Page 1 of 29
Int. J. Mass Spectrom. Manuscript submitted to Prof. V.M. Bierbaum Special Issue
Guest Editors: Profs Mary Rodgers and Peter Armentrout
Handling Editor: Prof. Michael Bowers
Submission Date: March 1, 2014 through April 30, 2014.
REVISED Version: 1 July 2014
Bis(dimethylphosphino)methane-Ligated Silver
Chloride, Cyanide and Hydride Cluster Cations:
Synthesis and Gas-Phase Unimolecular Reactivity‡
Alex J. Clark a,b, Athanasios Zavras a,b, George N. Khairallah a,b* and Richard A.J. O’Hair a,b*
a
School of Chemistry and Bio21 Molecular Science and Biotechnology Institute, The
University of Melbourne, Parkville, Victoria 3010, Australia
b
The University of Melbourne, Parkville, Victoria 3010, Australia
‡ Dedicated to Prof. Ronnie Bierbaum on the occasion of her 65th Birthday and in recognition
of her important contributions to gas-phase ion chemistry and service to the American
Society of Mass Spectrometry.
* Correspondence should be addressed to Richard O’Hair ([email protected]), George
Khairallah ([email protected])
Abstract
Page 2 of 29
Electrospray ionization mass spectrometry (ESI/MS) of mixtures of AgNO3 or AgBF4
with the capping ligand Me2P-(CH2)-PMe2 (dmpm, L) in a solution of 1:1
methanol:chloroform or acetonitrile revealed the formation of: dinuclear clusters [Ag2L2]2+,
[Ag2(L-H)L]+, and [Ag2ClL2]+; trinuclear clusters primarily as [Ag3Cl2L3]+; and the
tetranuclear cluster [Ag4Cl3L3]+. Addition of NaBH4 to these solutions of dmpm-protected
clusters results in the formation of the hydride cluster cations [Ag2HL2]+, [Ag3HL3]2+,
[Ag3HL4]2+, [Ag3H(CN)L3]+ and [Ag3HClL3]+ as determined by ESI/MS. Use of NaBD4
confirmed that the borohydride is the source of the hydride in these clusters.
The gas-phase unimolecular chemistry of selected clusters was examined in a LTQFT Hybrid Linear Ion Trap (LIT) Mass Spectrometer. Low-energy collision induced
dissociation (CID) was performed on the clusters [Ag3Cl2L3]+, [Ag3HClL3]+, [Ag3H(CN)L3]+,
[Ag3HL3]2+, [Ag3HL4]2+ and their nascent fragments. Neutral ligand loss, core fission and
ligand activation were observed to depend on the cluster stoichiometry. Thus [Ag3XYL3]+
(X,Y = Cl2; X = H, Y = Cl or CN) mainly fragment via ligand loss, [Ag3XYL2]+ undergoes
both ligand loss and core fission, [Ag3XYL]+ undergoes core fission and [Ag2YL]+ undergoes
ligand activation. Small yields of reductive elimination of HY are observed for [Ag3HYL2]+
(Y = Cl and CN). Density functional theory calculations were used to calculate the energetics
of the optimized structures for all parent, fragment ions and neutrals and to estimate the
reaction endothermicities. Generally there is reasonable agreement between the most
abundant product ion formed and the predicted endothermicity, except for the reductive
elimination reactions. Finally, electron induced dissociation (EID) of [Ag3HL3]2+ and
[Ag3HL4]2+ gave similar fragmentation channels to CID.
Running title: Bis(dimethylphosphino)methane Ligated Silver Cluster Cations.
Page 3 of 29
Keywords: Ligand-protected silver clusters; bis-phosphine; Me2P(CH2)PMe2; gas-phase; mass
spectrometry; collision-induced dissociation; electron-induced dissociation; DFT calculations; silver
hydrides
1. Introduction:
Intense interest in developing routes for the synthesis of ligand-capped coinage metal
clusters is driven by a desire to produce monodisperse nanoclusters [1] for applications in
materials science and catalysis [2]. Electrospray ionization mass spectrometry (ESI/MS) has
proven to be valuable in monitoring the solution-phase growth [3,4] of coinage metal
nanoclusters and to direct their bulk synthesis [3r,5] as well as to monitor their solution and
gas phase reactivity [3,5]. For example, these MS studies have revealed that the growth of
bis-phosphine capped gold nanoclusters, upon reduction of Au(I) salts by sodium
borohydride, is complex and depends on a range of factors including: (i) the solvent(s), (ii)
the type of reductant, (iii) the gold salt, (iv) the steric and geometric constraints of the bisphosphine and, (v) the molar ratio of the Au(I) salt to the bis-phosphine.
Previous ESI/MS studies of the mixture formed upon reaction of silver salts with
sodium borohydride in the presence of bis(diphenylphosphino)methane (dppm) revealed the
formation of the following silver hydride cluster cations: [Ag3H(dppm)3]2+,
[Ag3HCl(dppm)3]+ and [Ag10H8(dppm)6]2+.[3r,6] This prompted the mass spectrometry
directed synthesis and structural characterization of the clusters [Ag3HCl(dppm)3]BF4,
[Ag3Cl2(dppm)3]BF4 and [Ag3H(dppm)3](BF4)2 . In addition, the gas-phase unimolecular
chemistry of [Ag3H(dppm)3]2+, [Ag3HCl(dppm)3]+ and [Ag3Cl2(dppm)3]+, examined using
low-energy collision induced dissociation (CID) and electron-induced dissociation (EID)
Page 4 of 29
experiments, highlighted how the cluster structure influences the competition between ligand
loss and cluster fragmentation.
Here we carry out related studies using a less sterically demanding ligand,
bis(dimethylphosphino)methane, Me2P-(CH2)-PMe2 (henceforth referred to as dmpm or L).
The lesser bulk of this ligand raises the possibility of forming different clusters, as well as
making DFT calculations amenable to determine potential structures of cluster ions and their
fragmentation products.
2. Experimental:
The following suppliers were used to purchase chemicals, which we used without further
purification: (i) Aldrich: bis(dimethylphosphino)methane (dmpm, L) (97%), sodium
borodeuteride (98%), silver tetrafluoroborate (98%), (ii) Chemsupply: silver nitrate (99%),
(iii) Ajax Finechem: sodium borohydride (97%), (iv) Merck: chloroform and methanol (AR
grade for synthesis and HPLC grade for ESI/MSn experiments), (v) Burdick & Jackson:
acetonitrile (HPLC grade).
Cluster synthesis: Caution is required in handling dmpm, which is unstable in air. Thus, all
syntheses were performed either in a nitrogen purged glove bag or a glove box. Nanoclusters
were produced by dissolving a silver salt (AgNO3, 34 mg, 0.20 mmol or AgBF4, 39 mg, 0.20
mmol) in 20 mL of solvent (acetonitrile or a 1:1 methanol:chloroform mixture), followed by
addition of dmpm (32 µL, 0.20 mmol). After allowing this solution to stir for ca. 5 minutes,
sodium borohydride (36 mg, 1.0 mmol) was added. Following the addition of sodium
borohydride, and depending on the conditions, some solutions remained clear, while others
became coloured on a spectrum from pale yellow to deep red. In order to determine the
Page 5 of 29
source of hydride in the clusters, the same procedure was followed, but by replacing sodium
borohydride with sodium borodeuteride (42 mg, 1.0 mmol).
Mass spectrometry: Mass spectrometry experiments were conducted by diluting solutions
from the silver cluster synthesis above with either methanol or acetonitrile, to a concentration
typically between 10-50 μM. The diluted solution was injected at a sample flow rate of 5 μL.
min-1 into the Finnigan ESI source of a LTQ FT Hybrid Linear Ion Trap (LIT) Mass
Spectrometer (Thermo, Bremen, Germany) described in detail elsewhere [7]. Typical
electrospray source conditions involved needle potentials of 3.8 – 5.2 kV with the capillary
temperature set at ca. 250 °C. The tube lens voltage was set to 40.0 V, and the capillary
voltage was set to 20.0 V. For the unimolecular reactions, the silver cluster cation of interest
was mass selected with a window of 15 m/z units and then subjected to either CID or EID. In
CID experiments, the normalized collision energy was selected to deplete the parent ion to a
relative abundance generally less than 20% for an activation Q of 0.25 and activation time of
30 ms with the scan mass range set between the low m/z cut-off for an ion trap and m/z 2000.
For high-resolution mass analysis and the EID experiments, the silver cluster cations were
transferred to the FT-ICR cell (< 1.5x10-9 Torr) via the ion optics transfer region (~2x10-7
Torr). The EID experiments were carried out as previously described [7]. Briefly, an
indirectly heated emitter cathode located downstream of the FT-ICR cell was used to supply
the FT-ICR cell with low-energy electrons whose energy is determined by the potential
difference between the emitter cathode with an EID offset of -3.2 V and the grid positioned in
front of the cathode, which is variable. The silver cluster cations were bombarded with
electrons of energy 11 - 14 eV for 70 ms.
Page 6 of 29
Density functional theory calculations: Structures of key cluster ions observed by ESI/MS
and subjected to CID were optimized using the Gaussian 09B01 program package.[8]
Frequencies were calculated to confirm each structure as a local minimum on the potential
energy surface. Calculations were performed at the B3LYP [9,10] level of theory using a
hybrid basis set of SDD (Ag) and 6-31+G(d) (all other atoms). All calculated structures are
contained in the Supporting material, while Fig. 1 compares the structures of [Ag3HClL3]+
and [Ag3Cl2L3]+ determined via X-ray crystallography for L = dppm to those optimized from
the DFT calculations for L = dmpm. Thus, by comparing the core metals, the average Ag-Ag
bond length appears to be longer in the case of dmpm (3.61 and 3.05 Å) when compared to
the equivalent bonds in dppm (3.21 and 2.90 Å). When comparing the ligands, we can find
that the average Ag-P bond lengths were similar in all cases (2.44 ≤ Ag-P ≤ 2.53 Å), as were
the Ag-Cl bond lengths, which ranged from 2.69 Å in the case of [Ag3Cl2(dppm)3]+ to 2.86
Å in the case of [Ag3HCl(dppm)3]+. Finally, the bite angles (P-C-P) of the dmpm and dppm
ligands was compared and found to have comparable values (111° < P-C-P < 116°), with
those for dmpm being slightly more obtuse than dppm.
Page 7 of 29
Structure Ag-Ag
Ag-P
3.05
2.53
dmpm (a)
3.61
2.49
(b)
2.90
2.44
dppm (c)
3.21
2.45
(d)
Values are the average length of all appropriate bonds.
Ag-Cl
2.83
2.80
2.86
2.69
Ag-H
1.95
1.89
-
Fig. 1. Comparison of the structures of (a) [Ag3HClL3]+ and (b) [Ag3Cl2L3]+ (L = Me2P(CH2)-PMe2), determined from DFT calculations carried out at the B3LYP/SDD6-31+G(d)
level of theory, with (c) [Ag3HClL*3]+ and (d) [Ag3Cl2L*3]+ (L* = Ph2P-(CH2)-PPh2),
determined via X-ray crystallography of the salts [Ag3HClL*3]BF4 and [Ag3Cl2L*3]BF4. Key
bond lengths in the table are given in angstroms. The phenyl/methyl groups and counter
anions (for dppm crystal structures) are omitted for clarity.
Page 8 of 29
3. Results & Discussion:
3.1 Types of cationic silver clusters formed with dmpm:
The following sections describe: (i) the Ag(I) species formed upon mixing silver salt
(AgNO3 or AgBF4) and dmpm in solution (1:1 MeOH/CHCl3 or MeCN), (ii) the clusters
formed after addition of sodium borohydride to the mixture, and (iii) the types of clusters
formed with dmpm in comparison to those formed with dppm.
3.1.1 Cationic clusters formed upon addition of dmpm to either AgNO3 or AgBF4 in
solution:
100
a)
+
Relative intensity (%)
80
+
[Ag2ClL2]
[Ag3Cl2L3]
523
803
+
60
[Ag(LO)2]
40
411
+
[Ag4Cl3L3]
20
947
0
200
100
300
400
500
600
700
800
900
1000
b)
80
60
2+
[Ag2L2]
244
+
[Ag2ClL2]
40
523
20
0
200
300
400
+
[Ag2(NO3)L2]
550
500
600
+
[Ag3Cl2L3]
803
700
800
900
1000
m/z
Fig. 2. Selected ESI/MS spectra of: (a) a solution of AgNO3 and dmpm in 1:1 MeOH/CHCl3.
(b) a solution of AgBF4 and dmpm in MeCN. Spectra were obtained after ca 5 minutes of
mixing. The m/z values shown are of the most intense peak in the cluster.
Page 9 of 29
Mixing an Ag(I) salt and dmpm (L) in a solution primarily resulted in the formation
of dinuclear and trinuclear clusters, often incorporating halide ions. As has been noted by
other workers [11] the dmpm ligand is challenging to work with due to its pyrophoric nature
and stench, and can yield variations in the outcomes for the synthesis of transition metal
complexes. Indeed, we found that different syntheses yielded different relative ion intensities
depending, amongst others, on the exact concentration of species in solution, time the
reaction mixture was analyzed and the inertness of the reaction atmosphere used in the
synthesis. In some cases, if the dmpm was briefly to exposed to oxygen prior to mixing with
the silver salt, partial oxidation occurred, as evidenced by the formation of L+16 ions in the
ESI/MS (e.g. [Ag(LO)2]+ in Figure 2a).
Fig. 2 and Supplementary material Fig. S1 are typical representations of the ions
observed and the assignments confirmed via high-resolution mass spectrometry (HRMS) and
a consideration of the isotope distribution (data not shown). Using 1:1 methanol:chloroform
as solvent, the most abundant clusters observed were chloride-containing, appearing
primarily as [Ag2ClL2]+ (m/z 523) and [Ag3Cl2L3]+ (m/z 802) (Fig. 2a), with traces of the
larger cluster [Ag4Cl3L3]+. The presence of these chloride peaks is likely from trace HCl
impurities in the chloroform. Changing the solvent system from 1:1 MeOH/CHCl3 to
acetonitrile, resulted in a substantial decrease in the relative abundance of chloride clusters.
In acetonitrile (Fig. 2b and Supplementary material Fig. S1) the observed silver clusters
included: [Ag2L2]2+ (m/z 244), its deprotonated homologue [Ag2(L-H)L]+ (m/z 487),
[Ag2L3]2+ (m/z 312), [Ag2(BF4)L2]+ (m/z 575), [Ag2ClL2]+ (m/z 523), [Ag2(NO3)L2]+ (m/z 550)
and [Ag3Cl2L3]+ (m/z 803) amongst others. Additionally, cyanide-containing clusters were
observed including: [Ag3(CN)2L3]+ (m/z 783) and [Ag3(CN)ClL3]+ (m/z 794). This latter
observation suggests that the C-CN bond in acetonitrile has been activated, likely by the
presence of silver. The observation of metal cyanide clusters in the gas phase has been
Page 10 of 29
previously reported, and included the silver cyanide cluster cations [AgnCNn-1]+ (n = 1-4)
[12]. In addition, previous reports on the activation of C-CN bonds by a metal includes for
instance: (a) the reaction of acetonitrile with a zerovalent nickel bis-(dialkylphosphino)ethane
fragment (alkyl = methyl, isopropyl) which proceed via a C-CN bond activation in THF [13]
and; (b) the observation of the Ag4CN+ cluster upon CID of a complex formed when mixing a
silver salt and glycine [14] .
3.1.2 Formation of cationic hydride-containing clusters upon addition of NaBH4:
ESI/MS analysis after addition of sodium borohydride to the silver/ligand mixture in a
1:1 methanol:chloroform solutions (Fig. 3a), consistently showed the formation of peaks
corresponding to the hydride-containing species [Ag2HL2]+ (m/z 489), [Ag3HClL3]+ (m/z 769)
and [Ag3HL3]2+ (m/z 366), amongst others. Chloride containing peaks, as well as peaks due to
oxidized ligand are also observed. Similarly, for acetonitrile solutions (Fig. 3b), addition of
NaBH4 causes the formation of a range of species whose abundance varied substantially over
time. In addition to the three hydride containing clusters formed for MeOH:CHCl3,
acetonitrile solutions also generated [Ag3HL4]2+ (m/z 434) as well as the cyanide-containing
cluster [Ag3H(CN)L3]+ (m/z 758). Consistent with other bisphosphine-protected silver
clusters [3r,6], there was no observation of partial cluster reduction following addition of
borohydride (e.g. [Ag3L3]+).
In order to determine the source of the hydrides in the clusters observed, the reaction
of sodium borodeuteride (NaBD4) with a solution of AgBF4 and dmpm in 1:1
methanol:chloroform was monitored. The resulting peaks corresponded to the deuteride
clusters [Ag2DL2]+, [Ag3DClL3]+ and [Ag3DL3]2+ (Supplementary material Fig. S2),
Page 11 of 29
confirming that NaBH4 is the source of the hydrides, analogous to previously observed
hydride clusters [3r,6].
100 a)
+
[Ag2ClL2]
80
2+
Relative intensity (%)
60
40
+
[Ag3HClL3]
523
[Ag3HL3]
366
803
20
0
200
+
[Ag3Cl2L3]
769
489
300
400
500
600
700
800
900
1000
100 b)
2+
[Ag3HL4]
80
2+
60
40
758
366
20
0
200
+
[Ag3H(CN)L3]
434
[Ag3HL3]
803
489
300
400
500
600
700
800
900
1000
m/z
Fig. 3. Selected LTQ mass spectra of solutions after addition of sodium borohydride. (a)
AgNO3 and dmpm in MeOH:CHCl3, ca 2 hours after addition of NaBH4. (b) AgBF4 and
dmpm in MeCN, ca 2 hours after addition of NaBH4. The m/z values shown are of the most
intense peak in the cluster.
3.1.3 Comparison to data on types of silver nanoclusters formed with dppm:
It is worth comparing the types of clusters formed for the reaction of silver salts with sodium
borohydride with either Ph2P-(CH2)-PPh2 (dppm) or Me2P-(CH2)-PMe2 (dmpm) capping
ligands (Table 1).
Page 12 of 29
In the case of dmpm, a richer variety of clusters are observed prior to the addition of NaBH4.
Whereas, the dinuclear clusters, [Ag2L2]2+ [15] and [Ag2(BF4)L2]+ are formed in both cases,
in the case of dmpm, clusters such as [Ag2L(L-H)]+ , the higher nuclearity cluster
[Ag4Cl3L3]+ as well as [Ag2L3]2+ were observed. This latter ion is likely to be the dication of
[Ag2L3](BF4)2 previously characterized via X-ray crystallography.[16]
Following the addition of sodium borohydride, the silver hydride clusters: [Ag3HL3]2+,
[Ag3HL4]2+ and [Ag3HClL3]+ were also observed with both ligands. However, the larger
cluster, [Ag10H8L6]2+, is only observed for dppm. The cyanide containing clusters are more
abundant in the case of dmpm.
Table 1: Comparison of the types of silver nanoclusters observed by ESI/MS which are formed via
the addition of sodium borohydride to Ag(I) salts in the presence of bis-phosphine ligand as a function
of the ligand. A ✓ indicates that the ion was observed (and reported). A blank box indicates that the
ion was not observed (reported). dmpm = bis(dimethylphosphino)methane, dppm =
bis(diphenylphosphino)methane.
Cluster
Pre NaBH4 addition
[Ag2L2]2+
[Ag2(BF4)L2]+
[Ag2L(L-H)]+
[Ag2L3]2+
[Ag(MeCN)L]+
[Ag4Cl3L3]+
Post NaBH4 addition
[Ag3HL3]2+
[Ag3HL4]2+
[Ag4Cl3L3]+
[Ag3H(CN)L3]+
[Ag3HClL3]+
[Ag3Cl2L3]+
[Ag10H8L6]2+
L = dmpm
L = dppm
Page 13 of 29
3.2 Gas-phase unimolecular fragmentation reactions of silver nanoclusters via CID:
Three types of fragmentation channels were observed in the CID spectra of
[Ag3Cl2L3]+; [Ag3HClL3]+; [Ag3H(CN)L3]+; [Ag3HL3]2+ (Fig. 4), including: (i) loss of a
neutral dmpm ligand (e.g.; Fig. 4a, c.f. eq. 1), (ii) fission of the cluster core with loss of
AgX, where X = Cl, CN, or H (e.g.; Fig. 4c, c.f. eq. 6), and (iii) fragmentation of the dmpm
ligand to generate a neutral MePCH2 fragment (e.g.; c.f. eq. 13). The fragmentations
observed together with their DFT calculated energies are presented in the section below. The
major fragmentation pathways observed together with the DFT calculated minimum energy
structure of each fragment and the enthalpy of each reaction are given in schemes 1-4.
100
a)
+
[Ag3Cl2L2]
80
667
60
40
20
Relative intensity (%)
+
[Ag3Cl2L]
+
[Ag2ClL2]
+
[Ag3Cl2L3]
531
523
803
0
100
*
b)
+
[Ag3Cl2L]
80
531
60
40
20
+
[Ag3Cl2L2]
667
[Ag2ClL]
[Ag3Cl2L]+. The mass selected precursor
387
0
100
c)
80
+
[Ag2ClL]
387
60
+
40
[AgL]
243
+
[Ag3Cl2L]
20
0
200
300
400
trinuclear silver dmpm clusters: (a)
[Ag3Cl2L3]+, (b) [Ag3Cl2L2]+, and (c)
*
+
Fig. 4. Low energy CID spectra for the
*
531
500
600
m/z
700
800
ion is designated by a *.
Page 14 of 29
In the case of [Ag3Cl2L3]+ (m/z 803),the CID spectrum (Fig. 4a) is dominated by the
loss of a ligand to generate [Ag3Cl2L2]+ (m/z 667) (eq. 1). Smaller fragments corresponding to
loss of 2 ligands ([Ag3Cl2L]+ (m/z 531)) (eq. 2) and loss of a ligand + AgCl ([Ag2ClL2]+ (m/z
523)) are also observed (eq. 3 and 4). This channel can be considered as a sequential loss of a
ligand (L) followed by loss of AgCl (eq. 3) or loss of a ligated AgCl (eq. 4). The calculated
enthalpies reveal that loss of one ligand (eq. 1) requires the least energy (98 kJ/mol), in
agreement with the experimental observation that this channel is the most abundant. In
addition, the calculated enthalpy for the loss of LAgCl (eq. 4) is less endothermic than loss of
2 ligands, also in line with experimental observations.
[Ag3Cl2L3]+
→ [Ag3Cl2L2]+ + L
→ [Ag3Cl2L]+ + 2L
→ [Ag2ClL2]+ + L + AgCl
→ [Ag2ClL2]+ + LAgCl
98 kJ/mol
234 kJ/mol
253 kJ/mol
113 kJ/mol
(1)
(2)
(3)
(4)
The main product ion, [Ag3Cl2L2]+, (Fig. 4b) gives rise to the loss of a ligand ([Ag3Cl2L]+ (m/z 531) (eq. 5); loss of AgCl ([Ag2ClL2]+ (m/z 523) (eq. 6)) and; loss of a ligand +
AgCl ([Ag2ClL]+ (m/z 387) (eq. 7 and 8)). The theoretically calculated enthalpies in this case,
indicate that the major fragment observed ([Ag3Cl2L]+ ) required the least energy (136 kJ/mol
versus 155 and 160 kJ/mol). In contrast, whereas loss of a ligand + AgCl (eq. 7 and 8) is less
favoured thermodynamically (160 kJ/mol), it is more prominent than loss of AgCl (eq. 6)
(155 kJ/mol). This discrepancy could potentially be due to a higher energy transition state
(kinetic barrier) associated with the loss of AgCl due to the breaking of metal-metal bonds
and rearrangement of the cluster. Another potential explanation for this observation, is the
possibility that L+AgCl loss occurs by two separate mechanisms; a one-step mechanism (loss
Page 15 of 29
of AgClL, eq. 8) and, in a two step mechanism, as a secondary fragment (loss of L in eq. 5,
then loss of AgCl in eq. 9).
[Ag3Cl2L2]+
→ [Ag3Cl2L]+ + L
→ [Ag2ClL2]+ + AgCl
→ [Ag2ClL]+ + L + AgCl
→ [Ag2ClL2]+ + LAgCl
136 kJ/mol
155 kJ/mol
301 kJ/mol
160 kJ/mol
(5)
(6)
(7)
(8)
Further CID experiments were performed on [Ag3Cl2L]+ (Fig. 4c) generating
fragments at m/z 387 corresponding to [Ag2ClL]+ concomitant with the loss of AgCl (eq. 9)
and m/z 243 corresponding to [AgL]+ concomitant with the loss of neutral Ag2Cl2 (or 2 AgCl)
(eqs. 10 and 11). Calculated enthalpies clearly show that the favoured experimental
fragmentation channel required the least energy (165 versus 236 kJ/mol).
[Ag3Cl2L]+
→ [Ag2ClL]+ + AgCl
→ [AgL]+ + Ag2Cl2
→ [AgL]+ + 2AgCl
165 kJ/mol
236 kJ/mol
384 kJ/mol
(9)
(10)
(11)
Finally, the silver dimer product in eq. 9 ([Ag2ClL]+) was mass selected for a further
stage of CID (Supplementary material Fig. S4c). Products corresponding to the loss of AgCl
([AgL]+ m/z 243) (eq. 12) and ligand fragmentation with the loss of MePCH2 and AgCl
([AgPMe3]+ m/z 183) (eq. 13) were observed, with the former being preferred both
theoretically and experimentally.
[Ag2ClL]+
→ [AgL]+ + AgCl
→ [Ag(PMe3)]+ + AgCl + MePCH2
220 kJ/mol
362 kJ/mol
(12)
(13)
The major fragmentation pathways of [Ag3Cl2L3]+ and its products are highlighted in scheme
1.
Page 16 of 29
+97.7 kJ/mol

-L
[Ag3Cl2L3]+
+136 kJ/mol

-L
[Ag3Cl2L2]+
+165 kJ/mol

-AgCl
[Ag3Cl2L]+
+220 kJ/mol

-AgCl
[Ag2ClL]+
[AgL]+
Scheme 1. DFT calculations on the major fragmentation pathway of [Ag3Cl2L3]+ and its
products under conditions of CID. The neutral losses and enthalpies of reactions are listed
under and above the corresponding arrow.
[Ag3HClL3]+ (m/z 769) produces similar fragmentation channels, with its CID
spectrum (Supplementary material Fig. S3) dominated by the loss of a ligand to generate
[Ag3HClL2]+ (m/z 633) (eq. 14) and a less intense fragment corresponding to loss of 2 ligands
([Ag3HClL]+ (m/z 497)) (eq. 15) also occur. DFT calculated enthalpies of these reactions
show that the loss of 1 ligand requires the least energy (64 kJ/mol), consistent with this being
the major channel.
[Ag3HClL3]+ → [Ag3HClL2]+ + L
→ [Ag3HClL]+ + 2L
64 kJ/mol
181 kJ/mol
(14)
(15)
Page 17 of 29
CID of [Ag3HClL2]+ is dominated by ligand loss to yield [Ag3HClL]+ (m/z 497) (eq.
16). Minor ions corresponding to loss of AgH to generate [Ag2ClL2]+ (m/z 523) (eq. 17) and
loss of (AgH + L) to generate [Ag2ClL]+ (m/z 387) (eq. 18) were observed, in addition to loss
of HCl to yield the reduced cluster [Ag3L2]+ (m/z 595) (eq. 19) containing mixed-valent Ag(I)
and Ag(0). DFT calculations reveal that the major fragment ion at m/z 497 is the least
endothermic channel (117 kJ/mol), with the 2 other observed channels requiring more energy
(144 and 202 kJ/mol), in agreement with experiments. In contrast, although the loss of HCl
appears to be only a few kJ/mol more endothermic that the loss of ligand, the reductive
elimination was observed to be a very minor pathway. Given that the hydride and chloride
ions are calculated to be on opposite faces of the Ag3 triangle, the minor elimination of HCl is
likely due to a kinetic barrier associated with rearrangement of the cluster to a geometry
suitable for reductive elimination.
[Ag3HClL2]+ → [Ag3HClL]+ + L
→ [Ag2ClL2]+ + AgH
→ [Ag2ClL]+ + AgH + L
→ [Ag3L2]+ + HCl
→ [Ag2ClL]+ + LAgH
(20)
117 kJ/mol
(16)
144 kJ/mol
(17)
290 kJ/mol
(18)
129 kJ/mol
(19)
202 kJ/mol
CID of [Ag3HClL]+, (Fig. 5), results in loss of AgH to generate [Ag2ClL]+ (m/z 387)
(eq. 21) and loss of AgCl to generate [Ag2HL]+ (m/z 353) (eq. 22), with the former
dominating, just as has been reported for the related dppm cluster [22], suggesting a
preference for loss of a silver hydride rather than a silver chloride. Minor peak corresponding
to the loss of both AgH and AgCl ([AgL]+ (m/z 243)) (eq. 23 and 24) is also observed.
Theory predicts that loss of AgH is the least endothermic requiring 173 kJ/mol, whereas loss
of AgCl requires 204 kJ/mol. The channel generating [AgL]+ is predicted be the most
endothermic requiring 392 kJ/mol. This latter result may suggest that it is likely a secondary
Page 18 of 29
fragment. These calculated enthalpies are consistent with the experimental relative
abundances observed experimentally. CID of [Ag2ClL]+) generated the same fragments to
those seen in eqs 12 and 13 (Supplementary material Fig. S3).
[Ag3HClL]+
→ [Ag2ClL]+ + AgH
→ [Ag2HL]+ + AgCl
→ [AgL]+ + Ag2HCl
→ [AgL]+ + AgH+ AgCl
173 kJ/mol
204 kJ/mol
428 kJ/mol
392 kJ/mol
(21)
(22)
(23)
(24)
The major fragmentation pathways of [Ag3HClL3]+ and its products are highlighted in
Scheme 2.
+63.7 kJ/mol

-L
[Ag3HClL3]+
+117 kJ/mol

-L
[Ag3HClL2]+
+173 kJ/mol

-AgH
[Ag3HClL]+
[Ag2ClL]+
Scheme 2. DFT calculations on the major fragmentation pathway of [Ag3HClL3]+ and its
products under conditions of CID. The neutral losses and enthalpies of reactions are listed
under and above the corresponding arrow.
CID of [Ag3H(CN)L3]+ results in similar fragmentation reactions (eqs. 25-38) to those
occurring for [Ag3HClL3]+. Reductive elimination of HCN from [Ag3H(CN)L2]+ (eq. 27) is
predicted to be the most thermodynamically favoured pathway (37 kJ/mol), however, it is a
Page 19 of 29
very minor channel experimentally. Conversely, the loss of a ligand (eq. 28) from this same
ion ([Ag3H(CN)L2]+) is the least thermodynamically favourable (411 kJ/mol), but the most
intense experimentally. This suggest that kinetic barriers are prominent in the fragmentation
of this cluster.
+72.1 kJ/mol

-L
[Ag3H(CN)L3]+
+137 kJ/mol

-AgH
[Ag3H(CN)L]+
[Ag3H(CN)L2]+
[Ag2(CN)L]+
+410 kJ/mol

-L
+212 kJ/mol

-AgCN
[AgL]+
Scheme 3. DFT calculations on the major fragmentation pathway of [Ag3H(CN)L3]+ and its
products under conditions of CID. The neutral losses and enthalpies of reactions are listed
under and above the corresponding arrow.
[Ag3H(CN)L]+ fragments in a similar fashion to the chloride complex [Ag3HClL]+ (Fig. 5). In
this case both AgH and AgCN losses are observed (eqs. 33-36), with AgH loss as the major
fragmentation pathway (eq. 33) (Fig. 5a) similar to what was previously observed.[6] DFT
calculations suggest that loss of AgCN should be favoured, as this pathway is less
Page 20 of 29
endothermic. However, the contradicting experimental observations suggest that a higherenergy transition state for AgCN loss is likely.
Figure 5: Competition between AgH and AgX loss in CID of [Ag3H(X)L]+: (a) X = CN; (b) X = Cl.
The mass selected precursor ion is designated by a *. The m/z values shown are of the most
intense peak in the cluster.
[Ag3H(CN)L3]+
[Ag3H(CN)L2]+
→ [Ag3H(CN)L2]+ + L
→ [Ag3H(CN)L]+ + 2L
→ [Ag3L2]+ + HCN
→ [Ag3H(CN)L]+ + L
→ [Ag2HL]+ + AgCN + L
→ [Ag2HL]+ + LAgCN
72 kJ/mol
483 kJ/mol
37 kJ/mol
410 kJ/mol
297 kJ/mol
155 kJ/mol
(25)
(26)
(27)
(28)
(29)
(30)
Page 21 of 29
→ [Ag3(CN)L]+ + AgH + L
→ [Ag3(CN)L]+ + LAgH
273 kJ/mol
184 kJ/mol
(31)
(32)
[Ag3H(CN)L]+
→ [Ag2(CN)L]+ + AgH
→ [Ag2HL]+ + AgCN
→ [AgL]+ + AgH + AgCN
→ [AgL]+ + Ag2H(CN)
137 kJ/mol
113 kJ/mol
391 kJ/mol
58 kJ/mol
(33)
(34)
(35)
(36)
[Ag2(CN)L]+
→ [AgL]+ + AgCN
212 kJ/mol
+
→ [Ag(PMe3)] + AgCN + MePCH2 354 kJ/mol
(37)
(38)
The main fragmentation channel observed in the CID spectrum of [Ag3HL4]2+ (m/z
434) (Supplementary materials Fig. S5) was ligand loss to yield [Ag3HL3]2+ (m/z 366) (eq.
39), a species also directly observed via ESI/MS. A minor channel corresponding to charge
separation was also observed (eq. 40). Although this channel seems to be favoured
thermodynamically (-45 kJ/mol), a kinetic (Coloumb) barrier must be associated with this
pathway. Thus, in addition to breaking Ag-Ag bonds, this ion pair formation requires ligands
to detach from the silver ions for bond breaking to occur. This may not be preferred since: (i)
the electron-deficient Ag3 ring leads to strong ligand interactions with the phosphorus π
electrons, and b) the ligands are arranged so that the ring is held together by entropic
contributions when at least 3 ligands are present. Conversely however, loss of the single
ligand observed experimentally (eq. 39), seems acceptable because the side of the triangle in
[Ag3HL4]2+ (c.f. structure in Scheme 4) carrying the two dmpm ligands would be
experiencing steric strain.
[Ag3HL4]2+
→ [Ag3HL3]2+ + L
→ [Ag2HL2]+ + [AgL2]+
90 kJ/mol
-45 kJ/mol
(39)
(40)
CID of [Ag3HL3]2+ results in cluster core fission (eq. 41) with generation of the
complementary ion pair ([Ag2HL]+ and [AgL2]+), which, based on DFT calculations
Page 22 of 29
predicted to be thermodynamically favourable (6 kJ/mol). Scheme 4 below represents a
summary of the ions observed upon CID of [Ag3HL3]2+ and its fragments, along with the
calculated enthalpies for these processes.
[Ag3HL4]2+
-L  +90
kJ/mol
+6
kJ/mol

[Ag2HL]+
[Ag3HL3]2+
[AgL2]+
+188
kJ/mol

-AgH
+150
kJ/mol

-L
[AgL]+
[AgL]+
Scheme 4. DFT calculations on the major fragmentation pathway of [Ag3HL4]2+, [Ag3HL3]2+
and its major products under conditions of CID. The neutral losses and enthalpies of reactions
are listed under and above the corresponding arrow.
[Ag3HL3]2+
[Ag2HL]+
[AgL2]+
→ [Ag2HL]+ + [AgL2]+
→ [AgL]+ + AgH
→ [AgL]+ + L
6 kJ/mol
188 kJ/mol
150 kJ/mol
3.3 Gas-phase unimolecular fragmentation reactions of doubly charged silver
nanoclusters via EID:
(41)
(42)
(43)
Page 23 of 29
The interactions of the doubly charged silver trimers ([Ag3HL4]2+ and [Ag3HL3]2+)
with electrons were studied. Attempts to react them with low energy electrons to study the
electron-capture dissociation process were unsuccessful. Therefore, the electron energy was
increased to ca 11-14 eV, generating the electron induced dissociation (EID) spectra in Fig. 6
and Supplementary material Fig. S6 (eqs. 44-49 and 50-53). In both ions, fragments similar to
those in the CID spectra were observed (eqs 39, 40 and 41 versus eqs 44, 47 and 50). Other
fragments corresponding to protonation of the ligand (LH+) (e.g.; eqs. 45 and 52) as well as
formation of complimentary ion pairs (eqs. 51) and those concomitant with ligand losses
(eqs. 45, 46, 48, 53) were observed.
Relative intensity (%)
100
2+
[Ag3HL4]
434
80
+
[Ag2HL2]
*
489
60
+
[Ag3L2]
40
20
+
597
[Ag2HL]
+
[AgL]
243
353
367
379
0
250
300
350
400
450
500
550
600
650
m/z
Figure 6. EID mass spectrum of [Ag3HL4]2+. The mass selected precursor ion is designated
by a *. The m/z values shown are of the most intense peak in the cluster.
[Ag3HL4]2+ + e-
→ [Ag3HL3]2+ + L
→ [Ag3L2]+ + LH+ + L
→ [Ag3L]+ + LH+ + 2L
(44)
(45)
(46)
Page 24 of 29
[Ag3HL3]2+
+ e-
→ [Ag2HL2]+ + [AgL2]+
→ [Ag2HL]+ +[AgL2]+ + L
→ [AgL]+ + [Ag2,H,L3]
(47)
(48)
(49)
→ [AgL2]+ + [Ag2HL]+
→ [Ag2HL2]+ + [AgL]+
→ [Ag3L2]+ + LH+
→ [Ag3L]+ + LH+ + L
(50)
(51)
(52)
(53)
4. Conclusions
ESI/MS has been used to probe silver cluster ions formed upon mixing dmpm and an
Ag(I) salts (AgNO3 or AgBF4) followed by addition of NaBH4. The solvent is not innocuous,
with 1:1 methanol:chloroform solutions, generating chloride containing clusters and,
acetonitrile solutions producing cyanide containing clusters. Upon addition of sodium
borohydride, a range of hydride containing clusters was observed. The largest hydride
clusters contained 3 core silver ions in contrast to the case when dppm was used as a ligand
where an Ag10 core was prominent.
When the singly charged trinuclear silver hydride cluster cations were subjected to
CID, loss of the neutral ligand dominates and is coupled to minor secondary fragmentations.
Core fission via loss of AgX (X = H, Cl or CN) only becomes significant for the mono
ligated clusters [Ag3XL]+, while ligand activation only occurs for [Ag2XL]+. The DFT
calculations reveal changes to the cluster geometries upon loss of ligands. Upon sequential
ligand loss from [Ag3XYL3]+ to [Ag3XYL2]+ and then to [Ag3XYL]+, the X and Y anions
move from their positions as face-capping on opposite faces of the silver triangle (c.f. Fig. 1)
to edge-capping on the sides newly exposed by the loss of ligands (e.g.; schemes 1 and 2).
We speculate that this geometry change may be responsible for promoting core fission.
Page 25 of 29
CID on the doubly charged cluster [Ag3HL3]2+, induces cluster core fission and
charge separation as the observed fragmentation channel. EID experiments on this cluster
reveal similar fragmentation pathways to those for the CID, in addition to channels forming
complimentary ion pairs.
5. Acknowledgements:
We thank the Australian Research Council (ARC) for financial support through DP1096134 and the
ARC CoE program. We also thank Dr Stephen Best for access to a glovebox.
6. References:
(1)
(a) R. Jin, Y. Zhu, H. Qian, Quantum-Sized Gold Nanoclusters: Bridging the Gap between
Organometallics and Nanocrystals, Chemistry: A European Journal 17 (2011) 6584-6593; (b)
T. Tsukuda, Toward an Atomic-Level Understanding of Size-Specific Properties of Protected
and Stabilized Gold Clusters, Bulletin of the Chemical Society of Japan 85 (2012) 151-168.
(2)
(a) Corain, G. Schmid, N Toshima (Eds.), Metal Nanoclusters in Catalysis and Materials
Science: The Issue of Size Control, Elsevier, Amsterdam, 2008; (b) Y. Zhang, X. Cui, F. Shi,
Y. Deng, Nano-Gold Catalysis in Fine Chemical Synthesis, Chemical Reviews 112 (2012)
2467–2505; (c) M. Stratakis, H. Garcia, Catalysis by Supported Gold Nanoparticles: Beyond
Aerobic Oxidative Processes, Chemical Reviews 112 (2012) 4469–4506; (d) Y. Lu, W. Chen,
Sub-nanometre sized metal clusters: from synthetic challenges to the unique property
discoveries, Chemical Society Reviews, 41 (2012) 3594-3623.
(3)
(a) M.F. Bertino, Z.-M. Sun, R. Zhang, L.-S. Wang, Facile Syntheses of Monodisperse UltraSmall Au Clusters, Journal of Physical Chemistry B 110 (2006) 21416-21418; (b) D. E.
Bergeron, J. W. Hudgens, Ligand Dissociation and Core Fission from Diphosphine-Protected
Gold Clusters, Journal of Physical Chemistry C 111 (2007) 8195-8201; (c) J. S. Golightly, L.
Gao, A. W. Castleman, Jr., D. E. Bergeron, J. W. Hudgens, R. J. Magyar, C. A. Gonzalez,
Page 26 of 29
Impact of Swapping Ethyl for Phenyl Groups on Diphosphine-Protected Undecagold, Journal
of Physical Chemistry C 111 (2007) 14625-14627; (d) D. E. Bergeron, O. Coskuner, J. W.
Hudgens, C. A. Gonzalez, Ligand Exchange Reactions in the Formation of DiphosphineProtected Gold Clusters, Journal of Physical Chemistry C 112 (2008) 12808-12814; (e) J. M.
Pettibone, J. W. Hudgens, Synthetic Approach for Tunable, Size-Selective Formation of
Monodisperse, Diphosphine-Protected Gold Nanoclusters, Journal of Physical Chemistry
Letters 1 (2010) 2536-2540; (f) Y. Shichibu; K. Konishi, HCl-induced nuclearity convergence
in diphosphine-protected ultrasmall gold clusters Small 6 (2010) 1216-1220; (g) J. M.
Pettibone, J. W. Hudgens, Gold Cluster Formation with Phosphine Ligands: Etching as a SizeSelective Synthetic Pathway for Small Clusters? ACS Nano 5 (2011) 2989-3002; (h) J. W.
Hudgens, J. M. Pettibone, T. P. Senftle, R. N. Bratton, Reaction Mechanism Governing
Formation of 1,3-Bis(diphenylphosphino) propane-Protected Gold Nanoclusters, Inorganic
Chemistry 50 (2011) 10178-10189; (i) J. M. Pettibone, J. W. Hudgens, Reaction network
governing diphosphine-protected gold nanocluster formation from nascent cationic platforms,
Physical Chemistry Chemical Physics 14 (2012) 4142-4154; (j) J. M. Pettibone, J. W. Hudgens,
Predictive Gold Nanocluster Formation Controlled by Metal-Ligand Complexes, Small 8
(2012) 715-725; (k) P. S. D. Robinson, T.-L. Nguyen, H. Lioe, R. A. J. O’Hair, G. N.
Khairallah, Synthesis and Gas-Phase Uni- and Bi- Molecular Reactivity of Bisphosphine
Ligated Gold Clusters, [AuxLy]n+, International Journal of Mass Spectrometry 330-332 (2012)
109-117; (l) Y. Shichibu, K. Suzukia, K. Konishi, Facile synthesis and optical properties of
magic-number Au13 clusters, Nanoscale 4 (2012) 4125-4129; (m) A. Zavras, G. N. Khairallah,
R. A. J. O’Hair, Bis(diphenylphosphino)methane Ligated Gold Cluster Cations: Synthesis and
Gas-Phase Unimolecular Reactivity, International Journal of Mass Spectrometry 354-355
(2013) 242-248; (n) S. Wang, X. Meng, A. Das, T. Li, Y. Song, T. Cao, X. Zhu, M. Zhu, R. Jin,
A 200-fold Quantum Yield Boost in the Photoluminescence of Silver- Doped AgxAu25-x
Nanoclusters: The 13th Silver Atom Matters, Angewandte Chemie International Edition 53
(2014) 2376 –2380.; (o) Z. Wu, E. Lanni, W. Chen, M. E. Bier, D. Ly and R. Jin, Journal Of
Page 27 of 29
the American Chemical Society 131 (2009) 16672-16674.; (p) T. Udayabhaskararao, M. S.
Bootharaju and T. Pradeep, Nanoscale 5 (2013) 9404-9411; (q) R. Langer, B. Breitung, L.
Wuensche, D. Fenske and O. Fuhr, Zeitschrift für anorganische und allgemeine Chemie 637
(2011) 995-1006; (r) A. Zavras, G. N. Khairallah, T. U. Connell, J. M. White, A. J. Edwards, P.
S. Donnelly, R. A. J. O’Hair, “Synthesis, Structure and Gas-Phase Reactivity of a Silver
Hydride Complex [Ag3((Ph2P)2CH2)3(μ3-H)Cl]BF4.”, Angewandte Chemie International
Edition (2013) 8391-8394.
(4)
K.M. Harkness, D.E. Cliffel, J.A. McLean, Characterization of thiolate-protected gold
nanoparticles by mass spectrometry, Analyst 135 (2010) 868-874.
(5)
Y. Kamei, Y. Shichibu, K. Konishi, Generation of Small Gold Clusters with Unique
Geometries through Cluster-to-Cluster Transformations: Octanuclear Clusters with Edgesharing Gold Tetrahedron Motifs, Angewandte Chemie International Edition 50 (2011) 74427445.
(6)
A. Zavras, G. N. Khairallah, T. U. Connell, J. M. White, A. J. Edwards, P. S. Donnelly, R. A. J.
O’Hair “Synthesis, Structural Characterization and Gas-Phase Unimolecular Reactivity of the
Silver Hydride Nanocluster [Ag3((PPh2)2CH2)3(µ3-H)](BF4)2.”, Inorganic Chemistry –
Submitted.
(7)
(a) L. Feketeová, G. N. Khairallah and R. A. J. O’Hair, Intercluster Chemistry of Protonated
and Sodiated Betaine Dimers Upon Collision Induced Dissociation and Electron Induced
Dissociation., European Journal of Mass Spectrometry 14 (2008) 107-110; (b) E. J. H. Yoo, L.
Feketeová, G.N. Khairallah and R. A. J. O’Hair, Unimolecular chemistry of doubly protonated
zwitterionic clusters, Journal of Physical Chemistry A 115 (2011) 4179-4185.
(8)
M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G.
Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P.
Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K.
Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T.
Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers,
Page 28 of 29
K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C.
Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B.
Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J.
Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski,
G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas, J. B.
Foresman, J. V. Ortiz, J. Cioslowski, and D. J. Fox, Gaussian 09 Revision B.01 (2009)
Gaussian, Inc., Wallingford CT.
(9)
A. D. Becke, Density-functional thermochemistry. III. The role of exact exchange, Journal of
Chemical Physics 98 (1993) 5648-5652.
(10) P. J. Stephens, F. J. Devlin, C. F. Chablowski, M. J. Frisch, Ab initio calculation of vibrational
absorption and circular dichroism spectra using density functional force fields, Journal of
Chemical Physics 98(45) (1994) 11623-11627.
(11) M. E. Slaney, D. J. Anderson, M. J. Ferguson, R. McDonald and M. Cowie,
Bis(diethylphosphino)methane As a Bridging Ligand in Complexes of Ir2, Rh2, and IrRh:
Geminal C–H Activation of α-Olefins, Organometallics 31 (2012) 2286-2301.
(12) Ian G. Dance , Philip A. W. Dean , Keith J. Fisher, Gas-Phase Metal Cyanide Chemistry:
Formation, Reactions, and Proposed Linear Structures of Copper(I) and Silver(I) Cyanide
Clusters, Inorganic Chemistry 33(26) (1994) 6261-6269.
(13) Tülay A. Ateşin , Ting Li , Sébastien Lachaize , William W. Brennessel , Juventino J. García ,
and William D. Jones, Experimental and Theoretical Examination of C−CN and C−H Bond
Activations of Acetonitrile Using Zerovalent Nickel, Journal of the American Chemical Society
129(24) (2007) 7562-7569.
(14) G. N. Khairallah, R. A. J. O’Hair, Gas phase synthesis and reactivity of Agn+ and Agn- H+
cluster cations, Dalton Transactions (2005) 2702-2712.
(15) H.H.Karsch and U.Schubert, Complexes with alkyl-substituted phosphinomethanes and
methanides. III. Structural chemistry of three-membered bridging ligands. III.
Page 29 of 29
Bis(dimethylphosphino)methane as a bridging ligand in a dimeric silver(I) complex, Zeitschrift
fuer Naturforschung, Teil B: Anorganische Chemie, Organische Chemie 37 (1982) 186-189.
(16) E.Fournier, A.Decken and P.D.Harvey, Small metal-containing oligomers using M2(dmpm)x
and M2(dmb)y luminescent building blocks (M = Cu, Ag; x = 2, 3; y = 1, 2), European Journal
of Inorganic Chemistry 22 (2004) 4420-4429.