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Synthesis, Structural Characterization, and Gas-Phase Unimolecular Reactivity of Bis(diphenylphosphino)amino Copper Hydride Nanoclusters [Cu3(X)(μ3‑H)((PPh2)2NH)3](BF4), Where X = μ2‑Cl and μ3‑BH4 Jiaye Li,† Jonathan M. White,† Roger J. Mulder,‡ Gavin E. Reid,†,§ Paul S. Donnelly,† and Richard A. J. O’Hair*,† †

School of Chemistry and Bio21 Molecular Science and Biotechnology Institute, University of Melbourne, 30 Flemington Road, Parkville, Victoria 3010, Australia ‡ CSIRO Manufacturing, Bayview Avenue, Clayton, Victoria 3168, Australia § Department of Biochemistry and Molecular Biology, University of Melbourne, 30 Flemington Road, Parkville, Victoria 3010, Australia S Supporting Information *

ABSTRACT: An electrospray ionization mass spectrometry (ESI-MS) survey of the types of cationic copper clusters formed from an acetonitrile solution containing a 1:1:20 mixture of tetrakis(acetonitrile)copper(I) tetrafluoroborate [Cu(MeCN)4(BF4)], bis(diphenylphosphino)amine (dppa = (Ph2P)2NH = L), and NaBH4 revealed a major peak, which based on both the accurate masses and isotope distribution was assigned as [Cu 3(BH4)(H)(L)3]+. This prompted synthetic efforts resulting in isolation of the dppa ligated trinuclear copper hydride nanoclusters, [Cu3(μ2-Cl)(μ3-H)(L)3](BF4) and [Cu3(μ3-BH4)(μ3-H)(L)3](BF4), which were subsequently structurally characterized using high resolution ESI-MS, X-ray crystallography, NMR, and IR spectroscopy. The Xray structures reveal a common structural feature of the cation, in which the three copper(I) ions adopt a planar trinuclear Cu3 geometry coordinated on the bottom face by a μ3-hydride and surrounded by three dppa ligands. ESI-MS of [Cu3(μ2-Cl)(μ3H)(L)3](BF4) and [Cu3(μ3-BH4)(μ3-H)(L)3](BF4) produces [Cu3(μ2-Cl)(μ3-H)(L)3]+ and [Cu3(μ3-BH4)(μ3-H)(L)3]+. The unimolecular gas-phase ion chemistry of these cations was examined under multistage tandem mass spectrometry conditions using collision-induced dissociation (CID). CID of both cations proceeds via ligand loss to give [Cu3(μ3-H)(X)(L)2]+, which is in competition with BH3 loss in the case of X = BH4. DFT calculations on the fragmentation of [Cu3(μ3-BH4)(μ3-H)(LMe)3]+ suggest that BH3 loss produces the hitherto elusive [Cu3(μ3-H)(μ2-H)(LMe)3]+, which undergoes further fragmentation via H2 loss. CID of the deuterium labeled cluster [Cu3(μ3-D)(μ3-BD4)(L)3]+ reveals that the competing losses of ligand and BD3 yield [Cu3(μ3-BD4)(μ3-D)(L)2]+ and [Cu3(D)2(L)3]+ as primary products, which subsequently fragment via further losses of BD3 or a ligand to give [Cu3(D)2(L)2]+. The coordinated hydrides in the latter ion are activated toward elimination of D2 to give [Cu3(L)2]+. Loss of HD and 2HD are minor channels, consistent with higher DFT predicted endothermicities to form [Cu3(D)(L)(L-H)]+ and [Cu3(L-H)2]+.

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alkenes, alkynes, or carbonyl compounds.9 Other applications of copper hydrido complexes include as cavity hosts for encapsulating main group anions and hydrogen anions,10 as hydrogen storage materials,11 as CO2 activation catalysts12−14 and as precursors for preparing high nuclearity copper nanostructures.2,15−17 The first copper hydrides with ancillary or supporting ligands were synthesized in the 1970s.18,19 Typical protocols for the synthesis of copper hydrides involve the treatment of solutions containing copper(I) salts and ligands with reducing reagents

INTRODUCTION

Copper hydrides are among the oldest class of transition metal hydrides, with Wurtz in 1844 isolating a pyrophoric “CuH” red solid that adopts the Wurtzite crystal structure.1 Since then polynuclear copper(I) hydrido complexes (“copper hydrides”, LxCuyHn) have been shown to play an important role as reactive intermediates in catalytic and stoichiometric transformations of organic substrates.2 The hexanuclear cluster (R3PCuH)6, known as Stryker’s reagent, is used for the chemoselective reduction of α,β-unsaturated carbonyl substances.3 Copper hydrides also play roles in the cycloaddition reactions of alkynes and azides,4 and in the enantioselective hydroamination,5 hydroalkylation6,7 and hydrobromination8 of © XXXX American Chemical Society

Received: July 19, 2016

A

DOI: 10.1021/acs.inorgchem.6b01696 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Examples of Polynuclear Copper Hydride Clusters Isolated and Characterized via X-ray Crystallographya

a

Other ligands have been omitted to emphasize the binding mode of the coordinated hydride.

(e.g., borohydrides, silanes, H2)11,15,20 or via ligand exchange reactions.21 Various copper hydrides with low and high nuclearity, stabilized by phosphine, carbene, and pyridine ligands, have been prepared, including dimers (Cu2),12,22,23 trimers (Cu 3 ), 20 , 24 hexamers (Cu 6 ), 2 0, 25 heptamers (Cu7),10,11,26 octamers (Cu8),11,26,27 and nanosized particles.15,16,21,28−32 Based on the fact that high nuclearity nanoparticles17 are formed when the amount of borohydride in the reaction solutions is increased, Liu and co-workers have reasoned that copper hydrides could be the key intermediates en route to the formation of large nanoparticles.2,15,16 While borohydrides are known to react with copper salts to form ligated mononuclear copper borohydrides, LnCu(BH4),33,34 there have been no reports on the isolation and structural characterization of borohydride stabilized polynuclear copper hydride complexes, although several copper hydride complexes have been reported (Scheme 1). Herein, we present the synthesis and structural characterization of the [Cu3(μ3-H)(μ3-BH4)(L)3]BF4 (1) and [Cu3(μ3H)(μ2-Cl)(L)3]BF4 (2) hydride complexes, where L = bis(diphenylphosphino)amine, dppa. X-ray crystallographic studies of cluster 1 have revealed a trinuclear copper center capped by a (μ3-BH4) on one face and a (μ3-H) on the other, a structural analogue to the silver cluster [Ag3(μ3-H)(μ3BH4)LPh3]BF4 (L = bis(diphenylphosphino)methane, dppm) published recently.35,36

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Figure 1. (A) ESI-MS (Vsource = 3.4 kV) of an acetonitrile solution of (eq 1) [Cu(MeCN)4]BF4, (eq 1) bis(diphenylphosphino)amine (L), and (eq 20) NaBH4 after 5 min of mixing. (B) FT-ICR HRMS of [Cu 3(BH4 )(H)(L) 3]+; (C) theoretical isotope distribution of [Cu3(BH4)(H)(L)3]+.

how changes in the ligand can influence cluster structure and reactivity. Cluster 2, obtained as a side product from the same reaction solution that yielded cluster 1, contains a related trinuclear Cu3 center. Although no attempts were made to establish the mechanism for the formation of 2, it might be formed from the reaction of a trinuclear cluster such as 1 with an impurity (e.g., HCl) in dichloromethane, as noted for the formation of [Ag3(μ3-H)(μ3-Cl)LPh3]BF4,35−37 or, alternatively, a reactive copper species might activate a C−Cl bond of dichloromethane. X-ray Crystallography Structure of Cluster 1. The single crystals of cluster 1 have been isolated from a slowly diffusing dichloromethane and diethyl ether solution. The structure of cluster 1 was determined by X-ray crystallography, and non-hydrogen atoms have been solved and refined anisotropically by full-matrix least-squares using the SHLEX97 program39 as implemented in the Olex2 software.40 The refined crystal structure of cluster 1 adopts a P21/n space group, shown in Figure 2. It is acknowledged that X-ray crystallography is not well suited to determining the presence of hydride ligands. Neutron diffraction studies on chemically related copper32 and silver clusters36 confirmed the presence of hydride, and the 1H NMR and IR data (vide inf ra) for the present complexes provide strong support of the assumptions made in defining the presence and position of the hydride in the molecular structures determined by X-ray crystallography. The structure of cluster 1 contains a planar trinuclear Cu3 cationic unit (Figure 2), which is bonded by three bidentate dppa ligands via Cu−P bonds, forming three Cu2P2N rings. The phenyl rings of dppa ligands are arranged above and below the Cu3 plane, forming two hydrophobic regions. A μ3-H and a μ3-BH4 cap from opposite sites of the Cu3 plane, respectively.

RESULTS AND DISCUSSION

Syntheses of [Cu3(μ3-BH4)(μ3-H)(L)3](BF4) (1) and [Cu3(μ2-Cl)(μ3-H)(L)3](BF4) (2). Electrospray ionization mass spectrometry (ESI-MS) can be used to direct the synthesis of cluster complexes by providing insight into the composition of reaction mixtures under different reaction conditions.35−37 ESIMS analysis of an acetonitrile solution containing a 1:1:20 mixture of tetrakis(acetonitrile)copper(I) tetrafluoroborate [Cu(MeCN)4(BF4)], dppa, and NaBH4 after 5 min of addition of NaBH4 gave the spectrum shown in Figure 1A. Based on both the accurate masses and isotope distribution observed under HRMS conditions (Figure 1B), the main peak observed at m/z 1362.1843 (calcd: 1362.1821) is assigned as the cluster [Cu3(BH4)(H)(L)3]+ (L = dppa). The observation of this cluster motivated us to attempt a bulk synthesis for full structural characterization. Cluster 1 was prepared from the reaction of stoichiometric amounts of [Cu(MeCN)4(BF4)] with NaBH4 in the presence of L (Scheme 2) in dichloromethane. We chose dppa as it is flexible in mono- or bidentate coordination modes, offering opportunities to isolate clusters with different nuclearities,38 and N-substituted derivatives could be explored to examine B


Article

Inorganic Chemistry

Scheme 2. Synthesis of the Trinuclear Hydride Clusters [Cu3(μ3-BH4)(μ3-H)(L)3](BF4), Cluster 1, and [Cu3(μ2-Cl)(μ3H)(L)3](BF4) (dppa = (PPh2)2NH, L), Cluster 2

longer than those in other trinuclear copper hydrides, [Cu3(dcpm)3(μ3-H)]2+ (dCu−H ≈ 1.67 Å)24 and [(dppbz)CuH]3 (dCu−H ≈ 1.6 Å).20 Structure of Cluster 2. Cluster 2 crystallized in the P1 space group (Figure 3). The crystal structure of cluster 2 adopts

Figure 2. Ortep-3 derived structure of cluster 1 cation with 30% ellipsoid possibilities. Phenyl hydrogen atoms, amine hydrogen atoms, and anion (BF4−) are omitted for clarity. Distances in (Å): Cu1−Cu3 = 2.6164(5), Cu1−Cu2 = 2.6706(5), Cu2−Cu3 = 2.6785(5), Cu1−B1 = 2.584(2), Cu2−B1 = 2.598(2), Cu3−B1 = 2.549(2), Cu1−P5 = 2.2624(6). The noncoordinating BF4 counteranion and disordered CH2Cl2 solvent molecules are not shown.

Figure 3. Ortep-3 derived structure of cluster 2 cation with 30% ellipsoid possibilities. Phenyl hydrogen atoms, amine hydrogen atoms and anion (BF4−) are omitted for clarity. Distances in Å: Cu1−Cu3 = 2.574(2), Cu1−Cu2 = 2.738(2), Cu2−Cu3 = 2.598(2), Cu1−Cl1 = 2.447(3), Cu3−Cl1 = 2.366(3), Cu1−P2 = 2.255(3), Cu2−P4 = 2.205(3), Cu2−P5 = 2.227(3). The noncoordinating BF4 counteranion and disordered CH2Cl2 solvent molecules are not shown.

As shown in Figure 2, the Cu3P6 unit deviates from planarity by 2−15°, as defined by the dihedral angle composed of P−Cu− Cu−P. The Cu−Cu distances are within the range 2.6−2.7 Å, comparable to Cu−Cu distances in other polynuclear copper hydride complexes (vide inf ra). Che and co-workers have reported the preparation of a number of trinuclear copper hydrides, with the cation as [Cu3(dcpm)3(μ3-H)]2+ (dcpm = bis(dicyclohexylphosphino)methane).24 The typical Cu−Cu distance of cluster 1 is significantly shorter than that in [Cu3(dcpm)3(μ3-H)]2+ (dCu−Cu = 2.88 Å), which could indicate the role of BH4− as a supporting ligand, resulting in a shorter Cu−Cu distance. Recently, Norton et al. synthesized a copper hydride [(dppbz)CuH]3 (dppbz = bis(diphenylphosphino)benzene), where a hydride is bonded with two Cu atoms in μ2 fashion and the average Cu−Cu distance is 2.58 Å, slightly shorter than those in cluster 1.20 The average Cu−B bond distance is 2.58 Å in cluster 1, which is significantly longer than the typical Cu−B bond distances in mononuclear copper−BH4 complexes (dCu−B = 2.2 Å),41,42 This suggests a weaker interaction between BH4− and the Cu3 moiety in cluster 1, which may explain the facile loss of BH3 from cluster 1 during CID experiments (vide inf ra). The measured Cu−H distances of μ3-H hydride in cluster 1 are within the range 1.7−1.8 Å,

a similar trinuclear Cu3 framework as cluster 1, with a Cu3 unit coordinated by three dppa and capped by a μ3-H (with similar caveats regarding the assumptions made for the presence of hydride). There is a μ2-Cl atom bridging two Cu atoms of an edge of the Cu3 unit, leading to two sets of Cu−Cu distances (dCu−Cu = 2.7 and 2.6 Å). The Cu1−Cu3 (d = 2.574 (2) Å) distance is shorter than that in (o-(iPr2P)C6H4)3BiCu3(μ2-Cl)3, which also contains a μ2-Cl atom.43 It appears that this distance is the shortest Cu−Cu distance reported for polynuclear copper complexes coordinated by a μ2-Cl atom and phosphine ligands. The Cu−Cl bond length of cluster 2 is comparable to that in [Cu3(μ2-Cl)2(dpmp)2][BF4] (dCu−Cl = 2.26 Å) (dpmp = bis(diphenylphosphinomethyl)phenylphosphine).44 Comparison of Cu−Cu Distances and Cu-Coordinated Anion Distances of Other Trinuclear Copper Complexes. A growing number of trinuclear copper clusters containing a Cu3(L)3 core with one, 3, or two, 4 and 5, anionic capping ligands have been reported in the literature.45−58 Alkynes and C


Article

Inorganic Chemistry halogen atoms are common capping ligands that adopt the μ3 coordination mode with Cu3 units in these complexes (Table S5). The average Cu−Cu distances in trinuclear copper complexes are from 2.3 to 3.2 Å, where halogen capped complexes show longer Cu−Cu bonds. The average Cu−Cu bond distances of clusters 1 and 2 (i) fall in this range and show similar bonding features and (ii) are slightly longer than the short Cu(I)−Cu(I) nonbonding internuclear distance of 2.453 found in Cotton’s classic Cu2(hpp)2 complex.45

Figure 4. 1H NMR spectra of cluster 1 and cluster 2. Deuterochloroform (CDCl3) was used as the solvent for the measurement in a 500 MHz NMR spectrometer.

NMR Spectroscopy of 1 and 2. In order to characterize clusters 1 and 2 in solution, nuclear magnetic resonance (NMR) spectroscopy was carried out on the following different nuclei: 1H, 13C, 11B, 31P, and 29F. The 1H NMR of clusters 1 and 2 in deuterochloroform solutions are shown in Figure 4. The spectroscopic data are fully consistent with the proposed structures of clusters 1 and 2 (Scheme 2). In cluster 1 (Figure 4A), the hydride signal has been split into a septet (2JPH of 9.8 Hz) by the six adjacent phosphorus atoms of the phosphine ligands, which collapses to a singlet in the 31P decoupled 1H NMR spectrum (Figure S1), suggesting that the trimer remains intact in solution on the NMR time scale. The chemical shift of the μ3-H hydride was recorded at δ 2.05 ppm, close to that of the Cu3(μ3-H) hydride (δ 2.0 ppm) found in the related [Cu3(dcpm)3(μ3-H)]2+ complex (dcpm = bis(dicyclohexylphosphino)methane).24 Moreover, the hydride 1 H NMR resonance falls between the regions found for μ4-H hydride (δ = 3.5 to δ = 6.5 ppm)11,59 and for μ2-H hydride (δ = 0.6 ppm) in [(dppbz)CuH]3 (dppbz =1,2-bis(diphenylphosphino)benzene).20 This again suggests that the hydride in cluster 1 maintains a μ3-H coordination mode in solution. The 1H NMR resonance of the μ3-BH4 group falls in the region of δ 0.9 − δ 0.4 ppm as a quartet, indicating the splitting effect of the 11B nucleus. The 11B decoupled 1H NMR resonance was recorded at δ 0.6 ppm as a singlet (Figure S2). Although there are no other examples of polynuclear borohydrido copper hydrides in the literature, it is common to see single-nuclear phosphino Cu(I) complexes with (μ(1−3)BH4) coordination, where the chemical shifts of copper coordinated BH4 fall in the δ 0−1.0 ppm range.60−64 The 1H NMR resonance of the coordinated borohydride anion found in

the related trinuclear silver cluster [Ag3(μ3-H)(μ3-BH4)LPh3]BF4 was located in the region δ 0.2−0.9 ppm, suggesting that the chemical shift is influenced by the mode of coordination (μ3-BH4) in these trinuclear coinage metal clusters.35 The 31P{1H} NMR resonance of cluster 1 was recorded at δ 50.3 ppm (Figure S3). This is low-field shifted compared to other non-BH4 coordinated Cu3(μ3-H) hydrides (δ 11.3 ppm) reported in the literature, reflecting the effect of the electron deficient character of the boron center of the μ3-BH4 group in cluster 1,20 but is similar to the 31P {1H} NMR chemical shift with that of [Ag3(μ3-H)(μ3-BH4)LPh3]BF4, found at δ 0.56 ppm.35 11B {1H} NMR spectroscopy revealed two resonances of cluster 1 at δ −1.1 ppm and δ −50.8 ppm (Figure S4), attributed to BF4− and BH4−, respectively, consistent with those of its structural analogue, [Ag3(μ3-H)(μ3-BH4)LPh3]BF4.35 The 19 F {1H} NMR spectrum (Figure S5) exhibits a single resonance at −155 ppm for the free BF4− counterion.35 The 1H NMR data of cluster 2 (Figure 4B) are consistent with the proposed structure in Scheme 2, composed of a Cu3 unit with three dppa ligands and one μ3-H. The septet 1H NMR resonance of μ3-H hydride of cluster 2 was found at δ 2.05 ppm, close to that of cluster 1, showing similar coordination environments in solution. Similarly, the 31P{1H}, 11 1 B{ H}, and 19F {1H} NMR resonances of cluster 2 were measured at δ 49.7 ppm, δ −0.94, and δ −154.7 ppm, respectively (Figures S6−S8), which are similar to those of cluster 1. It is interesting to note that the 31P {1H} NMR spectrum only shows a singlet resonance, which suggests that cluster 2 adopts a symmetrical cationic structure in solution at D


Article

Inorganic Chemistry

labeled cluster [Cu3(D)(BD4)L3]+ 1D to unravel which protons are associated with hydrogen loss channels. The single-isotope mass selected cluster cations 1 ([63Cu3(10BH4)(H)(L)3]+ m/z 1359) and 1D ([63Cu3(10BD4)(D)(L)3]+ m/z 1364) fragment under low-energy CID conditions to give two major peaks (Figure 6 and Figure

room temperature, where a similar structure arrangement has been observed in [Cu3(μ3-Cl)2(μ-dppa)3]+.65 IR Spectroscopy of 1 and 2. Attenuated total reflectance (ATR)-Fourier transform infrared spectroscopy (FTIR) has been widely applied to study metal−hydride and metal− borohydride interactions. For example, a number of metal borohydrides (Mx(BH4)y, M = alkaline metals, alkaline earth metals, or transition metals) have been recently investigated by ATR-FTIR.66,67 The ATR-FTIR spectra of cluster 1 is shown in Figure 5A, while that of cluster 2 is given in Figure S9. Two

Figure 5. (A) ATR-FTIR of cluster 1, which was immersed in silicone oil before measurement. The ATR-FTIR spectrum of silicon oil is shown as a red line. (B) DFT calculated IR spectrum of cluster 1 at the level of M06/6-31+G(d, p)/LANL2DZ. No scaling factors have been used. Simulated IR spectrum was plotted using GaussianView with the resolution of 4 cm−1. va, va′, and va″ are relevant to Cu3 bonded B−H stretching vibration; vb corresponds to free B−H stretching vibration; vc are the vibrations of μ3-H. Figure 6. ESI-CID-MSn single isotope selection spectra of cluster 1: (A) MS2 of [Cu3(BH4)(H)L3]+, m/z 1359.1840; p1 [Cu3L3]+ or [Cu3(H)(L)2(L-H)]+ (m/z 1344.1330(1344.1330)); p2 [Cu3(H)2L2]+ (m/z 961.0339(961.0338)). (B) MS3 of [Cu 3(H)2 L 3] +, m/z 1346.1438; p3 [Cu 3(H) 2L 2] + (m/z 961.0329(961.0338)); p4 [Cu3L2]+ or [Cu3(H)(L)(L-H)]+ (m/z 959.0175(959.0181)); p5 [Cu3(L-H)2]+ (m/z 957.0015(957.0025)). (C) MS3 of [Cu3(BH4)(H)L2]+ m/z 974.0893; p6 [Cu3(H)2L2]+ (m/z 961.0325(961.0338)); p7 [Cu3L2]+ or [Cu3(H)(L)(L-H)]+ (m/z 959.0176(959.0181); p8 [Cu3(L-H)2]+ (m/z 957.0021(957.0025)). The mass-selected precursor ion is represented by the *. Spectra are truncated and expanded to show the regions of interest. Simulated accurate m/z values are displayed in parentheses. [63]Cu, [10]B, [12]C isotopes were mass selected. A collision energy of 26−27 eV was used to fragment ions during CID experiments. The resolving power of the Orbitrap was set to 500,000.

broad bands were observed for cluster 1 at 2737 and 2046 cm−1, which can be attributed to two different stretching modes of B−H bonds associated with its two different coordinating environments in solid state cluster 1.66 The broadening of those bands could suggest the interaction of hydrogen atoms of borohydride with a Cu3 center. To better understand the bands associated with the coordinated μ3-BH4 in cluster 1, DFT calculations were carried out (Figure 5B). There are three calculated bands at ν 2222, 2229, 2230 cm−1 which are assigned to the stretching vibrations of the three B−H bonds of μ3-BH4 coordinated to the Cu3 in cluster 1, whereas the band at 2488 cm−1 is due to the free B−H bond (Figure S10). The bands at ν 1108−1132 cm−1 are due to the vibrations associated with the (μ3-H)Cu moiety (Figure S11).68 There are no absorption bands between 2000 and 3000 cm−1 of cluster 2, which is consistent with its proposed structure as [Cu3(μ3-H)(μ2-Cl)(L)3](BF4). Finally, the calculated bands between 1000 and 1050 cm−1 can be assigned to the vibrations of the BF4 counterion of cluster 1 and cluster 2.37 Unimolecular Gas-Phase Chemistry of [Cu3(X)(H)L3]+ (X = BH4 and Cl). CID has been used to examine the competition between bisphosphine ligand loss and fragmentation reactions involving the coordinated anionic ligands in the unimolecular reactions of [Ag3 (μ3-H)(μ3-BH4 )LPh3 ]BF 4, [Ag3(μ3-H)LPh3](BF4)2 and [Ag3(μ3-H)(μ3-Cl)LPh3]BF4.35−37 Since the unimolecular reaction of analogous polynuclear copper hydride complexes have not been previously examined, we wanted to examine related competing reactions for the cluster cations 1 and 2.69,70 We have also used the deuterium

S19) arising from loss of BX3 (where X = H or D) to yield [Cu3(X)2(L)3]+ (m/z 1346 for 1 and m/z 1348 for 1D, eq 1) or the loss of a neutral ligand (L) to give [Cu3(BX4)(X)(L)2]+ (m/z 974 for 1 and m/z 979 for 1D, eq 2). In contrast, the structural analogue of cluster 1 [Ag3(μ3-H)(μ3-BH4)L′3]+ solely fragments via loss of L (where L = dppm).35 Loss of BH3 was only observed from further CID on [Ag3(BH4)(H)L′2]+ to give [Ag3(H)2L′2]+, which suggests a weaker BH4−Cu3 interaction in cluster 1 than that in [Ag3(μ3-H)(μ3-BH4)L′3]+ (vide supra).35 Minor peaks due to the formation of [Cu3(L)3]+, [Cu3(X)(L)2(L-H)]+, and [Cu3(X) 2(L) 2]+ are observed (Figure S19). Although the neutral products are not detected in our experiments and we cannot rule out sequential reactions, the observed cations may arise from the losses of the combined neutrals as shown in eqs 3−5. E


Article

Inorganic Chemistry [Cu3(X)(BX4)L3]+ → [Cu3(X)2 (L)3 ]+ + BX3

(1)

→[Cu3(BX4)(X)(L)2 ]+ + L

(2)

→[Cu3(L)3 ]+ + BX3 + X 2

(3)

→[Cu3(X)(L)2 (L‐H)]+ + BX3 + XH

(4)

+

→[Cu3(X)2 (L)2 ] + BX3 + L

(5)

+

Mass selected [Cu3(X)2(L)3] fragments via ligand loss to yield [Cu3(X)2(L)2]+ (Figure 6B and eq 6). In addition, subsequent losses of “2H” and “4H” from this cluster are observed to give [Cu3(H)2(L)2 − 2H]+ (m/z 959) and [Cu3(H)2(L)2 − 4H]+ (m/z 957). Single-isotope selected [63Cu3(10BD4)(D)(L)3]+ CID-MSn experiments (Figure S19) revealed a dominant loss of D2 from [63Cu3(D)2(L)3]+ to form [Cu3L2]+ (m/z 959, eq 7, Figure S19B). Losses of HD and 2HD to form [Cu3(D)(L)(LH)]+ (m/z 960, eq 8) and [Cu3(L-H)2]+ (m/z 957, eq 9) are only minor reaction channels. [Cu3(X)2 L3]+ → [Cu3(X)2 (L)2 ]+ + L

(6)

→[Cu3(L)2 ]+ + L + X 2

(7)

+

→[Cu3(X)(L)(L − H)] + L + XH

(8)

→[Cu3(L‐H)2 ]+ + L + 2XH

(9)

Figure 7. LTQ-CID-MSn spectra of cluster 2: (A) [Cu3(Cl)(H)L3]+ m/z 1382, MS2; (B) [Cu3(Cl)(H)L2]+ m/z 997, MS3; (C) [Cu3(Cl)(H)(L)]+ m/z 612, MS4; (D) [Cu2(Cl)(L)]+ m/z 548, MS5. In these experiments a mass selection window of 18 m/z was used to isolate the full range of boron, carbon, and copper isotopes, and the most intense peak of these isotope clusters is represented by the m/z value. * refers to the mass-selected precursor ion.

+

CID of [Cu3(X)(BX4)(L)2] led to the formation of a product mixture: [Cu3(X)2(L)2]+ and [Cu3(H)2(L)2 − 2H]+, arising from loss of BX3 (eq 10) and a subsequent loss of “2H”. Singleisotope selected [63Cu3(10BD4)(D)(L)3]+ CID-MSn experiments (Figure S19C) revealed a dominant loss of D2 to form [Cu3L2]+ (m/z 959, eq 11). Once again, loss of HD and 2HD to form [Cu3(H)(L)(L-H)]+ (m/z 960, eq 12) and [Cu3(LH)2]+ (m/z 957, eq 13) are only minor reaction channels. Recent studies on the gas-phase fragmentation of the larger dppm ligated silver hydride cluster dication [Ag10H8L4]2+ also revealed how ligand loss acts as a trigger to activate the coordinated hydrides toward loss of hydrogen.71

(eq 16); (ii) [CuL]+ is formed directly from [Cu3(Cl)(H)(L)2]+ (Figure 7B, eq 17). There are two possible pathways for the formation of [Cu2(L)(L-H)]+ (m/z 895) in the CID spectrum of [Cu3(Cl)(H)(L)2]+: (i) combined loss of CuCl and H2 (eq 18); (ii) combined loss of CuH and HCl (eq 19). Although these pathways cannot be distinguished experimentally, the losses involve a coordinated anionic ligand (hydride or chloride) as well as a proton from the dppa. The deprotonation of N−H bond of a dppa ligand has been observed before,38 and related losses involving a coordinated anionic ligand and an acidic heteroatom proton of a coordinated ligand have been recently described.72 Finally, the binuclear complex [Cu2(Cl)(L)]+ fragments via loss of CuCl (Figure 7D, eq 21).

[Cu3(X)(BX4)L 2]+ → [Cu3(X)2 L 2]+ + BX3

(10)

→[Cu3L 2]+ + BX3 + X 2

(11)

[Cu3(Cl)(H)L3]+ → [Cu3(Cl)(H)L 2]+ + L

(14)

→[Cu3(X)(L)(L‐H)]+ + BX3 + XH

(12)

[Cu3(Cl)(H)L 2]+ → [Cu3(Cl)(H)(L)]+ + L

(15)

→[Cu3(L‐H)2 ]+ + BX3 + 2XH

(13)

→[Cu 2(Cl)(L)]+ + CuH + L

(16)

→[Cu(L)]+ + [Cu 2(Cl)(H)L]

(17)

→[Cu 2(L)(L‐H)]+ + CuCl + H 2

(18)

Multistage mass spectrometry experiments have also been carried out for cluster cation 2 (Figure 7 and eqs 14−21). HRMS of the product ions was used to assign their formulas based on a comparison of the experimental and simulated exact masses and isotope distributions (Figures S13−17). Lowenergy mass selected CID of cluster 2 led to the formation of [Cu3(Cl)(H)(L)2]+ (m/z 997) via ligand loss (Figure 7A, eq 14). CID of [Cu3(Cl)(H)(L)2]+ produced a number of copper containing cations, including: [Cu3(Cl)(H)(L)]+ (m/z 612) via ligand loss (eq 15); [Cu2(L)(L-H)]+ (m/z 895), [Cu2(Cl)(L)]+ (m/z 546), [CuL]+ (m/z 448), and an unidentified peak at m/z 687 (Figure 7B). It is interesting to note that CID on [Cu3(Cl)(H)(L)]+ (m/z 612) proceeded via CuH elimination to form [Cu2(Cl)(L)]+ (m/z 546) (Figure 7C, eq 20), while no [CuL]+ (m/z 448) was observed. This suggests that (i) [Cu2(Cl)(L)]+ is formed via the combined loss of L and CuH

→[Cu 2(L)(L‐H)]+ + CuH + HCl +

(19) +

[Cu3(Cl)(H)(L)] → [Cu 2(Cl)(L)] + CuH +

+

[Cu 2(Cl)(L)] → [Cu(L)] + CuCl

(20) (21)

Density Functional Theory (DFT) Calculations on the Competition between dppa Ligand Loss and Fragmentation Reactions Involving Coordinated BH4 and H. The experimentally observed sequential activation of a coordinated BH4 via BH3 losses (eqs 1 and 6) and coordinated hydrides via losses of a dppa ligand (eqs 2 and 3) to trigger two different hydrogen loss pathways (eqs 4 and 7 versus eqs 5 and 8) are of F


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Inorganic Chemistry

have been replaced by methyl group (CH3) to form the bis(dimethylphosphino)amine (dmpa = (Me2P)2NH = LMe) ligated system, thereby saving on computing time. The optimized geometry for [Cu3(BH4)(H)LMe3]+ (Figure 8A) is directly related to the core structure from the X-ray structure for 1, confirming that changing the phosphine substituent from Ph to Me has little effect on the core structure.73 Loss of BH3 from [Cu3(BH4)(H)(LMe)3]+ requires an energy of 45.7 kcal/mol to form [Cu3(H)2(LMe)3]+, which is slightly (∼5 kcal/mol) higher than that in the pathway to produce [Cu3(BH4)(H)(LMe)2]+ via ligand loss. This is consistent with the CID experiment, where both pathways are observed in the CID spectrum of [Cu3(BH4)(H)(LMe)3]+ (Figure 6A), with the less endothermic pathway leading to the major product ion. It is interesting to note that in the calculated structure of [Cu3(BH4)(H)(LMe)2]+ the μ3-BH4 ligand changes its coordination mode from binding to the face of three Cu atoms to the two Cu atoms of the edges from which a dmpa ligand was removed, suggesting that the BH4 ligand compensates for loss of coordination by the two phosphine atoms (Figure 8C). This is different from the calculated structure of [Ag3(BH4)(H)LMe2]+, where BH4 still favors the ligand (dppm) bonded Ag atoms.35 In the calculated structure of [Cu3(H)2(LMe)3]+, one hydride resides in a μ3-H coordination mode, whereas the other hydride prefers a μ2-H coordination with two Cu atoms (Figure 8B). Higher energies are required to fragment [Cu3(BH4)(H)(LMe)3]+ or [Cu3(H)2(LMe)3]+ via the combined losses of BH3 and “2H” (40.8 kcal/mol versus 56.3 and 66.4 kcal/mol) or L and “2H” (35.9 kcal/mol versus 51.4 and 61.5 kcal/mol) to produce [Cu3(L)3]+ or the isomeric [Cu3(H)(LMe)2(LMe-H)]+. The former reductive elimination process is preferred both experimentally from the deuterium labeling studies (Figures S19B and S19C) and from the DFT predicted thermochemistry and yields a mixed-valence Cu3 (0 and +1 oxidation states), suggesting electron transferred from the coordinated hydride to Cu atoms.20

interest as simple gas-phase models for (1) metal mediated activation of BH4 for hydrogen storage applications; (2) the growth of copper nanoparticles since they provide a link between copper clusters with coordinated borohydrides, copper hydride clusters, and all metal copper clusters.2,15−17 Thus, for these reasons we have used DFT calculations at the M06/6311G+(d,p)/LanL2DZ level of theory to examine the structural changes (Figure 8) occurring for the sequential fragmentation

Figure 8. DFT calculated models (M06/6-31+G(d,p)/LANL2DZ) for the key cations used to calculate the reaction endothermicities given in Table 1 for eqs 1−13.

reactions of [Cu3(BH4)(H)(L)3]+ (eqs 1−8) and to compare the DFT calculated energetics with the relative product yields (Table 1). As in our previous study on the related silver cluster, [Ag3(BH4)(H)LPh2]+,35 the phenyl rings of the dppa ligands

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Table 1. Summary of Key Experimental and DFT Calculated Data Associated with the Fragmentation Reactions of [Cu3(BX4)(X)LMe3]+, [Cu3(X)2LMe3]+, and [Cu3(BX4)(X)LMe2]+a product ion formed from reaction

rel yield from CID expts, %

DFT calcd reaction endothermicity, kcal/mol

[Cu3(X)(BX4)LMe3]+ (Parent Ion) [Cu3(X)2(L )3] (eq 1) 14 [Cu3(X)(BX4)(LMe)2]+ (eq 73 2) [Cu3(LMe)3]+ (eq 3) 2 [Cu3(X)(LMe)2(L-H)]+ (eq 4) 11 [Cu3(X)2(LMe)2]+ (eq 5) [Cu3(X)2LMe3]+ (Parent Ion) [Cu3(X)2(LMe)2]+ (eq 6) 73 [Cu3(LMe)2]+ (eq 7) 25 [Cu3(X)(LMe)(LMe-H)]+ (eq 8) [Cu3(LMe-H)2]+ (eq 9) 2 [Cu3(X)(BX4)LMe2]+ (Parent Ion) [Cu3(X)2LMe2]+ (eq 10) 95 [Cu3L2]+ (eq 11) 4 [Cu3(X)(LMe)(LMe-H)]+ (eq 12) 1 [Cu3(LMe-H)2]+ (eq 13) Me

+

CONCLUSION An ESI/MS survey of the types of cationic copper clusters formed when a copper salt is mixed with the dppa ligand and NaBH4 prompted efforts directed at the bulk synthesis of these clusters for subsequent structural studies. The trinuclear copper metal clusters 1 and 2 were successfully isolated. X-ray crystallography revealed that they share similar structural features. Low-energy CID experiments and DFT calculations provide insights into the thermal decomposition reactions of coordinated BH4 and H ligands. These results suggest that cluster 1 can be regarded as a model for activation of BH4 to produce coordinated hydrides, which in turn can liberate H2 to produce metallic clusters.

45.7 40.7 67.5 71.6 81.5

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40.8 56.3 66.4

EXPERIMENTAL SECTION

General Methods. All the manipulations were carried out using standard Schlenk techniques under an atmosphere of high purity dinitrogen. THF, DCM, hexane, and diethyl ether were dried by passing through solvent purification systems. Cu(MeCN)4BF4 was prepared based on a literature method.74 Other reagents are commercially available from Ajax Finechem Pty. Ltd., Strem Chemicals Inc., and Sigma-Aldrich. 1H, 13C{1H}, 31P{1H}, 9B{1H}, and 19F NMR were performed on a Bruker Avance 400 NMR spectrometer (400.13 MHz 1H frequency) equipped with a 5 mm triple resonance broadband probe (BB/2H−1H/19F) or on a model FT-NMR 500 spectrometer (Varian, Palo Alto, CA, USA). Chemical shifts of 1H and 13 C experiments were referenced to the residual protonated solvent

93.4 35.9 51.4 61.5 88.4

a For DFT calculated structures of reactant and product ions see Figure 8.

G


Article

Inorganic Chemistry signal peaks (CDCl3) at δ 7.26 and 77.16 ppm, respectively. FT-IR spectra were collected on a PerkinElmer spectrometer, and cluster 1 was immersed in silicone oil for this measurement. The Synthesis of [Cu3(BH4)(H)(L)3][BF4] (1). [Cu(MeCN)4][BF4] (2.0 g, 6.4 mmol) was dissolved in THF (30 mL) and dichloromethane (30 mL) in a Schlenk tube, to which bis(diphenylphosphino)amine (2.45 g, 6.4 mmol) was added. The colorless reaction mixture was cooled to −80 °C, and sodium borohydride (0.24 g, 6.4 mmol) was added. The reaction mixture was then allowed to warm up from −80 °C to room temperature and continuously stirred for 7 h at room temperature. The volatiles were removed in vacuo, to leave a purple/brown solid. The solid was extracted with dichloromethane, and this mixture was filtered. Exchange of vapors with diethyl ether and the filtrate resulted in crystals of 1 suitable for X-ray crystallography (0.7 g, 22%). ATR-FTIR (cm−1): 3247 (br), 2373 (w), 2064 (br), 1434 (s), 1306 (br), 1187 (w), 1100 (s) (BF4), 1051 (br), 930 (br), 734 (s). The Synthesis of [Cu3(H)(Cl)(L)3][BF4] (2). A similar procedure was used for the preparation of cluster 2. The cluster 2 raw product was collected by removing the DCM and diethyl ether of the filtrate in vacuo. Cluster 2 was purified by dissolving it in normal acetone followed by addition of hexane to induce precipitation. The single crystals of 2 were isolated from a DCM and diethyl ether solution and examined by X-ray crystallography. Yield of 2: 0.6 g, 19%. ATR-FTIR (cm−1): 3247 (w), 3055 (br), 2404 (w), 1725 (s), 1186 (w), 1098 (s) (BF4), 1061 (br), 1025 (w), 997 (w), 900 (s). NMR Spectroscopy of 1. 1H NMR (500 MHz, CDCl3): δ 7.32− 7.00 (m, 60H, PhH), 3.58 (s, 3H, NH), 2.08 (m, 1H, Cu3(μ3-H)), 0.93−0.43 (m, 4H, BH4). 13C NMR (126 MHz, CDCl3): δ 135.96 (d, J = 55.9 Hz), 132.74−131.46 (m), 130.82 (d, J = 26.6 Hz), 128.96 (d, J = 13.2 Hz). 11B NMR (128 MHz, CDCl3): δ −1.10, −50.79. 19F NMR (376 MHz, CDCl3): δ −155.00. 31P NMR (162 MHz, CDCl3): δ 50.30. NMR Spectroscopy of 2. 1H NMR (500 MHz, CDCl3): δ 7.33− 6.83 (m, 60H, PhH), 3.57 (s, 3H, NH), 2.04 (m, 1H, Cu3(μ3-H)). 13C NMR (126 MHz, CDCl3): δ 137.31−134.70 (m), 132.02 (d, J = 12.6 Hz), 130.82 (d, J = 25.6 Hz), 128.96 (d, J = 19.2 Hz). 11B NMR (128 MHz, CDCl3): δ −0.88. 19F NMR (376 MHz, CDCl3): δ −154.64. 31P NMR (162 MHz, CDCl3): δ 50.37. Mass Spectrometry Experiments. Mass spectra were recorded using a Finnigan hybrid linear quadrupole (LTQ) Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. The cluster complexes 1 and 2 were individually prepared as 100 μM acetonitrile solutions that were introduced into the mass spectrometer via a syringe pump set at a flow rate of 5 μL min−1 to the ESI capillary. For the optimum signal intensity of the target ions, the ESI conditions were tuned, with typical parameters as follows: spray voltage, 2.5−5.0 kV; capillary temperature, 250 °C; nitrogen sheath gas pressure, 10 (arbitrary units); capillary voltage, 32−36 V; and tube lens voltage, 85−90 V. Selected ions were transferred to the FT-ICR cell for accurate mass measurement with the use of selected ion monitoring (SIM) and selected reaction monitoring (SRM). The unimolecular fragmentation of clusters 1 and 2 was examined via CID. The massselected precursor ion was depleted to 10−20% using a normalized collision energy typically between 15−25% and a mass selection window of 18 m/z to isolate the full range of isotopes due to boron, carbon and copper isotopes. For the all 13C, 63Cu, and 10B single-isotope, mass-selected [63Cu3(H)(10BH4)(L)3]+ and [63Cu3(D)(10BD4)(L)3]+ CID-MSn experiments, samples (20 μM in acetonitrile) were introduced into an Orbitrap Fusion Lumos mass spectrometer (Thermo Scientific, San Jose, California) via electrospray ionization (ESI) at a flow rate of 10 μL/min, using a spray voltage of 3.5 kV, heated capillary temperature of 250 °C, a sheath gas pressure of 5, and RF lens set at 50%. ESI-MS spectra were acquired from m/z 300−2000, at a mass resolving power of 250,000 and an AGC target of 1.0E5. For MSn experiments, precursor ions were mass selected using the isolation quadrupole (±0.2 m/z) and then subjected to ion trap CID using an activation q value of 0.25. Collision energies were individually optimized for each

ion of interest. Spectra shown are the average of 50 scans, acquired using a mass resolving power of 500,000. X-ray Crystallography. Intensity data for compound 1 and 2 was collected on an Oxford Diffraction SuperNova CCD diffractometer using Cu Kα radiation, and the temperature during data collection was maintained at 130.0(1) K using an Oxford Cryostream cooling device. The structure was solved by direct methods, using the SHELXS program, and the refinement was carried out with SHELXL program full-matrix least-squares on F2 using the Olex2 software.40,75 All nonhydrogen atoms were refined anisotropically. The hydrogen atoms on the phenyl rings of dppa ligands were geometrically calculated positions and refined using a riding model. The thermal ellipsoid plots were generated using the program ORTEP-3.76 Solvent masks as implemented in Olex2 were applied for 1 and 2 to account for the unassigned electron densities in the pores.77 Crystal data for cluster 1 (M = 1449.35 g/mol): monoclinic, space group P21/n (No. 14), a = 18.4210(4) Å, b = 22.4885(3) Å, c = 21.3690(4) Å, β = 113.774(2)°, V = 8101.1(3) Å3, Z = 4, T = 292.56(10) K, μ(Cu Kα) = 2.410 mm−1, Dcalc = 1.188 g/cm3, 51047 reflections measured (5.99° ≤ 2Θ ≤ 153.656°), 16820 unique (Rint = 0.0375, Rsigma = 0.0425) which were used in all calculations. The final R1 was 0.0427 (I > 2σ(I)), and wR2 was 0.1209. Crystal data for cluster 2 (M = 1469.07 g/mol): triclinic, space group P1 (No. 1), a = 13.800(3) Å, b = 15.400(3) Å, c = 18.500(4) Å, α = 79.00(3)°, β = 80.00(3)°, γ = 74.00(3)°, V = 3679.2(15) Å3, Z = 2, T = 130.00(10) K, μ(MoKα) = 1.077 mm−1, Dcalc = 1.326 g/cm3, 15692 reflections measured (2.784° ≤ 2Θ ≤ 54.124°), 15690 unique (Rint = 0.0000, Rsigma = 0.0397) which were used in all calculations. The final R1 was 0.0666 (I > 2σ(I)), and wR2 was 0.2020. DFT Calculations. All the calculations were carried out using Gaussian0978 at the level of M0679 with 6-31+G(d,p)80 for nonmetal atoms and LANL2DZ81−83 for copper atoms. The initial input geometry for [Cu3(BH4)(H)LMe3]+ was constructed by taking the coordinates from the X-ray structure of 1 and replacing the phenyl groups of the dppa ligands by methyl groups to produce the related dmpa ligands. The resultant structure was fully optimized. The input geometries of the fragment cluster ions shown in Figure 8 were prepared by sequentially removing a dmpa ligand and BH3 from [Cu3(BH4)(H)LMe3]+ and fully optimizing the resultant structures. These in turn were modified to calculate all the remaining structures. Frequency calculations were carried out on all species to confirm that they correspond to minima (i.e., no imaginary frequencies). The results of the crystallographic studies are provided as Supporting Information and have been deposited at the CCDC under numbers 1488227 and 1488228.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01696. Mass spectra, spectroscopic characterization, figures and bond angles, all Cartesian coordinates of DFT calculated structures, and full citation of ref 76 (PDF) Crystallographic data for 1 (CIF) Crystallographic data for 2 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*Fax: (+) 61 3 9347 8124. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Australian Research Council for financial support (DP150101388) and Athanasios Zavras for help with the MS H


Article

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experiments in the initial stages of the project. The DFT calculations were carried out using the HPC facility of The University of Melbourne (pMelb0317 and punim0018) and at the National Computing Infrastructure (we thank Prof. Allan Canty for access).

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