Ligand-Centered Borenium Reactivity in Triaminoborane-Bridged

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Ligand-Centered Borenium Reactivity in Triaminoborane-Bridged Diphosphine Complexes Kyounghoon Lee,† Clara Kirkvold,‡ Bess Vlaisavljevich,*,‡ and Scott R. Daly*,† †

Department of Chemistry, The University of Iowa, E331 Chemistry Building, Iowa City, Iowa 52242, United States Churchill-Haines Laboratories, Department of Chemistry, The University of South Dakota, Vermillion, South Dakota 57069, United States

‡

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S Supporting Information *

ABSTRACT: Borenium ions (i.e., three-coordinate boron cations) are known to promote a wide variety of stoichiometric and catalytic reactions because of their high Lewis acidity. As demonstrated by the growing number of chemically reactive borane ligands, there is considerable interest in developing ligands with highly electrophilic boron sites that promote multisite reactivity in metal complexes. However, there are currently few examples of ligand-centered borenium ions, especially with ligands that form coordination complexes with a wide range of metals. Here we report borenium-like reactivity on a highly versatile diphosphine ligand. Treating (PhTBDPhos)NiCl2 (1) with strong Bronsted acids such as HBF4·Et2O, HOTf, or HNTf2 resulted in fluoride or chloride abstraction from BF4− or NiCl2, respectively, to form trans N−H and B−X bonds on the ligand backbone. HCl addition to the bridgehead N−B bond is reversible, and the reactivity depends on the identity of the supporting counteranions, as observed when treating [(PhTBDPhos)NiCl]2X2, where X = NTf2− (3), OTf− (4), or BArF4− (5), with HCl. The reaction of 4 with HNTf2 instead of HCl yielded NMR evidence of the latent borenium cation in solution and showed how poor nucleophiles such as triflate bind to the borenium ion in the solid state. Remarkably, replacing the chloride ligands in 1 with chelating and less-labile thiolates or catecholates, or changing the phosphorus substituents (phenyl to isopropyl), attenuates the reactivity on the ligand backbone. Density functional theory was used to quantify the reaction free energies, and the theoretical results corroborate the experimental observations. Given the broad utility of diphosphines in homogeneous catalysis and the known benefits of strong Lewis acid promotors in many catalytic reactions, we anticipate that the results will provide new opportunities for dual-site reactivity involving boron ligands and metals.

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INTRODUCTION Reactive ligand platforms containing trisubstituted boranes have proven to be effective for promoting new types of smallmolecule reactivity and catalysis with transition metals.1−10 Borane ligands often participate in reactions by capturing nucleophiles or stabilizing activated substrates, and they generally fall into one of two classes depending on the boron’s location and configuration with respect to the metal. Ligands with inner-sphere boranes that form dative Z-type M → B bonds with electron-rich metals have been shown to cleave H2 with borane assisting in hydride capture,11−17 and similar 1,2addition reactivity has been observed with a wide variety of E− H and E−X substrates.16−22 Ligands with outer-sphere boranes have proven to be equally useful for applications such as the hydrogenation of metal-bound CO, optical sensing of fluoride and cyanide, and hydrazine capture.10,23−29 While boranes have been explored extensively as reactive ligand moieties, there are few reported examples with related borenium ions (Chart 1).30−34 Borenium ions are threecoordinate boron cations composed of two anionic substituents and a neutral two-electron donor.35 They are strong © XXXX American Chemical Society

Chart 1. Comparison of Trisubstituted Boranes and Borenium Ions (Left) and Examples of Ligand-Centered Borenium Ions (Right)

Lewis acids because of the positive charge on boron, and their high Lewis acidity has been exploited with great effect in stoichiometric and catalytic reactions.36−38 This makes borenium ions very attractive for incorporation into reactive ligand platforms, but examples have been limited to ferrocenetype complexes containing coordinatively saturated metal centers. For example, Jaekle and co-workers prepared and Received: June 8, 2018

A

DOI: 10.1021/acs.inorgchem.8b01601 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

highly electrophilic ligand-centered borenium ion. The results represent rare examples of borenium reactivity on a diphosphine, a versatile class of ligands used heavily in homogeneous and cross-coupling catalysis. Furthermore, the experimental and theoretical results demonstrate how the ligand-centered borenium ion reactivity can be controlled by simply modifying ancillary ligands and the substituents attached to phosphorus.

structurally characterized ferrocenylborenium complexes by halide abstraction of the corresponding chloroborane adducts (Chart 1).39 Brunker et al. employed a similar strategy by abstracting halides or hydrides from azaferrocene−boranes to isolate several borenium cations.40 It was also reported that a tethered ruthenium(II) thiolate complex forms putative borenium ions upon reaction with secondary boranes.41 Single-crystal X-ray diffraction (XRD) and NMR data, however, appear to be more consistent with metal−ligand cooperative bonding of the boranes across Ru−thiolate bonds, as described by Sellmann and co-workers.42 Recently, we reported a new class of outer-sphere borane ligands called TBDPhos and described their preliminary reactivity with nickel and palladium.43 TBDPhos ligands are diphosphines bridged by a chelated triaminoborane called 1,8,10,9-triazaboradecaline (TBD).44 We discovered that (PhTBDPhos)MCl2 (where M = Ni or Pd) selectively reacts with H2O, alcohols, or hydrated nBu4NF to yield trans H−O or H−F addition across the bridgehead N−B bond on the TBD backbone (Scheme 1). This ligand-centered reactivity

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RESULTS AND DISCUSSION Borenium Reactivity. The first evidence of ligandcentered borenium reactivity with TBDPhos was discovered in reactions of (PhTBDPhos)NiCl2 (1) with HBF4·Et2O (Scheme 2). It has been reported that borenium ions have sufficient Lewis acidity to abstract fluoride from BF4− despite the high Lewis acidity of BF3.40 As an introduction, the Lewis acidity is typically measured using the Gutmann−Beckett or Childs methods to derive an experimental acceptor number (AN), where higher AN values indicate higher Lewis acidity.46−48 Both methods rely on the measurement of changes in the NMR chemical shift of a Lewis base upon dative acid−base adduct formation (OPEt3 for the Gutmann− Beckett method and crotonaldehyde for the Childs method). The experimental AN for BF3 in CDCl3 is 84, whereas the only known value for a triaminoborane was obtained for neat B(NMe2)3 with an AN = 9.45 The AN value for 1 was too low to be measured in CDCl3 using the Gutmann−Beckett method,43 but we suspect that the borane AN is similar to that of B(NMe2)3. Despite the very low Lewis acidity of PhTBDPhos, the reaction of 1 with 2 equiv of HBF4·Et2O in CH2Cl2 at room temperature resulted in fluoride abstraction from BF4− and a net trans HF addition across the bridgehead N−B bond in the TBD backbone. Two 11B NMR resonances assigned to fourcoordinate boron in TBDPhos and BF4− were observed in the 11 B NMR spectrum at δ 1.9 and −0.7, respectively. The 19F NMR spectrum revealed two corresponding peaks at δ −163.8

Scheme 1. Previously Reported Reactivity Studies with (PhTBDPhos)MCl2 Complexes43

was somewhat unusual given the nonexistent Lewis acidity of triaminoboranes,45 and our studies suggested that protonation of the bridgehead nitrogen in TBD was important for overcoming the low Lewis acidity at boron. Here we report evidence that the ligand-centered reactivity in TBDPhos complexes can stem from latent formation of a

Scheme 2. Reactivity Studies of 1 with Bronsted Acids and Silver Salts

B

DOI: 10.1021/acs.inorgchem.8b01601 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry (N3B−F) and −151.8 (BF4−), and a sharp resonance appeared at δ 69.9 in the 31P NMR spectrum. Single-crystal XRD data revealed that one chloride was displaced to yield a chloridebridged nickel dimer with outer-sphere BF4− counteranions ({[(PhTBDPhos-HF)Ni]2(μ-Cl)2}(BF4)2 (2-HF); Figure 1).

Table 2. 11B and 31P NMR Resonances for Compounds Reported Hereina 11

compound

31

B

23.9 1.9 24.7 24.7 6.2 24.7 6.0 1.1 2.6 24.6 5.3 24.6 24.4 24.0 24.3

1 2-HF 3 3-HCl 4 4-HCl 4-H2O 4-MeOH 5 5-HCl 6 7 8 9

P{1H}

solvent

70.1 69.9 67.8 70.4 71.2 67.8 70.2 71.7 64.5 68.3 69.8 97.8 79.0 78.6 71.6

CDCl3 CDCl3 CD3CN CH2Cl2 CH2Cl2 CD3CN CH2Cl2 CHCl3 CH3OH CD3CN CH2Cl2 CDCl3 CDCl3 CDCl3 CDCl3

Figure 1. Molecular structure of 2-HF with thermal ellipsoids at the 35% probability level. Phenyl groups, BF4− counteranions, cocrystallized solvent molecules, and hydrogen atoms attached to carbon atoms were omitted from the figure.

Chemical shifts are reported in δ units in ppm relative to BF3·Et2O (11B) and 85% H3PO4 (31P).

The structure confirmed that H−F was added across the bridgehead N−B bond to form trans N−H and B−F bonds. The B−F and N−B bond distances of 1.419(5) and 1.611(6) Å (Table 1) are similar to those previously for the reported fluoride-bound TBDPhos complex {[(PhTBDPhos-HF)Ni]2(μOH)2}Cl2 (Scheme 1). We next attempted to isolate the ligand-centered borenium ion by treating 1 with other strong Bronsted acids containing less-reactive counteranions. The addition of HNTf2 to a solution of 1 in CH2Cl2 yielded quantitative conversion to a new complex, as determined by NMR analysis of the mixture (Table 2). The 31P NMR resonance for 1 at δ 70.1 shifted slightly downfield to δ 71.2, and the 11B NMR resonance at δ 23.9 shifted to δ 6.2. This latter peak appeared downfield relative to other TBDPhos complexes with four-coordinate boron (δ 3.2−1.0).43 XRD analysis of single crystals grown from the reaction mixture, however, revealed that the crystals contained a mixture of two compounds superimposed in almost the same position in the unit cell. Both compounds were chloride-bridged nickel(II) dimers with outer-sphere NTf2− anions to balance the remaining charge on each Ni2+, but one had HCl bound trans across the bridgehead N−B bond ({[(PhTBDPhos-HCl)Ni]2(μ-Cl)2}(NTf2)2 (3-HCl)), whereas the other did not ({[(PhTBDPhos)Ni]2(μ-Cl)2}(NTf2)2 (3)). The crystallographic data were successfully modeled with ca. 40% and 60% of 3-HCl and 3, respectively,

and elemental analysis data collected on multiple crystals were close to this distribution. Remarkably, the observed mixture of 3-HCl and 3 is consistent with their calculated free-energy difference of 0.7 kcal/mol in dichloromethane (vide infra). The B−Cl bond distance of 1.96(2) Å in 3-HCl is longer than the N3B−Cl distances reported in subporphyrins,49−53 chlorotris(pyrazolyl)borates, 5 4 , 5 5 and related compounds,18,19,56 which typically range from 1.82 to 1.89 Å. Despite the reaction mixture showing single resonances corresponding to 3-HCl in the 31P and 11B NMR spectra prior to crystallization, NMR analysis of the isolated crystals revealed two sets of resonances consistent with the presence of both 3HCl and 3 in the modeled crystallographic data. The 31P NMR spectrum showed two resonances at δ 71.2 (3-HCl) and 70.4 (3), and the 11B NMR spectrum showed resonances at δ 6.2 (3-HCl) and 24.7 (3) in CH2Cl2. The difference in the products before and after reaction workup suggested that 3HCl slowly loses HCl to form 3 upon isolation. To test this hypothesis, we prepared 3 and {[(PhTBDPhos)Ni]2(μ-Cl)2}(OTf)2 (4) by treating 1 with AgNTf2 and AgOTf, respectively. Gratifyingly, the addition of 2 equiv of HCl in CH2Cl2 to 3 and 4 yielded an immediate reaction and the reappearance of the four-coordinate boron resonance in the 11 B NMR spectra consistent with 3-HCl and {[(PhTBDPhosHCl)Ni]2(μ-Cl)2}(OTf)2 (4-HCl). The 11B NMR data for the reaction of 3 with HCl matched those observed for 3-HCl, and

a

Table 1. Selected Bond Distances and Angles from Single-Crystal XRD Data M−P B−N B−N(P) P−N B−X P−M−P ∑NBN

1a

2-HFb

3

3-HCl

4

4-HCl

5

6c

7

8

9

2.1463(6) 2.1556(6) 1.408(3) 1.454(3) 1.454(3) 1.672(2) 1.678(2)

2.151(1) 2.164(1) 1.611(6) 1.504(6) 1.517(6) 1.644(3) 1.649(4) 1.419(5) 89.66(4) 330.9(3)

2.1492(9) 2.1699(9) 1.401(4) 1.447(4) 1.469(4) 1.660(2) 1.665(2)

2.1434(7) 2.1578(7) 1.59(2) 1.46(1) 1.47(1) 1.651(8) 1.700(7) 1.96(2) 89.99(3) 342(2)

2.149(2) 2.151(2) 1.391(9) 1.457(8) 1.470(8) 1.657(5) 1.668(5)

2.141(2) 2.147(1) 1.530(8) 1.477(8) 1.490(8) 1.653(4) 1.654(4) 2.010(8) 92.81(6) 340.5(5)

2.139(1) 2.141(1) 1.402(5) 1.446(6) 1.452(6) 1.662(3) 1.662(4)

2.159(2)

2.1457(9) 2.1603(9) 1.415(4) 1.455(5) 1.450(4) 1.676(3) 1.690(3)

2.1203(6) 2.1292(6) 1.416(3) 1.450(3) 1.456(3) 1.680(2) 1.683(2)

2.1370(5) 2.1520(5) 1.402(2) 1.455(2) 1.457(2) 1.684(2) 1.678(1)

90.20(3) 359.9(3)

97.60(2) 360.0(2)

94.97(2) 359.9(2)

92.91(2) 359.9(2)

88.49(3) 359.8(3)

89.84(6) 360.0(6)

93.27(4) 360.0(4)

1.40(1) 1.455(7) 1.671(5)

95.05(9) 360.0(5)

a

Values obtained from ref 43. bAverage values were used. cValues from the nondisordered half were used. C

DOI: 10.1021/acs.inorgchem.8b01601 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry similar 31P and 11B NMR resonances were observed for 4-HCl. A broad 11B NMR resonance was observed at δ 6.0, and a sharp 31P NMR resonance was observed at δ 70.2. These resonances were consistent with those obtained for 4-HCl prepared by treating 1 with HOTf, as shown in Scheme 2. As with 3-HCl, 4-HCl slowly releases HCl during workup, but crystals with HCl bound were isolated and the structure was confirmed by single-crystal XRD (Figure 2).

Figure 3. Calculated solution structure of 4-HCl showing ion pairing and hydrogen bonding between triflate and the N−H group on the protonated ligand. Calculated at the M06-L-D3/def2-TZVP//M06L/def2-SVP level of theory using a dichloromethane SMD solvent model. The N−H···OTf distances are 1.755 and 1.777 Å. See the Supporting Information for details.

MeOH) and {[(PhTBDPhos-H2O)Ni](μ-OH)2}Cl2 (MeOH = methanol) was required to attenuate H2O and alcohol loss from the TBD backbone during NMR data collection, especially overnight 13C NMR data acquisitions, and we believe this additional stability may be attributed to NEt3 hydrogen bonding with the added N−H proton on the TBD backbone (i.e., N−H···NEt3).43 Spectroscopic Evidence of the Latent Borenium Ion and Comparison to Known Examples. In a further attempt to isolate the borenium cation, we treated the dimer 4 with HNTf2 and HOTf. We reasoned that starting with the dimer would prevent chloride loss from nickel and subsequent binding to boron, as observed in similar reactions with 1. Treating 4 with HOTf yielded a broad singlet in the 11B NMR spectrum at δ 15.7 and a 31P NMR resonance at δ 73.7. However, attempts to grow single crystals from the reaction mixture only resulted in isolation of 4-HCl obtained presumably from decomposition of putative 4-HOTf (Scheme 3). In contrast, treating 4 with HNTf2 revealed solution NMR data consistent with those for borenium cation formation; the

Figure 2. Molecular structure of 4-HCl with thermal ellipsoids at the 35% probability level. Phenyl groups, triflate counteranions, and hydrogen atoms attached to carbon atoms were omitted from the figure.

Attempts to use other Bronsted acids revealed that the rate of HCl loss from TBDPhos depends on the identity of the counteranion. Unlike reactions with HNTf2 and HOTf, treating solutions of 1 with Brookhart’s acid, [H(OEt2)2]B[(3,5-(CF3)2C6H3)4] (HBArF4),57 resulted in a rapid elimination of HCl to form {[(PhTBDPhos)Ni]2(μ-Cl)2}(BArF4)2 (5), as confirmed by single-crystal XRD studies. NMR analysis of single crystals of 5 revealed a sharp singlet in the 31P NMR spectrum at δ 68.3 and two resonances in the 11B NMR spectrum: a broad feature at δ 24.6 corresponding to threecoordinate boron in TBDPhos and a sharp multiplet at δ −6.2 assigned to the BArF4− anion. To determine whether HCl could be added to the TBD backbone, we treated 5 with 2 equiv of HCl in CH2Cl2. Unlike 3 and 4, which react to near-completion with HCl to form 3HCl and 4-HCl, 11B NMR analysis of the reaction mixture with 5 revealed a mixture of three- and four-coordinate TBD resonances at δ 23.9 and 5.3, respectively. The broad 11B NMR resonance at δ 5.3 is similar to those observed for 3-HCl and 4-HCl, and so we assign this transient species as 5 with HCl added across the backbone (5-HCl). The difference in the rate of HCl addition and loss with 3HCl and 4-HCl compared to 5-HCl appears to stem from the weak coordinating ability of NTf2− and OTf−. Both are capable of hydrogen bonding and coordination to metals via the oxygen and nitrogen atoms,58−62 and we suspected that these interactions stabilize HCl on the TBD backbone. Indeed, the structure of 4-HCl revealed an N−H···OTf distance of 1.95 Å, which is in the typical range of hydrogen-bonding distances63 (a similar N−H···FBF3 hydrogen bonding with a distance of 1.84 Å is observed in the structure of 2-HF). In contrast, BArF4− cannot participate in these interactions to any appreciable extent. Density functional theory (DFT) calculations using continuum solvation described in the following sections support these observations and indicate that ion pairing is strongest for anions that can hydrogen bond with the N−H group on the ligand (Figure 3). Further evidence of this stabilization comes from our previous reactivity studies with 1. The addition of NEt3 to (PhTBDPhos-MeOH)NiCl2 (1-

Scheme 3. Reaction of 4 with HNTf2 and HOTf

D

DOI: 10.1021/acs.inorgchem.8b01601 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry P NMR resonance of 4 shifted to δ 75.2, and the 11B NMR resonance disappeared. This is reported to occur for borenium ions in solution due to enhanced quadrupolar relaxation and equilibrium binding of triflate to the borenium in solution, as shown in Scheme 4.64,65 Indeed, XRD analysis of crystals 31

that NEt3 should not be necessary in similar reactions of 3−5 with H2O or alcohols. Indeed, unlike 1, which slowly reacts with H2O over several days in the absence of NEt3, a CHCl3 solution of 4 and H2O yielded a quantitative reaction after several minutes, as determined by 11B and 31P NMR analysis of the reaction mixture (Scheme 5). A 11B NMR resonance

Scheme 4. Previously Reported Examples of Equilibrium Binding of Triflate to Borenium Ions65

Scheme 5. Reactivity Studies of 4 with H2O and MeOH

grown from PhF/pentane revealed highly disordered structures with unacceptable R factors for publication, but the data were sufficient to show that triflate binds to boron in the solid state to form 3-HOTf (Figure S2). A comparison of the 11B and 31P NMR shifts for the dimers with different counteranions reveals systematic trends consistent with the weakening of boron−anion interactions at the TBD backbone. The 11B NMR resonances shift to lower field and broaden as the interaction weakens from B−F to B−Cl to B−OTf (Table 3), and the 31P NMR resonances show a stepwise shift downfield. Table 3. Comparison of 11B and 31P NMR Resonances for Dimeric PhTBDPhos Complexes with Different Boron and Outer-Sphere Anionsa anion boron 2-HF 4-HCl 4-HOTf 3-HOTf

−

F Cl− OTf− OTf−

NMR

outer-sphere −

BF4 OTf− OTf− NTF2−

11

B

1.9 (sharp) 6.0 (broad) 15.7 (broad) none

31

P

69.9 70.2 73.7 75.2

consistent with four-coordinate boron was observed at δ 1.1, and a single 31P NMR resonance was observed at δ 71.7. These values are remarkably close to the 11B and 31P NMR resonances reported previously for {[(PhTBDPhos-H2O)Ni]2(μ-OH)2}Cl2 at δ 1.0 and 70.8, respectively. The 1H NMR spectrum revealed an upfield resonance at δ −4.54 characteristic of the bridging Ni−OH reported, reported previously as {[(PhTBDPhos-H2O)Ni]2(μ-OH)2}Cl2 at δ −4.48.43,66 We assigned the new product as {[(PhTBDPhosH2O)Ni]2(μ-OH)2}(OTf)2 (4-H2O) based on a comparison of the NMR data. Unfortunately, 4-H2O slowly decomposes over the course of several days, which thwarted attempts at structural characterization. Similar reactivity was observed for 4 with MeOH (Scheme 5). Dissolving 4 in MeOH or MeOD without NEt3 yielded an immediate transformation to a product tentatively assigned as {[(PhTBDPhos-MeOH)Ni]2(μ-Cl)2}(OTf)2 (4-MeOH) based on the newly formed 11B NMR resonance at δ 2.6 and a 31P NMR resonance at δ 64.5. For comparison, 11B and 31P NMR resonances for 1-MeOH in MeOD are δ 2.7 and 70.3, respectively. However, crystals grown by vapor diffusion of Et2O into the 4-MeOH reaction mixture yielded only 1MeOH, as confirmed by NMR analysis and single-crystal XRD studies. It appears that 1-MeOH may be obtained by ligand redistribution of 4-MeOH, but attempts to isolate or identify other products from the mother liquor were unsuccessful. The observed reactivity of 4 with H2O and MeOH without additives such as NEt3 raised questions about how changing the phosphorus substituents from phenyl to isopropyl would

Chemical shifts are reported in δ units in ppm relative to BF3·Et2O (11B) and 85% H3PO4 (31P). a

Given the stepwise change in the NMR data in response to decreased anion coordinating ability to boron, we postulated that using an acid with a more noncoordinating counteranion than OTf− or NTf2− such as Brookhart’s acid would allow us to isolate the borenium ion. However, treating solutions of 3, 4, or 5 with HBArF4 in CH2Cl2 yielded no significant reaction, as determined by 31P and 11B NMR analysis of the reaction mixtures. While somewhat unexpected, the results mirror the attenuated ligand-centered HCl reactivity of 5 with BArF4− counteranions compared to 3 (NTf2−) and 4 (OTf−). Overall, it is clear from these studies that the identity of the counteranion plays an important role in the observed ligandcentered reactivity of PhTBDPhos. Influence of the Structure and Phosphorus Substituents on Ligand-Centered Reactions. Most of the Ph TBDPhos complexes reported here and previously43 with Ni2+ and Pd2+ lose at least one chloride to form dimeric solidstate structures with bridging hydroxyl or chloride ligands during ligand-centered reactions.43 Notably, the reactions of 1 with H2O and alcohols reported by us previously also required the addition of NEt3 to proceed.43 While the role of NEt3 in these previously reported reactions is still under investigation, the observed reactivity of the dimers 3−5 with HCl suggested E

DOI: 10.1021/acs.inorgchem.8b01601 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

three complexes from the [HNEt3]Cl coproduct by washing them with H2O in air. DFT Calculations. DFT calculations were used to quantify the free energies associated with the PhTBDPhos reactions for comparison to the experimental results. DFT calculations were performed at the M06-L-D3/def2-TZVP//M06-L/def2-SVP level of theory using a solvation model based on density (SMD) for dichloromethane. Thermal corrections were applied using a quasi-harmonic approximation first proposed by Grimme.67,68 We first present the free energies associated with chloride loss from nickel and dimerization of the resulting monomer as a function of the noncoordinating counteranion (BF4−, NTf2−, and OTf−). A relatively high overall reaction energy of +23.7 kcal/mol was calculated for removing a single chloride ligand (entry 1, Table 4), but the addition of a proton source such as hydronium or diethyloxonium makes chloride loss exergonic because of the formation of HCl (entries 2 and 3). No significant free-energy change occurs upon the addition of BF4− and NTf2− to the resulting [(TBDPhos)NiCl]+ cation once chloride is removed (entries 4 and 5), but the reaction with OTf− is exergonic (−6.2 kcal/mol) because OTf− binds directly to nickel (entry 6). Subsequent dimerization of the monomers with BF4−, NTf2−, and OTf− counteranions are all exergonic regardless of the counteranion once chloride is chemically removed (entries 7−9), which is consistent with our experimental results. We next evaluated the free energies associated with the addition of HF and HCl across the bridgehead N−B bond in the dimers 2−4 for comparison to the experimental reactions summarized in Scheme 2. The addition of HF to the dimer 2 with BF4− anions is highly exergonic with ΔG = −36.2 kcal/ mol (entry 10). In contrast, the addition of HCl to dimers 3 and 4 is almost thermoneutral, which accounts for the reversible binding of HCl and our ability to experimentally isolate both 3 and 3-HCl from the corresponding reaction solutions (Scheme 2). The addition of H2O and MeOH to the dimer 4 with OTf− counteranions was slightly endergonic at +1.5 and +2.3 kcal/mol, respectively (entries 13 and 14). Both reactions are observed experimentally (Scheme 5), but the higher free energy of reaction may account for their instability with respect to decomposition relative to 4-HCl. As a final check, we modeled the change in free energy for treating 1 directly with HNTf2 and HOTf to form the dimers 3-HCl and 4-HCl, as shown in Scheme 2 (entries 15 and 16). Both calculated reactions, which account for chloride loss from nickel, dimerization, and the addition of generated HCl across the N−B bond, are thermodynamically favorable at −9.8 kcal/ mol. We next considered the reactivity of the bridgehead N−B bond on the TBDPhos backbone in the monomer 1 for a comparison to the calculated results with the dimers 2−4. To provide benchmarks to experimentally observed reactivity, free-energy calculations were performed for the reaction of 1 with MeOH to form 1-MeOH, as previously reported (entry 17). The reaction is exergonic at −1.1 kcal/mol and is more favored than the same reaction with the dimer 4 to form 4MeOH (+2.3 kcal/mol; entry 14). This energy difference appears to account for why we were only able to isolate 1MeOH from reactions of the dimer 4 with MeOH to form 4MeOH (Scheme 5). We next evaluated the reactions of H2O, HCl, and HF with 1 (entries 18−20). All three reactions are exergonic, with HF addition being the most favored as

affect the reactivity at the TBD backbone. We previously showed that ( iPr TBDPhos)NiCl2, which has isopropyl substituents attached to phosphorus, has no appreciable reactivity with H2O, alcohols, and fluoride, even in the presence of NEt3.43 To investigate whether this was true for the corresponding dimer, we prepared {[(iPrTBDPhos)Ni]2(μCl)2}(OTf)2 (6) by treating (iPrTBDPhos)NiCl2 with AgOTf. After its dimeric structure was confirmed using single-crystal XRD (Figure 4), 6 was treated with H2O and MeOH. In

Figure 4. Molecular structure of 6 with thermal ellipsoids at the 35% probability level. Hydrogen atoms were omitted from the figure.

striking contrast to phenyl-substituted 4, less than 15% of 6 was reacted 24 h after the addition of H2O or MeOH. It is not yet clear why we see different ligand-centered reactivity in the monomers and dimers and with different substituents attached to the phosphorus. Nevertheless, these empirical observations show that they can have a dramatic effect on the ligandcentered reactivity. Investigations of Ancillary Ligand Variations on the Ligand-Centered Reactivity. We next investigated how replacing the chloride ligands in 1 with anionic ligands that would not be easily displaced affected the reactivity at the TBD backbone. Chloride ligands in 1 were replaced with chelating and more strongly coordinating 1,2-benzenedithiolate (7), catecholate (8), and thiophenolate (9) by treating 1 with the corresponding thiols and catechol in the presence of NEt3 (Scheme 6). The structures of 7−9 were determined by singleScheme 6. Synthesis of (PhTBDPhos)NiL2a

a

L2 = C6H4S2 (7), C6H4O2 (8), and (C6H5S)2 (9).

crystal XRD (Figures 5 and S3), and NMR spectra and elemental analysis confirmed their proposed formulations (Table 2). Remarkably, PhTBDPhos in none of these compounds reacted with H2O, MeOH, or nBu4NF·(H2O)n. 31 P and 11B NMR analysis of the reaction mixture revealed that compounds 7 and 8 remained unchanged after 24 h, whereas 9 only showed signs of decomposition. As a further testament to their remarkable lack of reactivity, we subsequently isolated all F

DOI: 10.1021/acs.inorgchem.8b01601 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 5. Molecular structures of 7 and 9 with thermal ellipsoids at the 35% probability level. Hydrogen atoms and cocrystallized solvent molecules were omitted from the figures.

Table 4. Calculated ΔG for Reactions (kcal/mol) Shown at the M06-L-D3/def2-TZVP//M06-L/def2-SVP Level of Theory Using a Dichloromethane SMD entry

reaction

ΔG

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

(PhTBDPhos)NiCl2 (1) → [(PhTBDPhos)NiCl]+ + Cl− 1 + EtO2H+ → [(PhTBDPhos)NiCl]+ + HCl + Et2O 1 + H3O+ → [(PhTBDPhos)NiCl]+ + HCl + H2O [(PhTBDPhos)NiCl]+ + BF4− → [(PhTBDPhos)NiCl]BF4 [(PhTBDPhos)NiCl]+ + NTf2−→[(PhTBDPhos)NiCl]NTf2 [(PhTBDPhos)NiCl]+ + OTf− → [(PhTBDPhos)NiCl]OTf 2[(PhTBDPhos)NiCl]BF4 → 2 (dimer) 2[(PhTBDPhos)NiCl]NTf2 → 3 (dimer) 2[(PhTBDPhos)NiCl]OTf → 4 (dimer) 2 + 2HF → 2-HF 3 + 2HCl → 3-HCl 4 + 2HCl → 4-HCl 4 + 2H2O → 4-H2O 4 + 2MeOH → 4-MeOH 2(PhTBDPhos)NiCl2 + 2HNTf2 → 3-HCl 2(PhTBDPhos)NiCl2 + 2HOTf → 4-HCl 1 + MeOH → 1-MeOH 1 + H2O → 1-H2O 1 + HCl → 1-HCl 1 + HF → 1-HF 7 + HF → 7-HF 8 + HF → 8-HF 9 + HF → 9-HF 7 + MeOH → 7-MeOH 8 + MeOH → 8-MeOH 9 + MeOH → 9-MeOH

+23.7 −7.0 −19.0 +0.2 −0.8 −6.2 −23.4 −27.6 −14.1 −36.2 +0.7 −0.2 +1.5 +2.3 −9.8 −9.8 −1.1 −0.5 −1.3 −11.0 −5.8 −6.6 −4.6 +13.7 +12.9 +4.3

is favorable, none of our experiments use HF directly (it was obtained from reactions with HBF4·Et2O or hydrated nBu4NF). Hence, for a better comparison to the experiment, calculations were performed for 7−9 with MeOH (entries 24− 26). All three reactions are endergonic at +13.7, +12.9, and +4.3 kcal/mol, respectively, which corroborates their lack of reactivity relative to the identical reaction to form 1-MeOH from 1 (−1.1 kcal/mol; entry 17).

■

CONCLUSION

In summary, we reported reactivity and spectroscopic evidence consistent with latent borenium ion formation on a nickelbound diphosphine ligand. Despite the lack of measurable Lewis acidity of the TBD backbone, treating solutions of 1 with HBF4·Et2O resulted in fluoride abstraction from BF4− and net trans HF addition across the bridgehead N−B bond. Further evidence of ligand-centered borenium ions was afforded by treating 1 with HNTf2 and HOTf, which resulted in chloride loss from nickel and the subsequent formation of trans N−H and B−Cl bonds on the TBD backbone. Similar reactivity was observed when [(PhTBDPhos)NiCl]2X2, where X = NTf2− (3), OTf−(4), or BArF4− (5), was treated with HCl. Treating 4 with HNTf2 instead of HCl yielded evidence consistent with latent formation of the borenium cation in solution. Another key finding from our experiments is that the ligandcentered reactivity on TBDPhos is controlled by the ancillary ligands and substituents attached to phosphorus. We showed previously that changing the phosphorus substituents (phenyl vs isopropyl) inhibits ligand-centered reactivity in ( iPr TBDPhos)NiCl2 and ( iPr TBDPhos)PdCl 2 . Here, we showed that this reactivity can also be suppressed by replacing the chloride ligands in 1 with chelating and less labile thiolates or catecholate (7−9). DFT calculations suggest that the attenuated reactivity with 7−9 is due, in part, to unfavorable reaction free energies caused by the different ancillary ligands. Overall, given the broad utility of diphosphines in homogeneous catalysis69 and the known benefits of strong Lewis acid promotors in many catalytic reactions, we believe the ligand-centered reactivity in TBDPhos could provide new opportunities for two-site reactivity involving boron and

expected. The experimental reaction products of 1 with H2O and nBu4NF·(H2O)x are hydroxide-bridged dimers, which suggests that dimerization with bridging hydroxide ligands provides an additional thermodynamic driving force in the observed reactions with nickel. As a final comparison, we calculated the free energies of reactions of 7−9 with HF and MeOH. All three reactions are calculated to be exergonic in reactions with HF (entries 21− 23), albeit less so than 1 with HF (entry 20). Although the calculations show that net HF addition to the TBD backbone G

DOI: 10.1021/acs.inorgchem.8b01601 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

CH2−CH2−CH2), 47.8 (s, NCH2), 49.6 (s, NCH2), 118.6 (s, Ph), 121.8 (s, Ph), 125.0 (s, Ph), 128.5−129.4 (m, Ph), 131.5−132.5 (m, Ph). 19F NMR (CDCl3, 20 °C): δ −78.8 (s, NTf2). 31P{1H} NMR (CH2Cl2, 20 °C): δ 71.2 (s). IR (ATR, cm−1): 3160 w, 3065 vw, 2855 vw, 1586 w, 1575 vw, 1558 vw, 1539 w, 1520 m, 1481 vw, 1472 w, 1456 vw, 1435 m, 1382 w, 1249 w, 1224 vw, 1217 vw, 1188 vs, 1176 vw, 1127 s, 1095 s, 1047 s, 1032 vw, 995 m, 935 vw, 916 m, 895 m, 832 m, 787 m, 750 vw, 741 s, 715 m, 690 s, 662 m, 635 m, 625 vw, 616 m, 607 vw. Alternative Synthesis of 3. To a stirring suspension of 1 (0.35 g, 0.55 mmol) in CH3CN (20 mL) was added a solution of AgNTf2 (0.22 g, 0.57 mmol) in CH3CN (10 mL). The reaction formed a white precipitate, and the color of the solution changed from dark orange to red-orange. The mixture was stirred overnight, filtered, and evaporated to dryness under vacuum. The oily product was dissolved in CH2Cl2 (5 mL) and precipitated with excess Et2O (30 mL) to yield an analytically pure red-orange powder. XRD-quality crystals were obtained from CH2Cl2 solutions of 3 by vapor diffusion with Et2O. Yield: 0.25 g (52%). Anal. Calcd for C64H64B2Cl2F12N8Ni2O8P4S4: C, 43.6; H, 3.66; N, 6.35. Found: C, 43.3; H, 3.72; N, 6.21. 1H NMR (CD3CN, 20 °C): δ 1.56 (m, CH2−CH2−CH2, 4H), 2.81 (m, NCH2, 4H), 2.90 (t, NCH2, 4H), 7.42−7.53 (m, Ph, 8H), 7.56−7.65 (m, Ph, 4H), 7.65−7.78 (m, Ph, 8H). 11B NMR (CD3CN, 20 °C): δ 24.7 (br s, fwhm = 260 Hz). 13C{1H} NMR (CD3CN, 20 °C): δ 26.6 (s, CH2−CH2−CH2), 48.4 (s, NCH2), 48.5 (s, NCH2), 130.0 (vt, Ph, JPC = 5.1 Hz), 132.9 (s, Ph), 133.2 (vt, Ph, JPC = 5.1 Hz). 19F NMR (CD3CN, 20 °C): δ −80.3 (s, NTf2). 31P{1H} NMR (CD3CN, 20 °C): δ 67.8 (s). IR (ATR, cm−1): 3227 vw, 3051 w, 2927 w, 2853 w, 1585 vw, 1571 vw, 1514 m, 1480 vw, 1471 vw, 1434 s, 1383 w, 1350 s, 1325 m, 1274 m, 1198 s, 1179 s, 1138 m, 1096 s, 1055 m, 1031 m, 997 w, 966 w, 912 w, 883 m, 851 vw, 833 w, 787 m, 743 s, 711 vw, 693 vs, 668 vw, 649 vw, 643 vw, 634 vw, 628 vw, 609 s. {[(PhTBDPhos)Ni]2(μ-Cl)2}(OTf)2 (4). To a stirring solution of 1 (0.50 g, 0.79 mmol) in CH2Cl2 (50 mL) was added a solution of AgOTf (0.20 g, 0.78 mmol) in CH2Cl2 (10 mL). The reaction mixture formed a white precipitate, and the color of the solution changed from dark orange to red-orange. The mixture was stirred overnight and filtered. The filtrate was reduced to ca. 30 mL, and 4 was precipitated from the solution with excess Et2O. XRD-quality crystals were grown by vapor diffusion of Et2O into a solution of 4 in CH2Cl2. Yield: 0.50 g (85%). Anal. Calcd for C62H64B2Cl2F6N6Ni2O6P4S2: C, 49.6; H, 4.30; N, 5.60. Found: C, 49.6; H, 4.35; N, 5.45. 1H NMR (CD3CN, 20 °C): δ 1.57 (m, CH2− CH2−CH2, 4H), 2.82 (m, NCH2, 4H), 2.90 (t, NCH2, 4H), 7.42− 7.53 (m, Ph, 8H), 7.53−7.64 (m, Ph, 4H), 7.64−7.77 (m, Ph, 8H). 11 B NMR (CD3CN, 20 °C): δ 24.3 (br s, fwhm = 330 Hz). 13C{1H} NMR (CD3CN, 20 °C): δ 26.6 (s, CH2−CH2−CH2), 48.4 (s, NCH2), 48.5 (s, NCH2), 130.0 (vt, Ph, JPC = 5.3 Hz), 132.9 (s, Ph), 133.2 (vt, Ph, JPC = 5.1 Hz). 19F NMR (CD3CN, 20 °C): δ −79.0 (s, OTf). 31P{1H} NMR (CD3CN, 20 °C): δ 67.8 (s). IR (ATR, cm−1): 3052 w, 2996 vw, 2939 w, 2846 w, 1586 vw, 1570 w, 1539 vw, 1518 w, 1507 m, 1477 s, 1434 s, 1384 m, 1364 m, 1351 m, 1322 m, 1281 m, 1257 vs, 1222 w, 1213 m, 1176 vw, 1146 m, 1097 s, 1028 vs, 1014 w, 997 m, 963 w, 933 vw, 908 m, 883 m, 862 vw, 832 m, 796 vw, 786 m, 744 s, 710 m, 691 vs, 635 vs, 632 s, 617 s, 607 m. {[(PhTBDPhos)Ni]2(μ-Cl)2}(BArF4)2 (5). To a stirring suspension of 1 (0.10 g, 0.16 mmol) in CH2Cl2 (5 mL) was added a solution of [H(OEt2)2][BArF4] (0.16 g, 0.16 mmol) in CH2Cl2 (3 mL). The reaction mixture transformed into an orange-red solution upon mixing. The solution was stirred overnight, filtered, and reduced to ca. 5 mL. Vapor diffusion with Et2O yielded orange blocks. Yield: 0.16 g (70%). In a separate reaction, we showed that crystals can also be grown by vapor diffusion of pentane into solutions of 5 in PhF. Anal. Calcd for C124H88B4Cl2F48N6Ni2P4·C4H10O: C, 51.2; H, 3.29; N, 2.80. Found: C, 51.3; H, 3.57; N, 2.70. 1H NMR (CD3CN, 20 °C): δ 1.55 (m, CH2−CH2−CH2, 4H), 2.81 (m, NCH2, 4H), 2.88 (t, NCH2, 4H), 7.42−7.53 (m, Ph, 8H), 7.53−7.64 (m, Ph, 4H), 7.64−7.77 (m, Ph, 20H). 11B NMR (CD3CN, 20 °C): δ −6.2 (m, BArF4), 24.6 (br s, fwhm = 300 Hz). 13C{1H} NMR (CD3CN, 20 °C): δ 26.6 (s, CH2− CH2−CH2), 48.4 (s, NCH2), 48.5 (s, NCH2), 118.7 (m, BArF4, 3JFC =

metals. We are currently investigating the mechanism of these reactions, and a full mechanistic report will be forthcoming.

■

EXPERIMENTAL SECTION

General Considerations. Reactions were carried out under an atmosphere of N2 or Ar using glovebox or standard Schlenk techniques unless stated otherwise. Glassware used for air-free reactions were dried in an oven at 150 °C for at least 1.5 h and allowed to cool under vacuum before use. Solvents used under anhydrous conditions were dried and deoxygenated using a Pure Process Technologies Solvent Purification System. Fluorobenzene was distilled over CaH2 and stored over preactivated 3 Å molecular sieves. [H(OEt2)2][BArF4], (PhTBDPhos)NiCl2 (1), and (iPrTBDPhos)NiCl2 were prepared as described previously.43,57 Other reagents and deuterated solvents were purchased from commercial vendors and used as received. 1 H, 19F, and 31P NMR data were recorded on a Bruker AVANCE300 or DPX-300 instrument operating at 300 MHz for 1H, 282.2 MHz for 19F, and 121.4 MHz for 31P. 11B and 13C NMR data were acquired on a Bruker AVANCE-400 or DRX-400 instrument operating at 128.3 and 75.5 MHz, respectively. Chemical shifts are reported in δ units in ppm referenced to residual solvent peaks (1H and 13C) or to 0.05% C6H5CF3 in C6D6 (19F; δ −62.9), 85% H3PO4 (31P; δ 0.0), and BF3·Et2O (11B; δ 0.0). Microanalysis data (CHN) were collected using an EAI CE-440 elemental analyzer at the University of Iowa. IR spectra were collected on a Thermo Scientific Nicolet iS5 spectrometer using an attenuated-total-reflectance (ATR) accessory in an N2-filled glovebox. High-resolution electron impact mass spectrometry (HR-EI MS) spectra were recorded on a Waters GCT Premier instrument using time-of-flight, and fragment ions (M = molecule; L = ligand) were assigned based on a comparison to calculate natural abundance isotopic distributions. {[(PhTBDPhos-HF)Ni]2(μ-Cl)2}(BF4)2 (2-HF). To a stirring solution of 1 (0.10 g, 0.16 mmol) in CH2Cl2 (10 mL) was added HBF4·Et2O (42 μL, 0.31 mmol). The color of the reaction mixture changed from dark orange to red-orange. The solution was stirred overnight, filtered, and concentrated to ca. 5 mL. Red-orange needles were obtained by vapor diffusion with Et2O. Yield: 0.088 g (79%). Anal. Calcd for C60H66B4Cl2F10N6Ni2P4·CH2Cl2: C, 48.8; H, 4.56; N, 5.60. Found: C, 48.6; H, 4.38; N, 5.59. 1H NMR (CDCl3, 20 °C): δ 1.20 (m, 2H), 1.75 (m, 2H), 2.58 (m, 2H), 2.81 (m, 2H), 3.08−3.29 (m, 4H), 5.52 (s, NH, 1H), 7.13−7.28 (m, Ph, 6H), 7.28−7.50 (m, Ph, 14H). 11B NMR (CDCl3, 20 °C): δ −0.7 (s, BF4−), 1.9 (s, N3BF). 13C{1H} NMR (CDCl3, 20 °C): δ 26.2 (s, CH2−CH2−CH2), 46.2 (s, NCH2), 50.1 (s, NCH2), 128.4 (m, Ph), 129.2 (m, Ph), 130.9 (s, Ph), 131.3 (s, Ph), 131.6 (m, Ph), 132.7 (m, Ph). 19F NMR (CDCl3, 20 °C): δ −151.8 (s, BF4−), −163.8 (br s, fwhm = 130 Hz, N3BF). 31P{1H} NMR (CDCl3, 20 °C): δ 69.9 (s). IR (ATR, cm−1): 3220 w, 3053 w, 2939 w, 2882 w, 1480 w, 1457 vw, 1435 m, 1372 m, 1293 m, 1243 m, 1191 w, 1152 m, 1116 m, 1093 vw, 1082 w, 1044 vs, 995 w, 980 w, 954 m, 909 m, 881 s, 836 m, 810 m, 755 s, 739 vw, 729 w, 711 w, 692 vs, 632 vw, 627 s, 616 m, 611 vw, 604 m. {[(PhTBDPhos)Ni]2(μ-Cl)2}(NTf2)2 (3) and {[(PhTBDPhos-HCl)Ni]2(μCl)2}(NTf2)2 (3-HCl). To a stirring suspension of 1 (0.10 g, 0.16 mmol) in CH2Cl2 (5 mL) was added a solution of HNTf2 (0.044 g, 0.16 mmol) in CH2Cl2 (5 mL). The color of the reaction mixture immediately changed from dark orange to red-orange. The mixture was stirred overnight and filtered, and the filtrate was reduced to ca. 5 mL. Vapor diffusion with Et2O yielded orange blocks over the course of 1 day. Yield: 0.12 g (83%). In an alternative synthesis, the reaction was carried out in PhF, and the product was crystallized by vapor diffusion with pentane. Single-crystal XRD, elemental analysis, and NMR spectra on both sets of crystals revealed them to be a mixture of 3 and 3-HCl. Anal. Calcd for C64H64B2Cl2F12N8Ni2O8P4S4·HCl: C, 42.7; H, 3.64; N, 6.23. Found: C, 42.7; H, 3.63; N, 6.14. NMR details for 3-HCl (those for 3 are described below). 1H NMR (CDCl3, 20 °C): δ 1.33 (m), 1.74 (m), 2.74 (m), 3.09 (m), 3.46 (m), 5.50 (s, NH, 1H), 7.20−7.50 (m, Ph). 11B NMR (CH2Cl2, 20 °C): δ 6.2 (br s, fwhm = 300 Hz, N3BCl). 13C{1H} NMR (CDCl3, 20 °C): δ 25.8 (s, H

DOI: 10.1021/acs.inorgchem.8b01601 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Anal. Calcd for C36H36BN3NiO2P2·0.5CH2Cl2: C, 61.2; H, 5.20; N, 5.86. Found: C, 61.4; H, 5.38; N, 6.23. 1H NMR (CDCl3, 20 °C): δ 1.65 (m, CH2−CH2−CH2, 4H), 2.81−2.92 (m, NCH2, 8H), 6.20 (m, Ph, 4H), 7.31−7.46 (m, Ph, 12H), 7.77−7.85 (m, Ph, 8H). 11B NMR (CDCl3, 20 °C): δ 24.0 (br s, fwhm = 530 Hz). 13C{1H} NMR (CDCl3, 20 °C): δ 26.5 (s, CH2−CH2−CH2), 46.6 (s, NCH2), 48.4 (s, NCH2), 113.2 (s, Ph), 114.2 (s, Ph), 128.2 (vt, Ph, JPC = 5.4 Hz), 130.4 (s, Ph), 130.9 (vt, Ph, JPC = 27 Hz), 133.0 (vt, Ph, JPC = 5.5 Hz), 163.2 (s, Ph). 31P{1H} NMR (CDCl3, 20 °C): δ 78.6 (s). MS (EI) [fragment ion, relative abundance]: m/z 108 [C6H4O2, 28], 322 [L − P − 2Ph, 36], 379 [M − C6H4O2 − PPh2 − H, 12], 430 [L − Ph, 7], 488 [M − C6H4O2 − Ph, 1], 564 [M − C6H4O2 − H, 2], 673 [M, 20]. IR (ATR, cm−1): 3052 w, 2995 w, 2939 w, 2846 w, 1586 vw, 1569 w, 1554 vw, 1517 w, 1506 m, 1477 vw, 1434 s, 1389 s, 1382 s, 1364 m, 1350m, 1321 m, 1307 w, 1295 w, 1283 m, 1257 vs, 1225 m, 1212 m, 1205 m, 1176 m, 1160 w, 1115 vw, 1098 vs, 1071 vw, 1063 vw, 1028 s, 1014 m, 998 m, 933 m, 906 m, 893 w, 886 w, 861 m, 830 w, 795 s, 786 s, 762 m, 746 s, 729 vs, 691 vs, 653 vw, 633 m, 620 m. (PhTBDPhos)Ni(C6H5S)2 (9). To a stirring solution of 1 (0.50 g, 0.79 mmol) in CH2Cl2 (40 mL) in air was added thiophenol (0.16 mL, 1.56 mmol) and NEt3 (1 mL). The resulting dark-brown solution was stirred overnight, and excess pentane was added to precipitate a purple solid. The solid was collected by filtration, washed with H2O in air, and evaporated to dryness under vacuum overnight. Dark-purple needles were obtained by vapor diffusion of Et2O into a solution of 8 in CH2Cl2. Yield: 0.46 g (75%). Anal. Calcd for C42H42BN3NiP2S2: C, 64.3; H, 5.40; N, 5.36. Found: C, 64.2; H, 5.23; N, 5.38. 1H NMR (CDCl3, 20 °C): δ 1.49 (m, CH2−CH2−CH2, 4H), 2.76−2.84 (m, NCH2, 8H), 6.40−6.48 (m, Ph, 4H), 6.52−6.60 (m, Ph, 6H), 7.36− 7.49 (m, Ph, 12H), 7.89−8.03 (m, Ph, 8H). 11B NMR (CDCl3, 20 °C): δ 24.3 (br s, fwhm = 600 Hz). 13C{1H} NMR (CDCl3, 20 °C): δ 26.2 (s, CH2−CH2−CH2), 47.6 (s, NCH2), 48.6 (s, NCH2), 121.5 (s, Ph), 126.1 (s, Ph), 128.3 (t, Ph, JPC = 5.0 Hz), 130.1 (s, Ph), 131.5 (s, Ph), 132.8 (t, Ph, JPC = 4.7 Hz), 133.1 (s, Ph), 144.6 (s, Ph). 31P{1H} NMR (CDCl3, 20 °C): δ 71.6 (s). IR (ATR, cm−1): 3137 vw, 3053 m, 3001 vw, 2985 vw, 2944 vw, 2931 w, 2877 vw, 2858 vw, 1574 s, 1533 m, 1517 s, 1466 s, 1446 w, 1431 s, 1381 s, 1363 m, 1348 w, 1322 s, 1298 vw, 1283 s, 1222 w, 1209 s, 1171 s, 1151 w, 1116 w, 1095 s, 1083 m, 1055 vw, 1021 vs, 997 w, 964 w, 925 vw, 908 s, 884 s, 844 vw, 826 s, 798 vw, 790 vw, 777 m, 752 m, 741 vw, 733 vs, 712 w, 689 vs, 662 w, 653 vw. Reaction of 3 with HCl. To a stirring solution of 3 (0.10 g, 0.057 mmol) in CH2Cl2 (5 mL) was added a solution of 2 N HCl in Et2O (60 μL, 0.120 mmol). NMR data collected on the reaction mixture after 1 h revealed resonances consistent with 3-HCl. 11B NMR (CH2Cl2, 20 °C): δ 6.2 (br s, fwhm = 250 Hz). 31P NMR (CH2Cl2, 20 °C): δ 70.9 (s). A small peak assigned to unreacted 3 was observed at δ 70.4 (s). Reaction of 4 with HCl. To a stirring solution of 4 (0.12 g, 0.080 mmol) in CH2Cl2 (10 mL) was added 2 N HCl in Et2O (80 μL, 0.160 mmol). NMR data collected on the reaction mixture after 1 h revealed resonances consistent with those of 4-HCl. 11B NMR (CH2Cl2, 20 °C): δ 6.0 (br s, fwhm = 300 Hz). 31P NMR (CH2Cl2, 20 °C): δ 70.2 (s). These data are consistent with those for 4-HCl obtained by treating 1 with HOTf. Reaction of 5 with HCl. To a stirring solution of 5 (0.23 g, 0.079 mmol) in CH2Cl2 (5 mL) was added a solution of 2 N HCl in Et2O (80 μL, 0.160 mmol). NMR analysis of the reaction mixture after 1 h revealed a mixture of 5 and new resonances assigned to 5-HCl. 11B NMR (CH2Cl2, 20 °C): δ 23.9 (br s, fwhm = 300 Hz, 5), 5.3 (br s, fwhm = 250 Hz, 5-HCl), −6.9 (m, BArF4). 31P NMR (CH2Cl2, 20 °C): δ 69.8 (s, 5-HCl), 69.6 (s, 5). Reaction of 4 with HNTf2. To a stirring solution of 4 (0.10 g, 0.067 mmol) in CH2Cl2 (5 mL) was added a solution of HNTf2 (0.038 g, 0.14 mmol) in CH2Cl2 (3 mL). The brown-orange solution turned red-orange and was allowed to stir for 2 h. NMR analysis of the reaction mixture revealed no distinguishable peak in the 11B NMR spectrum, whereas a single 31P NMR resonance was observed at δ 75.2.

3.9 Hz), 125.5 (q, BArF4, 1JFC = 270 Hz), 129.8 (m, BArF4, 2JFC = 31.7 Hz), 130.0 (vt, Ph, JPC = 5.5 Hz), 132.9 (s, Ph), 133.3 (vt, Ph, JPC = 4.9 Hz), 135.7 (s, BArF4), 162.6 (q, BArF4, JBC = 50 Hz). 19F NMR (CD3CN, 20 °C): δ −62.9 (s, BArF4). 31P{1H} NMR (CD3CN, 20 °C): δ 68.3 (s). IR (ATR, cm−1): 2974 vw, 2870 vw, 1609 w, 1586 vw, 1534 w, 1518 m, 1473 vw, 1450 vw, 1438 w, 1384 w, 1353 s, 1326 m, 1274 vs, 1215 m, 1182 vw, 1159w, 1116 vw, 1094 m, 1025 s, 997 m, 933 m, 918 w, 882 m, 838 m, 791 w, 744 s, 709 m, 692 s, 681 s, 671 s, 645 m, 638 m, 625 s, 614 m, 601 m. {[(iPrTBDPhos)Ni]2(μ-Cl)2}(OTf)2 (6). To a stirring solution of (iPrTBDPhos)NiCl2 (0.10 g, 0.20 mmol) in CH2Cl2 (5 mL) was added a solution of AgOTf (0.052 g, 0.20 mmol) in CH2Cl2 (5 mL). The reaction mixture formed a white precipitate, and the color of the solution rapidly changed from purple to brown. The reaction mixture was stirred for 3 h, filtered, and concentrated to ca. 5 mL. Vapor diffusion with Et2O yielded dark-green-brown blocks over the course of 2 days. Yield: 0.11 g (90%). Anal. Calcd for C38H80B2Cl2F6N6Ni2O6P4S2: C, 37.1; H, 6.56; N, 6.84. Found: C, 37.0; H, 6.57; N, 6.74. 1H NMR (CDCl3, 20 °C): δ 1.50 (br s, CH3− CH−CH3, 12H), 1.64 (d, CH3−CH−CH3, 12H), 1.82 (m, CH2− CH2−CH2, 4H), 2.57 (br s, CH3−CH−CH3, 4H), 2.93 (t, NCH2, 4H), 3.26 (m, NCH2, 4H). 11B NMR (CDCl3, 20 °C): δ 24.6 (br s, fwhm = 520 Hz). 13C{1H} NMR (CDCl3, 20 °C): δ 19.1 (d, CH3− CH−CH3), 26.7 (s, CH2−CH2−CH2), 28.3 (m, CH3−CH−CH3), 45.2 (s, NCH2), 48.1 (s, NCH2). 19F NMR (CDCl3, 20 °C): δ −78.5 (s, OTf). 31P{1H} NMR (CDCl3, 20 °C): δ 97.8 (br s, fwhm = 190 Hz). IR (ATR, cm−1): 2965 m, 2933 m, 2871 m, 1515 s, 1461 m, 1435 w, 1393 w, 1384 w, 1366 wm, 1353 w, 1325 m, 1263 vs, 1221 m, 1209 s, 1173 vw, 1165 vw, 1139 vs, 1092 m, 1063 m, 1025 vs, 967 w, 933 w, 905 vw, 878 s, 827 s, 784 w, 773 m, 750 m, 699 vw, 691 vw, 686 vw, 640 s, 635 vs, 628 vw, 618 s, 610 s. (PhTBDPhos)Ni(C6H4S2) (7). To a stirring solution of 1 (0.97 g, 1.5 mmol) and benzene-1,2-dithiol (0.21 g, 1.5 mmol) in CH2Cl2 (100 mL) was added excess NEt3 (1 mL). The color of the reaction mixture changed from dark orange to brown. The solution was stirred overnight and concentrated to ca. 30 mL, and the solids were precipitated out by the addition of pentane (200 mL). The resulting brown powder was collected by filtration, washed with H2O in air, and dried under vacuum overnight. The powder was dissolved in CH2Cl2, and orange-yellow blocks were obtained by vapor diffusion with Et2O. Yield: 0.81 g (74%). Anal. Calcd for C36H36BN3NiP2S2: C, 61.2; H, 5.14; N, 5.95. Found: C, 61.5; H, 5.41; N, 6.35. 1H NMR (CDCl3, 20 °C): δ 1.62 (m, CH2−CH2−CH2, 4H), 2.83 (m, NCH2, 4H), 2.94 (t, NCH2, 4H), 6.66−6.73 (m, Ph, 2H), 7.16−7.28 (m, Ph, 10H), 7.28− 7.38 (m, Ph, 4H), 7.55−7.63 (m, Ph, 8H). 11B NMR (CDCl3, 20 °C): δ 24.4 (br s, fwhm = 640 Hz). 13C{1H} NMR (CDCl3, 20 °C): δ 26.6 (s, CH2−CH2−CH2), 47.6 (s, NCH2), 48.5 (s, NCH2), 120.8 (s, Ph), 127.3 (s, Ph), 127.9 (t, Ph, JPC = 5.2 Hz), 130.3 (s, Ph), 131.6 (vt, Ph, JPC = 26 Hz), 133.0 (t, Ph, JPC = 5.6 Hz), 149.1 (t, Ph, JPC = 10 Hz). 31 1 P{ H} NMR (CDCl3, 20 °C): δ 79.0 (s). MS (EI) [fragment ion, relative abundance]: m/z 183 [PPh2, 10], 276 [L − 3Ph, 21], 353 [L − 2Ph, 34], 379 [M − P − 3Ph − 2S, 11], 411 [M − P − 3Ph − S, 17], 430 [L − Ph, 100], 520 [M − P − 2Ph, 16], 596 [M − S − Ph, 1], 628 [M − Ph, 1], 705 [M, 74]. IR (ATR, cm−1): 3052 vw, 3042 w, 2984 w, 2957 vw, 2935 vw, 2915 w, 2864 w, 2838 w, 1586 vw, 1569 vw, 1558 vw, 1547 w, 1506 s, 1478 m, 1458 m, 1438 s, 1431 m, 1418 m, 1388 s, 1362 m, 1351 m, 1319 s, 1288 vs, 1234 vw, 1223 vw, 1215 m, 1205 vw, 1179 s, 1149 vw, 1130 vw, 1097 vs, 1090 vw, 1026 s, 999 s, 938 vw, 918 w, 902 m, 890 m, 874 w, 849 vw, 826 s, 783 s, 747 vs, 739 vs, 707 m, 691 vs, 667 w, 653 w, 633 m, 625 vw, 616 w, 596 s, 591 s, 576 vw, 564 s. (PhTBDPhos)Ni(C6H4O2) (8). To a stirring solution of 1 (0.20 g, 0.31 mmol) and benzene-1,2-diol (0.035 g, 0.32 mmol) in CH2Cl2 (20 mL) was added excess NEt3 (1 mL). The color of the solution immediately changed from dark orange to green. The reaction was stirred overnight and reduced to ca. 5 mL under vacuum, and solids were precipitated by the addition of pentane (30 mL). The precipitate was collected by filtration, washed with H2O in air, and dried under vacuum overnight. X-ray-quality crystals were obtained by vapor diffusion of Et2O into a solution of 8 in CH2Cl2. Yield: 0.18 g (85%). I

DOI: 10.1021/acs.inorgchem.8b01601 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Reaction of 4 with HOTf. To a stirring solution of 4 (0.12 g, 0.080 mmol) in CH2Cl2 (10 mL) was added HOTf (14 μL, 0.160 mmol). The color of the solution turned to red-orange. NMR data were collected on the solution after 1 h. 11B NMR (CH2Cl2, 20 °C): δ 15.7 (br s, fwhm = 630 Hz). 31P NMR (CH2Cl2, 20 °C): δ 73.7 (s). Vapor diffusion of Et2O into the solution yielded orange crystals that were revealed to be 4-HCl by single-crystal XRD analysis of multiple crystals. Reaction of 4 with H2O. A solution of 4 (0.050 g, 0.033 mmol) in CDCl3 (10 mL) was layered with H2O (3 mL) in air. The reaction mixture was agitated a few times and allowed to stand for 10 min. The CDCl3 layer was filtered through a column of Celite and analyzed by NMR spectroscopy. 1H NMR (CDCl3, 20 °C): δ −4.54 (s), 1.27 (m), 2.09 (m), 2.46 (m), 2.84 (m), 2.97 (m), 3.24 (m), 5.97 (s), 7.03− 8.02 (m, Ph). 11B NMR (CDCl3, 20 °C): δ 1.1 (s). 31P NMR (CDCl3, 20 °C): δ 71.7 (s). The compound slowly decomposes over time, which prevents further characterization. Reaction of 4 with MeOH. Compound 4 (0.050 g, 0.033 mmol) was dissolved in MeOH (10 mL) in air to yield an orange-red solution. After 1 h, 11B and 31P NMR analysis of the solution revealed single resonances at δ 2.6 and δ 64.5, respectively. Vapor diffusion with Et2O yielded orange blocks after 1 day. Single-crystal XRD and NMR data revealed the blocks to be previously reported 1-MeOH.43 Reaction of 6 with H2O. A solution of 6 in CDCl3 was layered with H2O and agitated several times in air. The H2O layer became cloudy, while the clear CDCl3 layer transformed from dark brown to purple. After 24 h, the 11B NMR spectrum of the reaction solution revealed resonances at δ 24.3 (three-coordinate B) and 1.3 (fourcoordinate B) in a 20:1 ratio. A sharp resonance was observed at δ 88.9 in the 31P NMR spectrum with two smaller features in the baseline. Reaction of 6 with MeOD. 6 was dissolved in MeOD in air to yield an orange-brown solution. After 24 h, the 11B NMR spectrum of the solution revealed resonances at δ 24.9 (three-coordinate B) and 3.3 (four-coordinate B) in a 7:1 ratio. Two 31P NMR resonances corresponding to the starting material and new species were observed at δ 92.5 and 86.4, respectively. Reactions of 7, 8, or 9 with H2O, MeOH, or [nBu4N]F·(H2O)n. Each reaction was performed using the following standard procedures. H2O reactions: A solution of 7, 8, or 9 (0.10 g) in CHCl3 (10 mL) with NEt3 (1 mL) was layered with H2O (5 mL) in air. It was vigorously stirred for 1 day. MeOH reactions: A solution or suspension of 7, 8, or 9 (0.10 g) in MeOH (10 mL) with NEt3 (1 mL) was vigorously stirred for 1 day. [nBu4N]F·(H2O)n reactions: To a stirring solution of 7, 8, or 9 (0.10 g) in CH2Cl2 (10 mL) was added a 1 M solution of nBu4NF·(H2O)n in tetrahydrofuran. The reaction was vigorously stirred for 1 day. 11 B and 31P NMR analysis of the reaction mixtures of 7 and 8 with H2O, MeOH, and nBu4NF·(H2O)n revealed only the starting material. The reactions of 9 with the same reagents resulted in decomposition, as determined by resonances observed at δ 20−30 in the 31P NMR spectra. These were previously assigned to PhTBDPhos decomposition.43 Crystallographic Studies. Single crystals were obtained by vapor diffusion with Et2O and CH2Cl2 (2-HF, 4, 4-HCl, 6, 7, 8, and 9) or pentane and PhF (3/3-HCl and 5) and mounted on a MiTeGen micromount using ParatoneN oil in air. The data collection, structural solution, and refinement were carried out as reported previously.70 Briefly, the structures were solved with direct methods (SHELXT), and non-hydrogen atoms were confirmed with least-squares methods (SHELXL).71 The positions of all hydrogen atoms were idealized, and they were allowed to ride on the attached carbon or nitrogen atoms. The final refinement included anisotropic temperature factors on all non-hydrogen atoms. A solvent mask was applied to the refinements of 4 and 8 due to the presence of highly disordered solvent molecules in the difference maps. Structure solution and refinement were performed, and the publication figure was generated with Olex.272 The details in data collection and refinement are available in Tables S4 and S5.

Computational Details. Geometry optimizations were performed using the Gaussian 16 software program package.73 The M06-L functional was employed for all optimizations and computations of harmonic vibrational frequencies using an ultrafine grid and the def2-SVP basis set.74,75 Single-point-energy calculations were performed on the optimized structure using the M06-L functional and Grimme’s D3 dispersion correction (M06-L-D3) and the def2-TZVP basis set for all atoms (M06-L/def2-SVP//M06-LD3/def2-TZVP). See Table S3 for select reactions where a full optimization and vibrational analysis using the def2-TZVP basis set for all atoms to show the basis set size is sufficient. To include the effect of solvation on our calculations (dichloromethane), the SMD solvent model was employed.76 Calculated free energies can be effected by the presence of low energy modes that are not welldescribed by the harmonic approximation. Using the approach of Grimme and co-workers,67 vibrational frequencies smaller than 100 cm−1 are replaced with 100 cm−1. A standard state correction has also been applied to the free energies assuming 1 M concentrations for all species in solution at 298.15 K. These corrections were treated as implemented in the GoodVibes package.77

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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.inorgchem.8b01601. Tabulated crystallographic and DFT data, NMR spectra, and molecular structures of 3, 3-OTf, and 8 (PDF) Accession Codes

CCDC 1847491−1847500 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 Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Scott R. Daly: 0000-0001-6229-0822 Author Contributions

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 This work was generously supported by the American Chemical Society’s Petroleum Research Fund (55989-DNI3) and the University of Iowa Center for Health Effects of Environmental Contamination. We thank Dale Swenson for collecting the single-crystal XRD data.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b01601 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b01601 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b01601 Inorg. Chem. XXXX, XXX, XXX−XXX