Synthesis , structure and gas-phase reactivity of the mixed silver hydride borohydride nanocluster [ Ag 3 ( μ 3H ) ( μ 3-BH 4 ) L Ph 3 ] BF 4 ( L Ph = bis ( diphenylphosphino ) methane ) †

Borohydrides react with silver salts to give products that span multiple scales ranging from discrete mononuclear compounds through to silver nanoparticles and colloids. The cluster cations [Ag3(H)(BH4)L3] + are observed upon electrospray ionization mass spectrometry of solutions containing sodium borohydride, silver(I) tetrafluoroborate and bis(dimethylphosphino)methane (L) or bis(diphenylphosphino)methane (L). By adding NaBH4 to an acetonitrile solution of AgBF4 and L , cooled to ca. −10 °C, we have been able to isolate the first mixed silver hydride borohydride nanocluster, [Ag3(μ3-H)(μ3-BH4)L Ph 3]BF4, and structurally characterise it via X-ray crystallography. Combined gas-phase experiments (L and L) and DFT calculations (L) reveal how loss of a ligand from the cationic complexes [Ag3(H)(BH4)L3] + provides a change in geometry that facilitates subsequent loss of BH3 to produce the dihydride clusters, [Ag3(H)2Ln] + (n = 1 and 2). Together with the results of previous studies (Girod et al., Chem. – Eur. J., 2014, 20, 16626), this provides a direct link between mixed silver hydride/borohydride nanoclusters, silver hydride nanoclusters, and silver nanoclusters.


Structural characterization of (1) by ESI/MS, NMR and IR spectroscopy and X-ray crystallography
The crystals of (1) formed in the bulk synthesis were first analysed via ESI/MS in both the positive and negative ion mode.The former gave an almost identical mass spectrum to that shown in Fig. 1a (data not shown), while the latter gave an abundant signal due to the BF 4 − counter ion (ESI Fig. S3 †).
We next attempted to characterise 1 via various NMR experiments (ESI Fig. S5-S16 †). 1 was dissolved into cold CD 3 CN to produce a saturated solution and this solution immediately introduced into the pre-cooled NMR probe at −15 °C.The 1 H NMR spectrum collected at −15 °C displayed a very broad multiplet centred at 0.5 ppm, attributed to coordinated BH 4 , from which no fine structure could be resolved (ESI Fig. S5 †).This may be due to (i) the fluxional nature of the binding of BH 4 , (ii) the complex splitting patterns due to spin-active nuclei and isotopomers of silver ( 107/109 Ag) and boron ( 10/11 B) and (iii) the influence of the quadrupolar 10 B nucleus.This signal collapsed into a broad singlet at 0.5 ppm under 11  Heating the sample from −15 °C to +25 °C in the NMR probe enabled the collection of 1 H data at various temperatures.The most obvious change in the spectra with time at −15 °C and then upon heating was the increase in the intensity of the singlet at 4.56 ppm which is attributable to dissolved H 2 and the corresponding reduction in the intensity of the coordinated hydride signal (ESI Fig. S16 †).
This series of experiments required the preparation of several different samples, as it was noted that 1 appears to undergo decomposition/reactions in these highly concentrated solutions, ultimately resulting in precipitation of a black material after approximately 3 hours at 25 °C.This rapid change in solution, even at −15 °C, precluded the overnight or longer acquisition of a 109 Ag NMR spectrum.
The Ag(1)-Ag(1) distances that connect the edges of the triangle are 2.9100(3) Å.The hydride H which lies on a crystallographic 3-fold axis is 0.96 Å displaced from the plane defined by the triangular silver(I) core and is coordinated to all silver(I) ions as a µ 3 -bridging ligand with a Ag(1)-H distance of 1.93(3) Å and Ag(1)-H-Ag(1) angle of 97.5(3)°.Relative to the µ 3 -hydride H, the opposing face of the trinuclear core has a µ 3 borohydride with distorted tetrahedral geometry, the boron lies on the 3-fold axis of symmetry and three symmetry related hydrides (H1a) coordinate to the three silver atoms.Related µ 3 -borohydride binding to the metal triangle of Fe(CO) 3 fragments has been reported for the trinuclear cluster [Fe 3 (µ-H)-(µ 3 -BH 4 )(CO) 9 ]. 27The boron-hydrogen bond lengths for BH 4 Given that AgBH 4 is known to undergo thermal decomposition reactions that liberate BH 3 (eqn (2)), 1,23 we were interested in examining whether such reactions occur in the gas phase for isolated, stoichiometrically well defined cluster cations.Thus, CID was carried out in a 2D linear ion trap to probe the lowenergy fragmentation pathways of [Ag Energy-resolved CID (ERCID) was used in a 3D ion trap to determine whether the product ions in Fig. 3a are due to primary fragmentation pathways of [Ag 3 (H)(BH 4 )L Ph 3 ] + or secondary fragmentation of primary fragment ions (ESI Fig. S18 †).The onset of ligand loss (eqn ( 5)) begins at ca. 0.6 V and continues to steadily increase up until 0.8 V (ESI Fig. S18 †).An increase in the collision voltage beyond this point results in the consumption of [Ag 3 (H)(BH 4 )L Ph 2 ] + (m/z 1109) and the increase of [Ag 3 (H) 2 L Ph 2 ] + (m/z 1095).These results suggest that the primary product ion upon CID of [Ag 3 (H)(BH 4 )L Ph 3 ] + arises from ligand loss and that ions of Unimolecular gas-phase chemistry of [Ag 3 (H)(BH 4 )L 2 ] + The primary product ions formed via ligand loss (eqn (5), Fig. 3) were mass selected and subjected to CID in the 2D ion trap (Fig. 4).Mass selection and subsequent CID of [Ag Once again, ERCID was used in a 3D ion trap to determine which of the product ions observed in Fig. 4a were primary (ESI Fig. S19 †).[Ag 3 (H) 2 L Ph 2 ] + (m/z 1095) begins to appear at 0.5 V upon CID [Ag 3 (H)(BH 4 )L Ph 2 ] + (m/z 1109), which corresponds to BH 3 loss (eqn ( 6)).A minor primary fragmentation channel assigned to neutral ligand loss (eqn (7)) is observed at around 0.6 V.Although decomposition reactions of coordinated ligands in metal complexes and clusters have been well studied in the gas-phase, 30,31 this appears to be the first experimental report on the gas-phase decomposition of a coordinated BH 4 − ligand via BH 3 loss.This reaction is related to that described by Wiberg and Henle (eqn (2)). 1 Unimolecular gas-phase chemistry of [Ag 3 (H)(BH 4 )L] + [Ag 3 (H)(BH 4 )L Ph ] + (m/z 723), formed via sequential ligand losses (eqn ( 4) and ( 6)), was mass selected and allowed to undergo CID.The sole fragmentation channel observed is due to the loss of neutral BH 3 (eqn ( 8)) (ESI Fig. S20 †).
Unimolecular gas-phase chemistry of [Ag 3 (H) 2 L n ] + The dihydride clusters [Ag 3 (H) 2 L Ph n ] + where n = 2, 1 were subjected to ERCID using a 3D ion trap (ESI Fig. S21 and S22 †).The major primary fragmentation of [Ag 3 (H) 2 L Ph 2 ] + (m/z 1095) occurs via ligand loss (eqn ( 9)), with an onset requiring ca.0.6 V.In contrast, The major primary fragmentation of [Ag 3 (H) 2 L Ph ] + (m/z 708) involves AgH loss, as previously described, 32 with the onset of fragmentation occurring at ca. 0.4 V.  12)), we turned to DFT calculations to examine the structures and energetics of the reactants and products of eqn ( 11) and ( 12) for the case of clusters containing L Me ligands.The initial geometry for [Ag 3 (H)(BH 4 )L Me 3 ] + was that related to the core structure from the X-ray structure for 1.Thus changing the phosphine substitutent from Ph to Me has little effect on the core structure.To calculate fragment ion structures, either BH 3 or L Me was removed and the resultant fragment was allowed to fully optimise.¶k 4][35][36][37] As expected, a transition metal centre with a d 10 electron configuration has four empty orbitals.Two of these four are always available for ligand coordination to give linear complexes [ML 2 ] + .However, the availability of the other two orbitals depends on the identity of the L ligands.For example, if the L-M-L bond angle in [ML 2 ] + using the bidentate ligands is forced to be bent, the two extra orbitals become available and as a result the tetrahedral complex [ML 4 ] + is formed.Also, the monodentate L ligands with relatively weak σ-donor abilities increase the possibility of all four orbitals on the metal centre being available.
In [Ag 3 (H)(BH 4 )L 3 ] + , the presence of the three bidentate phosphine ligands render all the four empty orbitals on three Ag centres susceptible to coordination.In such a case, the cluster has 3 × 4 = 12 available orbitals.Six of these twelve orbitals are occupied by the phosphine ligands.Three of them are involved in interaction with the hydride ligand (a µ 3 -bridging ligand) via a four-centre two-electrons bonding mode., where the corresponding anti-bonding skeletal molecular orbitals are also fully occupied. 38ith regards to the unimolecular fragmentation chemistry, the calculations indicate that the first loss of the ligand, L (eqn ( 12)), from [Ag 3 (H)(BH 4 )L Me 3 ] + results in transferring the hydride to Ag 2 and causes this centre to adopt a mainly linear structure with a very weak interaction with Ag 1 (Fig. 5).The presence of the very strong σ donating hydride ligand on the Ag 2 centre makes the two empty orbitals on Ag 2 less available and thus does not allow BH 4 − to strongly interact with them.
In this case, BH 4 − is only able to interact with the Ag 1 and Ag 3 centres to give a µ 2 -coordination mode (Fig. 5).If we ignore the weak interactions between Ag centres in [Ag 3 (H)(BH 4 )L Me 2 ] + , the Ag 1 and Ag 3 centres can be considered as three coordinate centres.For [Ag 3 (H)(BH 4 )L Me 2 ] + , the Ag 2 -H σ orbital interacts with one of the empty orbitals on Ag 3 and creates a 3-centre 2-electron bond between two silver(I) ions via a µ 2 -bridging hydride ligand.
From loss of a second L Me ligand, bonding in the product [Ag 3 (H)(BH 4 )L Me ] + can be viewed as interaction of the linear complex [BH 4 -Ag 3 -H] − with [Ag 2 L Me ] 2+ (Fig. 5).In this cluster, an Ag 3 -H σ orbital interacts with an empty orbital on Ag 2 , and BH 4 − bridges Ag 3 to Ag 1 through three of its B-H bonds.There-fore, each Ag(I) centre in [Ag 3 (H)(BH 4 )L Me ] + experiences a linear two coordinated environment.Our calculations show that, in excellent agreement with the CID data, the BH 3 loss becomes easier as the number of ligands, L, decreases.The loss of the ligand, L, increases the electron deficiency of the metal centres, leading to the stronger coordination of BH 4 − to the Ag centres, as evident from the shorter Ag-H(BH 4 ) and longer B-H bond distances in [Ag 3 (H)-(BH 4 )L Me ] + (Fig. 5).In other words, ligand loss enhances the acidity of Ag centres and makes them more prone to compete with BH 3 for hydride abstraction.The better the competition, the easier the BH 3 loss.In contrast to BH 3 loss (eqn ( 11)) neutral ligand loss (eqn ( 12)) from [Ag 3 (H)(BH 4 )L Me ] + is more difficult than that from [Ag 3 (H)(BH 4 )L Me 3 ] + and [Ag 3 (H)(BH 4 )L Me 2 ] + , supported by the DFT calculations.This difference can be rationalised in terms of the molecular orbital approach. 39In general, the HOMO of d 10 complexes (ML n ) with a coordination number greater than two (n > 2) suffers from a slight anti-bonding interaction between L and M, leading to weakening of the M-L bonds.However, this anti-bonding interaction disappears in linear d 10 -ML 2 complexes, causing the M-L bonds in ML 2 to be much stronger than those in ML 3 and ML 4 .As mentioned above, clusters [Ag 3 (H)(BH 4 )L Me 3 ] + and [Ag 3 (H)(BH 4 )L Me 2 ] + have the Ag centres not present as two-coordinate, and thus the loss of the ligand, L, from these clusters is relatively easy.By contrast, all the Ag centres in [Ag 3 (H)(BH 4 )L Me ] + are mainly two-coordinate, thereby not having the relevant anti-bonding interaction, forming very strong M-L bonds.

Conclusions
The sodium borohydride induced reduction of silver(I) salts to form nanoparticles has been described as a "black-box" synthesis. 19While it is now well established that there are different growth stages, [18][19][20] the actual molecular species associated during growth to nanoparticles and the mechanisms for growth are not fully understood.By studying the formation and reactions of small ligand protected nanoclusters, we are able to better understand the fundamental interactions between silver salts and borohydride.
We have previously shown that BH 4 − is a source of hydride .This is the first silver nanocluster containing a "captured" borohydride anion, and may have relevance to binding of BH 4 − to silver nanoparticle surfaces, 20,21 or bulk silver metal surfaces. 40Given that nanoclusters such as [{Ag 7 (µ 4 -H)(E 2 P(OR) 2 } 6 ] are precursors to further growth into silver nanoparticles, 21 it will be interesting to establish whether the nanoclusters [Ag 3 (µ 3 -H)-(µ 3 -Cl)L Ph 3 ]BF 4 , 9 [Ag 3 (µ 3 -H)L Ph 3 ](BF 4 ) 2 , 10 and [Ag 3 (µ 3 -H)-(µ 3 -BH 4 )L Ph 3 ]BF 4 can further grow into silver nanoparticles.The current and previous 32 gas-phase experiments and DFT calculations on [Ag 3 (H) 2−x (BH 4 ) x L n ] + clusters provide a direct link between mixed hydride/borohydride silver clusters (x = 1), dihydride silver clusters (x = 0) 32 and silver clusters via discrete unimolecular reactions occurring for isolated clusters (Scheme 2).Thus CID triggers loss of the ligand, L (eqn ( 12)), resulting in a change in the binding mode(s) of the H and BH 4 ligands (Fig. 5).Perhaps related reactions occur at the surfaces of silver nanoparticles, which might drive the development of catalysts for hydrogen storage applications. 41

Experimental
Synthesis of solution phase silver clusters for MS analyses Silver(I) tetrafluoroborate (1.9 mg, 0.010 mmol) and bis(diphenylphosphino)methane (3.8 mg, 0.010 mmol) in 20 mL acetonitrile were added to a 25 mL Quickfit Erlenmeyer flask equipped with a magnetic stirrer and stopper.The solution was cooled to ca.−10 °C by immersing the reaction flask in an ice/water bath above the solvent level.All reagents were kept in the dark and flasks covered in foil.Sodium borohydride (5.7 mg, 0.150 mmol) was added as a powder and the solution changed colour from clear to light yellow.Silver(I) tetrafluoroborate (194 mg, 1.0 mmol) and bis(diphenylphosphino)methane (384 mg, 1.0 mmol) in 100 mL acetonitrile were added to a 250 mL Quickfit round bottomed flask equipped with a magnetic stirrer and stopper.The solution was cooled to −10 °C by immersing the reaction flask in an ice/water bath above the solvent level.All reagents were kept in the dark and flasks covered in foil.Sodium borohydride (57.0 mg, 1.50 mmol), was added as a powder and the solution changed colour from clear to light yellow over ca. 5 minutes.The solution was filtered after stirring for 3 hours and frozen solid by immersing the flask in liquid nitrogen.While frozen, 100 mL of diethylether was added and the flask moved to the fridge.After 72 hours crystalline material was formed and characterised by X-ray crystallography.

Mass spectrometry
Mass spectra were recorded using a Finnigan hybrid linear quadrupole (LTQ) Fourier transform ion cyclotron resonance (FTICR) mass spectrometer.The silver clusters prepared in the solution phase were diluted to 50 µM and introduced into the mass spectrometer via a syringe pump set at a flow rate of 5 µL min −1 to the ESI capillary.The ESI conditions used, for optimum intensity of the target ions, typically were: spray voltage, 4.2-5.0kV, capillary temperature, 250 °C, nitrogen sheath gas pressure, 5 (arbitrary units), capillary voltage 25 V, tube lens voltage 15 V. Selected ions were transferred to the FTICR cell for accurate mass measurement with the use of selected ion monitoring (SIM) and selected reaction monitoring (SRM) to obtain the most reliable results.The unimolecular fragmentation of silver clusters was examined via CID.The mass-selected precursor ion was depleted to 10-20% using a normalised collision energy typically between 20-25% and a mass selection window of 15 Th to isolate the full range of isotopes due to boron and silver isotopes.
Energy resolved CID experiments were carried out using a Finnigan 3D ion trap (LCQ) mass spectrometer.The method of Broadbelt was adapted. 42The silver clusters were diluted to 50 µM and introduced into the mass spectrometer via a syringe pump set at 5 µL min −1 through a Finnigan ESI source.The source conditions used for optimum intensity of the target ions were: spray voltage 4.5-5.1 kV, capillary temperature 200 °C, nitrogen sheath gas pressure, 50 (arbitrary units), capillary voltage 30 V, tube lens voltage −55 V.The massselected precursor ion was isolated with a mass selection window of 15 Th.The normalised collision energy (NCE) was increased incrementally by 1.0% typically starting from a NCE where no fragmentation is observed, until reaching the NCE required for depleting the precursor ion to <5% relative intensity.The NCE was converted to an amplitude of the resonance excitation RF voltage (tick amp) as described in the ESI.† The relative intensity of precursor and product ions were plotted as a function of the increasing amplitude to determine: (i) the onset of precursor fragmentation and (ii) the assignment of product ions as primary or secondary fragments of the massselected silver cluster.

Crystallography
Intensity data for compound 1 was collected on an Oxford Diffraction SuperNova CCD diffractometer using Cu-Kα radiation, the temperature during data collection was maintained at 130.0(1) using an Oxford Cryostream cooling device.The structure was solved by direct methods and difference Fourier synthesis. 43The thermal ellipsoid plot was generated using the program ORTEP-3 44 integrated within the WINGX 45

Density functional theory
Computational details. [46][47][48][49][50][51][52][53] Gaussian 09 46 was used to fully optimise all the structures reported in this paper at the M06 level of density functional theory. 47,48The effective-core potential of Hay and Wadt with a double-ξ valence basis set (LANL2DZ) was chosen to describe Ag.The 6-31G(d) basis set was used for other atoms.Polarization functions were also added for Ag (ξf = 1.611).This basis set combination will be referred to as BS1.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 of theory.BS2 utilises the def2-TZVP basis set on all atoms.Effective core potentials including scalar relativistic effects were used for silver atom.We have used the corrected potential energies obtained from the M06/ BS2//M06/BS1 calculations throughout the paper unless otherwise stated.

Fig. 1
Scheme 1 Reactions of BH 4 − with silver salts to give products spanning multiple scales.(a)-(f ) represent discrete isolated species whose structures have been determined via X-ray crystallography.The scale indicates ionic interactions or the outermost diameter of each silver cluster not inclusive of ligands.Counterions and hydrogen atoms have been omitted for clarity.
Fig.S14 †).The 1 H-decoupled13 C NMR spectrum displays resonances attributable to coordinated phosphine ligand (ESI Fig.S15 †).Heating the sample from −15 °C to +25 °C in the NMR probe enabled the collection of 1 H data at various temperatures.The most obvious change in the spectra with time at −15 °C and then upon heating was the increase in the intensity of the singlet at 4.56 ppm which is attributable to dissolved H 2 and the corresponding reduction in the intensity of the coordinated hydride signal (ESI Fig.S16 †).This series of experiments required the preparation of several different samples, as it was noted that 1 appears to undergo decomposition/reactions in these highly concentrated solutions, ultimately resulting in precipitation of a black material after approximately 3 hours at 25 °C.This rapid

Computational study of BH 4 −
decomposition triggered via ligand loss in the clusters [Ag 3 (H)(BH 4 )L Me n ] + (n = 1-3) To better understand how the number of diphosphine ligands, n, in the clusters, [Ag 3 (H)(BH 4 )L n ] + , influence the competition between decomposition of the ligated BH 4 − (eqn (11)) versus loss of a ligand (eqn (
Finally, the last three orbitals on Ag centres overlap with three filled B-H σ orbitals of BH 4 − , leading to coordination of BH 4 − in µ 3 -form.The 50 valence electron [Ag 3 (H)(BH 4 )L 3 ] + cluster is not expected to have direct metal-metal interactions, consistent with other related M 3 L 6 clusters where M has a d 10 electron configuration. 35This is highlighted by an examination of its HOMO, which suffers from the Ag-Ag anti-bonding interactions derived from the silver d xz orbitals (Fig. 6).The short Ag-Ag bond distances in [Ag 3 (H)(BH 4 )L 3 ] (2.971-2.993Å) can be mainly rationalised by the presence of the hydride ligand that creates the four-centre two-electron bonds with the Ag centres.A similar metal-metal bond distance was also observed by Harvey et al. in the [Pd 3 (H 2 PCH 2 PH 2 ) 3 (CO)(H)] − cluster (2.932 Å)

Scheme 2
Scheme 2 Direct link established between mixed hydride/borohydride clusters (Black), dihydride clusters (Blue) and "all metal" clusters (Red) based on gas-phase unimolecular fragmentation reactions of mass selected clusters reported here using CID and in ref. 32 using laserinduced dissociation (LID).