Gas-Phase IonâMolecule Reactions of Copper Hydride Anions [CuH2

Article pubs.acs.org/IC

Gas-Phase Ion−Molecule Reactions of Copper Hydride Anions [CuH2]− and [Cu2H3]− Athanasios Zavras,† Hossein Ghari,§ Alireza Ariafard,*,‡,§ Allan J. Canty,‡ 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 ‡ The School of Physical Sciences, University of Tasmania, Private Bag 75, Hobart, Tasmania 7001, Australia § Department of Chemistry, Faculty of Science, Central Tehran Branch, Islamic Azad University, Shahrak Gharb, Tehran, Iran S Supporting Information *

ABSTRACT: Gas-phase reactivity of the copper hydride anions [CuH2]− and [Cu2H3]− toward a range of neutral reagents has been examined via multistage mass spectrometry experiments in a linear ion trap mass spectrometer in conjunction with isotope labeling studies and Density Functional Theory (DFT) calculations. [CuH2]− is more reactive than [Cu2H3]−, consistent with DFT calculations, which show it has a higher energy HOMO. Experimentally, [CuH2]− was found to react with CS2 via hydride transfer to give thioformate (HCS2−) in competition with the formation of the organometallic [CuCS2]− ion via liberation of hydrogen; CO2 via insertion to produce [HCuO2CH]−; methyl iodide and allyl iodide to give I− and [CuHI]−; and 2,2,2trifluoroethanol and 1-butanethiol via protonation to give hydrogen and the product anions [CuH(OCH2CF3)]− and [CuH(SBu)]−. In contrast, the weaker acid methanol was found to be unreactive. DFT calculations reveal that the differences in reactivity between CS2 and CO2 are due to the lower lying π* orbital of the former, which allows it to accept electron density from the Cu center to form the initial three-membered ring complex intermediate, [H2Cu(η2-CS2)]−. In contrast, CO2 undergoes the barrierless side-on hydride transfer promoted by the high electronegativity of the oxygen atoms. Side-on SN2 mechanisms for reactions of [CuH2]− with methyl iodide and allyl iodide are favored on the basis of DFT calculations. Finally, the DFT calculated barriers for protonation of [CuH2]− by methanol, 2,2,2trifluoroethanol, and 1-butanethiol correlate with their gas-phase acidities, suggesting that reactivity is mainly controlled by the acidity of the substrate.

■

INTRODUCTION

Given that solvent can influence reactivity of copper hydride complexes,16 and the relationship between cluster size, n, and reactivity of the cuprate complexes, [MCuH2]n, has never been examined previously, we have used the multistage mass spectrometry capabilities of a linear ion trap mass spectrometer17 to examine the fundamental gas-phase reactivity of neutral substrates with the mass selected hydrido cuprate complexes, [CuH2]−12,13 and [Cu2H3]−. While a range of transition metal hydride cations18 and anions19 have been formed in the gas phase, typically as the products of X−H (X = H, C, etc.) bond activation reactions, fewer studies have used ion−molecule reactions to examine the reactivity of transition metal hydride ions with organic substrates.20 Thus, we chose neutral substrates representative of different classes of reactivity: CS2 and CO2 for their ability to undergo insertion reactions into M−H bonds; methyl iodide and allyl iodide to investigate their reduction to hydrocarbons; and the acids CF3CH2OH and BuSH to investigate protonation of the

Copper(I) hydride, first isolated by Wurtz in 1844, is a pyrophoric red solid that adopts a Wurtzite crystal structure. In the quest to “tame” CuH by using ligands to create soluble copper hydrides as selective reagents in organic synthesis,2 two main approaches have been adopted. The first involves the use of Lewis bases, B, such as pyridine3 or phosphines4 to generate complexes of the type [BCuH]n.5 This approach has led to the successful generation of the widely used Stryker’s reagent,6 and more recently Lipshutz’s “CuH in a bottle”.7 The second approach has involved the generation of hydrido cuprate complexes of the types [MCuH2]n (where M = Li or K)8,9 and [LiRCuH]n.10 Related hydrido cuprates have been prepared in the absence of alkali metal cations as part of a series of matrix isolation studies aimed at examining the formation of transition metal hydrides.11 In particular, Andrews and co-workers have characterized a series of copper hydrides including [CuH2]−12,13 and [Cu2H]−.14,15 The former anion was suggested as a potential hydrogen carrier for hydrogen storage applications.12 1

© XXXX American Chemical Society

Received: September 5, 2016

A

DOI: 10.1021/acs.inorgchem.6b02145 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

dyotropic rearrangements,24 we found that the copper hydride anions [CuH2]− and [Cu2H3]− can be prepared by (i) mass selecting the copper formate anions [Cu(O2CH)2]− (Figure S2a and S2b) and [Cu2(O2CH)3]− (Figure S3a−c), formed under gentle electrospray ionization conditions (Figure S1) from an acetonitrile solution containing cuprous oxide and formic acid or its deuterium labeled isotopologue, DCO2D (Scheme 1a), and subjecting these ions to multiple stages of collision-induced dissociation (CID) where sequential decarboxylation reactions of the coordinated formates lead to the formation of coordinated hydrides; and (ii) sequential fragmentation reactions of precursor copper formate anions under “in source” CID.25 [CuH2]−, 1, and [Cu2H3]−, 2, prepared via the former method, were individually mass selected and allowed to undergo ion−molecule reactions (IMRs) with a volatile neutral substrate in a MS4 or MS5 experiment, respectively (Scheme 1b and c). Of particular interest was to determine which of these cuprates more readily undergoes hydride transfer toward neutral substrates, which is related to their relative “hydricity”.26,27 Ion−Molecule Reactions of [CuH2]− and [Cu2H3]−. The gas-phase ion−molecule reactions of the cuprate anions [CuH2]− (Figure S4 and Table S1) and [Cu2H3]− (Figure S5 and Table S2) were explored with a range of neutral substrates including CS2 and CO2, methyl iodide and allyl iodide, and the acids CF3CH2OH and CH3(CH2)3SH. While [CuH2]− was found to react with all of these substrates, [Cu2H3]− was found to be generally less reactive (Table 1). Because the nature of the products formed is substrate specific, in the next sections we discuss the results of experiments by individual class of substrates. Reactions of [CuH2]− and [Cu2H3]− with CS2. Two ionic products are observed for the ion−molecule reaction of [63CuH2]− (m/z 65) with CS2 (Figure 1a). The major reaction pathway proceeds via hydride transfer to produce [HCS2]− (m/ z 77) and 63CuH (eq 1). The minor reaction pathway results in the formation of the organometallic ion [CuCS2]− (m/z 139) via the reductive elimination of H2 (eq 2), a reaction related to

coordinated hydrides. Where possible, comparisons are made with published reactivity of the same substrates toward the bare hydride anion21 and dimethyl cuprate, [CuMe2]−.22 Finally, DFT calculations were used to examine potential mechanisms to account for the observed reactivity of [CuH2]−.

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RESULTS AND DISCUSSION Gas-Phase Formation of [CuH2]− and [Cu2H3]−. Electrospray ionization mass spectrometry (ESI−MS) together with various combinations of multistage mass spectrometry (MSn) experiments were used to prepare hydrido cuprate complexes and study their reactivity toward a range of substrates (Scheme 1). While copper hydride anions have been previously prepared Scheme 1. Multistage Mass Spectrometry (MSn) Experiments for the Gas-Phase Preparation of Hydrido Cuprate Complexes via Collision-Induced Dissociation (CID)a

a (a) [Cux(O2CH)x+1]− (x = 1,2), (b) [CuH2]−, 1, and (c) [Cu2H3]−, 2, and their reaction(s) with various neutral substrates to yield anionic products via ion−molecule reactions (IMRs).

via sequential decarboxylation reactions of aliphatic copper carboxylates followed by β-hydride fragmentation reactions23 or

Table 1. Gas-Phase Kinetics Associated with the Ion−Molecule Reactions of [63CuH2]− (m/z 65) and [63Cu2H3]− (m/z 129) with Various Neutral Reagents ion 63

neutral reagent −

[ CuH2]

CS2 CO2 CH3I C3H5I

[63Cu2H3]−

CF3CH2OH BuSH CS2 CO2 CH3I C3H5I CF3CH2OH BuSH

reaction channel; branching ratio out of 100%; eq no.a −

−

[HCS2] + CuH; 91%; eq 1; [CuCS2] + H2 ; 9%; eq 2 [CuH(O2CH)]− ; 100%; eq 15 I− + CuH + CH4; 81%; eq 16 [CuH(I)]− + CH4; 19%; eq 17 I− + CuH + C3H6; 53%; eq 22 [CuH(I)]− + C3H6; 47%; eq 23 [CuH(OCH2CF3)]− + H2; 100; eq 32 [CuH(SBu)]− + H2; 100%; eq 36 [Cu2S2]•− + CH3•; 100%; eq 12 [Cu2H2(O2CH)]−; 100%; eq 15 N.R.f [Cu2H2(I)]− + C3H6; 100%; eq 31 [Cu2H2(OCH2CF3)]− + H2; 100%; eq 35 [Cu2H2(SBu)]− + H2; 100%; eq 39

φe

kexptb,c,d −10

(2.2 ± 0.4) × 10 (2.8 ± 0.4) × 10−13 (4.7 ± 0.3) × 10−10

19 ± 4 (4 ± 0.5) × 10−2 34 ± 2

(1.1 ± 0.1) × 10−9

68 ± 7

± 0.6) ± 1.2) ± 0.2) ± 0.1) N.R.f (1.3 ± 0.1) (5.4 ± 1.0) (1.4 ± 0.2)

(7.1 (9.3 (1.7 (1.3

× × × ×

10−10 10−9 10−11 10−13

× 10−10 × 10−12 × 10−11

45 580 (1.7 (2 N.R.f 10 7 × 10−2 1

± ± ± ±

20 7640 0.1) × 10−1 0.1) × 10−2

±1 ± 0.1

Equation no. is the full equation listed in the text. bMean ± standard deviation (n = 3). cIn units of cm3 molecules−1 s−1. dRates measured for the reaction of neutral reagents with the hydrido cuprate anions [CuH2]− or [Cu2H3]− were determined from triplicate experiments. eReaction efficiency, φ = (kexpt/kADO) × 100. The kADO is the theoretical ion−molecule collision rate constant obtained from the average-dipole orientation (ADO) theory,28 which is calculated using the Colrate program.29 fN.R. = no reaction at the longest reaction time (10 000 ms) examined at a concentration of methyl iodide of 2.9 × 109 molecules cm−3. a

B


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Inorganic Chemistry [CuMe2]− + CS2 → [MeCS2 ]− + CuMe → [MeCS2 CuMe]−

30

the oxidation of organocuprates. These assignments were confirmed via the following isotope labeling studies: (i) [63CuHD]− (m/z 66, Figure 1b), formed from a solution containing mixtures of deuterium labeled and unlabeled formic acid, reacted with CS2 to give [HCS2]− (m/z 77, eq 3), [DCS2]− (m/z 78, eq 4), and the minor product [63CuCS2]− (m/z 139, eq 5); (ii) [65CuH2]− (m/z 67) reacted with CS2 (Figure 1c) to give [HCS2]− (m/z 77, eq 6) and [65CuCS2]− (m/z 141, eq 7); and (iii) [63CuD2]− (m/z 67) reacts with CS2 (Figure 1d) to give [DCS2]− (m/z 78, eq 8) and [63CuCS2]− (m/z 139, eq 9).

→ [63CuCS2 ]− + H 2

(12)

[63/65Cu 2H3]− + CS2 → [63/65Cu 2S2 ]•− + CH•3

(13)

(2) (3)

→ [DCS2 ]− + 63CuH

(4)

→ [63CuCS2 ]− + HD

(5) (6)

→ [65CuCS2 ]− + H 2

(7)

[63CuD2 ]− + CS2 → [DCS2 ]− + 63CuD

(8)

→ [63CuCS2 ]− + D2

[63Cu 2H3]− + CS2 → [63Cu 2S2 ]•− + CH•3

Reactions of [CuH2]− and [Cu2H3]− with CO2. In contrast to the reactivity of [63CuH2]− toward CS2, [63CuH2]− reacts very slowly (2.8 × 10−13 cm3 molecules−1 s−1, reaction efficiency of 0.04%) with CO2 to only produce [63CuH(O2CH)]− (m/z 109, Figure 2a, eq 14). This reaction proceeds via a direct insertion pathway resulting in carbon dioxide capture and transformation into a coordinated formate anion.34,35 Related insertion reactions between CO2 and copper hydrides play key roles in copper-catalyzed transformations of CO2.36 Although the bare hydride anion reacts slowly with CO2

(1)

[63CuHD]− + CS2 → [HCS2 ]− + 63CuD

[65CuH 2]− + CS2 → [HCS2 ]− + 65CuH

(11)

The observed hydride transfer reaction (eq 1) is related to the reaction of [CuMe2]− with CS2, which proceeds via methyl anion transfer (eq 10).22e In contrast, [CuMe2]− also forms an adduct with CS2 (eq 11), but does not form the organometallic ion, [CuCS2]− (cf., eq 2). The ion−molecule reaction of [63Cu2H3]− with CS2 is much slower (1.7 × 10−11 cm3 molecules−1 s−1, reaction efficiency of 0.17%, Table 1) than that of the mononuclear cuprate and proceeds via an entirely different pathway to produce a single product ion at m/z 190, formulated as the mixed valence dinuclear copper sulfide cluster [63Cu2S2]•− (Figure S6a, eq 12). By mass selecting the [63/65Cu 2 H3 ]− (m/z 131) isotopologue, a mass shift of 2 Da to m/z 192 is observed (Figure S6b, eq 13), thus confirming the formulation. We infer that a neutral methyl radical is also formed, and thus the reaction between [Cu2S2]•− and CS2 is likely to be multistep and mechanistically complex. Finally, [Cu2S2]•− has been previously observed in the gas phase32 and via computational chemistry.33

Figure 1. Ion−molecule reaction of various mass selected isotopologues of [CuH2]− with CS2 ([CS2] ion trap = 7.2 × 109 molecules cm−3) at an activation time of 200 ms: (a) [63CuH2]− m/z 65; (b) [63CuHD]− m/z 66; (c) [65CuH2]− m/z 67; and (d) [63CuD2]− m/z 67. The most intense peak in the ion is represented by the m/z value. *Represents the mass selected precursor ion.

[63CuH 2]− + CS2 → [HCS2 ]− + 63CuH

(10)

(9)

−

The rate of reaction between [CuH2] and CS2 is 2.2 × 10−10 cm3 molecules−1 s−1, corresponding to a reaction efficiency of 19% (Table 1). The kinetic isotope effect (KIE) for the hydride transfer reaction of [63CuHD]− (m/z 66) with CS2, determined by dividing the integrated ion counts of [HCS2]− (m/z 77) by [DCS2]− (m/z 78), AH/AD, was found to be 0.98.31 The fact that the KIE is very close to unity suggests that the rate-determining step does not involve the breaking and forming of bonds to H/D.

Figure 2. LTQ mass spectra obtained for the ion−molecule reaction of hydrido cuprate anions with CO2 ([CO2] ion trap = 2.2 × 1012 molecules cm−3) at an activation time of 600 ms: (a) [CuH2]− m/z 65; and (b) [Cu2H3]− m/z 129. The most intense peak in the ion is represented by the m/z value. *Represents the mass selected precursor ion. For these experiments, the helium bath gas cylinder was replaced with a helium cylinder seeded with 1.03% carbon dioxide (see Experimental Section for further details). C


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Inorganic Chemistry to form the formate anion,21 it is noteworthy that the bare formate anion is not observed in reactions of [63CuH2]− (experiments were carried out to attempt to detect it at m/z 45). Finally, the copper hydride [63CuH2]− can be regenerated when [63CuH(O2CH)]− (m/z 109) is mass selected and subjected to collision-induced dissociation (Figure S2b). [63CuH 2]− + CO2 → [63CuH(O2 CH)]−

[CuMe2]− + C3H5I → I− + CuMe + C4 H8 → [MeCuI]− + C4 H8 63

Reactions of [CuH2]− and [Cu2H3]− with CH3I and CH2CHCH2I. [63CuH2]− reacts with methyl iodide (Figure S7a) to yield two ionic products, I− and [63CuHI]−. The observation of the latter ion suggests that the reaction of methyl iodide with [63CuH2]− proceeds via cross-coupling to yield methane and [63CuH] and I− (eq 16) as well as [63CuH(I)]− (eq 17). The ion−molecule reaction of methyl iodide with [63CuH2]− reacts at a rate of 4.7 × 10−10 cm3 molecules−1 s−1. Sequential replacement of methides by hydrides in cuprates enhances reactivity toward methyl iodide, with the following reactivity order being observed: [CuH2]− > [MeCuH]− > [CuMe2]−.22c

→ [63CuH(I)]− + CH4

(17)

[63CuHD]− reacts with methyl iodide to give I− as well as [ CuH(I)]− and [63CuD(I)]− (Figure S9). The observation of the latter two isotopologues suggests that I− is formed by both eqs 18 and 19.

(19)

→ [63CuHI]− + CH3D

(20)

→ [63CuDI]− + CH4

(21)

(22)

→ [63CuH(I)]− + C3H6

(23)

(28)

→ [63CuHI]− + C3H5D

(29)

→ [63CuDI]− + C3H6

(30)

−

(31) −

Acid−Base Reactions of [CuH2] and [Cu2H3] with Alcohols and 1-Butanethiol. We next examined whether the coordinated hydrides of [63CuH2]− could be protonated to liberate H2.38 Methanol, with a gas-phase acidity (ΔHacid) of 382 kcal mol−1,39 was found to be unreactive (data not shown). In contrast, the bare hydride anion rapidly deprotonates methanol, highlighting that coordination of the hydride to the copper center decreases its basicity.21 The stronger acid trifluoroethanol (ΔHacid = 361.7 kcal mol−1)39 reacted with [63CuH2]− via an acid−base reaction to form the anionic product [63CuH(OCH2CF3)]− (m/z 163, eq 32, Figure S11a). The rate of this protonation reaction was measured to be 7.1 × 10−10 cm3 molecules−1 s−1, which corresponds to a reaction efficiency of 45%. 2,2,2-Trifluoroethanol reacts with [63CuHD]− to protonate both the D and the H sites to form [63CuH(OCH2CF3)]− (eq 33) and [63CuD(OCH2CF3)]− (eq 34), respectively (Figure S12). The KIE for this reaction can be directly determined from the ion abundances and was found to be 0.85. Related reverse isotope effects have been observed for the protonation of iron−hydride complexes by acids.38 The binuclear cuprate undergoes an acid base reaction with trifluorethanol to form the anionic product [Cu2H3(OCH2CF3)]− (m/z 227, eq 35, Figure S11b), albeit at a slower rate (5.4 × 10−12 cm3 molecules−1 s−1, reaction efficiency of 0.07%).

The ion−molecule reaction of [63CuH2]− with allyl iodide was slightly faster with a reaction rate of 1.1 × 10−9 cm3 molecules−1 s−1, corresponding to a reaction efficiency of 68%. Two ionic products, I− and [63CuHI]−, are also observed (Figure S8a), with the latter product ion suggesting formation of propene via a cross-coupling pathway to produce I− (m/z 127) and [63CuH] (eq 22) as well as [63CuHI]− (eq 23). In contrast, not only is [CuMe2]− an order of magnitude less reactive toward allyl iodide, but also produces I− (eq 24), [MeCuI]− (eq 25), as well as [C3H5CuI]− (eq 26).22d The formation of the latter homocoupling product arises from oxidative addition to give a Cu(III) η3-allyl intermediate. The lack of such a product in the reaction of [CuH2]− suggests that such an intermediate may not be formed in the case of [CuH2]−. [63CuH 2]− + C3H5I → I− + 63CuH + C3H6

→ I− + 63CuD + C3H6

[63Cu 2H3]− + C3H5I → [63Cu 2H 2(I)]− + C3H6

(18)

→ I− + 63CuD + CH4

(27)

Although the isotope effects associated with product channels involving formation of I− could not be determined due to the fact that neutral species containing H or D are not detected (i.e., eqs 18 versus 19 and eqs 27 versus 28), by comparing the product ion abundances of [63CuH(I)]− (eq 20 or 29) and [63CuD(I)]− (eq 21 or 30), kinetic isotope effects of 1.51 and 0.9 are calculated for the reactions of [63CuHD]− with methyl iodide and allyl iodide, respectively. Changes in order from normal to inverse isotope effects have been reported for hydride transfer from tungsten hydrides to Ph3C+BF4−.37 [63Cu2H3]− is unreactive toward methyl iodide at ion activation times of up to 10 000 ms (Figure S7b). However, in contrast with the trends in reactivity of methyl iodide and allyl iodide toward [63CuH2]−, allyl iodide reacts with [63Cu2H3]− (m/z 129) to yield the product ion [63Cu2H2I]− (m/z 255, Figure S8b) at a reaction rate of 1.3 × 10−10 and a reaction efficiency of 10%. The observation of the latter ion suggests the formation of propene via a cross-coupling pathway (eq 31).

63

[63CuHD]− + CH3I → I− + 63CuH + CH3D

(26) −

[63CuHD]− + C3H5I → I− + 63CuH + C3H5D

(15)

(16)

−

[ CuHD] reacts with allyl iodide to give I as well as [63CuH(I)]− and [63CuD(I)]− (Figure S10). The observation of the latter two isotopologues suggests that I− is formed by eqs 27 and 28.

(14)

[63CuH 2]− + CH3I → I− + 63CuH + CH4

(25)

→ [C3H5CuI]− + C2H6

[63Cu2H3]− (m/z 129) also reacts via CO2 capture (Figure 2b, eq 15), but at approximately one-half the rate (1.3 × 10−13 cm3 molecules−1 s−1, reaction efficiency of 0.02%) of [63CuH2]−. [63Cu 2H3]− + CO2 → [63CuH 2(O2 CH)]−

(24)

D


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Inorganic Chemistry

Figure 3. Energy profile obtained from DFT calculations, which shows the following: (A) The relationship between [CuH2]− and [Cu2H3]− and their respective isomers. Key bond lengths are given in angstroms. The HOMO for (B) 1A; and (C) 2A. The relative Gibbs and potential energies (in parentheses) obtained from the M06/BS2//M06/BS1 calculations are given in kcal mol−1.

Figure 4. Energy profile obtained from DFT calculations for competing mechanisms for the reaction of [CuH2]− with CS2: (a) hydride transfer pathway (red); (b) addition/H2 elimination to form [CuCS2]− (black); and (c) adduct formation (blue, not observed experimentally). The relative Gibbs and potential energies (in parentheses) obtained from the M06/BS2//M06/BS1 calculations are given in kcal mol−1.

E


Article

Inorganic Chemistry [63CuH 2]− + CF3CH 2OH → [CuH(OCH 2CF3)]− + H 2

DFT Calculated Mechanisms for the Reactions of [CuH2]− with CS2 and CO2. DFT calculations were carried out for the reactions on [63CuH2]− (Figure 4) with CS2 to establish the likely mechanisms associated with the hydride transfer reaction (eq 1) and the reaction giving rise to the organometallic ion, [63CuCS2]− (eq 2). The lowest energy pathway for initial attack by [63CuH2]−, 1A, on CS2 involves coordination of the carbon atom at the copper center via TS1A−2 to form the three-membered ring, [H2Cu(η2-CS2)]−, 2. Related three-membered ring structures are formed in the reactions of Cu atoms47 and metal complexes48 with CS2. Sideon transfer of a hydride via TS1A−5 is not competitive, consistent with previous calculations on the related side-on transfer of CH3− from [CuMe2]− to CS2, which was found to be a high energy process.22e The initially formed [(H)2Cu(η2CS2)]−, 2, can either fragment via H2 loss or via hydride transfer to form [HCS2]−. The pathway for H2 loss from 2 results in the formation of a σ bond between the hydrides, TS2−3, to give the ion−molecule complex [H2Cu(η2-CS2)]−, 3, which is then able to lose a molecule of H2, TS3−4, to give the observed organometallic ion [CuCS2]− (m/z 139), 4. The hydride transfer pathway from 2 occurs via electrophilic attack of the carbon by a hydride, TS2−5, to yield hydridothioformato cuprate, 5, which can isomerize via TS5−6 to 6. Even though H2 loss is predicted to be thermodynamically favored, [HCS2]− formation is kinetically favored due to both TS2−5 and TS5−6 being lower in energy than TS2−3 and TS3−4. Although the formation of CuH and [HCS2]− is predicted to be slightly endothermic, at the higher CCSDT level of theory, formation of these products is predicted to be exothermic by 4.5 kcal mol−1 (Figure S15). Finally, the fact that the isotope effect for the formation of [HCS2]− is close to unity is consistent with TS1A−2 being the rate-determining step. DFT calculations were carried out (Figure 5) to establish the mechanism of the CO2 insertion reaction into [CuH2]− (eq 14). The electrophilic nature of the carbon atom of CO2 allows [CuH2]− to transfer a hydride to the carbon atom as it approaches. This transforms carbon dioxide to a formate anion, which is coordinated via an oxygen atom in the complex

(32)

[63Cu3H3]− + CF3CH 2OH → [CuH 2(OCH 2CF3)]− + H 2 (35) 63

−

[ CuH2] reacts with 1-butanethiol (ΔHacid = 353.7 kcal mol−1)39 at the collision rate40 to give [63CuH(S(CH2)3CH3)]− (m/z 153)41 and H2 (Figure S13a, eq 36). The reaction of [63CuHD]− with 1-butanethiol (Figure S14) produced [63CuH(S(CH2)3CH3)]− (eq 37) and [63CuD(S(CH2)3CH3)]− (eq 38, Figure S14), giving a KIE of 0.86, which is comparable to that of 2,2,2-trifluoroethanol. The binuclear cuprate undergoes a slower (1.4 × 10−11 cm3 molecules−1 s−1, reaction efficiency of 1%) acid−base reaction with 1-butanethiol to form the anionic product [Cu2H3(S(CH2)3CH3)]− (m/z 217, eq 39, Figure S13b). [63CuH 2]− + CH3(CH 2)3 SH → [63CuH(S(CH 2)3 CH3)]− + H 2

(36)

[63Cu 2H3]− + CH3(CH 2)3 SH → [63Cu 2H 2(S(CH 2)3 CH3)]− + H 2

(39) −

DFT Calculated Structures of [CuH2] and [Cu2H3]− and Their HOMOs. The formation of [Cu2H3]− via ESI−MS suggests that it is stabilized with respect to its dissociation to form [CuH2]− and CuH (eq 40), as indicated in Figure 3A. The most stable isomer of [Cu2H3]−, [(HCu)2H]− 2A, contains a three-centered (Cu−H−Cu) two-electron bond. The dissociation of [Cu2H3]− 2A, to yield CuH and [CuH2]− 1A, is endothermic and requires 57.8 kcal mol−1. A related ligated copper hydride cluster, [(LCu)2H]+, has been isolated by Sadighi’s group,42 and related halocuprates43 and organocuprates [(ArCu)2Ar]− (Ar = mesityl) have been structurally characterized.44 Other higher energy isomeric structures include linear [HCuHCuH]− 2B, and the T-shaped complex [H2CuCuH]− (2C).45 [Cu 2H3]− → [CuH 2]− + CuH

(40)

Given that organocuprate reactivity is governed by the orbital interactions between the HOMO of the cuprate and the LUMO of the electrophilic substrate,46 we were interested in establishing whether the enhanced reactivity of the mononuclear hydrido cuprate arises from differences in its HOMO. A comparison of the HOMOs of 1A, Figure 3B, and 2A, Figure 3C, reveals that the HOMO of 1A is higher in energy and is based on the hydride ligands. This contrasts with both the HOMO of dimethyl cuprate, which is based on the Cu center,46 and 2A, which is based on both the copper atoms and the terminal hydrides. Given the unique HOMO of [CuH2]−, we next examined the potential energy diagrams of various mechanisms associated with its reactions with all of the substrates examined experimentally.

Figure 5. Energy profile obtained from DFT calculations for the insertion of CO2 into [CuH2]−. The relative Gibbs and potential energies (in parentheses) obtained from the M06/BS2//M06/BS1 calculations are given in kcal mol−1. F


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Inorganic Chemistry [CuH(O2CH)]−, 7, that in turn can isomerize to the thermodynamically favored isomer 8 via TS7−8. It follows from the calculations that CS2 is activated through a different mechanism than CO2. The CS2 molecule, with a lower lying π* orbital, has the capability to accept electron density from the Cu center and thus binds strongly to Cu. In contrast, due to the relatively strong CO π bonds in CO2, the coordination of this molecule to Cu is less feasible; for example, attempts to optimize the corresponding three-membered ring complex [H2Cu(η2-CO2)]− failed. In support of this approach, we found that the analogous transition structure TS2−3 (Figure 4) for CO2 is very unstable with a Gibbs free energy of 44.8 kcal mol−1. On the other hand, CO2 is very reactive toward side-on hydride transfer and can undergo the relevant reaction without any activation barrier, whereas this is not the case for CS2. Because hydride transfer increases the electron density on the pendant oxygen atoms of CO2, the high electronegativity of the oxygen atoms helps to facilitate this process. The slow experimentally determined rate and efficiency of this insertion reaction can be reconciled with the calculated surface by noting that the initially formed insertion products 7 and 8 are “hot” and must be collisionally cooled with the helium bath gas for them to be trapped and subsequently detected in the mass spectrometry experiments. Thus, it is likely that a large proportion of these “hot” product ions undergo deinsertion to regenerate the reactant anion, [CuH2]−. DFT Calculated Mechanisms for the Reactions of [CuH2]− with Methyl Iodide and Allyl Iodide. Cuprates can react with methyl iodide and allyl iodide via a number of mechanisms, including side-on SN2 reactions or via oxidative addition followed by reductive elimination.22a−d The only viable pathway for the reaction of [CuH2]− with methyl iodide involves a side-on SN2 reaction as shown in Figure 6. The carbon atom of the methyl group directly approaches the hydride ligand via transition state TS1A−9, with a colinear arrangement of H−Cu---H---CH3---I, which then produces complex 9 between copper hydride, methane, and the iodide anion. Although 9 can dissociate to yield HCuHCH3 10 and I−, it is entropically favored for 9 to dissociate via loss of both methane and CuH to form the experimentally observed complex, [CuH(I)]− 11 (eq 18), and iodide anion (eq 17). All attempts to locate a transition state for oxidative addition via approach of the copper atom of [CuH2]− to the backside of the carbon atom of methyl iodide collapsed to the side-on transition state TS1A−10. In the case of [CuH2]− reacting with allyl iodide, the following mechanisms were examined via DFT calculations: (i) side-on SN2 and SN2′ reactions (Figure 7); and (ii) oxidative addition followed by reductive elimination (Figure S16). The transition states for the SN2 and SN2′ reactions of allyl iodide (TS1A−12 and TS1A−13) are similar to that of methyl iodide (TS1A−9) and also produce complexes 12 and 13 between copper hydride, propene, and the iodide anion. These complexes can dissociate via the formation of the organometallic [HCu(η2-C3H5)]−, 14, and I−, extrusion of propene, or loss of both propene and CuH to form the experimentally observed complex, [CuH(I)]− 15 (eq 23), and iodide anion (eq 22). The SN2 pathway has a slightly lower barrier than that for the SN2′ reaction, but both reactions might occur under the experimental conditions. Interestingly, a recent study on the reactions of N-heterocyclic carbene (NHC) copper hydride complexes, LCuH, with allyl bromides has found that the products of both SN2 and SN2′ pathways can occur and that

Figure 6. Energy profile obtained from DFT calculations for the sideon SN2 reaction of [CuH2]− with methyl iodide. The relative Gibbs and potential energies (in parentheses) obtained from the M06/BS2// M06/BS1 calculations are given in kcal mol−1.

which of these pathways dominates depends on the nature of the substrate, the solvent, as well as the NHC.49 The SN2 reaction for allyl iodide has a lower barrier than that for methyl iodide, consistent with a slightly faster reaction between allyl iodide and [CuH2]−. An oxidative-addition-reductive-elimination pathway was located and involves multiple transition states (Figure S16). While the initial transition state has an activation energy similar to those for the SN2 and SN2′ pathways, the DFT calculations predict that homocoupling should be the major pathway. Given that the product of homocoupling, [C3H5CuI]−, is not observed experimentally, this suggests that oxidative addition pathways are unlikely to be important. DFT Calculated Mechanisms for the Reactions of [CuH2]− with Acids. The DFT calculated surface for the reaction of [CuH2]−, 1A, with methanol proceeds via the initial formation of the ion−molecule complex, 16. The transition state, TS16−17, barrier for subsequent formation of H2 and the coordinated methoxide, 17, lies above the energy of separated reactants (Figure 8), consistent with the fact that no reaction is observed under the near thermal conditions of the ion trap.50 In contrast, the reaction of [CuH2]−, 1A, with 2,2,2-trifluoroethanol to give 18 and the subsequent formation of the coordinated alkoxide, 19, has a transition state barrier for the formation of H2, TS18−19, that is below the energy of the separated reactants. Because the barrier for proton transfer decreases as the acidity increases from CF3CH2OH (Figure 8) to CH3(CH2)3SH (Figure S17), there is a good correlation between the gas-phase acidity of the substrates and the DFT calculated activation G


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Inorganic Chemistry

Figure 7. Energy profile obtained from DFT calculations for the side-on SN2 reactions of [CuH2]− with allyl iodide. Left, SN2′; right, SN2. The relative Gibbs and potential energies (in parentheses) obtained from the M06/BS2//M06/BS1 calculations are given in kcal mol−1.

Using the DFT calculated transition states and substituting H for D gives KIEs of 0.97 and 0.99 associated with TS1A−9 and TS1A−9 for the reactions of [63CuHD]− with methyl iodide and allyl iodide, respectively (Figure S19). The former value is in poor agreement with the experimentally determined KIE of 1.51 (Table 2), highlighting that interpretation of the experimentally measured isotope effects is problematic in the case of reactions such as these, where the formation of iodide dominates the product channels. Finally, the protonation reactions both give inverse isotope effects. Because there is only one transition state associated with these reactions (TS18−19 in Figure 8b and TS26−27 in Figure S17), this is consistent with the DFT calculated transition states, which are found to be “late” (Figure 9). Related inverse isotope effects for protonation reactions of metal hydrides have also been proposed to occur via late transition states.38e Such late transition states benefit from the formation of the stronger H−D bond,52 giving rise to inverse isotope effects. Indeed, using the DFT calculated transition states and substituting H for D gives KIEs of 0.66 and 0.61 associated with TS18−19 and TS26−27 (Figure S19).

energies (Figure S18). This supports the view that the reactivity is mainly controlled by the acidity of the substrates. The kinetic isotope effects (0.85 and 0.86, respectively) are consistent with rearrangement processes involving hydride transfer in the ratedetermining step; for example, see TS18−19 for CF3CH2OH (Figure 8). Experimentally Determined Kinetic Isotope Effects and Their Relationship to the DFT Calculated Mechanisms. We now briefly revisit the kinetic isotope effects determined from the integrated ion abundances of the product ions formed in the reactions of [63CuHD]− with various substrates (Table 2). We note that care must be used in their interpretation because some reactions approach the collisioncontrolled limit, so competitive effects in reaction channels must be considered.51 This is challenging in the cases of the reactions of [63CuHD]− with CS2, methyl iodide, and allyl iodide, where the isotope effect for the other product channels could not be determined due to formation of ions that did not contain H or D (i.e., eqs 5, 18, 19, 27, and 28). The isotope effect for formation of [HCS2]−, which is the dominant reaction of CS2, is close to unity. This is consistent with DFT calculated potential energy diagram (Figure 4), where the ratedetermining step TS1A−2 does not involve breaking the Cu− H(D) bond.

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CONCLUSIONS Mass spectrometry experiments involving isotope labeling together with DFT calculations highlight that the hydrido H


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Inorganic Chemistry

Figure 8. Energy profile obtained from DFT calculations for the reaction of [CuH2]− with alcohols, ROH: (a) R = Me; (b) R = CF3CH2. The relative Gibbs and potential energies (in parentheses) obtained from the M06/BS2//M06/BS1 calculations are given in kcal mol−1.

cuprate [CuH2]− exhibits a rich substrate-dependent bimolecular reactivity. [CuH2]− readily reacts with methyl iodide and allyl iodide via substitution, a reaction that has been used in the condensed phase to reductively remove halogens from alkyl halides.8d,9 The insertion of CO2 into the Cu−H bond leads to the formation of coordinated formate, while CS2 reacts to give the dithioformate anion as the major product. The former reaction has been observed in solution for copper hydrides.36 Another difference between these two systems is that CS2 is capable of releasing H2 but CO2 is not. This finding can be rationalized by the lower energy π* orbitals on CS2, which promotes coordination to the metal center. The hydrido cuprate [CuH2]− has been suggested as a complex for hydrogen storage applications.12 Our studies have shown that there are two different chemically induced routes for the liberation of hydrogen from [CuH2]− via the following:

Table 2. Kinetic Isotope Effect (KIE) Associated with the Formation of Certain Product Ion Channels in the Ion− Molecule Reactions of [63CuHD]− (m/z 66) with Various Neutral Reagents ion 63

molecule −

[ CuHD]

CS2b CH3Ib C3H5Ib CF3CH2OH CH3(CH2)3SH

KIE = AH/ADa AH(eq 3)/AD(eq 4) = 0.98 AH(eq 21)/AD(eq 20) = 1.51 AH(eq 30)/AD(eq 29) = 0.9 AH(eq 34)/AD(eq 33) = 0.85 AH(eq 38)/AD(eq 37) = 0.86

a AH = ion count of the ionic product where the neutral molecule has reacted with the hydride (light isotope) via given eq no.; and AD = ion count of the ionic product where the neutral molecule has reacted with the deuteride (heavy isotope) via given eq no. bThe isotope effects associated with other product channels could not be determined due to the formation of ions that did not contain H or D.

Figure 9. DFT calculated transition states for the protonation reactions of [CuH2]− with (a) CF3CH2OH via TS18−19, and (b) CH3(CH2)3SH via TS26−27. I


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Inorganic Chemistry

extrapolation of plots of ln([hydrido cuprate]− intensity/total ions) versus RD. Rate constants were calculated by dividing the pseudo firstorder rate coefficient by the calculated concentration of carbon disulfide in the ion trap. Theoretical rates for the ion−molecule reactions of hydrido cuprates with neutrals were calculated using the Average Dipole Orientation (ADO) theory of Su and Bowers28 with the program COLRATE.29 Determination of Kinetic Isotope Effects. The kinetic isotope effect was determined for the ion−molecule reactions of the mass selected isotopologue [63CuHD]− (m/z 66) with carbon disulfide, methyl iodide, allyl iodide, 2,2,2-trifluoroethanol, and 1-butanethiol (see Table 2). The ion count for product ion produced from the activation of the Cu−H bond was divided by the ion count for the product ion produced by the activation of the Cu−D bond. Depending on the reaction, corrections were made for overlapping isotope contributions, that is, 2H, 13C, 17O, and 33S. Theoretical Methods. Gaussian 0955 was used to fully optimize all of the structures reported in this Article at the M06 level of density functional theory (DFT).56 The SDD basis set with Stuttgart potentials was used to describe Cu and I.57 The 6-31+G(d,p) basis set was used for other atoms. This basis set combination will be referred to as BS1. Frequency calculations were carried out at the same level of theory as those for the structural optimization. Transition structures were located using the Berny algorithm. Intrinsic reaction coordinate (IRC)58 calculations were used to confirm the connectivity between transition structures and minima. To further refine the energies obtained from the M06/BS1 calculations, we carried out single-point energy calculations for all of the structures with a larger basis set (BS2) at the M06 level. BS2 utilizes the def2-TZVP basis set on all atoms. An effective core potential including scalar relativistic effect was used for I atom.59 To estimate the corresponding Gibbs energies, ΔG, the corrections were calculated at the B3LYPD3BJ/BS1 level using the conditions of T = 298.15 K; P = 2 × 10−3 Torr, which reflect the operating conditions of the ion trap (T ≈ 298 K; P ≈ 2 × 10−3 Torr) and finally added to the single-point energies. We have used the corrected Gibbs free energies and the potential energies obtained from the M06/BS2//M06/BS1 calculations throughout this Article unless otherwise stated. Finally, the DFT calculated KIEs were determined by calculating the Gibbs free energies of the transition states where either of the Cu−H’s was replaced by Cu−D (Figure S19).

(i) Homocoupling of both hydride ligands triggered by a substrate, to give the substrate coordinated to copper. As mentioned above, only CS2 reacts to trigger such a loss via the formation of the organometallic ion [CuCS2]− (eq 2), but this is only a minor reaction channel. (ii) Protonation by an acid, AH, to give [CuHA]−. This reaction occurs for both 2,2,2-trifluoroethanol (eq 32) and 1butanethiol (eq 36). Related reactions have been proposed to occur in the solution for copper hydrides reacting with isopropanol.38d The observation of facile protonation of [CuH2]− by these acids suggests the possibility of using hydrido cuprates as catalysts to selectively transform formic acid into hydrogen and carbon dioxide (eq 41).53 Such studies are underway and will be reported in due course.

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HCO2 H → H 2 + CO2

(41)

EXPERIMENTAL SECTION

Materials. Chemicals from the following suppliers were used without further purification: Ajax Finechem, (i) copper(I) oxide, Cu2O; (ii) formic acid (98%), HO2CH; MERCK, (iii) acetonitrile, (98%), CH3CN; SIGMA, (iv) formic acid d2 (98%), DO2CD; (v) 1butanethiol, (99%), CH3(CH2)3SH; (vi) allyl iodide, CH2CHCH2I, (98%); (vii) methyl iodide, CH3I (99%); COREGAS, (viii) gaseous carbon dioxide, CO2 (1.03% in Ultra High Purity helium (99.995%)); CHEMSUPPLY, (ix) carbon disulfide, CS2 (99.9%); ACROS, (x) 2,2,2-trifluorethanol, CF3CH2OH (99.8%). Mass Spectrometry. Mass spectrometry experiments were conducted on a modified Finnigan hybrid linear triple-quadrupole (LTQ) Fourier transform ion cyclotron resonance (FTICR) mass spectrometer.54 Under ion−molecule reaction conditions, collisions with the helium bath gas quasi-thermalize the ions to room temperature.50 Both CID and IMR experiments were performed on this instrument.54 The solution used to prepare copper hydride anions in the gas phase was prepared as follows: In situ hydrido formate complexes for ESI−MS were typically generated by adding 10 mmol of copper(I) oxide to a 20 mL solution of acetonitrile, generating a reddish colored suspension. To this suspension was immediately added 20 mmol of formic acid to give a clear solution. This solution was diluted in acetonitrile to a copper(I) concentration of 50 μM and injected at a flow rate 3−5 μL min−1 into the Finnigan ESI source. ESI source conditions typically involved needle potentials of 3.0−5.0 kV to give a stable source current of ca. 0.5 μA and a nitrogen sheath gas pressure of 8 arbitrary units. The ion transfer capillary temperature was set to 275 °C. The tube lens voltage was set to −38 V, and the capillary voltage was set to −13 V. Unimolecular fragmentation studies involved the anion of interest being mass selected in the linear ion trap (LIT) and then subjected to CID, where the normalized collision energy was set between 10−25 arbitrary units to result in the mass selected precursor ion to be depleted to 10−20% with an activation Q of 0.25 and activation time of 30 ms. Kinetic Measurements. The kinetics for the reaction between the hydrido cuprate anions and neutral reagents were examined using the LTQ FT hybrid mass spectrometer. Ion−molecule reaction rates were measured by isolating the reactant ion and allowing it to react with neutral reagent at various activation times, similar to previously reported ion−molecule reactions.54 The neutral substrates were introduced at various concentrations into the ion trap via the helium inlet line. In the case of the reactions with carbon dioxide, the helium bath gas cylinder was replaced by a helium cylinder seeded with 1.03% CO2, and the entire mass spectrometer was allowed to re-equilibrate with this gas mixture for 15 min. Rates were measured by varying the time delay between isolation of the reactant ion and its mass analysis (“reaction delay”, RD). The decay of the reactant hydrido cuprate anions was monitored over at least eight values of RD. The intensity of the reactant ion was calculated by integration of its ion count within the mass-selected window. Pseudo first-order rates were estimated by

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02145. Mass spectra, pseudo-first-order kinetics, DFT calculated mechanisms, and Cartesian coordinates and total energies for all calculated structures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Richard A. J. O’Hair: 0000-0002-8044-0502 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. George Khairallah for useful discussions regarding the mass spectrometry experiments and the Australian Research Council for financial support DP150101388 (to R.A.J.O. and A.J.C.). We gratefully acknowledge the generous allocation of computing time from the J


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(f) Semmelhack, M. F.; Stauffer, R. D. Reductions with copper hydride. New preperative and mechanistic aspects. J. Org. Chem. 1975, 40, 3619−3621. (9) [KCuH2]n complexes: (a) Yoshida, T.; Negishi, E.-I. A novel copper-containing hydride species and its application to the reduction of organic substances. J. Chem. Soc., Chem. Commun. 1974, 762−763. (b) Masamune, S.; Rossy, P. A.; Bates, G. S. Reductive removal of halo and mesyloxy groups with a copper(I) complex. J. Am. Chem. Soc. 1973, 95, 6452−6454. (10) [LiRCuH]n complexes: (a) Boeckman, R. K.; Michalak, R. Reduction of α,β-unsaturated carbonyl compounds by ″ate″ complexes of copper(I) hydride. J. Am. Chem. Soc. 1974, 96, 1623−1625. (b) Masamune, S.; Bates, G. S.; Georghiou, P. E. Reactions of lithium alkyl and alkynyl cuprates. Selective removal of halo and mesyloxy groups and reduction of α,β-unsaturated ketones. J. Am. Chem. Soc. 1974, 96, 3686−3688. (c) House, H. O.; DuBose, J. C. Reduction as a side reaction arising from the thermal decomposition of lithium organocuprates to form copper hydride derivatives. J. Org. Chem. 1975, 40, 788−790. (11) Andrews, L. Matrix infrared spectra and density functional calculations of transition metal hydrides and dihydrogen complexes. Chem. Soc. Rev. 2004, 33, 123−132. (12) Andrews, L.; Wang, X. Infrared Spectra and Structures of the Stable CuH2−, AgH2−, AuH2−, and AuH4− Anions and the AuH2 Molecule. J. Am. Chem. Soc. 2003, 125, 11751−11760. (13) The gas-phase photo electron spectrum of [CuH2]− has also been reported: Calvi, R. M. D.; Andrews, D. H.; Lineberger, W. C. Negative ion photoelectron spectroscopy of copper hydrides. Chem. Phys. Lett. 2007, 442, 12−16. (14) Wang, X.; Andrews, L.; Manceron, L.; Marsden, C. Infrared Spectra and DFT Calculations for the Coinage Metal Hydrides MH, (H2)·MH, MH2, M2H, M2H−, and (H2) CuHCu in Solid Argon, Neon, and Hydrogen. J. Phys. Chem. A 2003, 107, 8492−8505. (15) The gas-phase photo electron spectrum of [Cu2H]− has also been reported: (a) Xie, H.; Li, X.; Zhao, L.; Liu, Z.; Qin, Z.; Wu, X.; Tang, Z.; Xing, X. Vibrationally Resolved Photoelectron Imaging of Cu2H− and AgCuH− and Theoretical Calculations. J. Phys. Chem. A 2013, 117, 1706−1711. (b) Vetter, K.; Proch, S.; Gantefoer, G.. F.; Behera, S.; Jena, P. Hydrogen mimicking the properties of coinage metal atoms in Cu and Ag monohydride clusters. Phys. Chem. Chem. Phys. 2013, 15, 21007−21015. (16) Sass, D. C.; Heleno, V. C. G.; Cavalcante, S.; da Silva Barbosa, J.; Soares, A. C. F.; Constantino, M. G. Solvent Effect in Reactions Using Stryker’s Reagent. J. Org. Chem. 2012, 77, 9374−9378. (17) (a) O’Hair, R. A. J. The 3D Quadrupole Ion Trap Mass Spectrometer as a Complete Chemical Laboratory for Fundamental Gas Phase Studies of Metal Mediated Chemistry. Chem. Commun. 2006, 1469−1481. (b) O’Hair, R. A. J.; Rijs, N. J. Gas Phase Studies of the Pesci Decarboxylation Reaction: Synthesis, Structure, and Unimolecular and Bimolecular Reactivity of Organometallic Ions. Acc. Chem. Res. 2015, 48, 329−340. (18) Armentrout, P. B.; Sunderlin, L. S. In Transition Metal Hydrides; Dedieu, A., Ed.; VCH: New York, 1992; pp 1−64. (19) (a) Squires, R. R. Gas-Phase transition-metal negative ion chemistry. Chem. Rev. 1987, 87, 623−646. (b) Damrauer, R. Organometallic Chemistry in the Flowing Afterglow: A Review. Organometallics 2004, 23, 1462−1479. (20) (a) Lane, K. R.; Squires, R. R. Formation of HCr(CO)3− from the remarkable reaction of hydride ion with benzenechromium tricarbonyl. Gas-phase reactions of a novel 14-electron metal anion complex. J. Am. Chem. Soc. 1985, 107, 6403−6404. (b) McDonald, R. N.; Schell, P. L. Gas-phase ligand substitution reactions with the 17electron transition-metal complexes (OC)4Fe−, (OC)5Cr−, and (OC)4MnH−. Organometallics 1988, 7, 1820−1827. (c) Khairallah, G. N.; O’Hair, R. A. J. Gas Phase Synthesis of Ag4H+ and its Mediation of C-C Bond Coupling of Allylbromide. Angew. Chem., Int. Ed. 2005, 44, 728−731. (d) Schlangen, M.; Schwarz, H. Thermal activation of methane by group 10 metal hydrides MH+: the same and not the same. Angew. Chem., Int. Ed. 2007, 46, 5614−5617. (e) Firouzbakht,

University of Tasmania and the National Computing Infrastructure. We thank the reviewers for their helpful insights.

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Inorganic Chemistry

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Inorganic Chemistry rates by up to a factor of 3−4 (see, for example: Balaj, O. P.; Balteanu, I.; Rossteuscher, T. T. J.; Beyer, M. K.; Bondybey, V. E. Catalytic oxidation of CO with N2O in gas-phase platinum clusters. Angew., Chem., Int. Ed. 2004, 43, 6519−6522. Anderson, M. L.; Ford, M. S.; Derrick, P. J.; Drewello, T.; Woodruff, D. P.; Mackenzie, S. R. Nitric Oxide Decomposition on Small Rhodium Clusters, Rhn± J. Phys. Chem. A 2006, 110, 10992−11000), the use of a point charge model for the ADO rates is an entirely reasonable approximation for a small system such as [CuH2]−. (41) The gas-phase photo electron spectrum of the related [CuH(SH)]− has been reported: Qin, Z.; Liu, Z.; Cong, R.; Xie, H.; Tang, Z.; Fan, H. Photoelectron imaging and theoretical study on the structure and chemical binding of the mixed- ligand M(I) complexes, [HMSH] - (M = Cu, Ag, and Au). J. Chem. Phys. 2014, 140, 114307/ 1−114307/7. (42) Wyss, C. M.; Tate, B. K.; Bacsa, J.; Gray, T. G.; Sadighi, J. P. Bonding and Reactivity of a μ-Hydrido Dicopper Cation. Angew. Chem., Int. Ed. 2013, 52, 12920−12923. (43) (a) Ko, Y. J.; Wang, H.; Pradhan, K.; Koirala, P.; Kandalam, A. K.; Bowen, K. H.; Jena, P. Superhalogen properties of CumCln clusters: Theory and experiment. J. Chem. Phys. 2011, 135, 244312/1−244312/ 7. (b) Subramanian, L.; Hoffmann, R. Bonding in Halocuprates. Inorg. Chem. 1992, 31, 1021−1029. and references cited therein (c) Chen, S.; Li, J.; Zhou, C.; Wu, J.; Tempel, D. J.; Henderson, P. B.; Brzozowski, J. R.; Cheng, H. Weak Chemical Complexation of PH3 with Ionic Liquids. J. Phys. Chem. B 2010, 114, 904−909. (44) (a) Bomparola, R.; Davies, R. P.; Hornauer, S.; White, A. J. P. Structural Characterization of Magnesium Organocuprates Derived from Grignard Reagents: CuI-Based Inverse Crown Ethers. Angew. Chem., Int. Ed. 2008, 47, 5812−5815. (b) Krieck, S.; Gçrls, H.; Westerhausen, M. Structural Diversity of Calcium Organocuprates(I): Synthesis of Mesityl Cuprates via Addition and Transmetalation Reactions of Mesityl Copper(I). Chem. - Asian J. 2010, 5, 272−277. (45) For discussions on metal−metal interactions in coinage metal complexes, see: (a) Gray, T. G.; Sadighi, J. P. In Molecular Metal− Metal Bonds; Liddle, S. T., Ed.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2015; Chapter 11, pp 397−428. (b) Schmidbaur, H. The aurophilicity phenomenon: A decade of experimental findings, theoretical concepts and emerging applications. Gold Bulletin 2000, 33, 3−10. (c) Schmidbaur, H.; Schier, A. A briefing on aurophilicity. Chem. Soc. Rev. 2008, 37, 1931−1951. (d) Schmidbaur, H.; Schier, A. Argentophilic Interactions. Angew. Chem., Int. Ed. 2015, 54, 746−784. (e) Cotton, F. A.; Feng, X.; Timmons, D. J. Further Study of Very Close Nonbonded CuI− CuI Contacts. Molecular Structure of a New Compound and Density Functional Theory Calculations. Inorg. Chem. 1998, 37, 4066−4069. (46) Yoshikai, N.; Nakamura, E. Mechanisms of Nucleophilic Organocopper(I) Reactions. Chem. Rev. 2012, 112, 2339−2372. (47) (a) Zhou, M.; Andrews, L. Reactions of Co, Ni, and Cu Atoms with CS2: Infrared Spectra and Density-Functional Calculations of SMCS, M-(η2-CS)S, M-CS2, and MCS2+ in Solid Argon. J. Phys. Chem. A 2000, 104, 4394−4401. (b) Dobrogorskaya, Y.; Mascetti, J.; Pápai, I.; Nemukhin, A.; Hannachi, Y. Theoretical Investigation of the Reactivity of Copper Atoms with Carbon Disulfide. J. Phys. Chem. A 2003, 107, 2711−2715. (48) For reviews on the reactions of CS2 with transition metal complexes, including insertion and formation of η2 complexes, see: (a) Yaneff, P. V. Thiocarbonyl and related complexes of the transition metals. Coord. Chem. Rev. 1977, 23, 183−220. (b) Werner, H. Novel coordination compounds formed from CS2 and heteroallenes. Coord. Chem. Rev. 1982, 44, 165−185. (c) Pandey, K. K. Reactivities of carbonyl sulphide (COS), carbon disulphide (CS2) and carbon dioxide (CO2) with transition metal complexes. Coord. Chem. Rev. 1995, 140, 37−114. (49) Nguyen, T. N. T.; Thiel, N. O.; Pape, F.; Teichert, J. F. Org. Lett. 2016, 18, 2455−2458. (50) Donald, W. A.; Khairallah, G. N.; O’Hair, R. A. J. The Effective Temperature of Ions Stored in a Linear Quadrupole Ion Trap Mass Spectrometer. J. Am. Soc. Mass Spectrom. 2013, 24, 811−815.

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