Zerovalent Nickel Compounds Supported by 1,2-Bis

Article pubs.acs.org/IC

Zerovalent Nickel Compounds Supported by 1,2-Bis(diphenylphosphino)benzene: Synthesis, Structures, and Catalytic Properties Michelle C. Neary, Patrick J. Quinlivan, and Gerard Parkin* Department of Chemistry, Columbia University, New York, New York 10027, United States S Supporting Information *

ABSTRACT: Zerovalent nickel compounds which feature 1,2-bis(diphenylphosphino)benzene (dppbz) were obtained via the reactivity of dppbz towards Ni(PMe3)4, which affords sequentially (dppbz)Ni(PMe3)2 and Ni(dppbz)2. Furthermore, the carbonyl derivatives (dppbz)Ni(PMe3)(CO) and (dppbz)Ni(CO)2 may be obtained via the reaction of CO with (dppbz)Ni(PMe3)2. Other methods for the synthesis of these carbonyl compounds include (i) the formation of (dppbz)Ni(CO)2 by the reaction of Ni(PPh3)2(CO)2 with dppbz and (ii) the formation of (dppbz)Ni(PMe3)(CO) by the reaction of (dppbz)Ni(CO)2 with PMe3. Comparison of the ν(CO) IR spectroscopic data for (dppbz)Ni(CO)2 with other (diphosphine)Ni(CO)2 compounds provides a means to evaluate the electronic nature of dppbz. Specifically, comparison with (dppe)Ni(CO)2 indicates that the o-phenylene linker creates a slightly less electron donating ligand than does an ethylene linker. The steric impact of the dppbz ligand in relation to other diphosphine ligands has also been evaluated in terms of its buried volume (%Vbur) and steric maps. The nickel center of (dppbz)Ni(PMe3)2 may be protonated by formic acid at room temperature to afford [(dppbz)Ni(PMe3)2H]+, but at elevated temperatures, effects catalytic release of H2 from formic acid. Analogous studies with Ni(dppbz)2 and Ni(PMe3)4 indicate that the ability to protonate the nickel centers in these compounds increases in the sequence Ni(dppbz)2 < (dppbz)Ni(PMe3)2 < Ni(PMe3)4; correspondingly, the pKa values of the protonated derivatives increase in the sequence [Ni(dppbz)2H]+ < [(dppbz)Ni(PMe3)2H]+ < [Ni(PMe3)4H]+. (dppbz)Ni(PMe3)2 and Ni(PMe3)4 also serve as catalysts for the formation of alkoxysilanes by (i) hydrosilylation of PhCHO by PhSiH3 and Ph2SiH2 and (ii) dehydrocoupling of PhCH2OH with PhSiH3 and Ph2SiH2.

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with the reactivity of Ni(PMe3)4 towards formic acid,15 we synthesized counterparts that feature the bidentate dppbz ligand. Dppbz is closely related to 1,2-bis(diphenylphosphino)ethane (dppe), with the principal difference being the presence of an o-phenylene rather than an ethylene linker. While not as commonly employed as dppe, dppbz has found applications for a variety of the transition metals.16,17 The majority of reactivity studies pertaining to nickel, however, have focused on divalent compounds.17 Therefore, it is noteworthy that access to zerovalent nickel compounds is provided by the reaction between Ni(PMe3)4 and dppbz, which gives sequentially (dppbz)Ni(PMe3)2 and Ni(dppbz)2, as illustrated in Scheme 1.18,19 The molecular structure of (dppbz)Ni(PMe3)2 has been determined by X-ray diffraction, as illustrated in Figure 1.20 As expected for a d10 metal center,21 the structure of (dppbz)Ni(PMe3)2 is based on a tetrahedral geometry, with the [(dppbz)Ni] chelate ring adopting an almost orthogonal conformation to the [Ni(PMe3)2] plane with a dihedral angle of 89.3°. However, in view of the constraints associated with coordination of the chelating dppbz ligand, the geometry is distorted from that of an ideal tetrahedron, as illustrated by the fact that the P−Ni−P bond angle associated with the dppbz ligand [91.23(2)°] differs significantly from the corresponding

INTRODUCTION Although the first zerovalent nickel compound, Ni(CO)4, was synthesized in 1890,1 decades passed before other zerovalent nickel compounds, e.g. Ni(PX3)4 (X = F, Cl, Br),2 Ni(CNR)4,3,4 Ni(CO)4-x(PR3)x,5 and Ni(PR3)4,6,7 were reported. Of these, zerovalent tertiary phosphine compounds, Ni(PR3)4, are of particular interest because the strong σ-donor and weak π-acceptor nature of PR3 create reactive metal centers that are capable of cleaving a variety of bonds, including Si−H,8 O−H,9 S−H,10 C−S,11 S−S,12 and C−X (X = F, Cl)13 bonds. Indeed, zerovalent nickel compounds participate in a variety of catalytic transformations such as cross-coupling, amination, hydrogenation, and hydrodefluorination.14 Moreover, we also recently demonstrated that Ni(PMe3)4 was capable of effecting the catalytic decarboxylation of formic acid to release H2.15 Here, we report the use of 1,2-bis(diphenylphosphino)benzene (dppbz)16 to afford a series of zerovalent nickel compounds that provide a means to assess the steric and electronic properties of the dppbz ligand and thereby provide a comparison with other bidentate phosphine ligands. In addition, we also compare aspects of the reactivity and catalytic properties of Ni(dppbz)2, (dppbz)Ni(PMe3)2, and Ni(PMe3)4.

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RESULTS AND DISCUSSION

Synthesis and Structures of Ni(dppbz)2 and (dppbz)Ni(PMe3)2. To extend our aforementioned study concerned © XXXX American Chemical Society

Received: October 13, 2017

A

DOI: 10.1021/acs.inorgchem.7b02636 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1

Figure 2. Molecular structure of Ni(dppbz)2 (modification A).

of the displacement of the Ni from the chelate ring as measured by the fold angle between the NiP2 and P2C2 planes.24 For example, modification B possesses almost orthogonal NiP2 planes (85.8°) and little puckering of the chelate rings (1.3 and 6.0°), whereas modification A exhibits a dihedral angle of 75.4° and fold angles of 8.1 and 26.9° (Table 1). Selected data for four-coordinate nickel compounds of diphosphine ligands that feature two phenyl groups on each phosphorus, namely Ni(dppX)2 and {Ni(dppX)2}2+, are summarized in the Supporting Information. Of these compounds, it is particularly appropriate to compare the structure of zerovalent Ni(dppbz)2 with that of its divalent counterpart, {Ni(dppbz)2}2+.17a In this regard, the coordination environment about nickel in {Ni(dppbz)2}2+ deviates considerably from tetrahedral and is distorted towards square planar, as illustrated by the magnitude of the τ4 (0.24) and τδ (0.23) geometry indices. Furthermore, the dihedral angle between the NiP2 planes of the two chelate rings of {Ni(dppbz)2}2+ (24.1°) is much smaller than that for Ni(dppbz)2 (75.4−85.8°). Interestingly, this distortion towards square-planar coordination is not as severe as that observed for other {Ni(dppX)2}2+ derivatives. For example, {Ni(dppv)2}2+ (0.0°)25,26 and {Ni(dppe)2}2+ (0.0°), which possess [C2H2] and [C2H4] linkers between the two PPh2 donors, lie on inversion centers such that the NiP4 moieties are rigorously planar; furthermore, {Ni(dmpe)2}2+ (3.4°) and {Ni(depe)2}2+ (0.0°), which feature methyl and ethyl substituents on phosphorus, possess geometries that may also be classified as square-planar.27 In contrast to these complexes, which exhibit a greater degree of planarity than does {Ni(dppbz) 2 } 2+ , the dihedral angle for {Ni(dmpp) 2 } 2+ (43.7°),27 which features a [C3H6] linker, is considerably larger. This variability in dihedral angles is in accord with the fact that whereas four-coordinate d10 nickel compounds show a strong preference for tetrahedral coordination, d8 nickel compounds exhibit both square-planar and tetrahedral extremes.21,28,29 In addition to the geometrical differences between Ni(dppbz)2 and {Ni(dppbz)2}2+, it is also pertinent to note that the Ni−P bond lengths of Ni(dppbz)2 (2.148−2.192 Å) are an average of 0.06 Å shorter than those of cationic {Ni(dppbz)2}2+ (2.216− 2.231 Å).17a Although this observation is interesting because metal−ligand bond lengths are often expected to decrease upon oxidation, as illustrated by comparison of M−Cl bond lengths in various pairs of transition-metal compounds with different oxidation states,30 a corresponding analysis for M−PR3 bonds indicates the opposite trend with M−PR3 bond lengths

Figure 1. Molecular structure of (dppbz)Ni(PMe3)2.

angle for the two PMe3 ligands [104.67(2)°]. Furthermore, the distortion is also illustrated by the fact that the four-coordinate τ4 (0.86) and τδ (0.85) geometry indices are smaller than the value of 1.00 for an idealized tetrahedron.22 Despite this distortion, however, the Ni−P bond lengths associated with the dppbz ligand [2.1325(6) Å and 2.1364(6) Å] are similar to those for the PMe3 ligands [2.1545(6) Å and 2.1692(6) Å], all of which are comparable to the mean value of Ni−P bond lengths [2.16 Å] for four-coordinate Ni(0) compounds, Ni(PX3)4, listed in the Cambridge Structural Database (CSD).23 For further comparison, structural data for other zerovalent tertiary phosphine nickel compounds that feature one bidentate diphenylphosphine ligand and two monodentate phosphine ligands, Ni(κ2-P2)(P)2, are summarized in the Supporting Information. The molecular structure of Ni(dppbz)2 has also been determined by X-ray diffraction studies on several different crystalline modifications, as illustrated in Figure 2 and the Supporting Information. Interestingly, while the modifications exhibit similar Ni−P bond lengths [2.1481(6)−2.1919(9) Å], noticeable differences are observed in (i) the dihedral angle between the NiP2 planes of the two chelate rings and (ii) the magnitude B

DOI: 10.1021/acs.inorgchem.7b02636 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Selected Metrical Data for Ni(dppbz)2 d(Ni−P1)/Å d(Ni−P2)/Å d(Ni−P3)/Å d(Ni−P4)/Å P1−Ni−P2/° P3−Ni−P4/° Ni(dppbz)2 (A)a Ni(dppbz)2 (B)a Ni(dppbz)2 (C)a {Ni(dppbz)2}(PF6)2b

2.1669(10) 2.1655(14) 2.1481(6) 2.228(5)

2.1919(9) 2.1526(15) 2.1517(6) 2.219(6)

2.1644(9) 2.1562(15) 2.1525(6) 2.216(5)

2.1726(9) 2.1499(15) 2.1486(6) 2.231(5)

87.49(3) 90.67(6) 90.98(2) 85.2(2)

89.87(3) 91.04(5) 90.51(2) 85.2(2)

[P1NiP2]/ [P3NiP4] dihedral angle/°

[P1NiP2] fold angle/°c

[P3NiP4] fold angle/°c

75.4 85.8 83.6 24.1

26.93 5.95 11.62 21.4

8.14 1.29 11.43 18.1

Modification A: Ni(dppbz)2. Modification B: Ni(dppbz)2·C6H6. Modification C: Ni(dppbz)2·0.5C6H6. bData taken from ref 17a. cThe fold angle is defined as the dihedral angle between the [PNiP] and [PCCP] planes. a

Figure 3. Geometry optimized structures of Ni(dppbz)2, (dppbz)Ni(PMe3)2, and Ni(PMe3)4 (hydrogen atoms not shown for clarity).

Scheme 2

increasing upon oxidation.30,31 Indeed, examination of structurally characterized compounds listed in the CSD indicates that the average Ni−P bond length for Ni(II) {Ni(PX3)4}2+ complexes (2.21 Å) is 0.05 Å longer than that for Ni(0) Ni(PX3)4 complexes (2.16 Å).23 The origin of the increase in the M−PR3 bond length has been rationalized in terms of both (i) π back-bonding from the metal to the P−R σ* antibonding orbitals being reduced upon oxidation30,32 and (ii) a reduction in the Pauli repulsion terms upon lengthening the M−P bond in the oxidized form.31a,33 Interconversion between (dppbz)Ni(PMe 3) 2 and Ni(dppbz)2/Ni(PMe3)4. 1H and 31P NMR spectroscopy demonstrates that solutions of (dppbz)Ni(PMe3)2 are stable at room temperature with respect to ligand redistribution and the formation of the homoleptic compounds Ni(dppbz)2 and Ni(PMe3)4. It is therefore interesting to note that Ni(dppbz)2 and Ni(PMe3)4 do not combine to form (dppbz)Ni(PMe3)2 under the same conditions. Density functional theory (B3LYP) geometry optimization calculations on (dppbz)Ni(PMe3)2, Ni(dppbz)2, and Ni(PMe3)4 (Figure 3), however, suggest that ligand redistribution between Ni(dppbz)2 and Ni(PMe3)4 to form (dppbz)Ni(PMe3)2 (Scheme 2) is almost thermoneutral.34 In support of this result, (dppbz)Ni(PMe3)2 is formed upon heating a mixture of Ni(dppbz)2 and Ni(PMe3)4 at 100 °C (Scheme 2); however, the reaction is very slow, to the extent that equilibrium is not achieved after 2 weeks. Synthesis of Carbonyl Derivatives, (dppbz)Ni(PMe3)(CO) and (dppbz)Ni(CO)2. In addition to substitution of the PMe3 ligands of (dppbz)Ni(PMe3)2 by dppbz (Scheme 1), the

Scheme 3

PMe3 ligands may also be replaced by CO to afford sequentially the monocarbonyl and dicarbonyl compounds (dppbz)Ni(PMe3)(CO) and (dppbz)Ni(CO)2 (Scheme 3). A more convenient synthetic method for (dppbz)Ni(CO)2, however, involves treatment of Ni(PPh3)2(CO)2 with dppbz (Scheme 3).35 Likewise, a more convenient approach for the synthesis of (dppbz)Ni(PMe3)(CO) is the reaction of (dppbz)Ni(CO)2 with PMe3 (Scheme 3). The molecular structures of (dppbz)Ni(PMe3)(CO) and (dppbz)Ni(CO)2 were determined by X-ray diffraction C

DOI: 10.1021/acs.inorgchem.7b02636 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4. Molecular structure of the two conformers of (dppbz)Ni(CO)2.

monodentate PR3 ligands on a metal center is typically predicted by using the Tolman electronic parameter (TEP), which is the frequency of the A1 symmetry ν(CO) vibrational mode of Ni(CO)3(PR3) complexes.6a,37−39 Specifically, as the donor ability of the PR3 ligand increases, ν(CO) decreases as a result of increased π-backbonding into the carbonyl ligand.40−42 The assignment of a substituent parameter (χi, cm−1) to each R group allows the TEP for a PR3 ligand to be predicted via the empirical expression ν(CO) = 2056.1 + Σχi, thereby providing a means to calculate the electronic properties of a ligand without requiring the synthesis of the corresponding Ni(CO)3(PR3) complex. Since this approach requires that the χi values for the individual R groups are known, an important extension of the TEP is the computationally derived ligand electronic parameter (CEP), which uses computed rather than experimental frequencies.43,44 Specifically, because the CEP uses only computed frequencies, it allows the electronic properties of a ligand (L) to be evaluated, even if the requisite Ni(CO)3L compound is unknown or unstable. Despite the widespread interest and applications of the Tolman electronic parameter for monodentate tertiary phosphine ligands, however, it is only more recently that a similar approach using nickel dicarbonyl compounds of the type [κ2-R2PXPR2]Ni(CO)2 was proposed as a general approach for evaluating the donor ability of bidentate phosphines.45−49 Such an investigation is warranted because the geometric nature of the linker is expected to exert an electronic effect due to the different natural bite angles of the bidentate ligands.50,51 In this regard, it was demonstrated that computed ν(CO)sym values for a series of [κ2-R2P(CH2)nPR2]Ni(CO)2 compounds may be expressed

Figure 5. Molecular structure of (dppbz)Ni(PMe3)(CO) (modification A).

(Figures 4 and 5), thereby completing the series of (dppbz)Ni(CO)x(PMe3)2-x (x = 0, 1, 2) compounds. Interestingly, the asymmetric unit of (dppbz)Ni(CO)2 contains two crystallographically independent molecules that differ in the conformations of the phenyl substituents.36 Despite these differences, there is little variation in the geometry of the metal center, as illustrated by the data listed in Table 2. In contrast, the two crystalline modifications (see Supporting Information) of (dppbz)Ni(PMe3)(CO) exhibit distinctly different P−Ni−C bond angles, although the structures are otherwise similar (Table 3). (dppbz)Ni(CO)2 and the Evaluation of the Steric and Electronic Properties of dppbz. The electronic impact of Table 2. Selected Metrical Data for (dppbz)Ni(CO)2 a

Ni1 Ni2a a

d(Ni−PA)/Å

d(Ni−PB)/Å

PA−Ni−PB/°

CA−Ni−CB/°

[PNiP]/[CNiC] dihedral angle/°

[PNiP] fold angle/°b

2.1860(15) 2.1834(15)

2.1823(15) 2.1856(15)

89.95(6) 90.02(6)

111.4(2) 114.0(2)

88.3 88.1

9.5 2.4

Two independent molecules in the asymmetric unit. bThe fold angle is defined as the dihedral angle between the [PNiP] and [PCCP] planes.

Table 3. Selected Metrical Data for (dppbz)Ni(PMe3)(CO) a

A Ba

d(Ni−P1)/Å

d(Ni−P2)/Å

P1−Ni−P2/°

P3−Ni−C1/°

[PNiP]/[CNiC] dihedral angle/°

[PNiP] fold angle/°b

2.1483(16) 2.1662(5)

2.1487(16) 2.1630(5)

90.85(6) 90.56(2)

104.86(19) 112.11(4)

89.0 86.3

2.0 8.6

a

Modification A: (bppbz)Ni(PMe3)(CO). Modification B: (bppbz)Ni(PMe3)(CO)·0.5C6H6. bThe fold angle is defined as the dihedral angle between the [PNiP] and [PCCP] planes. D

DOI: 10.1021/acs.inorgchem.7b02636 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry by the empirical relationship ν(CO)sym = 1982 + ΣχB − 4n, where χB is the substituent parameter and n is the number of carbon atoms in the linker (1 ≤ n ≤ 3), thereby indicating that the bidentate ligand becomes more electron donating as the length of the linker increases.46,52 Experimental ν(CO)sym and ν(CO)asym data for [κ2-Ph2PXPPh2]Ni(CO)2 derivatives are compiled in Table 4 and the

subsequent to the original report for (dppm)Ni(CO)253 indicate that the dppm−nickel carbonyl system is considerably more complex than that for other bidentate phosphines because the strain associated with a four-membered chelate ring results in the adoption of other coordination modes.56 Specifically, spectroscopic evidence has been presented for the existence of (κ1 -dppm)Ni(CO)3 , (κ 1-dppm)2 Ni(CO)2 , [Ni(μ-dppm)(CO)]2(μ-CO), and [Ni(μ-dppm)(CO)2]2,57−64 while dinuclear [Ni(μ-dppm)(CO)]2(μ-CO) has also been characterized by X-ray diffraction.60 These compounds have different IR spectroscopic signatures (see Supporting Information) and have been shown to interconvert in solution.60 Since no other spectroscopic evidence was presented to support the formulation of (dppm)Ni(CO)2, the purported IR data for (dppm)Ni(CO)2 may be incorrect.60 Furthermore, on the basis of an X-ray diffraction investigation, it has been noted that the counterpart with methyl substituents on phosphorus, namely (dmpm)Ni(CO)2,65 actually exists as dinuclear [Ni(μ-dmpm)(CO)2]2 with bridging dmpm ligands, rather than a monomer with a chelating dmpm ligand.66 As such, this observation questions further the IR spectroscopic assignment for (dppm)Ni(CO)2. Although it is not possible to use the reported data for (dppm)Ni(CO)2 to determine experimentally whether dppm is a better or worse donor than dppe in the nickel system, the molybdenum system, (dppX)Mo(CO)4, is amenable for such analysis because (dppm)Mo(CO)4 has been shown by X-ray diffraction to exhibit bidentate coordination of the dppm ligand.67 In this regard, the ν(CO) data for (dppm)Mo(CO)4 (2020, 1920, 1907, and 1879 cm−1) are very similar to the values for (dppe)Mo(CO)4 (2020, 1919, 1907, and 1881 cm−1),68 such that it is evident that there is actually little difference between a [CH2] and a [CH2CH2] linker. The steric properties of monodentate phosphine ligands have traditionally been expressed in terms of the Tolman cone angle37 and related concepts.69 However, the cone angle concept has much less applicability for bidentate phosphines, which bear little resemblance to a cone-like structure. For such ligands, a better estimate of the steric properties may be expressed in terms of the so-called buried volume (%Vbur), which corresponds to the portion of the volume of a sphere centered on the metal atom that is buried by overlap with the ligand atoms.70−72 The %Vbur values for the dppbz ligands in each of the nickel compounds described here are summarized in Table 5, which illustrates that, despite the different conformations of the phenyl substituents, the ligand presents overall similar steric demands in each complex. In addition to providing a quantitative measure of

Table 4. IR Spectroscopic Data for [κ2-Ph2PXPPh2]Ni(CO)2 and (Ph3P)2Ni(CO)2 ring size

νsym (cm−1)

νasym (cm−1)

solid or solution state

(dppm)Ni(CO)2 (dppe)Ni(CO)2

4 5

(dppv)Ni(CO)2 (dppbz)Ni(CO)2

5 5

(dppp)Ni(CO)2

6

(dppb)Ni(CO)2 (Ph3P)2Ni(CO)2

7

1997a 1997 1998 2006 2004 2007 2003 2007 2000 2000 1996 2002 1995

1939a 1936 1936 1945 1945 1950 1944 1952 1937 1941 1935 1944 1933

CHCl3 C2H4Cl2 nujol CHCl3 benzene nujol solid benzene CHCl3 benzene benzene benzene solid

ref 53 19 54 53 this 54 this this 53 this this this this

work work work work work work work

a

Data reported for (dppm)Ni(CO)2 are considered to be erroneous. See ref 60.

Supporting Information.53−55 On the basis of these data, it is evident that dppbz (2007 and 1952 cm−1) with an o-phenylene linker is slightly less electron donating than dppe (2004 and 1945 cm−1), which possesses an ethylene linker. Furthermore, comparison with (Ph3P)2Ni(CO)2 (2002 and 1944 cm−1) indicates that dppbz is also less electron donating than a pair of PPh3 ligands. Examination of the data presented in Table 4 also demonstrates how ν(CO) data for dicarbonyl compounds are influenced by the form of sample (i.e., solid state or solution, including the solvent). For example, ν(CO)sym for (dppe)Ni(CO)2 has been reported to have values of 1997 and 2006 cm−1. The magnitude of this difference becomes significant when it is considered that increasing the linker size by one methylene group is predicted to reduce ν(CO)sym by 4 cm−1.46 Indeed, the use of different experimental conditions has been proposed as a possible rationalization for why the experimental data for [κ2-R2P(CH2)nPR2]Ni(CO)2 do not correlate perfectly with the computed data.46 Therefore, to provide experimental data to relate the impact of linker size on ν(CO), we analyzed (dppe)Ni(CO)2, (dppp)Ni(CO)2, and (dppb)Ni(CO)2 in benzene solutions. Significantly, the ν(CO) values for (dppe)Ni(CO)2 (2004 and 1945 cm−1), (dppp)Ni(CO)2 (2000 and 1941 cm−1), and (dppb)Ni(CO)2 (1996 and 1935 cm−1) decrease upon increasing the chain length, and the magnitude of the change in ν(CO)sym per methylene group (4 cm−1) corresponds exactly to that predicted by the aforementioned relationship based on computational data, i.e. ν(CO)sym = 1982 + ΣχB − 4n.46 Despite this good agreement, however, the experimental data for (dppm)Ni(CO)2 reported in the literature (1997 and 1939 cm−1)53 are anomalous, with both values being lower than those for (dppe)Ni(CO)2. Examination of the literature suggests that this discrepancy may be a consequence of the values reported for (dppm)Ni(CO)2 being erroneous. In this regard, several investigations

Table 5. %Vbur Values for the dppbz Ligand in {Ni(dppbz)} Compounds %Vbur Ni(dppbz)2 (A) Ni(dppbz)2 (B) Ni(dppbz)2 (C) (dppbz)Ni(CO)2 (dppbz)Ni(PMe3)(CO) (dppbz)Ni(PMe3)2 a

E

47.4a 47.9 48.4a 49.2 48.4a 48.6 50.0a 50.3 50.8a 49.5 48.8

Values for two crystallographically independent dppbz ligands. DOI: 10.1021/acs.inorgchem.7b02636 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry the overall steric bulk via the value of %Vbur, finer details of the ligand profile are afforded by examination of steric maps that illustrate how the steric bulk is distributed about the metal center via the use of colored contours. Steric maps for (dppbz)Ni(CO)2 and (dppbz)Ni(PMe3)2 are illustrated in Figure 6, which

Table 6. Metrical and %Vbur Data for (dppm)Ni(CO)2, (dppe)Ni(CO)2, (dppp)Ni(CO)2, and (dppe)Ni(CO)2a P−Ni−P/° OC−Ni−CO/° (dppm)Ni(CO)2 (dppe)Ni(CO)2 (dppp)Ni(CO)2 (dppb)Ni(CO)2 a

75.9 89.2 94.6 99.9

116.2 114.7 115.0 114.5

[PNiP]/[CNiC] dihedral angle/°

%Vbur

89.2 89.0 89.9 87.3

41.9 47.8 52.9 54.2

Geometry optimized structures.

as illustrated in Figure 7.77 Examination of the %Vbur data (Table 6) for these compounds indicates that the steric impact of the phenyl substituents increases as the size of the linker increases. With respect to the origin of this effect, it is pertinent to note that increasing the number of methylene groups results in an increase in the P−Ni−P bond angle (Table 6), and that an increase in bond angle for diphosphines is generally regarded to result in greater steric interactions.78,79 However, it is important to emphasize that the steric impact on the metal center is not simply a consequence of changing the P−Ni−P bond angle. Specifically, examination of both the geometry optimized structures (Figure 7) and the steric maps (Figure 8) indicates that

Figure 6. Steric maps for the dppbz ligand in (dppbz)Ni(PMe3)2 and one of the conformations of (dppbz)Ni(CO)2 (PMe3 and CO ligands omitted for clarity).

show how the bulk is concentrated in the east and west hemispheres,73 as would be expected for a C2 symmetric ligand. To place the %Vbur values for the dppbz ligand in more context, we calculated the values for a series of geometry optimized (dppX)Ni(CO)2 compounds in which the size of the linker is progressively increased, namely (dppm)Ni(CO)2, (dppe)Ni(CO)2,74 (dppp)Ni(CO)2,75 and (dppb)Ni(CO)2,76

Figure 7. Geometry optimized structures of (dppm)Ni(CO)2, (dppe)Ni(CO)2, (dppp)Ni(CO)2, and (dppe)Ni(CO)2 (hydrogen atoms omitted for clarity).

Figure 8. Steric maps for (dppm)Ni(CO)2, (dppe)Ni(CO)2, (dppp)Ni(CO)2, and (dppe)Ni(CO)2 (CO ligands omitted for clarity). F

DOI: 10.1021/acs.inorgchem.7b02636 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Ni(PMe3)2H]+ is provided by the fact that several homoleptic [Ni(PR3)4H]+ and {Ni[P(OR)3]4H}+ complexes and also bidentate variants have been reported to exhibit similar chemical shifts.9a,89−95 Of these, only four complexes, namely [Ni(PMe3)4H][(2-ClC6H4O)3H2],9a [HNi(PPri2NPh2)2][BF4],91 [(dmpbz) 2 NiH][BF 4 ],85b and [NiH(8P Cy 2 N H H)(dppe)][BF4]2,96 have been structurally characterized by X-ray diffraction, but the position of the hydride ligand was only determined for the latter. Therefore, we determined the structure of [(dppbz)Ni(PMe3)2H]+ by using density functional theory geometry optimization (Figure 10), which demonstrates

torsional differences associated with rotation about the Ni−P bond also have an influence on the overall steric environment. Thus, while the [(CH2)xP2Ni] cores of [(dppm)Ni] and [(dppe)Ni] moieties have approximate C2 symmetry, such symmetry is absent for [(dppp)Ni] and [(dppb)Ni], in which the ligands adopt conformations that feature less symmetrically disposed phenyl substituents, with at least one phenyl group directed towards the metal center. Reactivity of (dppbz)Ni(PMe3)2 towards Formic Acid: Protonation and Catalytic Decarboxylation. The decarboxylation of formic acid is of much current interest with respect to its ability to serve as a hydrogen storage material,80 and there is particular emphasis on the use of earth abundant nonprecious metals to catalyze the release of hydrogen.81−84 In this regard, we previously demonstrated that Ni(PMe3)4 is able to effect catalytic release of H2 from formic acid at 80 °C.15,85,86 We have now demonstrated that the heteroleptic variant, (dppbz)Ni(PMe3)2, is also capable of such activity, with a turnover number of 10 over a period of ca. 16 h at 60 °C;87 however, the activity diminishes considerably after this period, and analysis of the reaction by 1H NMR spectroscopy demonstrates that the reaction is accompanied by the formation of Ni(dppbz)2 and (dppbz)Ni(PMe3)(CO). Precedent for the formation of a carbonyl derivative in such systems is provided by the observation that Cp*Mo(PMe3)3H reacts with formic acid to afford the carbonyl complex, Cp*Mo(PMe3)2(CO)H.83a While (dppbz)Ni(PMe3)2 does not remain intact upon heating with formic acid, the protonated species, [(dppbz)Ni(PMe3)2H]+, may be observed by 1H and 31P NMR spectroscopy at room temperature. For example, [(dppbz)Ni(PMe3)2H]+ is characterized by a signal at δ −14.36 ppm in C6D6, which appears as a triplet of triplets due to coupling to the two sets of phosphine ligands with 2JPH = 34 and 47 Hz. Interestingly, a 2D-EXSY88 study demonstrates that the hydride ligand of [(dppbz)Ni(PMe3)2H]+ undergoes exchange with formic acid (Figure 9). Additional support for the formation of [(dppbz)-

Figure 10. Geometry optimized structure of [(dppbz)Ni(PMe3)2H]+ (hydrogen atoms on carbon omitted for clarity).

that protonation occurs at one of triangular faces of the NiP4 tetrahedron, thereby resulting in a distorted trigonal bipyramidal structure in which the hydride occupies an axial position.93 The basicity of metal centers, as well as the corresponding pKa values and hydricities of the protonated derivatives, is of considerable relevance to reactions involving metal hydride compounds.97 As such, effort has been directed towards evaluating the impact of bite angle on the pKa values of [Ni(diphosphine)2H]+

Figure 9. 2D-EXSY experiment demonstrating the exchange of the hydride ligand of [(dppbz)Ni(PMe3)2H]+ with formic acid. G

DOI: 10.1021/acs.inorgchem.7b02636 Inorg. Chem. XXXX, XXX, XXX−XXX

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and materials chemistry.104 There has been recent emphasis in developing catalysts for this transformation that are based on nonprecious metals such as nickel82a,105−107 and, in this regard, both Ni(PMe3)4 and (dppbz)Ni(PMe3)2 are capable of achieving hydrosilylation of benzaldehyde (Scheme 4).108 For example, Ni(PMe3)4 catalyzes the double insertion of PhCHO into two of the Si−H bonds of PhSiH3 at room temperature to afford PhSiH(OCH2Ph)2; furthermore, upon heating to 80 °C, the triple insertion product, PhSiH(OCH2Ph)3, is obtained. Likewise, (dppbz)Ni(PMe3)2 catalyzes the double insertion of PhCHO into PhSiH3 at room temperature and triple insertion at 80 °C, albeit with a lower activity than that for Ni(PMe3)4. The triple insertion of carbonyl compounds into PhSiH3 is much less common than double insertion,109,110 and it is noteworthy that these nickel compounds can selectively achieve double and triple insertion as a function of temperature. Ni(PMe3)4 and (dppbz)Ni(PMe3)2 also catalyze the insertion of PhCHO into the Si−H bonds of Ph2SiH2 (Scheme 4), with the former being more active. At room temperature, both Ni(PMe3)4 and (dppbz)Ni(PMe3)2 afford primarily the single insertion product, Ph2SiH(OCH2Ph), whereas both catalysts afford Ph2Si(OCH2Ph)2 at 80 °C. Although the mechanisms of these transformations are unknown, we note that the initial step in reactions of PhSiH3 catalyzed by zerovalent nickel species has been proposed to involve oxidative addition of the Si−H bond,105j and that the insertion of carbonyl compounds into Ni−H bonds has been observed105a-d,111 and invoked105e as a step in nickel-catalyzed hydrosilylation.112 In addition to the synthesis of alkoxysilanes via hydrosilylation, there is also interest in their synthesis via dehydrocoupling of alcohols and silanes because it provides a means of releasing H2 on-demand and is, therefore, of significance to the “hydrogen economy”.109,113 Thus, it is noteworthy that Ni(PMe3)4 and (dppbz)Ni(PMe3)2 are also capable of forming alkoxysilanes via the dehydrocoupling of PhSiH3 and PhCH2OH (Scheme 5), with the former being the more active catalyst. The dehydrocoupling, however, is less efficient than the corresponding hydrosilylation reaction between PhSiH3 and PhCHO, as indicated by the fact that the principal products at room temperature are the mono- and dialkoxysilanes, PhSiH2(OCH2Ph)114 and PhSiH(OCH2Ph)2. Moreover, while Ni(PMe3)4 allows for formation of PhSi(OCH2Ph)3 at 80 °C, complete conversion is not observed. Dehydrocoupling of Ph2SiH2 and PhCH2OH by Ni(PMe3)4 is also less efficient than the corresponding hydrosilylation reaction. For example, Ph2SiH(OCH2Ph) is the principal product observed after heating at 80 °C overnight, and formation of significant quantities of Ph2Si(OCH2Ph)2 is observed only after several days at 80 °C. Ni(PMe3)4 and (dppbz)Ni(PMe3)2 also catalyze dehydrocoupling of (i) PhSiH3 and

Figure 11. 31P{1H} NMR spectra of a mixture of Ni(PMe3)4 (A) and (dppbz)Ni(PMe3)2 (B) in the absence (a) and presence (b−f) of increasing concentrations of formic acid, thereby demonstrating that protonation of Ni(PMe3)4 occurs before (dppbz)Ni(PMe3)2.

Scheme 4

compounds.92,93,98 To complement these studies, it is pertinent to examine hybrid systems that incorporate both bidentate and monodentate phosphine ligands. In this regard, titration of a mixture of (dppbz)Ni(PMe3)2 and Ni(PMe3)4 with formic acid demonstrates that formation of [Ni(PMe3)4H]+ occurs prior to formation of [(dppbz)Ni(PMe3)2H]+, as illustrated in Figure 11.99 A corresponding titration of a mixture of (dppbz)Ni(PMe3)2 and Ni(dppbz)2 with formic acid (see Supporting Information) demonstrates that protonation of (dppbz)Ni(PMe3)2 occurs to the exclusion of Ni(dppbz)2.100,101 Thus, the pKa values of the protonated derivatives increase in the sequence [Ni(dppbz)2H]+ < [(dppbz)Ni(PMe3)2H]+ < [Ni(PMe3)4H]+. Ni(PMe3)4 and (dppbz)Ni(PMe3)2 as Catalysts for Hydrosilylation and Dehydrocoupling. The catalytic hydrosilylation of carbonyl compounds to afford alkoxysilanes is not only of interest in terms of providing a means to reduce a substrate to an alcohol102 but is also of interest from the perspective that alkoysilanes have applications in organic synthesis103 Scheme 5

H

DOI: 10.1021/acs.inorgchem.7b02636 Inorg. Chem. XXXX, XXX, XXX−XXX

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resulting in a deep orange solution. The volatile components were removed in vacuo, and the orange residue was washed with acetonitrile (3 × 3 mL) and dried in vacuo to afford (dppbz)Ni(PMe3)2 as an orange powder (175 mg, 77% yield). Orange crystals suitable for X-ray diffraction were obtained via vapor diffusion of pentane into a benzene solution. Anal. Calcd for (dppbz)Ni(PMe3)2: C, 65.8%; H, 6.4%. Found: C, 66.0%; H, 6.2%. 1H NMR (C6D6): 0.87 [s, 18H of Ni(PMe3)2], 6.95 [m, 2H of C6H4], 7.04 [t, 4 p-H of C6H5, 3JH−H = 7], 7.12 [t, 8 m-H of C6H5, 3JH−H = 7], 7.55 [m, 2H of C6H4], 7.70 [m, 8 o-H of C6H5]. 13C{1H} NMR (C6D6): 22.05 [m, 6C of Ni(PMe3)2], 127.26 [s, 4 p-C of C6H5], 128.12 [s, 8 m-C of C6H5], 128.61, [s, 2C of C6H4], 132.07 [t, 8 o-C of C6H5, 2JP−C = 7], 133.29 [t, 2C of C6H4, 2 JP−C = 7], 143.20 [m, 4 i-C of C6H5], 150.05 [tt, 2C of C6H4, 1JP−C = 45, 3JP−C = 6]. 31P{1H} NMR (C6D6): −19.74 [t, Ni(PMe3)2, 2JP−P = 29], 46.57 [t, (dppbz)Ni, 2JP−P = 29]. IR (ATR, solid, cm−1): 3050 (w), 2956 (w), 2891 (m), 2801 (w), 1584 (m), 1478 (m), 1450 (w), 1431 (m), 1327 (w), 1292 (m), 1273 (m), 1254 (w), 1181 (w), 1156 (w), 1104 (m), 1084 (m), 1068 (w), 1027 (m), 998 (m), 933 (s), 838 (w), 758 (m), 738 (m), 692 (s), 661 (m), 620 (w), 531 (m), 514 (s), 493 (m), 452 (m), 420 (w). Synthesis of Ni(dppbz)2. A solution of (dppbz)Ni(PMe3)2 (10.5 mg, 0.016 mmol) and dppbz (7.1 mg, 0.016 mmol) in benzene (ca. 0.7 mL) in an NMR tube equipped with a J. Young valve was heated at 60 °C for 18 h. The volatile components were removed by lyophilization to afford Ni(dppbz)2 as an orange powder, which was washed with pentane (2 × 1 mL) and dried in vacuo (7.1 mg, 47% yield). Orange crystals of Ni(dppbz)2 suitable for X-ray diffraction were obtained via the slow evaporation of a solution in acetonitrile, while crystals of composition Ni(dppbz)2·0.5C6H6 and Ni(dppbz)2·C6H6 were obtained via vapor diffusion of pentane into benzene solutions. Anal. Calcd for Ni(dppbz)2·0.5C6H6: C, 76.4%; H, 5.2%. Found: C, 76.0%; H, 5.3%. 1H NMR (C6D6): 6.80 [s, 4H of C6H4], 6.84 [t, 16 m-H of C6H5, 3JH−H = 7], 6.92 [t, 8 p-H of C6H5, 3JH−H = 7], 7.37 [s, 16 o-H of C6H5], 7.42 [s, 4H of C6H4]. 13C{1H} NMR: 127.29 [s, 8 p-C of C6H5], 127.62 [16 m-C of C6H5], 128.65 [s, 4C of C6H4], 132.54 [m, 16 o-C of C6H5 and 4C of C6H4], 139.30 [m, 8 i-C of C6H5], 149.63 [m, 4C of C6H4]. 31P{1H} NMR (C6D6): 47.37. IR (ATR, solid, cm−1): 3051 (w), 2957 (w), 1810 (vw), 1584 (w), 1478 (m), 1448 (w), 1431 (m), 1302 (vw), 1249 (vw), 1182 (w), 1157 (w), 1103 (m), 1086 (m), 1027 (w), 999 (vw), 907 (vw), 846 (vw), 812 (vw), 758 (m), 738 (m), 690 (s), 666 (m), 618 (w), 516 (s), 482 (m), 453 (m), 440 (m), 417 (m). Synthesis of (dppbz)Ni(CO)2, (dppe)Ni(CO)2, (dppp)Ni(CO)2, and (dppb)Ni(CO)2. (dppbz)Ni(CO)2, (dppe)Ni(CO)2, (dppp)Ni(CO)2, and (dppb)Ni(CO)2 were obtained via the reaction of (Ph3P)2Ni(CO)2 with the respective diphosphine. For example, a mixture of (Ph3P)2Ni(CO)2 (100 mg, 0.156 mmol) and dppbz (73 mg, 0.16 mmol) was dissolved in benzene (ca. 5 mL) and heated at 60 °C for 2 h in a sealed ampoule to produce a clear yellow solution. After this period, the volatile components were removed in vacuo, and the oily residue was stirred overnight with pentane (ca. 3 mL). The resulting yellow suspension was centrifuged, and the solid was collected and washed with pentane (3 × 1.5 mL) and dried in vacuo to afford (dppbz)Ni(CO)2 as a pale yellow powder (62 mg, 71% yield). Yellow crystals of (dppbz)Ni(CO)2 suitable for X-ray diffraction were obtained via vapor diffusion of pentane into a benzene solution. Anal. Calcd for (dppbz)Ni(CO)2: C, 68.5%; H, 4.3%. Found: C, 68.8%; H, 4.4%. 1H NMR (C6D6): 6.86 [m, 2H of C6H4], 6.98 [m, 8 m-H of C6H5], 6.99 [m, 4 p-H of C6H5], 7.40 [m, 2H of C6H4], 7.51 [m, 8 o-H of C6H5]. 13C{1H} NMR (C6D6): 128.69 [t, 8 m-C of C6H5, 3 JP−C = 4], 129.32 [s, 4 p-C of C6H5], 130.02 [s, 2C of C6H4], 132.67 [t, 8 o-H of C6H5, 2JP−C = 7], 133.49 [t, 2C of C6H4, JP−C = 6], 137.59 [t, 4 i-C of C6H5, 1JP−C = 18], 146.23 [t, 2C of C6H4, JP−C = 40], 200.83 [t, 2C of CO, 2JP−C = 4]. 31P NMR (C6D6): 45.89. IR (ATR, solid, cm−1): 3057 (w), 2003 (s) [ν(CO)], 1944 (s) [ν(CO)], 1585 (w), 1479 (m), 1448 (w), 1434 (m), 1330 (w), 1304 (w), 1183 (w), 1160 (w), 1108 (m), 1094 (m), 1026 (w), 999 (w), 850 (w), 812 (w), 766 (w), 744 (s), 693 (s), 668 (m), 533 (s), 520 (s), 508 (s), 490 (s), 466 (m), 424 (m). (ii) (dppe)Ni(CO)2, (dppp)Ni(CO)2, and

MeOH and (ii) Ph2SiH2 and MeOH with similar results to the corresponding reactions of PhCH2OH.

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SUMMARY In summary, a series of zerovalent nickel compounds which feature the diphosphine, dppbz, namely (dppbz)Ni(PMe3)2, (dppbz)Ni(PMe3)(CO), (dppbz)Ni(CO)2, and Ni(dppbz)2, was synthesized and structurally characterized by X-ray diffraction. The electronic and steric properties of dppbz and other diphosphine ligands of the type Ph2PXPPh2 were evaluated in terms of ν(CO) and buried volume data for (κ2-Ph2PXPPh2)Ni(CO)2 compounds. Examination of the reactivity of Ni(dppbz)2, (dppbz)Ni(PMe3)2, and Ni(PMe3)4 towards formic acid at room temperature demonstrates that the ability to protonate the nickel centers in these compounds increases in the sequence Ni(dppbz)2 < (dppbz)Ni(PMe3)2 < Ni(PMe3)4, such that the pKa values of the protonated derivatives increase in the sequence [Ni(dppbz)2H]+ < [(dppbz)Ni(PMe3)2H]+ < [Ni(PMe3)4H]+. At elevated temperatures, (dppbz)Ni(PMe3)2 is capable of effecting catalytic release of H2 from formic acid. (dppbz)Ni(PMe3)2 and Ni(PMe3)4 also serve as catalysts for the formation of alkoxysilanes via (i) hydrosilylation of PhCHO by PhSiH3 and Ph2SiH2 and (ii) dehydrocoupling of PhCH2OH with PhSiH3 and Ph2SiH2.

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EXPERIMENTAL SECTION

General Considerations. All manipulations were performed using a combination of glovebox, high vacuum, and Schlenk techniques under a nitrogen or argon atmosphere.115 Solvents were purified and degassed by standard procedures. NMR spectra were measured on Bruker 300 DRX and Bruker Avance 500 DMX spectrometers. 1H NMR spectra are reported in ppm relative to SiMe4 (δ = 0) and were referenced internally with respect to the protio solvent impurity (δ = 7.16 for C6D5H and 1.72 for THF-d8).116 13C NMR spectra are reported in ppm relative to SiMe4 (δ = 0) and were referenced internally with respect to the solvent (δ = 128.06 for C6D6).116 31P NMR chemical shifts are reported in ppm relative to 85% H3PO4 (δ = 0) and were referenced electronically by using the 1H resonance frequency of SiMe4.117 Coupling constants are reported in Hertz. Infrared spectra were recorded on a PerkinElmer Spectrum Two spectrometer in attenuated total reflectance (ATR) mode and are reported in reciprocal centimeters. Ni(PMe3)4 was synthesized by the literature method,15 and other chemicals were obtained from Sigma-Aldrich (PMe3, CO, PhCH2OH, MeOH, PhCHO, HCO2H, and mesitylene), Alfa Aesar (Ph2SiH2), Strem Chemicals [dppbz, (PPh3)2Ni(CO)2], Cambridge Isotope Laboratories (H13CO2H), and Acros Organics (PhSiH3) and used as supplied. The calculation of the percent buried volumes (%Vbur) and the steric maps was determined by using SambVca 2.0 (https://www.molnac.unisa.it/OMtools/sambvca2.0/)70a for a sphere of radius 3.5 Å about the metal center and Bondi van der Waals radii scaled by a factor of 1.17. X-ray Structure Determinations. X-ray diffraction data were collected on a Bruker Apex II diffractometer. The structures were solved using direct methods and standard difference map techniques and were refined by full-matrix least-squares procedures on F2 with SHELXTL (version 2014/7).118 Crystallographic data were deposited with the Cambridge Crystallographic Data Centre (CCDC 1579980− 1579986). Computational Details. Calculations were carried out using DFT as implemented in the Jaguar 8.9 (release 15) suite of ab initio quantum chemistry programs.119 Geometry optimizations were performed with the B3LYP density functional using the LACVP** basis set, which were also used to determine Gibbs free energy values at 1 atm and 298.15 K. Cartesian coordinates and energies of the geometry optimized structures are provided in the Supporting Information. Synthesis of (dppbz)Ni(PMe3)2. A mixture of Ni(PMe3)4 (151 mg, 0.416 mmol) and dppbz (155 mg, 0.347 mmol) was dissolved in benzene (ca. 5 mL) and stirred for 3 h at room temperature, thereby I

DOI: 10.1021/acs.inorgchem.7b02636 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

heated at 60 °C and monitored via 1H and 13C{1H} NMR spectroscopy, thereby demonstrating the formation of 13CO2. Titration of a Mixture of Ni(PMe3)4 and (dppbz)Ni(PMe3)2 with Formic Acid. A solution of Ni(PMe3)4 (4.6 mg, 0.013 mmol) and (dppbz)Ni(PMe3)2 (8.3 mg, 0.013) mmol) in THF-d8 (ca. 0.7 mL) was treated with increasing amounts of HCO2H and monitored via 1 H and 31P{1H} NMR spectroscopy, thereby demonstrating that [Ni(PMe3)4H]+ forms before [(dppbz)Ni(PMe3)2H]+. Titration of a Mixture of (dppbz)2Ni and (dppbz)Ni(PMe3)2 with Formic Acid. A solution of (dppbz)2Ni (6 mg, 0.006 mmol) and (dppbz)Ni(PMe3)2 (4.8 mg, 0.007 mmol) in THF-d8 (ca. 0.7 mL) was treated with increasing amounts of HCO2H and monitored via 1H and 31 1 P{ H} NMR spectroscopy, thereby demonstrating the formation of [(dppbz)Ni(PMe3)2H]+,with no evidence for [(dppbz)2NiH]+. Reactivity of Ni(dppbz)2 towards Formic Acid. A solution of Ni(dppbz)2 (4.0 mg, 0.0042 mmol) in C6D6 (ca. 0.7 mL) in an NMR tube equipped with a J. Young valve was treated with HCO2H (5.0 μL, 0.13 mmol). The mixture was heated at 60 °C and monitored via 1H and 31P{1H} NMR spectroscopy over a period of 3 days, thereby demonstrating that H2 was not released. Hydrosilylation of Benzaldehyde by PhSiH3. (a) A solution of Ni(PMe3)4 (0.7 mg, 0.002 mmol), benzaldehyde (23.1 mg, 0.218 mmol), and PhSiH3 (6.9 mg, 0.064 mmol) in C6D6 (0.7 mL) in an NMR tube equipped with a J. Young valve was heated at 80 °C. The reaction was monitored by 1H NMR spectroscopy, thereby demonstrating the formation of PhSi(OCH2Ph)3 after several hours (TOF per Si−H bond = 45 h−1).120 (b) A solution of (dppbz)Ni(PMe3)2 (0.6 mg, 0.001 mmol), benzaldehyde (3.2 mg, 0.030 mmol), and PhSiH3 (1.0 mg, 0.009 mmol) in C6D6 (0.7 mL) in an NMR tube equipped with a J. Young valve was heated at 80 °C. The reaction was monitored by 1H NMR spectroscopy, thereby demonstrating the formation of PhSi(OCH2Ph)3 over a period of several days (TOF per Si−H bond = 6 day−1). (c) A solution of Ni(PMe3)4 (3.0 mg, 0.008 mmol), benzaldehyde (4.6 mg, 0.043 mmol), and PhSiH3 (15.5 mg, 0.146 mmol) in C6D6 (0.7 mL) in an NMR tube equipped with a J. Young valve was monitored by 1H NMR spectroscopy thereby demonstrating the formation of PhSiH(OCH2Ph)2 at room temperature (TOF per Si−H bond = 10 h−1). (d) A solution of (dppbz)Ni(PMe3)2 (5.0 mg, 0.008 mmol), benzaldehyde (13.0 mg, 0.123 mmol), and PhSiH3 (3.9 mg, 0.036 mmol) in C6D6 (0.7 mL) in an NMR tube equipped with a J. Young valve was monitored by 1H NMR spectroscopy, thereby demonstrating the formation of PhSiH(OCH2Ph)2 at room temperature (TOF per Si−H bond = 10 h−1). Hydrosilylation of Benzaldehyde by Ph2SiH2. (a) A solution of Ni(PMe3)4 (1.2 mg, 0.003 mmol), benzaldehyde (25.4 mg, 0.239 mmol), and Ph2SiH2 (20.2 mg, 0.110 mmol) in C6D6 (0.7 mL) in an NMR tube equipped with a J. Young valve was heated at 80 °C. The reaction was monitored by 1H NMR spectroscopy, thereby demonstrating the formation of Ph2Si(OCH2Ph)2 over a period of 18 h. (b) A solution of (dppbz)Ni(PMe3)2 (0.8 mg, 0.001 mmol), benzaldehyde (8.6 mg, 0.081 mmol), and Ph2SiH2 (6.9 mg, 0.037 mmol) in C6D6 (0.7 mL) in an NMR tube equipped with a J. Young valve was heated at 80 °C. The reaction was monitored by 1H NMR spectroscopy, thereby demonstrating the formation of Ph2SiH(OCH2Ph) and Ph2Si(OCH2Ph)2 over a period of 6 days. (c) A solution of Ni(PMe3)4 (3.1 mg, 0.009 mmol), benzaldehyde (9.9 mg, 0.093 mmol), and Ph2SiH2 (7.9 mg, 0.043 mmol) in C6D6 (0.7 mL) in an NMR tube equipped with a J. Young valve was monitored by 1H NMR spectroscopy. The formation of Ph2SiH(OCH2Ph) was observed after period of ca. 12 h at room temperature, and a 4:1 mixture of Ph2SiH(OCH2Ph) and Ph2Si(OCH2Ph)2 was observed after a period of 3 days. (d) A solution of (dppbz)Ni(PMe3)2 (6.1 mg, 0.009 mmol), benzaldehyde (10.7 mg, 0.101 mmol), and Ph2SiH2 (8.5 mg, 0.046 mmol) in C6D6 (0.7 mL) in an NMR tube equipped with a J. Young valve was monitored by 1H NMR spectroscopy, thereby demonstrating the formation of Ph2SiH(OCH2Ph) over the course of 3 days at room temperature.

(dppb)Ni(CO)2 for analysis by IR spectroscopy were generated by reaction of (Ph3P)2Ni(CO)2 with the respective diphosphine. Synthesis of (dppbz)Ni(PMe3)(CO). A solution of (dppbz)Ni(CO)2 (13 mg, 0.023 mmol) in benzene (ca. 1 mL) was cooled to −196 °C and treated with PMe3 (ca. 0.08 mmol) via vapor transfer. The mixture was heated at 80 °C for 2 days in a sealed tube. After this period, the clear yellow solution was lyophilized and the residue obtained was washed with pentane (5 × 1 mL) and extracted into benzene (2 mL). The benzene extract was lyophilized to give (dppbz)Ni(PMe3)(CO) as a yellow powder (12 mg, 85% yield). Yellow crystals of (dppbz)Ni(PMe3)(CO) suitable for X-ray diffraction were obtained via vapor diffusion of hexanes into a solution in benzene, while yellow crystals of (dppbz)Ni(PMe3)(CO)·0.5C6H6 were obtained from a solution in benzene. Anal. Calcd for (dppbz)Ni(PMe3)(CO): C, 67.0%; H, 5.5%. Found: C, 67.2%; H, 5.2%. 1H NMR (C6D6): 0.59 [d, 9H of PMe3, 2JP−H = 6], 6.87 [m, 2H of C6H4], 6.96 [t, 2 p-H of C6H5, 3JH−H = 7] 7.00 [t, 4 m-H of C6H5, 3JH−H = 7], 7.06 [t, 2 p-H of C6H5, 3JH−H = 7], 7.17 [t, 4 m-H of C6H5, 3JH−H = 7], 7.34 [m, 4 oH of C6H5], 7.60 [m, 2H of C6H4], 8.19 [m, 4 o-H of C6H5]. 13C{1H} NMR (C6D6): 19.58 [dt, 3C of Ni(PMe3), 1JP−C = 18, 3JP−C = 6], 127.70 [s, 2 p-C of C6H5], 128.35 [s, 4 m-C of C6H5], 128.58 [t, 4 m-C of C6H5, 3JP−C = 5], 129.35 [s, 2 p-C of C6H5], 129.51 [s, 2C of C6H4], 131.78 [t, 4 o-C of C6H5, 2JP−C = 7], 132.82 [t, 2C of C6H4, JP−C = 7], 133.53 [4 o-C of C6H5], 140.06 [td, 2 i-C of C6H5, JP−C = 15, JP−C = 7], 141.84 [2 i-C of C6H5, JP−C = 16, JP−C = 5], 147.94 [td, 2C of C6H5, JP−C = 42, 2JP−C = 5], 204.94 [1C of CO]. 31P{1H} NMR (C6D6): 51.04 [d, (dppbz)Ni, 2JP−P = 22], −16.98 [t, Ni(PMe3), 2JP−P = 22]. IR (ATR, solid, cm−1): 3051 (w), 2959 (w), 2894 (w), 2278 (vw), 2007 (w), 1905 (s) [ν(CO)], 1585 (w), 1479 (m), 1432 (m), 1329 (w), 1299 (w), 1277 (w), 1181 (w), 1162 (w), 1088 (m), 1070 (w), 1027 (w), 999 (w), 943 (m), 844 (w), 811 (w), 743 (m), 719 (m), 696 (s), 667 (m), 517 (s), 496 (s), 453 (m), 416 (m). Ligand Exchange between Ni(dppbz)2 and Ni(PMe3)4. A solution of Ni(dppbz)2 (2 mg, 0.0021 mmol) and Ni(PMe3)4 (1 mg, 0.0028 mmol) in C6D6 (ca. 0.7 mL) in an NMR tube equipped with a J. Young valve was heated at 100 °C for several weeks and monitored via 1H and 31P{1H} NMR spectroscopy, thereby demonstrating the slow and incomplete conversion to (dppbz)Ni(PMe3)2. Thermal Stability of (dppbz)Ni(PMe3)2. A solution of (dppbz)Ni(PMe3)2 (5 mg, 0.008 mmol) in C6D6 (ca. 0.7 mL) in an NMR tube equipped with a J. Young valve was heated to 100 °C and monitored via 1H and 31P{1H} NMR spectroscopy, thereby demonstrating minimal conversion to Ni(dppbz)2 over a period of several weeks. Reactivity of (dppbz)Ni(PMe3)2 towards CO. A solution of (dppbz)Ni(PMe3)2 (10 mg, 0.015 mmol) in C6D6 (ca. 0.7 mL) was treated with CO (1 atm). The mixture was heated to 60 °C and monitored via 1H and 31P{1H} NMR spectroscopy, thereby demonstrating the formation of (dppbz)Ni(PMe3)(CO), (dppbz)Ni(CO)2, and Ni(dppbz)2. Reactivity of (dppbz)Ni(PMe3)2 towards Formic Acid. (a) A solution of (dppbz)Ni(PMe3)2 (10 mg, 0.015 mmol) in C6D6 (ca. 0.7 mL) was treated with HCO2H (8.0 μL, 0.21 mmol) and monitored via 1H NMR spectroscopy, thereby demonstrating the formation of [(dppbz)Ni(PMe3)2H]+ [1H NMR (C6D6): 7.36 [br], 7.29 [br], 7.25 [br], 6.98 [br], 0.71 [br], −14.36 [m, 2JH−P = 34, 2JH−P = 47]. 31P{1H} NMR (C6D6): 64.36 [t, 2JP−P = 55], −25.95 [t, 2JP−P = 55]]. An EXSY experiment demonstrated exchange between [(dppbz)Ni(PMe3)2H]+ and HCO2H. (b) A solution of (dppbz)Ni(PMe3)2 in C6D6 (0.64 mL of 0.012 M, 0.0077 mmol) containing mesitylene (0.034 M, 0.022 mmol) as an internal standard was treated with HCO2H (5.0 μL, 0.13 mmol). The sample was heated at 60 °C and monitored via 1H and 31P{1H} NMR spectroscopy. After 16 h, a significant amount of ligand redistribution to form, inter alia, (dppbz)2Ni and [Ni(PMe3)4H]+, was evident; after 12 days, the primary species in solution were (dppbz)2Ni and (dppbz)Ni(PMe3)(CO), and H2 is also liberated. (c) A solution of (dppbz)Ni(PMe3)2 (5.0 mg, 0.0076 mmol) in C6D6 (ca. 0.7 mL) in an NMR tube equipped with a J. Young valve was treated with H13CO2H (2.8 μL, 0.74 mmol). The sample was J

DOI: 10.1021/acs.inorgchem.7b02636 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Dehydrocoupling of Benzyl Alcohol and PhSiH3. (a) A solution of Ni(PMe3)4 (3.1 mg, 0.009 mmol), benzyl alcohol (14.3 mg, 0.132 mmol), and PhSiH3 (4.3 mg, 0.040 mmol) in C6D6 (0.7 mL) in an NMR tube equipped with a J. Young valve was monitored by 1H NMR spectroscopy, thereby demonstrating the formation of a ca. 4:1 mixture of PhSiH(OCH2Ph)2 and PhSiH2(OCH2Ph) after a period of 1 day at room temperature. The sample was heated at 80 °C, resulting in the formation of a ca. 1:1.4 mixture of PhSi(OCH2Ph)3 and PhSiH(OCH2Ph)2 after a period of 1 day. (b) A solution of (dppbz)Ni(PMe3)2 (2.5 mg, 0.004 mmol), benzyl alcohol (7.1 mg, 0.066 mmol), and PhSiH3 (2.2 mg, 0.020 mmol) in C6D6 (0.7 mL) in an NMR tube equipped with a J. Young valve was monitored by 1H NMR spectroscopy, thereby demonstrating the formation of a ca. 1:1 mixture of PhSiH2(OCH2Ph) and PhSiH(OCH2Ph)2 after a period of 1 day at room temperature. Dehydrocoupling of Benzyl Alcohol and Ph2SiH2. (a) A solution of Ni(PMe3)4 (3.2 mg, 0.009 mmol), benzyl alcohol (11.8 mg, 0.111 mmol), and Ph2SiH2 (9.0 mg, 0.049 mmol) in C6D6 (0.7 mL) in an NMR tube equipped with a J. Young valve was heated at 80 °C. The reaction was monitored by 1H NMR spectroscopy, thereby demonstrating the formation of Ph2SiH(OCH2Ph) over a period of 22 h and Ph2Si(OCH2Ph)2 after a period of 3 days. (b) A solution of (dppbz)Ni(PMe3)2 (4.3 mg, 0.007 mmol), benzyl alcohol (8.0 mg, 0.075 mmol), and Ph2SiH2 (6.1 mg, 0.033 mmol) in C6D6 (0.7 mL) in an NMR tube equipped with a J. Young valve was heated at 80 °C. The reaction was monitored by 1H NMR spectroscopy, thereby demonstrating the formation of Ph2SiH(OCH2Ph) after a period of 22 h and Ph2Si(OCH2Ph)2 after a period of 3 days. Dehydrocoupling of Methanol and PhSiH3. (a) A solution of Ni(PMe3)4 (2.8 mg, 0.008 mmol), methanol (4.5 mg, 0.140 mmol), and PhSiH3 (4.5 mg, 0.041 mmol) in C6D6 (0.7 mL) in an NMR tube equipped with a J. Young valve was monitored by 1H NMR spectroscopy, thereby demonstrating the formation of PhSiH(OMe)2 after 2 days at room temperature. The sample was then heated at 80 °C, resulting in the formation of a ca. 1:6 mixture of PhSi(OMe)3 and PhSiH(OMe)2 after a period of a 20 h. (b) A solution of (dppbz)Ni(PMe3)2 (2.3 mg, 0.004 mmol), methanol (1.9 mg, 0.060 mmol), and PhSiH3 (1.9 mg, 0.018 mmol) in C6D6 (0.7 mL) in an NMR tube equipped with a J. Young valve was monitored by 1H NMR spectroscopy, thereby demonstrating the formation of a 1:1.3 mixture of PhSiH2(OMe) and PhSiH(OMe)2 after 2 days at room temperature. The sample was then heated at 80 °C, resulting in the formation of primarily PhSiH(OMe)2 after a period of a 20 h. Dehydrocoupling of Methanol and Ph2SiH2. (a) A solution of Ni(PMe3)4 (2.7 mg, 0.007 mmol), methanol (3.4 mg, 0.106 mmol), and Ph2SiH2 (6.6 mg, 0.036 mmol) in C6D6 (0.7 mL) in an NMR tube equipped with a J. Young valve was heated at 80 °C. The reaction was monitored by 1H NMR spectroscopy, thereby demonstrating the formation of a 2:1 mixture of Ph2SiH(OMe) and Ph2Si(OMe)2 after 20 h. (b) A solution of (dppbz)Ni(PMe3)2 (4.3 mg, 0.004 mmol), methanol (2.1 mg, 0.066 mmol), and Ph2SiH2 (4.2 mg, 0.023 mmol) in C6D6 (0.7 mL) in an NMR tube equipped with a J. Young valve was heated at 80 °C. The reaction was monitored by 1H NMR spectroscopy, thereby demonstrating the formation of Ph2SiH(OMe) after 20 h.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Gerard Parkin: 0000-0003-1925-0547 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the United States Department of Energy, Office of Basic Energy Sciences (Grant DE-FG02-93ER14339) for support of this research.121 M.C.N. acknowledges the National Science Foundation for a Graduate Research Fellowship under Grant DGE 11-44155.

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REFERENCES

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02636. Tables of structural and spectroscopic data, structural figures, NMR spectral data, and Cartesian coordinates for geometry optimized structures (PDF) Accession Codes

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DOI: 10.1021/acs.inorgchem.7b02636 Inorg. Chem. XXXX, XXX, XXX−XXX

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Crestani, M. G.; Muñ oz-Hernández, M.; Morales-Morales, D.; Warsop, B. A.; Jones, W. D.; García, J. J. Selective hydrogenation of the CO bond of ketones using Ni(0) complexes with a chelating bisphosphine. J. Mol. Catal. A: Chem. 2009, 309, 1−11. (h) Fischer, P.; Götz, K.; Eichhorn, A.; Radius, U. Decisive steps of the hydrodefluorination of fluoroaromatics using [Ni(NHC)2]. Organometallics 2012, 31, 1374−1383. (15) Neary, M. C.; Parkin, G. Nickel-catalyzed release of H2 from formic acid and a new method for the synthesis of zerovalent Ni(PMe3)4. Dalton Trans. 2016, 45, 14645−14650. (16) It is pertinent to note that, in addition to dppbz,a−f a variety of other abbreviations for 1,2-bis(diphenylphosphino)benzene have been used in the literature, including dppb,g−i bppb,j dbpe,k dppph,l dpebenz,m dpp-benzene,n bdp,o,p dpb,q and dp.r Of these, dppb is used widely in addition to dppbz but is not employed herein because it is also used as the abbreviation for 1,4-bis(diphenylphosphino)butane.s−u See, for example: (a) Eberhart, M. S.; Norton, J. R.; Zuzek, A.; Sattler, W.; Ruccolo, S. Electron transfer from hexameric copper hydrides. J. Am. Chem. Soc. 2013, 135, 17262−17265. (b) Williams, B. S.; Goldberg, K. I. Studies of reductive elimination reactions to form carbon-oxygen bonds from Pt(IV) complexes. J. Am. Chem. Soc. 2001, 123, 2576−2587. (c) Arisawa, M.; Suzuki, T.; Ishikawa, T.; Yamaguchi, M. Rhodium-catalyzed substitution reaction of aryl fluorides with disulfides: p-orientation in the polyarylthiolation of polyfluorobenzenes. J. Am. Chem. Soc. 2008, 130, 12214−12215. (d) Hatakeyama, T.; Kondo, Y.; Fujiwara, Y.; Takaya, H.; Ito, S.; Nakamura, E.; Nakamura, M. Iron-catalysed fluoroaromatic coupling reactions under catalytic modulation with 1,2-bis(diphenylphosphino)benzene. Chem. Commun. 2009, 1216−1218. (e) Culkin, D. A.; Hartwig, J. F. Carboncarbon bond-forming reductive elimination from arylpalladium complexes containing functionalized alkyl groups. Influence of ligand steric and electronic properties on structure, stability, and reactivity. Organometallics 2004, 23, 3398−3416. (f) Holloway, L. R.; Clough, A. J.; Li, J. Y.; Tao, E. L.; Tao, F.-M.; Li, L. A combined experimental and theoretical study of dinitrosyl iron complexes containing chelating bis(diphenyl)phosphinoX (X = benzene, propane and ethylene): X-ray crystal structures and properties influenced by the presence or absence of pi-bonds in chelating ligands. Polyhedron 2014, 70, 29−38. (g) Kaeser, A.; Mohankumar, M.; Mohanraj, J.; Monti, F.; Holler, M.; Cid, J.-J.; Moudam, O.; Nierengarten, I.; Karmazin-Brelot, L.; Duhayon, C.; Delavaux-Nicot, B.; Armaroli, N.; Nierengarten, J.-F. Heteroleptic copper(I) complexes prepared from phenanthroline and bis-phosphine ligands. Inorg. Chem. 2013, 52, 12140−12151. (h) Wei, F.; Zhang, T.; Liu, X.; Li, X.; Jiang, N.; Liu, Z.; Bian, Z.; Zhao, Y.; Lu, Z.; Huang, C. Efficient nondoped organic light-emitting diodes with Cu-I complex emitter. Org. Electron. 2014, 15, 3292−3297. (i) Osawa, M.; Kawata, I.; Igawa, S.; Hoshino, M.; Fukunaga, T.; Hashizume, D. Vapochromic and mechanochromic tetrahedral gold(I) complexes based on the 1,2-bis(diphenylphosphino)benzene Ligand. Chem. - Eur. J. 2010, 16, 12114−12126. (j) Li, R. X.; Li, X. J.; Wong, N. B.; Tin, K. C.; Zhou, Z. Y.; Mak, T. C. W. Syntheses and characterizations of iridium complexes containing bidentate phosphine ligands and their catalytic hydrogenation reactions to alpha,beta-unsaturated aldehydes. J. Mol. Catal. A: Chem. 2002, 178, 181−190. (k) Al-Noaimi, M.; Sunjuk, M.; El-khateeb, M.; Haddad, S. F.; Haniyeh, A.; AlDamen, M. cis-trans Isomerism in mixed-ligand ruthenium(II) complexes containing bis(phosphine) and azoimine ligands. Polyhedron 2012, 42, 66−73. (l) Sakurada, T.; Sugiyama, Y.; Okamoto, S. Cobaltcatalyzed cross addition of silylacetylenes to internal alkynes. J. Org. Chem. 2013, 78, 3583−3591. (m) Liu, C.; Liu, M.; Che, G.-B.; Su, B.; Wang, L.; Zhang, X.-X.; Zhang, S. High performance organic ultraviolet photodetectors based on novel phosphorescent Cu(I) complexes. Solid-State Electron. 2013, 89, 68−71. (n) Dierkes, P.; van Leeuwen, P. W. N. M. The bite angle makes the difference: a practical ligand parameter for diphosphine ligands. J. Chem. Soc., Dalton Trans. 1999, 1519−1529. (o) Vergote, T.; Gharbi, S.; Billard, F.; Riant, O.; Leyssens, T. Ketone hydrosilylation by Cu(I) diphosphine complexes: A kinetic study. J. Organomet. Chem. 2013, 745−746, 133−139. (p) Li, N.; Ou, J.; Miesch, M.; Chiu, P. Conjugate reduction and reductive L

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(20) For other examples of zerovalent nickel phosphine compounds that contain both bidentate and monodentate phosphines, see: Le Page, M. D.; Patrick, B. O.; Rettig, S. J.; James, B. R. 2-Pyridylphosphine and -diphosphine complexes of nickel(0), their reactivity (including aqueous solution chemistry), and some related, incidental methylphosphonium iodides. Inorg. Chim. Acta 2015, 431, 276−288. (21) Albright, T. A.; Burdett, J. K.; Whangbo, M.-H. Orbital Interactions in Chemistry, 2nd ed.; Wiley: Hoboken, NJ, 2013; Chapter 16. (22) τ4 = [360 − (α + β)]/141, where α and β are the two largest angles. τδ = τ4(β/α), where α > β. See: (a) Yang, L.; Powell, D. R.; Houser, R. P. Structural variation in copper(I) complexes with pyridylmethylamide ligands: Structural analysis with a new fourcoordinate geometry index, τ. Dalton Trans. 2007, 955−964. (b) Reineke, M. H.; Sampson, M. D.; Rheingold, A. L.; Kubiak, C. P. Synthesis and structural studies of nickel(0) tetracarbene complexes with the introduction of a new four-coordinate geometric index, τδ. Inorg. Chem. 2015, 54, 3211−3217. (23) Searches of the Cambridge Structural Database were performed with version 5.38. See: Groom, C. R.; Bruno, I. J.; Lightfoot, M. P.; Ward, S. C. The Cambridge Structural Database. Acta Crystallogr., Sect. B: Struct. Sci., Cryst. Eng. Mater. 2016, 72, 171−179. (24) Such puckering is not uncommon for dppbz compounds. For example, (dppbz)NiCl3 exhibits a puckering of 37.8°. See: Hwang, S. J.; Anderson, B. L.; Powers, D. C.; Maher, A. G.; Hadt, R. G.; Nocera, D. G. Halogen photoelimination from monomeric nickel(III) complexes enabled by the secondary coordination sphere. Organometallics 2015, 34, 4766−4774. (25) Williams, A. F. Structures of bis(1,2-diphenylphosphinoethane)nickel(II) dinitrate and bis(cis-1,2-diphenylphosphinoethene)nickel(II) diperchlorate. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1989, C45, 1002−1005. (26) Li, Y. Z.; Liu, J. C.; Qu, J. Q.; Wang, L. F.; Xia, C. G.; Niu, J. Z. Bis[1,2-bis(diphenylphosphino)ethene]nickel(II) diperchlorate. Acta Crystallogr., Sect. E: Struct. Rep. Online 2002, E58, m237−m238. (27) Berning, D. E.; Noll, B. C.; DuBois, D. L. Relative hydride, proton, and hydrogen atom transfer abilities of [HM(diphosphine)2]PF6 complexes (M = Pt, Ni). J. Am. Chem. Soc. 1999, 121, 11432− 11447. (28) Blanchard, S.; Neese, F.; Bothe, E.; Bill, E.; Weyhermuller, T.; Wieghardt, K. Square planar vs tetrahedral coordination in diamagnetic complexes of nickel(II) containing two bidentate piradical monoanions. Inorg. Chem. 2005, 44, 3636−3656. (29) Kilbourn, B. T.; Powell, H. M.; Darbyshire, J. A. C. The green form of bis(benzyldiphenylphosphine(II): an interallogen compound. Proc. Chem. Soc. 1963, 207−208. (30) (a) Orpen, A. G.; Connelly, N. G. Structural Systematics: Role of P-A σ* orbitals in metal-phosphorus π-bonding in redox-related pairs of M-PA3 Complexes (A = R, Ar, or, R = alkyl). Organometallics 1990, 9, 1206−1210. (b) Orpen, A. G.; Connelly, N. G. Structural evidence for the participation of P-X σ* orbitals in metal-PX3 bonding. J. Chem. Soc., Chem. Commun. 1985, 1310−1311. (31) For examples, see reference 27 and: (a) MacInnis, M. C.; DeMott, J. C.; Zolnhofer, E. M.; Zhou, J.; Meyer, K.; Hughes, R. P.; Ozerov, O. V. Cationic two-coordinate complexes of Pd(I) and Pt(I) have longer metal-ligand bonds than their neutral counterparts. Chem. 2016, 1, 902−920. (b) Adams, J. J.; Arulsamy, N.; Sullivan, B. P.; Roddick, D. M.; Neuberger, A.; Schmehl, R. H. Homoleptic trisdiphosphine Re(I) and Re(II) complexes and Re(II) photophysics and photochemistry. Inorg. Chem. 2015, 54, 11136−11149. (c) Longato, B.; Riello, L.; Bandoli, G.; Pilloni, G. Iridium(III, 0, and -I) complexes stabilized by 1,1′-bis(diphenylphosphino)ferrocene (dppf): Synthesis and characterization. Crystal structures of [Na(THF)5][Ir(dppf)2]· THF and [Ir(dppf)2]. Inorg. Chem. 1999, 38, 2818−2823. (d) Sattler, A.; Parkin, G. Formation of a cationic alkylidene complex via formal hydride abstraction: synthesis and structural characterization of [W(PMe3)4(η2-CHPMe2)H]X (X = Br, I). Chem. Commun. 2011, 47, 12828−12830.

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(41) For other spectroscopic methods to evaluate the donor properties of ligands see, for example: Teng, Q.; Huynh, H. V. Determining the electron-donating properties of bidentate ligands by 13 C NMR spectroscopy. Inorg. Chem. 2014, 53, 10964−10973. (42) Electrochemical potentials have also been used to parameterize ligand donor abilities. See: Lever, A. B. P. Electrochemical parametrization of metal-complex redox potentials, using the ruthenium(III) ruthenium(II) couple to generate a ligand electrochemical series. Inorg. Chem. 1990, 29, 1271−1285. (43) Perrin, L.; Clot, E.; Eisenstein, O.; Loch, J.; Crabtree, R. H. Computed ligand electronic parameters, from quantum chemistry and their relation to Tolman parameters, Lever parameters, and Hammett constants. Inorg. Chem. 2001, 40, 5806−5811. (44) For related computational studies, see: Gusev, D. G. Donor properties of a series of two-electron ligands. Organometallics 2009, 28, 763−770. (45) Hamann, B. C.; Hartwig, J. F. Systematic variation of bidentate ligands used in aryl halide amination. Unexpected effects of steric, electronic, and geometric perturbations. J. Am. Chem. Soc. 1998, 120, 3694−3703. (46) Lovitt, C. F.; Frenking, G.; Girolami, G. S. Donor-acceptor properties of bidentate phosphines. DFT study of nickel carbonyls and molecular dihydrogen complexes. Organometallics 2012, 31, 4122− 4132. (47) Fuse, M.; Rimoldi, I.; Cesarotti, E.; Rampino, S.; Barone, V. On the relation between carbonyl stretching frequencies and the donor power of chelating diphosphines in nickel dicarbonyl complexes. Phys. Chem. Chem. Phys. 2017, 19, 9028−9038. (48) Note that the Mo(CO)4L2 platform (reference 39a) may also be used to evaluate bidentate ligands. (49) A platform based on cis-[LL′Rh(CO)2] complexes has also been recently introduced for bidentate ligands. See: Canac, Y.; Lepetit, C. Classification of the electronic properties of chelating ligands in cis[LL′Rh(CO)2] complexes. Inorg. Chem. 2017, 56, 667−675. (50) The natural bite angle (also referred to as ligand bite angle) is the preferred chelating bond angle that is determined by ligand backbone constraints and not by any angular preferences of the metal. As such, it is not synonymous with P−M−P bite angle, which is simply the bond angle at the metal in the complex. See: (a) Casey, C. P.; Whiteker, G. T. The natural bite angle of chelating diphosphines. Isr. J. Chem. 1990, 30, 299−304. (b) Kuhl, O. The natural bite angle - Seen from a ligand’s point of view. Can. J. Chem. 2007, 85, 230−238. (c) van Leeuwen, P.; Kamer, P. C. J.; Reek, J. N. H.; Dierkes, P. Ligand bite angle effects in metal-catalyzed C-C bond formation. Chem. Rev. 2000, 100, 2741−2769. (d) Kamer, P. C. J.; van Leeuwen, P.; Reek, J. N. H. Wide bite angle diphosphines: Xantphos ligands in transition metal complexes and catalysis. Acc. Chem. Res. 2001, 34, 895−904. (51) In addition to the size of the linker influencing the electron donating properties of the ligand, it is pertinent to note that studies on a series of [κ2-Ph2PXPPh2]Ni(CO)2 compounds, which feature biaryl or biheteroaryl X linkers, demonstrate that these ligands have different electronic properties even though the linkers have the same size (4 atoms). See reference 47. (52) It is pertinent to emphasize that the spectroscopic probe of the electron donating ability of a ligand does not necessarily reflect the redox properties of a molecule which reflect the relative impact on two different states. In this regard, while the half-wave potentials for the Ni(II)/Ni(I) couple of a series of {Ni(diphos)2}2+ compounds becomes more positive as the ligand bite angle increases, there is little effect in the Ni(I)/Ni(0) couples. See reference 17a. (53) van Hecke, G. R.; Horrocks, W. D., Jr. Approximate force constants for tetrahedral metal carbonyls and nitrosyls. Inorg. Chem. 1966, 5, 1960−1968. (54) Tanaka, K.; Kawata, Y.; Tanaka, T. Improved route to nickel(0) carbonyl derivatives with tertiary phosphine. Chem. Lett. 1974, 3, 831− 832. (55) Hieber, W.; Zahn, E.; Ellermann, J. Das chemische verhalten von nickeltetracarbonyl gegenuber basischen systemen. Z. Naturforsch., B: J. Chem. Sci. 1963, 18, 589−594. N

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(90) Schunn, R. A. Stable nickel hydride complexes. Inorg. Chem. 1970, 9, 394−395. (91) Das, P.; Stolley, R. M.; van der Eide, E. F.; Helm, M. L. A NiII−bis(diphosphine)−hydride complex containing proton relays − Structural characterization and electrocatalytic studies. Eur. J. Inorg. Chem. 2014, 4611−4618. (92) See references 27 and (a) Miedaner, A.; DuBois, D. L.; Curtis, C. J.; Haltiwanger, R. C. Generation of metal formyl complexes using nickel and platinum hydrides as reducing agents. Organometallics 1993, 12, 299−303. (b) Berning, D. E.; Miedaner, A.; Curtis, C. J.; Noll, B. C.; Rakowski DuBois, M. C.; DuBois, D. L. Free-energy relationships between the proton and hydride donor abilities of [HNi(diphosphine)2]+ complexes and the half-wave potentials of their conjugate bases. Organometallics 2001, 20, 1832−1839. (93) For calculations pertaining to [Ni(PR3)4H]+, see: (a) Qi, X.-J.; Liu, L.; Fu, Y.; Guo, Q.-X. Ab initio calculations of pKa values of transition-metal hydrides in acetonitrile. Organometallics 2006, 25, 5879−5886. (b) Nimlos, M. R.; Chang, C. H.; Curtis, C. J.; Miedaner, A.; Pilath, H. M.; DuBois, D. L. Calculated hydride donor abilities of five-coordinate transition metal hydrides [HM(diphosphine)2]+ (M = Ni, Pd, Pt) as a function of the bite angle and twist angle of diphosphine ligands. Organometallics 2008, 27, 2715−2722. (c) Chen, S.; Rousseau, R.; Raugei, S.; Dupuis, M.; DuBois, D. L.; Bullock, R. M. Comprehensive thermodynamics of nickel hydride bis(diphosphine) complexes: A predictive model through computations. Organometallics 2011, 30, 6108−6118. (94) Four coordinate complexes, [Ni(PR3)3H]+, have also been reported. See, for example: Siedle, A. R.; Newmark, R. A.; Gleason, W. B. Protonation of phosphine complexes of zerovalent nickel, palladium, platinum, and ruthenium with fluorocarbon acids. Inorg. Chem. 1991, 30, 2005−2009. (95) For a review of nickel hydride compounds, see reference 82a. (96) Stolley, R. M.; Darmon, J. M.; Das, P.; Helm, M. L. Nickel bisdiphosphine complexes: Controlling the binding and heterolysis of H2. Organometallics 2016, 35, 2965−2974. (97) (a) Wiedner, E. S.; Chambers, M. B.; Pitman, C. L.; Bullock, R. M.; Miller, A. J. M.; Appel, A. M. Thermodynamic hydricity of transition metal hydrides. Chem. Rev. 2016, 116, 8655−8692. (b) Morris, R. H. Bronsted-Lowry acid strength of metal hydride and dihydrogen complexes. Chem. Rev. 2016, 116, 8588−8654. (98) Reference 92b and Curtis, C. J.; Miedaner, A.; Ellis, W. W.; DuBois, D. L. Measurement of the hydride donor abilities of [HM(diphosphine)2]+ complexes (M = Ni, Pt) by heterolytic activation of hydrogen. J. Am. Chem. Soc. 2002, 124, 1918−1925. (99) In contrast to [(dppbz)Ni(PMe3)2H]+, the hydride signal of [Ni(PMe3)4H]+ does not exhibit distinct coupling to phosphorus, presumably due to facile dissociation of PMe3. In this regard, reversible dissociation of P(OEt)3 from {Ni[P(OEt)3]4H}+ has been invoked in the mechanism of elimination of H2 in the presence of H+. See reference 89b. (100) The observation that Ni(dppbz)2 is not protonated by formic acid under these conditions is in accord with the fact that the pKa of [Ni(dppbz)2H]+ in acetonitrile is calculated to be 15.2 ± 1.5 (reference 93a), whereas carboxylic acids (MeCO 2H, 23.51; PhCO2H, 21.51) have greater values. See: Kaljurand, I.; Kutt, A.; Soovali, L.; Rodima, T.; Maemets, V.; Leito, I.; Koppel, I. A. Extension of the self-consistent spectrophotometric basicity scale in acetonitrile to a full span of 28 pKa units: Unification of different basicity scales. J. Org. Chem. 2005, 70, 1019−1028. (101) It is worth noting that the fluorinated derivative, Ni(dfepe)2, is not protonated by CF3CO2H, while the stronger acid, TfOH, results in irreversible loss of the dfepe ligand.a Furthermore, Ni[P(OEt)3]4 is not susceptible to protonation by MeCO2H.89b Ni(PPh3)4 does, however, react with acetic acid to liberate H2.b (a) Bennett, B. L.; White, S.; Hodges, B.; Rodgers, D.; Lau, A.; Roddick, D. M. (Fluoroalkyl)phosphine complexes of nickel(0) and cobalt(I). J. Organomet. Chem. 2003, 679, 65−71. (b) Saraev, V. V.; Kraikivskii, P. B.; Matveev, D. A.; Zelinskii, S. N.; Lammertsma, K. EPR study of the oxidation reaction

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States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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