), 4.98 (d, br, 4H, CH2), 2.57 (m, br, 2H, CH2), 1.75 (m, br, 2H, CH2). 13 C{1H} NMR (500 MHz, d6-DMSO): δ 133.80 (CH), 123.76 (CH), 110.70 (CH), 48.37 (CH2), 27.39 (CH3). ESI-HRMS ([M]+): m/z calcd for [C20H20Br2N4Ni+NH4]+ 549.9746, found 549.9760. Anal. Calcd for C20H20Br2N4Ni·CH3OH: C, 44.49; H, 4.27; N, 9.88. Found: C, 44.70; H, 4.03; N, 10.05. Synthesis of (MeCPropCMe)(bipy)nickel(II) Hexafluorophosphate (3a). Complex 2a (100 mg, 0.19 mmol) was suspended in methanol (25 mL). To this suspension was added 2,2′-bipyridine (32 mg, 0.20 mmol). This mixture was stirred at room temperature for 2 h, at which point it had clarified completely. To this solution was added sodium hexafluorophosphate (163 mg, 0.97 mmol), predissolved in 5 mL of MeOH, along with 20 mL of H2O. Precipitation proceeded over the next 2 h, at which point the solid pale yellow product was collected by filtration (136 mg, 0.17 mmol, 88% yield). Further purification can be accomplished, if necessary, by prolonged washing with water or liquid−liquid extraction with DCM and water. 1H NMR (400 MHz, d6-DMSO): δ 8.70 (d, 2H, CH), 8.39 (t, 2H, CH), 7.78 (m, 2H, CH), 7.67 (m, 2H, CH), 7.55 (t, 2H, CH), 7.43 (d, 2H, CH), 7.38 (m, 4H, CH) 5.90 (m, br, 2H, CH2), 5.03 (m, br, 2H, CH2), 4.61 (s, 6H,

Figure 4. DFT-calculated HOMO (left) and LUMO (center) plots of the dication of 3b and DFT-calculated SOMO (right) of the monocation of 3b.

Supporting Information). This is consistent with our assignment of the first reduction of these complexes as bipyridinebased. Notably, it was found that, if the initial coordinates of the geometry optimization were set to tetrahedral, simulating a Ni(I) state, this relaxed to a square-planar reduced-bipyridine state over the course of the geometry optimization. The HOMO and LUMO of the dication of 3b, as well as the SOMO of the monocation of 3b, are shown in Figure 4. In addition to the ligand-based LUMO, 3b was found to have a HOMO that is localized on the bis(NHC) ligand. Groundstate DFT calculations were also performed for 3a and 4a,b, and the frontier orbitals calculated for these systems are analogous to those calculated for 4a (see Figures S13−S15 in the Supporting Information). One might predict the presence of a large ligand to ligand charge transfer band in the visible spectrum on the basis of this electronic structure. However, the compounds are off-white solids and only compound 3a begins to show a slight yellow color at high concentrations in solution. The only visible absorbances found in the UV−visible spectra (Figures S9−S12 in the Supporting Information) are tails stemming from strong UV transitions. We attribute this lack of ligand to ligand absorbance to the orthogonality of the HOMO and LUMO in these systems. D

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Organometallics

CH2). 13C{1H} NMR (500 MHz, d6-DMSO): δ 172.66 (CNi), 166.71 (C(CH3)3), 156.01 (CH), 152.26 (CH), 134.33 (CH), 124.78 (CH), 124.46 (CH), 120.74 (CH), 111.42 (CH), 56.29 (C(CH3)3), 48.29 (CH2), 36.37 (CH2), 30.25 (CH2), 27.07 (CH2). ESI-HRMS ([M]2+): m/z calcd for [C38H44N6Ni]2+ 321.1485, found 321.1485. Anal. Calcd for C38H44N6NiP2F12: C, 48.90; H, 4.75; N, 9.00. Found: C, 49.30; H, 4.89; N, 8.84. UV−vis (MeCN): λmax/nm (ε/M−1 cm−1) 318 (21700 ± 1400), 306 (20100 ± 1400), 288 (17800 ± 1200), 270 (22600 ± 1500). Electrochemistry. Electrochemical experiments were performed using a BASi Epsilon potentiostat. Cyclic voltammograms (CVs) were performed under nitrogen in a one-compartment cell using a glassycarbon BASi working electrode (3 mm diameter), a platinum-wire counter electrode, and a nonaqueous silver reference electrode from BASi containing a silver wire in an acetonitrile solution of 0.1 M tetran-butylammonium hexafluorophosphate (TBAPF6) and 0.1 mM AgNO3. All experiments were performed in dry acetonitrile using 0.1 M TBAPF6 as the supporting electrolyte. Ferrocene was used as an internal reference. X-ray Crystallographic Studies. Single-crystal X-ray diffraction studies were carried out on a Bruker Kappa APEX-II CCD diffractometer equipped with molybdenum Kα radiation (λ = 0.710 73 Å). The crystals were mounted on a Cryoloop with Paratone oil, and data were collected under a nitrogen gas stream at 100 K using ω and φ scans. Data were integrated and scaled using the Bruker SAINT software program. Solution by direct methods (SHELXS) produced a complete phasing model consistent with the proposed structure. All non-hydrogen atoms were refined anisotropically by full-matrix least squares (SHELXL-97). All hydrogen atoms were placed using a riding model. Their positions were constrained relative to their parent atom using the appropriate HFIX command in SHELXL-97. Crystallographic data, structure refinement parameters, and additional notes on structure refinement are summarized in the Supporting Information. Spectroscopy. 1H NMR spectra were recorded on a Varian Hg 400 MHz spectrometer at room temperature. 13C NMR spectra were recorded on a Varian VX 500 MHz spectrometer at room temperature. Data were processed using ACD/NMR Processor software. Chemical shifts are reported relative to TMS (δ 0) and referenced against solvent residual peaks. Microanalyses were performed by NuMega Resonance Laboratories, Inc. High-resolution mass spectra were recorded on an Agilent 6230 Accurate-Mass TOFMS instrument. UV−vis spectra were recorded on a Shimadzu UV 3600 spectrometer. UV−visible absorbances were measured in a cell with a 1 mm path length and quartz windows. DFT Calculations. Restricted Kohn−Sham (RKS) calculations were performed in the ORCA software suite (version 3.0.3) using the B3LYP functional with the RIJCOSX approximation.38−42 All atoms were treated with the Ahlrichs DEF2-TZVP/J basis set.43−46 Dispersion corrections were applied using the Becke−Johnson damping scheme (D3BJ), and solvation was accounted for using the COSMO solvation model in acetonitrile.37,47 All structures were optimized starting from solid-state structures determined by X-ray crystallography and confirmed to be minima by frequency calculations performed at the same level of theory. Molecular graphics and analyses were performed with the UCSF Chimera package, which was developed by the Resource for Biocomputing, Visualization, and Informatics at the University of California, San Francisco (supported by NIGMS P41-GM103311).48

CH3), 2.03 (m, br, 2H, CH2). 13C{1H} NMR (500 MHz, d6-DMSO): δ 171.18 (CNi), 156.16 (CH), 151.82 (CH), 142.48 (CH), 135.68 (CH), 135.05 (CH), 128.88 (CH), 124.37 (CH), 124.05 (CH), 112.07 (CH), 111.17 (CH), 48.69 (CH2), 35.68 (CH2), 28.47 (CH3). ESIHRMS ([M]2+): m/z calcd for [C29H28N6Ni]2+ 259.0859, found 259.0860. Anal. Calcd for C29H28N6NiP2F12: C, 43.04; H, 3.49; N, 10.39. Found: C, 42.65; H, 3.88; N, 10.35. UV−vis (MeCN): λmax/nm (ε/M−1 cm−1) 321 (22000 ± 1300), 309 (18900 ± 1100), 283 (22600 ± 1400), 272 (25400 ± 1600). Synthesis of (CProp2C)(bipy)nickel(II) Hexafluorophosphate (3b). Complex 2b (127 mg, 0.24 mmol) was suspended in methanol (30 mL). To this suspension was added 2,2′-bipyridine (42 mg, 0.27 mmol). This mixture was stirred at room temperature for 2 h, at which point it had clarified completely. To this solution was added sodium hexafluorophosphate (132 mg, 0.79 mmol), predissolved in 5 mL of MeOH, along with 15 mL of H2O. Precipitation proceeded over the next 2 h, at which point the solid white product was collected by filtration (126 mg, 0.15 mmol, 65% yield). Further purification can be accomplished, if necessary, by prolonged washing with water or liquid−liquid extraction with DCM and water. 1H NMR (400 MHz, d6-DMSO) δ: 8.71 (d, 2H, CH), 8.40 (td, 2H, CH), 7.83 (m, 2H, CH), 7.73 (m, 4H, CH), 7.57 (m, 2H, CH), 7.32 (m, 4H, CH), 5.93 (t, 4H, CH2), 5.01 (d, 4H, CH2), 2.40 (m, br 2H, CH2), 1.87 (m, br 2H, CH2). 13C{1H} NMR (500 MHz, d6-DMSO) δ: 172.08 (CNi), 156.16 (CH), 151.52 (CH), 135.64 (CH), 135.07 (CH), 124.91 (CH), 124.33 (CH), 124.04 (CH), 120.81 (CH), 112.01 (CH), 111.10 (CH), 48.72 (CH2), 36.33 (CH2), 35.641 (CH2), 30.21 (CH2), 25.59 (CH2). ESI-HRMS ([M]2+): m/z calcd for [C30H28N6Ni]2+ 265.0861, found 265.0859. Anal. Calcd for C30H28N6NiP2F12·1.5H2O: C, 42.48; H, 3.68; N, 9.91. Found: C, 42.68; H, 4.08; N, 10.04. UV−vis (MeCN): λmax/nm (ε/M−1 cm−1) 320 (17700 ± 300), 309 (15700 ± 300), 287 (14300 ± 300), 270 (17500 ± 200). Synthesis of (MeCPropCMe)(tBu-bipy)nickel(II) Hexafluorophosphate (4a). Complex 2a (62 mg, 0.12 mmol) was suspended in methanol (20 mL). To this suspension was added 4,4′-di-tert-butyl2,2′-bipyridine (36 mg, 0.13 mmol). This mixture was stirred at room temperature for 2 h, at which point it had clarified completely. To this solution was added sodium hexafluorophosphate (117 mg, 0.70 mmol), predissolved in 5 mL of MeOH, along with 25 mL of H2O. Precipitation proceeded over the next 2 h, at which point the solid white product was collected by filtration (109 mg, 0.12 mmol, 99% yield). Further purification can be accomplished, if necessary, by prolonged washing with water or liquid−liquid extraction with DCM and water. 1H NMR (400 MHz, d6-DMSO): δ 8.79 (s, 2H, CH), 7.71 (m, 6H, CH), 7.46 (d, 2H, CH), 7.31 (m, 4H, CH), 5.89 (t, 2H, CH2), 4.99 (d, b, 2H, CH2), 2.39 (m, br, 2H, CH2), 1.86 (m, br, 2H, CH2), 1.42 (s, 18H, CH3). 13C{1H} NMR (500 MHz, d6-DMSO): δ 171.1 (CNi), 166.2 (C(CH3)3), 155.8 (CH), 151.1 (CH), 135.2 (CH), 134.7 (CH), 124.5 (CH), 123.9 (CH), 123.6 (CH), 120.4 (CH), 111.6 (CH), 110.7 (CH), 67.1 (C(CH3)3), 48.3 (CH2), 35.9 (CH2), 29.8 (CH 2 ), 25.2 (CH 3 ). ESI-HRMS ([M] 2+ ): m/z calcd for [C 37 H44 N 6 Ni] 2+ 315.1485, found 315.1486. Anal. Calcd for C37H44N6NiP2F12: C, 48.23; H, 4.81; N, 9.12. Found: C, 47.96; H, 5.20; N, 8.74. UV−vis (MeCN): λmax/nm (ε/M−1 cm−1) 319 (18000 ± 1000), 306 (15600 ± 800), 283 (18300 ± 900), 272 (21500 ± 1100). Synthesis of (CProp2C)(tBu-bipy)nickel(II) Hexafluorophosphate (4b). Complex 2b (158 mg, 0.29 mmol) was suspended in methanol (35 mL). To this suspension was added 2,2′-bipyridine (85 mg, 0.32 mmol). This mixture was stirred at room temperature for 2 h, at which point it had clarified completely. To this solution was added sodium hexafluorophosphate (158 mg, 0.94 mmol), predissolved in 5 mL of MeOH, along with 10 mL of H2O. Precipitation proceeded over the next 2 h, at which point the solid white product was collected by filtration (194 mg, 0.21 mmol, 71% yield). Further purification can be accomplished, if necessary, by prolonged washing with water or liquid−liquid extraction with DCM and water. 1H NMR (500 MHz, d6-DMSO): δ 8.72 (d, 2H, CH), 7.31 (m, 2H, CH), 7.83 (d, 2H, CH), 7.73 (m, 4H, CH), 7.57 (m, 2H, CH), 7.32 (m, 4H, CH), 5.93 (m, 4H, CH2), 5.01 (m, 4H, CH2), 2.40 (m, 2H, CH2), 1.87 (m, 2H,

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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.7b00847. Crystallographic data, additional information about cyclic voltammograms, DFT calculations, and additional characterization data (PDF) Cartesian coordinates for a calculated structure (XYZ) E

DOI: 10.1021/acs.organomet.7b00847 Organometallics XXXX, XXX, XXX−XXX

Article

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(15) Vinh Huynh, H.; Jothibasu, R. Eur. J. Inorg. Chem. 2009, 2009, 1926−1931. (16) Berding, J.; Lutz, M.; Spek, A. L.; Bouwman, E. Organometallics 2009, 28, 1845−1854. (17) Ibrahim, H.; Bala, M. D. New J. Chem. 2016, 40, 6986−6997. (18) Liu, B.; Zhang, Y.; Xu, D.; Chen, W. Chem. Commun. 2011, 47, 2883−2885. (19) Baker, M. V.; Skelton, B. W.; White, A. H.; Williams, C. C. J. Chem. Soc., Dalton Trans. 2001, 111−120. (20) Guo, J.; Lv, L.; Wang, X.; Cao, C.; Pang, G.; Shi, Y. Inorg. Chem. Commun. 2013, 31, 74−78. (21) Huynh, H. V.; Koh, C. H. M.; Nguyen, V. H. Dalton Trans. 2017, 46, 11318−11326. (22) Zell, T.; Feierabend, M.; Halfter, B.; Radius, U. J. Organomet. Chem. 2011, 696, 1380−1387. (23) Fischer, P.; Götz, K.; Eichhorn, A.; Radius, U. Organometallics 2012, 31, 1374−1383. (24) Schaub, T.; Doring, C.; Radius, U. Dalton Trans. 2007, 1993− 2002. (25) Reineke, M. H.; Sampson, M. D.; Rheingold, A. L.; Kubiak, C. P. Inorg. Chem. 2015, 54, 3211−3217. (26) Jean-Baptiste dit Dominique, F.; Gornitzka, H.; Hemmert, C. Organometallics 2010, 29, 2868−2873. (27) Thoi, V. S.; Chang, C. J. Chem. Commun. 2011, 47, 6578−6580. (28) Chiu, P. L.; Lai, C.-L.; Chang, C.-F.; Hu, C.-H.; Lee, H. M. Organometallics 2005, 24, 6169−6178. (29) Spencer, L. P.; Winston, S.; Fryzuk, M. D. Organometallics 2004, 23, 3372−3374. (30) Liao, C.-Y.; Chan, K.-T.; Zeng, J.-Y.; Hu, C.-H.; Tu, C.-Y.; Lee, H. M. Organometallics 2007, 26, 1692−1702. (31) Berding, J.; van Dijkman, T. F.; Lutz, M.; Spek, A. L.; Bouwman, E. Dalton Trans. 2009, 6948−6955. (32) Shi, Z.; Thummel, R. P. J. Org. Chem. 1995, 60, 5935−5945. (33) Shi, Z.; Thummel, R. P. Tetrahedron Lett. 1995, 36, 2741−2744. (34) Clark, M. L.; Grice, K. A.; Moore, C. E.; Rheingold, A. L.; Kubiak, C. P. Chem. Sci. 2014, 5, 1894−1900. (35) Smieja, J. M.; Kubiak, C. P. Inorg. Chem. 2010, 49, 9283−9289. (36) Sampson, M. D.; Nguyen, A. D.; Grice, K. A.; Moore, C. E.; Rheingold, A. L.; Kubiak, C. P. J. Am. Chem. Soc. 2014, 136, 5460− 5471. (37) Sinnecker, S.; Rajendran, A.; Klamt, A.; Diedenhofen, M.; Neese, F. J. Phys. Chem. A 2006, 110, 2235−2245. (38) Neese, F. J. Comput. Chem. 2003, 24, 1740−1747. (39) Kossmann, S.; Neese, F. Chem. Phys. Lett. 2009, 481, 240−243. (40) Neese, F.; Wennmohs, F.; Hansen, A.; Becker, U. Chem. Phys. 2009, 356, 98−109. (41) Izsák, R.; Neese, F. J. Chem. Phys. 2011, 135, 144105−14115. (42) Neese, F. Wiley Interdisciplinary Reviews: Computational Molecular Science 2012, 2, 73−78. (43) Schäfer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571−2577. (44) Schäfer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829−5835. (45) Andrae, D.; Häußermann, U.; Dolg, M.; Stoll, H.; Preuß, H. Theor. Chem. Acc. 1990, 77, 123−141. (46) Weigend, F. Phys. Chem. Chem. Phys. 2006, 8, 1057−1065. (47) Grimme, S.; Ehrlich, S.; Goerigk, L. J. Comput. Chem. 2011, 32, 1456−1465. (48) Pettersen, E. F.; Goddard, T. D.; Huang, C. C.; Couch, G. S.; Greenblatt, D. M.; Meng, E. C.; Ferrin, T. E. J. Comput. Chem. 2004, 25, 1605−1612.

(XYZ) (XYZ) (XYZ) (XYZ)

Accession Codes

CCDC 1587192−1587196 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 for C.P.K.: [email protected]. ORCID

Clifford P. Kubiak: 0000-0003-2186-488X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the W.M. Keck Center for Integrated Biology for use of their computing cluster, Dr. Curtis Moore and Dr. Milan Gembicky for assistance with X-ray crystallographic studies, and Dr. Yongxuan Su for HRMS analysis. This work was supported by the Air Force Office of Scientific Research through the MURI program under AFOSR Award FA9550-10-1-0572. T.M.P. acknowledges support from NSF CHE-1461632.

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REFERENCES

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DOI: 10.1021/acs.organomet.7b00847 Organometallics XXXX, XXX, XXX−XXX