New Routes for Multicomponent Atomically Precise Metal Nanoclusters.

Atomically precise metal nanoclusters (NCs), protected by a monolayer of ligands, are regarded as potential building blocks for advanced technologies. They are considered as intermediates between the atomic/molecular regime and the bulk. Incorporation of foreign metals in NCs enhances several of their properties such as catalytic activity, luminescence, and so on; hence, it is of high importance for tuning their properties and broadening the scope of applications. In most of the cases, enhancement in specific properties was observed upon alloying due to the synergistic effect. In the past several years, many alloy clusters have been synthesized, which show a tremendous change in the properties than their monometallic analogs. However, controlling the synthesis and tuning the structures of alloy NCs with atomic precision are major challenges. Various synthetic methodologies have been developed so far for the controlled synthesis of alloy NCs. In this perspective, we have highlighted those diverse synthetic routes to prepare alloys, which include co-reduction, galvanic reduction, antigalvanic reduction, metal deposition, ligand exchange, intercluster reaction, and reaction of NCs with bulk metals. Advancement in synthetic procedures will help in the preparation of alloy NCs with the desired structure and composition. Future perceptions concerning the progress of alloy nanocluster science are also provided.

Introduction

There is a rich and fascinating history for metals and their alloys.[1−3] Alloys have been known from ancient times; bronze (90% Cu and 10% Sn) was the first to be invented circa BC 3500. Nanoscience has provided a new direction to the subject of metals and their alloys.[4−6] The famous Lycurgus cup known since the 4th century AD is one of the earliest known applications of nanotechnology.[7] It is found to be composed of nanoparticles of Ag, Au, and Cu of 50–100 nm in size, dispersed in a glass matrix.[8] The cup changes its color in different light, greenish in reflected light, and reddish in transmitted light. The nanoalloy particles of different sizes in the cup are responsible for this color change. Michael Faraday observed the unusual behavior of finely divided metal particles in their colloids in 1857.[9] Their optical properties are now known to be due to surface plasmon excitations. Nanoparticles with monolayer protection were prepared over a century later, enabling the synthesis of solid powders of such freely dispersible particles. They also show similar optical characteristics. Nanoclusters (NCs), which are atomically precised, are the smallest analogues of such ligand-protected nanoparticles and they exhibit unique optical, electronic, dielectric, magnetic, and chemical properties.[10,11] These properties are strongly dependent on the composition and geometric variations. The combination of two or more metals in NCs modify several of their properties, often leading to enhancement in the desired properties such as luminescence, catalysis, etc.[12−14] Therefore, the main objective of multicomponent alloy NC science is to explore the rich variety of alloys and their interesting properties.

The synthesis of solution-phase Au NCs protected by phosphine ligands was started in 1969–1970 and a few phosphine-protected bi- and trimetallic NCs were also prepared.[15−19] After a long gap, in 2005, Shichibu et al. synthesized Au25(SG)18 (SG = glutathione) by ligand exchange from phosphine-protected Au NCs.[20] Glutathione was used for the synthesis of gold nanoparticles by Whetten et al.[21] After that, many NCs have been prepared with ligand protection. More than 300 monometallic NCs of Au, Ag, and Cu have been synthesized by various synthetic methods. However, the number of multimetallic NCs is less.[22−33] Among them, the number of bimetallic NCs is higher than trimetallic and tetrametallic ones.[34−42]

During the synthesis of alloy NCs, mixtures of products get formed. For unambiguous characterization of the formed NC, it is important to purify or separate the as-synthesized NCs. Several separation techniques such as gel electrophoresis[43] and size exclusion chromatography (SEC)[44] have made tremendous contributions to the science of metal NCs and their alloys. In 2005, Tsukuda et al. separated the mixture of glutathione-protected Au NCs such as Au10(SG)10, Au15(SG)13, Au18(SG)14, Au22(SG)16, Au22(SG)17, Au25(SG)18, Au29(SG)20, Au33(SG)22, and Au39(SG)24 using polyacrylamide gel electrophoresis (PAGE).[45] Other than SEC, high-performance liquid chromatography (HPLC) and thin-layer chromatography (TLC) are used widely for purification and separation of clusters. The composition of alloy NCs can be probed by high-resolution electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) mass spectrometry (MS).[42,46] In 2005, compositions of a series of glutathione-protected Au NCs separated by PAGE technique were determined mass-spectrometrically. Later, Whetten et al. unambiguously assigned the composition of the most popular NC in this family, Au25(PET)18 (PET = 2-phenylethanethiol), through ESI MS in 2007.[47] Although the composition of alloy NCs can be examined by MS, the exact structure and position of dopants can be understood only by using single-crystal X-ray diffraction (SCXRD). The structure of Au25(PET)18 was solved in 2008 and, in 2014, the first X-ray crystal structure of AgAu25–(PET)18 was determined.[22,47,48] Recently, electron diffraction has been used to resolve the structures of those NCs for which crystallization is difficult.[49] In addition to the experimental methods, computation has also contributed significantly for understanding the structures and properties of these clusters.[22,50−53]

To understand the origin of optical and electronic properties of NCs, the substitution of metal atoms by another element is worthwhile. For example, the emission intensity of rod-like [Au25(PPh3)10(SR)5Cl2]2+ can be enhanced 200-fold by substituting 13 Au atoms with Ag.[54] The doped NC, [Ag13Au12(PPh3)10(SR)5Cl2]2+, having the same structure and nuclearity exhibits 40% photoluminescence quantum yield. An interesting change in the UV–vis spectrum of Au25(SR)18 was seen after single Pd/Pt doping.[55−57] The change in electronic structure upon doping with Pd/Pt led to the enhancement of catalytic efficiency toward hydrogen evolution reaction than with the undoped parent cluster, Au25(SR)18. The stability of the NC was enhanced due to doping. Unlike Pd/Pt, multiple Au atoms were replaced by Ag atoms and, depending on the number of doped Ag atoms, the electronic and catalytic properties differ.[58,59] The silver analogue of Au25(SR)18, namely, Ag25(SR)18, also showed different optical and electronic properties upon doping with Pd, Pt, and Au atoms.[60,61] The yellow color of Ag25(SR)18 turned to greenish-yellow while being doped with Pd and the color turned dark green upon doping with Pt and Au. Also, the singly doped Ag25(SR)18 with Pd, Pt, and Au exhibited higher catalytic performance.[62] Doping can induce chirality in NCs. For example, the incorporation of three Ag atoms in Au18(2,4-DMBT)14 (DMBT = dimethylbenzenethiol) transformed the achiral NC into the chiral system.[63,64] Also, the doping of Ag in Au38(SR)24 induced chirality. Along with a significant change in circular dichroism (CD) spectrum, AgAu38–(SR)24 lowers the racemization temperature.[36] The doped foreign atoms prefer to occupy certain positions in the parent NC. Pd, Ni, Pt, and Cd atoms always prefer to occupy the center, whereas Ag and Au atoms can be doped in all the possible positions. But Cu atoms always prefer outer staple positions.[65] Hence, by proper combinations, more than one metal atom can be incorporated to make multicomponent alloy NCs.

In this perspective article, we focus on the various synthetic methods used for making multimetallic alloy NCs (Scheme ). At first, we discuss the classical methods used for the preparation of alloy NCs such as co-reduction and galvanic and antigalvanic reduction methods. After that, we discuss the other emerging procedures for the formation of alloy NCs, which are metal deposition, ligand exchange, intercluster reactions, and reaction between NCs and bulk metals. In the case of co-reduction method, the structure and composition cannot be controlled, while galvanic and antigalvanic exchange reaction methods are used for making a large number of alloy NCs, keeping the structural integrity the same as that of the monometallic analogues. Metal deposition certainly changes the composition; however, the total structure of the parent cluster remains unaltered. Ligand exchange method, which is a well-established method for the preparation of noble metal NCs, is now being used for the synthesis of alloy NCs with new atomicity and properties. Other than galvanic and antigalvanic reduction methods, a new synthetic method has been discussed in detail where reaction involving two different NCs (named as intercluster reaction) leads to the formation of structure-conserved alloy NCs. Such intercluster reactions between Ag and Au NCs are illustrated here. Similar to the reaction between two NCs, NC was found to react with bulk metals also. For more detailed information about the structures and properties of NCs, we direct the readers to several recent review articles.[13,61,66−70]

Scheme 1

Schematic Representation of Various Synthetic Methodologies of Alloy NCs

Co-reduction, galvanic reduction, antigalvanic reduction, metal deposition, ligand exchange, intercluster reaction, and reaction of NCs with bulk metal are illustrated.

Co-reduction Method

Co-reduction is a kind of Brust–Schiffrin method where, instead of one metal precursor, several metal precursors are mixed with appropriate protective ligands and the reduction is performed with suitable reducing agents. The method is also known as the direct reduction method. With the help of this synthetic procedure, a large number of bimetallic NCs and a few trimetallic NCs have been prepared. In 1984, the first phosphine-protected biicosahedral Au and Ag alloy NC was synthesized following co-reduction method by Teo and Keating. The obtained alloy NC contained 25 metal atoms with the molecular composition of [(PPh3)12Au13Ag12Cl6].[71] This work was followed by the synthesis of [(Ph3P)l0Au13Ag12Br8](PF6).[72] Then, the same group found new triicosaherdal and tetraicosahedral alloy NCs, [(p-Tol3P)12Au18Ag20Cl14] and [(Ph3P)12Au22Ag24Cl10], respectively.[73,74] In 1994, Brust et al. reported the synthesis of thiolate-protected Au NCs, which showed more stability than their phosphine-protected analogs.[75] Among thiolate-protected NCs, Au25(SR)18 is one of the most studied NCs due to its higher stability. In 2009, Murray et al. synthesized PdAu24(PET)18, which was found to have different electrochemical properties than its monometallic analog.[57] Ag, Cu, and Pt atoms were incorporated into Au25(PET)18 to make Au25–Ag(PET)18,[76] Au25–Cu(PET)18,[77] and PtAu24(PET)18[55]via this method. Later, PdAu24(DDT)18 (DDT = dodecanethiol) was purified by Negishi et al. using HPLC and the cluster was more stable than monometallic Au25(DDT)18.[78] Au25–Ag(DDT)18 was also prepared and the number of doped Ag atoms was dependent on the molar ratio of the precursor salts.[79] Ni-doped bi- and trimetallic clusters such as Ag4Ni and Ag12Au12Ni were achieved by this method, which can exhibit interesting magnetic properties.[18,80] Various other alloys with interesting properties can be synthesized easily. Such an example is the chiral alloy NC, Au13Cu2((2r,4r)/(2s,4s)-BDPP)3(SPy)6, which was synthesized using chiral ligand 2,4-bis(diphenylphosphino)pentane (BDPP).[81] In addition, alloys of larger-sized NCs were also obtained such as (Ag-Au)144(PET)60, Au144–Cu(SC6H13)60, [Au80Ag30 (PhC≡C)42Cl9]Cl, etc.[82−84] A large number of Ag-rich alloy NCs were also synthesized such as [Ag46Au24(TBHP)32]2+, (TOA)3AuAg16(TBBT)12, Au12Ag32(FTP)30, Ag28Cu12(2,4-DCBT)24, etc.[31,85−87] Among various Ag NCs, Ag25(DMBT)18 is one of the most studied Ag NCs due to its exceptional stability than other Ag NCs and it is structurally similar to Au25(PET)18. Different groups have synthesized its Pt-, Pd-, and Au-doped alloys such as PtAg24(2,4-DMBT)18, PdAg24(2,4-DMBT)18, and AgAu25–(PET)18.[60,88] Our group has reported Ni-doped Ag25, NiAg24(2,4-DMBT)18.[89] Synthesis of alloy NCs using metal precursors having large differences in their redox potentials is difficult. Hence, the incorporation of Fe and Cd by co-reduction method is not possible. NaBH4 is the most used reducing agent during the synthesis of bi- and trimetallic NCs. However, other reducing agents were used such as H2, which was used to prepare [Pt(H)(PPh3)(AuPPh3)7]2+.[90] A list of NCs prepared so far (until September 2020) is given in Table , which includes co-reduction method.

Table 1

List of Alloy NCs, Their Method of Synthesis, and Characterization Methods Used

no	core	ligand	alloy cluster	focus	method of synthesis	reference
1	Au24Cd	phenylethylthiol	Au24Cd(PET)18	crystal structure	antigalvanic exchange	(91,92)
2	Au24Hg	phenylethylthiol	Au24Hg(PET)18	mass spectrometry	antigalvanic exchange	(91,92)
3	Au15Ag3	2,4-dimethylbenzenethiol	Au15Ag3(2,4-DMBT)14	crystal structure	antigalvanic exchange	(63)
4	Au9M4 (M = Ag, Cu)	diphenylmethylphosphine	[Au9M4Cl4(PMePh2)8][C2B9H12]·CH2Cl2 (M = Ag, Cu)	FAB mass spectrometry	antigalvanic exchange	(93)
5	Au23–xCux	cyclohexanethiol	[Au23–xCux(CHT)16]−	crystal structure	antigalvanic exchange	(94)
6	M1AgxAu24–x (M = Cd/Hg)	phenylethanethiol	M1AgxAu24–x(PET)18 (M = Cd/Hg)	mass spectrometry	antigalvanic exchange	(41)
7	AgAu17	hexanethiol	AgAu17(HT)14	crystal structure	antigalvanic exchange	(95)
8	Au24–x–yAgxCuyPd	1-dodecanethiol	Au24–x–yAgxCuyPd(SC12H25)18	mass spectrometry	antigalvanic exchange	(96)
9	Au38Cu1	2-phenyethanethiol	Au38Cu1(2-PET)24	mass spectrometry	antigalvanic exchange	(97)
10	(Ag-Au)144	phenylethanethiol	(Ag-Au)144(PET)60	mass spectrometry	co-reduction	(83)
11	Au144–xCux	hexanethiol	Au144–xCux(HT)60	mass spectrometry	co-reduction	(82)
12	Ag4Ni2	dimercaptosuccinic acid	Ag4Ni2(DMSA)4	mass spectrometry	co-reduction	(80)
13	Ag4Pt2	dimercaptosuccinic acid	Ag4Pt2(DMSA)4	mass spectrometry	co-reduction	(98)
14	Ag4Pd2	dimercaptosuccinic acid	Ag4Pd2(DMSA)4	crystal structure	co-reduction	(99)
15	Au13Cux (x = 2, 4, 8)	pyridine-2-thiol, triphenylpyridine 4-tert-butylbenzenethiol pyridinediphenylphosphine	Au13Cu2(PPh3)6(Spy)6]+ Au13Cu4(PPh2Py)4(SC6H4-tert-C4H9)8]+ Au13Cu8(PPh2Py)12]+	crystal structures	co-reduction	(100)
16	Ag28Cu12	2,4-dichlorobenzenethiol	[Ag28Cu12(2,4-DCBT)24]−4	crystal structure	co-reduction	(85)
17	Au24Ag20	phenylalkynyl, 2-pyridylthiolate	Au24Ag20(2-Spy)4(PhC≡C)20Cl2	crystal structure	co-reduction	(101)
18	Ag24Pd	2,4-dichlorobenzenethiol	(PPh4)2[Ag24Pd(2,4-DCBT)18]	crystal structure	co-reduction	(60)
19	Ag24Pt	2,4-dichlorobenzenethiol	(PPh4)2[Ag25–xPtx(2,4-DC)18] (x = 1, 2)	crystal structure	co-reduction	(60)
20	Ag24Ni	2,4-dimethylbenzenethiol	(PPh4)2[Ag24Ni(SR)18]	mass spectrometry	co-reduction	(89)
21	Au80Ag30	phenylalkene	[Au80Ag30(PhC≡C)42Cl9]Cl	crystal structure	co-reduction	(84)
22	Au24Pd	phenylethanethiol	[Au24Pd(PET)18]−1	crystal structure	co-reduction	(57,102)
23	Au24Pt	phenylethanethiol	[Au24Pt(PET)18]	crystal structure	co-reduction	(55,102)
24	Au37Pd1	phenylethanethiol	Au37Pd1(PET)24	mass spectrometry	co-reduction	(56)
25	Au36Pd2	phenylethanethiol	Au36Pd2(PET)24	mass spectrometry	co-reduction	(56,103)
26	AgxAu38–x	phenylethanethiol	AgxAu38–x(PET)24	crystal structure	co-reduction	(36)
27	Ag46Au24	tert-butyl hydroperoxide	[Ag46Au24(TBHP)32]2+	crystal structure	co-reduction	(86)
28	Au12Ag12Ni	hexafluroantimonate, triphenylphosphine	[Au12Ag12Ni(PPh3)10Cl7][SbF6]	crystal structure	co-reduction	(18)
29	Au12Ag12Pt	triphenylphosphine	[Au12Ag12Pt(PPh3)10Cl7]Cl	crystal structure	co-reduction	(18)
30	Ag13Au12	diphenylmethylphosphine	(MePh2P)10Au12Ag13Br9	crystal structure	co-reduction	(104)
31	Au18Ag20	para-tolylphosphine	[(p-Tol3P)12Au18Ag20Cl14]	crystal structure	co-reduction	(7)
32	Au22Ag24	triphenylphosphine	[(Ph3P)12Au22Ag24Cl10]	crystal structure	co-reduction	(74)
33	AuAg19	O,O-diisopropyl dithiophosphate	[AuAg19{S2P(OnPr)2}12]	crystal structure	co-reduction	(105)
34	AuAg24	1,1-bis(diphenylphosphino)methane, cyclohexanethiol	[AuAg24(Dppm)3(CHT)17]2+	crystal structure	co-reduction	(106)
35	Pt2Ag23	triphenylphosphine	[Pt2Ag23Cl7(PPh3)10]	crystal structure	co-reduction	(107)
36	AuAg16	tert-butylbenzenethiol	(TOA)3AuAg16(TBBT)12	crystal structure	co-reduction	(87)
37	Ag26Pt	2-ethylbenzenethiol	Ag26Pt(2-EBT)18(PPh3)6	crystal structure	co-reduction	(108)
38	Au2Cu6	adamentanethiol, diphenylpyridinephosphine	Au2Cu6(S-Adm)6(PPh2Py)2	crystal structure	co-reduction	(109)
39	Au4Ag5	adamentanethiol, 1,1-bis(diphenylphosphino)methane	[Au4Ag5(dppm)2(SAdm)6]+	crystal structure	co-reduction	(110)
40	Au4Ag23	1,1′-bis(diphenylphosphino)ferrocene, tert-butylalkynyl	[Au4Ag23(C≡CBut)10Cl7(dppf)4]2+	crystal structure	co-reduction	(39)
41	Au5Ag24	1,1′-bis(diphenylphosphino)ferrocene, tert-butylbenzylnyl	[Au5Ag24(C≡CC6H4-p-But)16(dppf)4Cl4]3+	crystal structure	co-reduction	(39)
42	AuxAg29–x	1,3-benzenedithiol, triphenylphosphine	AuxAg29–x(BDT)12(PPh3)4	crystal structure, mass spectrometry	co-reduction	(35)
43	Au23–xAgx	cyclohexanethiol	Au23–xAgx(S-Adm)16	crystal structure	co-reduction	(111)
44	Au19Cd2	cyclohexanethiol	[Au19Cd2(CHT)16]−	crystal structure	co-reduction	(94)
45	Cu30Ag61	adamentanethiol, tetraphenylborate	[Cu30Ag61(SAdm)38S3](BPh4)	crystal structure	co-reduction	(112)
46	Au4Cu4	adamentanethiol, 1,1-bis(diphenylphosphino)methane	[Au4Cu4(Dppm)2(SAdm)5]Br	crystal structure	co-reduction	(113)
47	Au8Ag57	1,3-bis(diphenylphosphino)propane, cyclohexanethiol	[Au8Ag57(Dppp)4(CHT)32Cl2]Cl	crystal structure	co-reduction	(114)
48	Pt1Ag9	tris(4-fluorophenyl)phosphine	Pt1Ag9[P(Ph-F)3]7Cl3	crystal structure	co-reduction	(115)
49	Au130–xAgx	tert-butylbenzenethiol	Au130–xAgx(TBBT)55	crystal structure	co-reduction	(116)
50	Ag17Cu12	1,3-benznedithiol, triphenylphosphine	Ag17Cu12(BDT)12(PPh3)4	crystal structure	co-reduction	(117)
51	Au7Ag9	1,1′-bis(diphenylphosphino)ferrocene	[Au7Ag9(dppf)3(CF3CO2)7BF4]n	crystal structure	co-reduction	(118)
52	Ag20Cu12	2,4-dimethylbenzenethiol, 1,1-bis(diphenylphosphino)methane	[Ag20Cu12(2,4-DMBT)14(Dppm)6Br8]2+	crystal structure	co-reduction	(119)
53	Au9Ag12	1-adamantanethiol/tert-butylmercaptan, 1,1-bis(diphenylphosphino)methane	[Au9Ag12(SR)4(dppm)6X6]3+	crystal structure	co-reduction	(120)
54	PtAu8	triphenylphosphine	PtAu8(PPh3)8](NO3)2	crystal structure	co-reduction	(121)
55	Au12+nCu32	4-(trifluoromethyl)thiophenol	Au12+nCu32(SR)30+n]4– (n = 0, 2, 4, 6)	crystal structure	co-reduction	(122)
56	Cd1Au14	tert-butylthiol	Cd1Au14(StBu)12	crystal structure	co-reduction	(123)
57	Au8Ag3	triphenylphosphine	Au8Ag3(PPh3)7Cl3	crystal structure	co-reduction	(124)
58	Au36–xAgx	tert-butylbenzenethiol	Au36–xAgx(SPh-tBu)24	mass spectrometry	co-reduction	(125)
59	Au38–xCux	2,4-dimethylbenzenethiol	Au38–xCux(2,4-DMBT)24 (x = 0–6)	crystal structure	co-reduction	(126)
60	Au19Cu30	3-ethynylthiophene/ethynylbenzene	[Au19Cu30(C≡CR)22(Ph3P)6Cl2]	crystal structure	co-reduction	(127)
61	Au3Ag38	phenylethanethiol	[Au3Ag38(PET)24X5]2– (X = Cl or Br)	crystal structure	co-reduction	(128)
62	Au4Pt2	phenylethanethiol	Au4Pt2(PET)8	crystal structure	co-reduction	(129)
63	Au4Pd2	phenylethanethiol	Au4Pd2(PET)8	crystal structure	co-reduction	(130)
64	Au24Cu6	tert-butyl benzenethiol	Au24Cu6(TBBT)22	crystal structure	co-reduction	(131)
65	Au36–xCux	m-methylbenzenethiol	Au36–xCux(m-MBT)24 (x = 1–3)	crystal structure	co-reduction	(132)
66	Au13Cu2	1,3-bis(diphenylphosphino)propane, pyridine-2-thiol, (2r,4r)/(2s,4s)-2,4-bis(diphenylphosphino)pentane	Au13Cu2(DPPP)3(SPy)6, Au13Cu2((2r,4r)/(2s,4s)-BDPP)3(SPy)6	crystal structure	co-reduction	(81)
67	PtAu7	triphenylphosphine	[Pt(H)(PPh3)(AuPPh3)7]2+	crystal structure	co-reduction	(90)
68	Ag32Au12	flurobenzenethiol	Ag32Au12(FTP)30]4–	crystal structure	co-reduction, intercluster reaction	(31,133)
69	Au21–xAgx	tert-butylthiol	Au21–xAgx(TBT)15 (x = 4–8)	crystal structure	co-reduction, ligand exchange	(134)
70	Au21–xCux	tert-butylthiol	Au21–xCux(TBT)15 (x = 0, 1), Au21–xCux(TBT)15 (x = 2–5)	crystal structure	co-reduction, ligand exchange	(134)
71	Au25–xAgx (x = 6–8)	phenylethanethiol	[Au25–xAgx(PET)18]	crystal structure	co-reduction, antigalvanic exchange	(48,76,92,135,136)
72	Au25–xCux	phenylethanethiol	[Au25–xCux(PET)18]	mass spectrometry	co-reduction, antigalvanic exchange	(77,92,136)
73	Ag25–xAux	2,4-dichlorobenzenethiol/2,4-dimethylbenzenethiol	(PPh4)2[Ag25–xAux(SR)18]	crystal structure, mass spectrometry	co-reduction, galvanic exchange, intercluster reaction	(60,137,138)
74	Ag7Au6	mercaptosuccinic acid	Ag7Au6(H2MSA)	mass spectrometry	galvanic exchange	(37)
75	AuxAg50–x	tert-butyl benzylmercaptant 1,1-bis(diphenylphosphino)methane	AuxAg50–x(DPPM)6(SR)30 (R = TBBM)	crystal structure	galvanic exchange	(139)
76	AuAg24	6-mercaptohexanoic acid	AuAg24(MHA)18	mass spectrometry	galvanic exchange	(140)
77	Ag26Au/Ag24Au	2-ethylbenzenethiol	[Ag26Au(2-EBT)18(PPh3)6]+/[Ag24Au(2-EBT)18]−	crystal structure	galvanic exchange	(141)
78	Pt1Ag12Cu12Au4	adamentanethiol	Pt1Ag12Cu12Au4(S-Adm)18(PPh3)4	mass spectrometry	galvanic reduction	(34)
79	Au22Ir3	phenylethylthiol	Au22Ir3(PET)18	mass spectrometry	intercluster reaction	(142)
80	Au12Ag17	1,3-benzenedithiol, triphenylphosphine	Au12Ag17(BDT)12(PPh3)4	mass spectrometry	intercluster reaction	(89)
81	MAuxAg28–x (M = Ni/Pd/Pt)	1,3-benzenedithiol, triphenylphosphine	MAuxAg28–x(BDT)12(PPh3)4 (M = Ni/Pd/Pt)	mass spectrometry	intercluster reaction	(89)
82	Au20Ag5	captopril	Au20Ag5(Capt)18	mass spectrometry	intercluster reaction	(143)
83	Ag51–xAux	1,3-benzenedithiol	Ag51–xAux(BDT)19(PPh3)3	mass spectrometry	intercluster reaction	(144)
84	Ag13Au12	triphenylphosphine, phenylethanethiol	[Ag13Au12(PPh3)10(SR)5Cl2]2+	crystal structure	ligand exchange	(54)
85	PtAg28	1,3-benzenedithiol, triphenylphosphine	PtAg28(BDT)12(PPh3)4	crystal structure	ligand exchange	(145,146)
86	PdAg28	1,3-benzenedithiol, triphenylphosphine	PdAg28(BDT)12(PPh3)4	mass spectrometry	ligand exchange	(89)
87	NiAg28	1,3-benzenedithiol, triphenylphosphine	NiAg28(BDT)12(PPh3)4	mass spectrometry	ligand exchange	(89)
88	Au16Ag	adamentanethiol	[Au16Ag(S-Adm)13]	crystal structure	ligand exchange	(95)
89	Pt1Ag12	1,1-bis(diphenylphosphino)methane, 2,4-dimethylbenzenethiol	Pt1Ag12(dppm)5(SPhMe2)2	mass spectrometry	ligand exchange	(147)
90	Au4Cu5	cyclohexanethiol, 1,1-bis(diphenylphosphino)methane	[Au4Cu5(C6H11S)6(Dppm)2](BPh4)	crystal structure	ligand exchange	(113)
91	Au8Ag55	1,3-bis(diphenylphosphino)propane, cyclohexanethiol, tetraphenylborate	[Au8Ag55(Dppp)4(C6H11S)34](BPh4)2	crystal structure	ligand exchange	(114)
92	Pt1Ag28	adamentanethiol, triphenylphosphine	Pt1Ag28(S-Adm)18(PPh3)4	crystal structure	ligand exchange	(148)
93	Au1Ag22	adamentanethiol	[Au1Ag22(S-Adm)12]3+	crystal structure	ligand exchange	(149)
94	Au24–xAgx	tert-butyl benzylmercaptant	Au24–xAgx(TBBM)20	crystal structure	ligand exchange	(150)
95	PtAg28	hexanethiol	PtAg28(HT)18(PPh3)4	crystal structure	ligand exchange	(151)
96	Ag2Au25	phenylethanethiol	Ag2Au25(PET)18	mass spectrometry	metal deposition	(152)
97	MAu24 (M = Ag/Cu)	phenylethanethiol, triphenylphosphine	[MAu24(PPh3)10(PET)5Cl2]2+ (M = Ag/Cu)	crystal structure	metal deposition	(153)

Galvanic Reduction Method

The post-synthetic metal exchange reaction of a NC with a suitable metal precursor is one of the important methods for the preparation of alloy NCs. Among different metal exchange reactions, galvanic reduction reaction has become a very effective approach for making multimetallic alloy NCs as well as various anisotropic alloy nanoparticles. According to the classic galvanic theory, a metal ion of higher reduction potential in solution gets reduced and replaces a metal atom present in a material and the latter subsequently enters the solution after being oxidized. The process is explained in detail in Scheme . The metal activity sequence is Fe (−0.77 V) > Cd (−0.40 V) > Co (−0.28 V) > Ni (−0.25 V) > Cu (+0.34 V) > Hg (+0.79 V) > Ag (+0.80 V) > Pd (+0.95 V) > Pt (+1.2 V) > Au (+1.50 V). Various metal atoms have been incorporated in the Ag NCs using galvanic replacement procedure to make silver-rich multimetallic alloy NCs. Such an example is the atomically precise doping of Au atom in Ag25(DMBT)18 to make Ag24Au(DMBT)18 (reduction potentials of Ag+/Ag and Au+/Au are 0.80 and 1.69 V, respectively).[137] It was found that co-reduction led to the synthesis of Ag25–Au(DMBT)18 (where x = 1–8) and, therefore, it is difficult to make single crystals. The availability of single crystal will help in understanding the position of the dopant and, hence, synthesis of a single product is important, which can be achieved by this method. Ag24Au(DMBT)18 exhibits higher stability and enhanced photoluminescence than monometallic Ag25(DMBT)18. For the synthesis of Ag24Au(DMBT)18, an already synthesized Ag25(DMBT)18 was treated with AuClPPh3, which changed the color of the solution from brown to green. The product was characterized using ESI MS and SCXRD. The structure of bimetallic Ag24Au(DMBT)18 is similar to the monometallic one. During galvanic exchange reaction, the structure of monometallic NC remains unaltered in their multimetallic analogue; however, Kang et al. reported shape-altered synthesis of alloy NCs using this method.[40] The incorporation of Au atoms in PtAg24(DMBT)18 resulted in the formation of shape-unaltered trimetallic PtAuAg24–(DMBT)18 when Au-DMBT was used as the precursor, while the use of AuBrPPh3 led to the formation of shape-altered trimetallic Pt2Au10Ag13(PPh3)10Br7. In this case, Br– peeled away the PtAg12 core from the staple motifs and transformed it into a biicosahedron, which was stabilized by PPh3 ligand instead of DMBT. Trimetallic PtCuxAg28−x(BDT)12(PPh3)4 and tetrametallic Pt1Ag12Cu12Au4(S-Adm)18(PPh3)4 (S-Adm = adamentanethiol) NCs were synthesized by galvanic replacement procedure. Several bimetallic NCs have been prepared via this method, which are listed in Table .

Scheme 2

Schematic Representation of Galvanic and Antigalvanic Exchange Reaction Processes Using Au and Ag as Examples

Antigalvanic Exchange Reaction

Alloying or doping in Au25(SR)18 was performed largely with different metals such as Ag, Cu, Pd, Pt, Ni, Cd, Hg, and Ir. According to the classic galvanic reduction method, Au NCs are unable to react with the less noble metal atoms such as Ag, Cu, Cd, etc. (reduction potentials of Au+/Au, Ag+/Ag, Cu2+/Cu, and Cd2+/Cd are +1.50, +0.80, +0.34, and −0.40 V, respectively). However, Murray et al. showed the reduction of Ag ions by Au NCs, which was followed by the replacement of Au atoms with Ag, leading to the formation of AgAu25–(SR)18 alloy NCs.[135] This reaction was named as antigalvanic reduction, which is a unique property of Au and Ag NCs and nanoparticles of core size below 3 nm.[136] In 2012, Wu performed the reaction between Au25(PET)18 and Ag ions and the formed bimetallic products were characterized using laser desorption ionization (LDI) MS. To prove that antigalvanic exchange reaction is not a unique property of Au25(PET)18 NC, approximately 2–3 nm nanoparticles were treated with AgNO3 solution. The incorporated Ag was detected by XPS and, after the reaction, the binding energy of Ag indicated the incorporation of neutral Ag, which confirmed the reduction of Ag ions by Au nanoparticles. A similar experiment was performed on 3 nm-sized Ag nanoparticles using Cu salt and incorporation of neutral Cu in Ag nanoparticles was observed using XPS. This proved that Cu ions can be reduced by more noble Ag atoms. Antigalvanic reduction reaction is feasible due to the enhanced reducing ability when the metal is in nanoscale form. One of the important driving factors for antigalvanic reduction is the protective ligands on the surface of nanoparticles or NCs. It was proposed that the partial negative charge present on the surface ligands plays a crucial role in the reduction of more reactive ions. This observation led Wu et al. to modify the sequence of metal activity as Fe > Ni > Pd > Au (∼3 nm) > Cu > Ag.[92,136] After that, Zhu et al. showed the incorporation of monovalent Cu/Ag and bivalent Cd/Hg in Au25(SR)18 NCs.[91] Cd and Hg atoms were getting doped at the center of the icosahedral core, while Cu and Ag atoms were incorporated on the surface of the icosahedron. Similar to galvanic exchange reaction, in the case of antigalvanic exchange, dopants are mostly getting incorporated in the parent NCs without changing their structures and compositions. However, in some NC systems, the replacement with a foreign metal atom can lead to structural transformations. For example, the doping of Ag and Cu atoms in Au25(SR)18, Au38(PET)24, Au36(SR)24, Au144(PET)60, etc., and doping of Cd and Hg in Au25(SR)18 resulted in the formation of alloy NCs with preserved composition and structure.[82,91,97,132] On the other hand, doping of Ag atoms in Au23(CHT)16 (CHT = cyclohexylthiol) NCs first led to the formation of Au23–Ag(CHT)16, which then got converted to Au25–Ag(CHT)18.[38,154] The Au-rich trimetallic and tetrametallic NCs M1AgAu24–(SR)18 (M = Cd/Hg)[41] have also been synthesized by this effective method. A list of multicomponent NCs made via antigalvanic reduction is presented in Table .

Metal Deposition Method

The metal deposition or intramolecular metal exchange is one of the emerging alloying methods. Li et al. reported the synthesis of Ag2Au25(SR)18via reaction of Au25(SR)18 with AgNO3[152] where Ag atoms occupied the outer staple. But, after treatment with excess ligand, Ag atoms diffused within the icosahedral core, which led to the formation of AgAu24(SR)18. The reaction occurred in two steps as shown in Scheme A. The first step is the self-metal exchange, which is the intramolecular metal exchange. During the first step, one of the Ag atoms, which were in the outer staple (denoted in blue color in Scheme A), got exchanged with one of the icosahedral Au atoms. Thus, after exchange, one Ag atom was on the inner icosahedral surface and another one in the outer staple. This was followed by the second step where the surface Au and Ag atoms were detached from the surface of the cluster and resuted in the synthesis of AgAu24(SR)18. This process was called the metal stripping process. The same process was observed by Zheng et al. in AuAg24(MHA)18 NCs.[140]

Scheme 3

Mechanistic pathway of the conversion of Ag2Au25(SR)18 to AgAu25(SR)18 and Proposed Mechanisms of Cu and Ag Atom Deposition in [Au24(PPh3)10(PET)5Cl2]+.

(A) Mechanistic pathway of the conversion of Ag2Au25(SR)18 to AgAu25(SR)18. Color codes: red, yellow, and cyan colors denote S, Au, and Ag atoms, respectively. (B, C) Proposed mechanisms of Cu and Ag atom deposition in [Au24(PPh3)10(PET)5Cl2]+, which resulted in the synthesis of [MAu24(PPh3)10(PET)5Cl2]2+ (M = Cu/ Ag). Color codes: yellow, orange, pink, and navy blue colors denote Au atoms, while cyan, gray, red, and green colors denote Cu, Ag, S, and Cl, respectively.

Adapted from refs (152) and (153). Copyright 2018 Nanomaterials and 2017 Nature

Using high-resolution ESI MS, they monitored real-time diffusion of Au atoms from the staple motif to the icosahedral surface and then to the center of the icosahedron to attain thermodynamic stability. The metal deposition method can be extrapolated into a hollowing and refilling process where foreign metal atoms are incorporated to make alloy NCs. Single Cu and Ag atoms were added successfully by Wang et al. in [Au24(PPh3)10(PET)5Cl2]+, which led to the synthesis of [MAu24(PPh3)10(PET)5Cl2]2+ (M = Ag/ Cu).[153] In this case, the −SR group and Cl atom played important roles. At first, a foreign atom got attached to the S/ Cl atom present in the waist/apex positions, respectively. Due to this interaction, the Au–S/Au–Cl bond collapsed and the Au atom migrated to the central position, which reduced the energy of the system as shown in Scheme B,C. Mainly, the Ag atom was doped at the apex site, while Cu occupied both apex and waist positions as the Ag–Cl bond is stronger than Ag–SR, unlike that of Cu–Cl that has the same binding energy as of Cu–SR.

Ligand Exchange Method

Ligand exchange is an efficient method for the synthesis of atomically precise NCs in which one NC is converted to another, with the same or different nuclearity in the presence of foreign ligands.[68] The ligand exchange process where the structure of the starting NC undergoes structural conversion is named as ligand exchange-induced structural transformation (LEIST) by Jin’s group.[155] This is a fast-evolving method in NC chemistry, which enhances the range of applications of NCs by introducing different functional ligands. LEIST method has been widely used for making a large number of monometallic NCs; however, it is now also being used for the preparation of multimetallic NCs. Biicosahedral Pt2Ag23(PPh3)10Cl7 was converted to monoicosahedral PtAg24(DMBT)18 and PtAg28(BDT)12(PPh3)4 on addition of DMBT and BDT ligands, respectively.[145,147] PtAg28(S-Adm)18(PPh3)4 was synthesized by the conversion of PtAg24(DMBT)18 after addition of Adm-SH and PPh3.[148] Similarly, MAg28(BDT)12(PPh3)4 (M = Ni/Pd/Pt) was synthesized via the LEIST method starting from MAg24(DMBT)18 (M = Ni/Pd/Pt) (see Figure ).[89] Ligand exchange method also helped make the n-hexanethiol (HT)-protected NC, PtAg28(HT)18(PPh3)4 from PtAg28(S-Adm)18(PPh3)4, keeping the structure unaltered.[151] Moreover, Zhu et al. showed the synthesis of highly luminescent Pt1Ag12(dppm)5(DMBT)2 from feebly luminescent Pt2Ag23(PPh3)10Cl7 by introducing dppm along with DMBT.[147] Further, Kang et al. reported a new alloy NC, AgAu16(S-Adm)13 starting from AgAu17(CHT)14 by this methodology.[95] Also, AgAu24–(TBBT)16 was synthesized from AgAg23–(HT)16 upon treatment with TBBT. It has been proposed that the drastic change in the nuclearity of the NC is due to the change in the electron-withdrawing and electron-donating effect of ligands, which depends on the position of the functional groups. The noncovalent interactions and steric hindrance are also crucial factors for this transformation. However, the effect of ligand on the nuclearity and structure of the NC is unclear.

Figure 1

(A) Formation of MAg28(BDT)12(PPh3)4 from MAg24(DMBT)18 (M = Pd, Pt, Ni) NCs. Green, pink, yellow, and orange denote M (Pd, Pt, Ni), Ag, S, and P atoms, respectively. Blue color denotes C and H atoms of thiol ligands. (B) Conversion of AgAu17(CHT)14 to AgAu16(S-Adm)13. The colors pink, green, and yellow denote Ag, Au, and S atoms, respectively. Blue color denotes C and H atoms of thiol ligands.

Intercluster Reaction Method

Besides various synthetic methods, the reaction between two NCs has become an emerging method to obtain multimetallic alloy NCs. We are discussing this method more elaborately than others as it has not been reviewed previously, in the context of alloy clusters. Krishnadas et al. found the chemical reaction between two atomically precise NCs, which resembles the reaction between two organic molecules. They first reported the reaction between pure Ag and Au NCs, Ag44(FTP)30 and Au25(FTP)18, respectively, which led to the formation of Au-doped Ag44(FTP)30 and Ag-doped Au25(FTP)18 bimetallic NCs.[138] These intercluster reactions are controlled by both kinetics and thermodynamics. The extent of reaction and its mechanism were studied in detail using MS. Negative-mode MALDI MS at two different reaction times at the Au25(FTP)18 side is shown in Figure . The blue spectrum corresponding to MALDI MS after 1 h of reaction showed the incorporation of up to 5 Ag atoms, which resulted in the formation of Au25–Ag(FTP)18, while the insertion of up to 13 Ag atoms was observed after 3 h of reaction (red spectrum). As both reacting NCs were protected by the same protecting ligand, no ligand exchange was observed in the MS. The doping of Ag atoms into Au25(SR)18 was found to be more facile via this protocol where up to 20 Ag atoms can be incorporated. The total number of atoms and the overall charge state remain preserved in the formed alloy NCs during this kind of substitution reaction. Similar to the Au25 region, the reaction also occurred in the Ag44 region as shown in Figure .[133] At first, a mixture of bimetallic NCs was formed having the composition, AuAg44–(FTP)30 where x = 1–12. After 1 h, this mixture resulted in the formation of Au12Ag32(FTP)30 exclusively (Figure A). The reaction was monitored by time-dependent UV–vis spectroscopy as shown in Figure B. It showed a significant spectral change with time and the final spectrum was completely different than that of Ag44(FTP)30 as well as Au25(FTP)18, confirming the formation of the bimetallic NC Au12Ag32(FTP)30. Although tools like MS, UV–vis absorption spectroscopy, NMR, etc., help in understanding the extent of doping, the precised structural insight can be obtained only from X-ray crystallography. In the absence of a single crystal, one can understand the doping position by computations as the structures of the monometallic NCs remain preserved to a large extent in the multimetallic NCs during such reactions. The single-crystal X-ray structure of Ag44(FTP)30 consists of a Ag12 hollow icosahedron inside a Ag20 dodecahedron, which is covered by a Ag12(FTP)30 outer shell. On the other hand, Au25(FTP)18 is made up of a Ag13 central icosahedron protected by a Ag12(FTP)18 surface motif. In the case of the reaction involving Ag44(FTP)30 and Au25(FTP)18, the total energy of the substituted product was found to be the most negative when Au and Ag atoms occupied the icosahedral surface of Ag44(FTP)30 and Au25(FTP)18, respectively. Hence, it was concluded that the substitution of metal atoms during the intercluster reaction was an energy-driven process. It was proposed that the intercluster reaction proceeds through bond breaking due to metallophilic and noncovalent interactions between ligands or redox reactions. The fragments generated after bond breaking behaved as nucleophiles, which led to the incorporation of another metal atom similar to that of the metal exchange reactions. The formation of an adduct of two NCs (due to noncovalent interactions between ligands and metallophilic interactions) was observed in ESI MS during the reaction between two structurally similar Au and Ag NCs, Au25(PET)18 and Ag25(DMBT)18, as shown in Figure .[156] The adduct vanished within 5 min of mixing and a series of peaks were observed due to the formation of AuAg(SR)18. This adduct formation suggested that the intercluster reaction is bimolecular. The structure of this adduct was calculated using DFT, which manifested that bond lengths of both NCs were longer than that of individual NCs. Also, the bond angles of staples of both NCs changed significantly in the adduct. Further chemical and structural transformation might occur during the reaction after adduct formation. In the alloy NCs AuAg(SR)18, the number of doped Ag and Au atoms can be varied from 1 to 24 by varying the molar ratio of reactant NCs. Therefore, the desired number of dopants can be inserted using intercluster reaction. Similar to the previous reaction, in this case, the total number of metal and ligand in alloy NCs is identical with that of the unreacted NCs. Regarding the dopant position, the computational study revealed that the total energy of the reaction will be the most favorable when a Au atom occupies the central position of the icosahedron in Ag25(DMBT)18, while a Ag atom occupies the icosahedral surface of Au25(PET)18. Not only Ag but also Ir metal incorporation in Au25(SR)18 was obtained via intercluster reaction, which otherwise is difficult to achieve by other methods. Bhat et al. discussed the intercluster reaction between an Ir NC, Ir9(PET)6, and a Au NC, Au25(PET)18, which led to the formation of Ir3Au22(PET)18.[142] In this reaction, only one single product got formed, which was then separated from the unreacted NCs by thin-layer chromatography (TLC), and the purity was checked with ESI MS. The DFT study revealed that the Ir atoms occupy the center and surface of the icosahedral core.

Figure 2

MALDI MS of reaction between Au25(FTP)18 and Ag44(FTP)30 in negative ion mode after (A) 1 h and (B) 3 h of the reaction at the Au25 region. Reprinted from ref (138). Copyright 2016 American Chemical Society.

Figure 3

(A) Time-dependent ESI MS and (B) time-dependent UV–vis absorption spectra of the reaction between Ag44(FTP)30 and Au25(FTP)18 at the Ag44 region. Adapted from ref (133). Copyright 2017 American Chemical Society.

Figure 4

ESI MS of the mixture of [Ag25(DMBT)18]− and [Au25(PET)18]−, resulting in the formation of the intermediate adduct [Ag25Au25(DMBT)18(PET)18]2– whose theoretical and experimental isotopic distributions match with each other. Reprinted from ref (156). Copyright 2016 Nature.

In these reactions, the use of two monothiol-protected NCs also led to ligand exchange; although, the total number of thiol ligands remains the same. As DMBT and PET are of the same mass, no ligand exchange could be observed in ESI MS data but ligand exchange between FTP and PET was observed. Due to high mobility, ligands undergo spontaneous exchange. The mobility of ligands can be decreased by using bidentate ligands such as dithiols. Ghosh et al. showed the reactions of bidentate thiol-protected NCs, Ag51(BDT)19(PPh3)3 and Ag29(BDT)12(PPh3)4, with Au25(PET)18 and noticed the exchange between Ag and Au atoms and no exchange occurred between the ligands.[144] The reaction rate was observed to be slow, which may be due to the presence of a rigid bidentate ligand that rigidifies the surface of the NC. They found the incorporation of three Au atoms in Ag51(BDT)19(PPh3)3 and Ag29(BDT)12(PPh3)4. Later on, the reaction between Ag29(BDT)12(PPh3)4 and Au25(PET)18 was studied in detail, which showed the incorporation of a maximum of 12 Au atoms in Ag29(BDT)12(PPh3)4, while only up to 7 Ag atoms were getting doped in Au25(PET)18, unlike the previously mentioned intercluster reactions.[89] This was due to the use of a very high concentration of Au25(PET)18 (five times higher) in comparison to Ag29(BDT)12(PPh3)4 to increase the reaction rate, which was otherwise very slow due to the rigid surface. This proved that the rate of reactions strongly depends on the metal–ligand interface. The oligomeric ML surface staples were well defined in these NCs, and in some of these clusters, the oligomeric staple motifs formed interlocked rings as in Ag25(SR)18 and Au25(SR)18. The structure of these kinds of NCs had been compared to the Borromean rings, as proposed by Natarajan et al., and NCs were referred to as aspicules (meaning shielded molecules).[157] According to this structural alignment, breaking of one ring led to the destruction of the entire structure of these NCs, which resulted in the generation of ML fragments behaving as nucleophiles in the substitution reaction. This process led to the incorporation of 1–24 Ag and Au atoms in the respective NCs to make bimetallic ones. This was not the same in the case of dithiol-protected NCs such as in Ag29(BDT)12(PPh3)4, which comprised two types of staples: Ag3S6 and Ag-P, unlike that of the Borromean ring structure. These staples were difficult to break as they were stabilized by dithiol, and hence, it was proposed that Au25(SR)18 NCs were interacting with Ag29(BDT)12(PPh3)4via weak van der Waal interactions at the less congested Ag3S6 staples as shown in Scheme . This interaction assisted the metal exchange between two NCs at the outer staple, leading to the synthesis of bimetallic NCs. Then, the doped Ag and Au atoms underwent intramolecular galvanic and antigalvanic exchange to occupy the energetically most stable positions, which were the icosahedral surface positions.

Scheme 4

Proposed Interactions between Ag and Au Atoms of Ag29(BDT)12(PPh3)4 and Au25(PET)18 (A), Which Led to the Formation of Kinetically Controlled Au12Ag17(BDT)12(PPh3)4 at the Outer Surface, Which Then Diffuses Inside the Icosahedron Core to Form a Thermodynamically Stable Structure.

Color codes: Green and pink (both light and dark shades) denote Ag atoms, yellow and purple denote Au atoms, cyan denotes S atoms, and orange denotes P atoms.

Adapted with permission from ref (89). Copyright 2020 American Chemical Society

Intercluster reaction was then expanded to make trimetallic NCs.[89] We showed the reaction of bimetallic MAg28(BDT)12(PPh3)4 (where M = Ni/Pd/Pt) with monometallic Au25(PET)18, which produced a mixture of trimetallic MAuAg28–(BDT)12(PPh3)4 and bimetallic AgAu25–(PET)18. Unlike Ag29(BDT)12(PPh3)4, the use of doped Ag29(BDT)12(PPh3)4 exhibited higher reactivity; however, it was slower as compared to the monothiol-protected ones. A time-dependent ESI MS of the reaction sequence between PdAg28(BDT)12(PPh3)4 and Au25(PET)18 (1:5 molar ratio) is shown in Figure , which manifests the extent of reaction with time. With increasing time, the number of doped Au atoms increased and it formed a stable single trimetallic NC PdAu12Ag16(BDT)12(PPh3)4, while doping up to 7 Ag atoms was seen in Au25(PET)18 forming a mixture of bimetallic AgAu25–(PET)18 (where x = 1–7) clusters. Similar to PdAu12Ag16(BDT)12(PPh3)4, trimetallic PtAu12Ag16(BDT)12(PPh3)4 NC too got formed by intercluster reaction between PtAg28(BDT)12(PPh3)4 and Au25(PET)18. Also, NiAuAg28–(BDT)12(PPh3)4 was obtained by intercluster reaction between NiAg28(BDT)12(PPh3)4 and Au25(PET)18. One important aspect to notice here was that the centrally doped Ni, Pd, and Pt atoms in Ag29(BDT)12(PPh3)4 did not get transferred to Au25(PET)18 to make corresponding bi- or trimetallic NCs. This observation supported the above mechanism, which depicted the involvement of metal–ligand interface during the intercluster reaction and not the central atom. Hence, based on this, we calculated the structure of trimetallic MAu12Ag16(BDT)12(PPh3)4 (M = Ni/Pd/Pt), where M atom was at the center of the icosahedral core and 12 Au atoms occupied the icosahedral surface positions.

Figure 5

ESI MS of reaction between PdAg28(BDT)12(PPh3)4 and Au25(PET)18 (1:5 molar ratio) at different time intervals. (A) Reaction at the PdAg28(BDT)12(PPh3)4 side and (B) the reaction at the Au25(PET)18 side. The formed clusters are [PdAuAg28–(BDT)12]4– and [AgAu25–(PET)18]−. The charged species are not mentioned in the text for simplicity. The red asterisk in (A) is due to thiolates produced during the reaction. Adapted with permission from ref (89). Copyright 2020 American Chemical Society.

There have been other attempts of intercluster reactions by other groups such as Xia et al., who reported the synthesis of Au20Ag5(Capt)18 by the reaction between Au25(Capt)18 and Ag30(Capt)18.[143] The product was purified using polyacrylamide gel electrophoresis (PAGE). While both the reactant NCs were non-emissive, the alloy cluster exhibited intense red emission. MALDI MS was used to determine the composition of as-synthesized alloy NCs.

The reaction of nanocluster has also been observed with nanoparticles by Xia et al.[143] They found the doping of Cu atoms by the reaction of Au25(SR)18 with Cu nanoparticles of ∼1.4 nm. This gave a new turn of the reaction involving nanoclusters. Similarly, Bose et al. have observed doping of Ag atoms in atomically precise Au25(PET)18 nanoclusters using polydispersed Ag nanoparticles.[158]

Nanocluster-Bulk Reactions

To understand the mechanism of intercluster reactions and the role of metal–ligand interfaces in greater detail, Kazan et al. conducted a reaction of NCs with surfaces of bulk metals.[159] They used Au25(PET)18 and Au38(PET)24 NCs to react with Ag, Cu, and Cd foils before and after functionalization with thiols. The doping rate was observed to be different for the treated and untreated foils. Treated Ag foils exhibited a faster reaction rate, which decreased with time, while the untreated one showed a lower reaction rate at first and slowly increased after a certain time. This observation indicated that pre-functionalized thiols on the foils play a crucial role in the reaction, which indeed emphasized the importance of metal–ligand interfaces during the reaction between NCs (the mechanism is shown in Scheme ).[160] In the case of Cu and Cd foils, the reaction did not occur with the bare metal foils. Insertion of a few Cu and Cd atoms can be seen during the reaction with the pre-treated foils. This new class of reactions of NCs with bulk metals opens up new opportunities for making different alloy NCs.

Scheme 5

Pictorial Illustration of the Reaction between Au25(PET)18 with Ag Foil. Adapted with permission from ref (159). Copyright 2019 Royal Society of Chemistry

Conclusions and Future Perspectives

In the following, we list below several possibilities to expand this science.

Diversity: About 100 bimetallic alloy NCs are known till now; however, only about 10 alloy NCs with more than one heteroatom have been reported. To the best of our knowledge, only two tetrametallic NCs ([Pt1Ag12Cu12Au4(S-Adm)18(PPh3)4] and Au24–AgCuPd(SC12H25)18) have been reported. Hence, incorporation of more metals in a given cluster core, such as pentametallic and beyond, will be worth exploring. This may lead to high entropy alloy NCs, which might show improved properties, in areas such as catalysis. Finding some methods to control the number of doped atoms in an alloy NC are also important.

The reaction of a NC with another NC of a different metal or even bulk metal is an efficient method for preparing bimetallic and trimetallic NCs. The reactions between a few NCs have been studied. There are more than 150 NCs whose structures have been solved and many more NCs are reported, whose structures are yet to be understood. Hence, intercluster reactions using these NCs can be studied to expand their chemistry. Reactions of NCs can be studied with other nanostructures and different bulk metals, which might give a new twist to the area.

Structures and properties: Over 100 multicomponent atomically precise alloy NCs comprising Au, Ag, Ni, Cu, Hg, Ir, Cd, Pd, and Pt have been synthesized till date using different synthesis methods. Most of these have been characterized with mass spectrometry, and about 50 have crystal structures. As a glaring gap, Fe and Co doping has not been accomplished yet in such clusters and these alloy clusters can be of potential interest due to their magnetic properties. Incorporation of lanthanides in NCs to make alloys will be an exciting study due to their interesting magnetic and photophysical properties. Even though Ni-doped NCs such as NiAu24, NiAg24, and NiAg28 have been synthesized, their magnetic properties have not been studied so far. These alloy NCs can show multiple phenomena such as circularly polarized luminescence, magneto-fluorescence, etc., which will help to broaden their applications.

Structures of multimetallic NCs will provide details of their optical and electronic properties. Unlike bimetallic NCs, the structure of tri- and tetrametallic NCs are yet to be studied extensively. Mainly, two kinds of alloy NCs are observed, (i) keeping the structure and composition the same, a specific number of foreign atoms are doped and (ii) the structure and compositions are different and the extent of doping depends on the molar ratio of the precursors used. In both the cases, it is crucial to get a single product either by controlling the synthesis method or by isolating the cluster of interest. The isolation of a specific isomer will be highly interesting, which can induce the possibility of crystallization.

Dynamics and mechanism: The exchange of metal atoms and the ligands between two different NCs manifest the dynamic nature of the outer staple motifs of NCs and protecting ligands. The investigation of this chemical event will be interesting. Also, a detailed study of the metal–ligand interface by theoretical calculations can reveal important information. In addition, a mechanistic study of the interactions between two NCs in the transition state is essential to have a deeper understanding of intercluster reactions. Although interaction of clusters leading to dimers have been observed in mass spectrometry, these species have not been trapped. Cluster reactions go through such an intermediate and several such intermediates have been identified. However, none of them have been seen. With the advancements in cryo-electron microscopy, it might be possible to observe such species in solution. If that becomes possible, it will lead to a new understanding of atom transfer processes, how chemical bonds are modified, etc. The existence of Borromean rings may be understood from such studies.

Bulk properties: The availability of larger crystals will enable the measurements of various physical properties of NC solids such as electrical conductivity, mechanical properties, etc., which can open a new paradigm in NC-based research. A systematic effort in the properties of clusters has to occur. The mechanical response of a few monometallic NC solids has been studied; however, this new field need to be explored using various multimetallic NC solids.

Assemblies and superstructures: Supramolecular assemblies of NCs with different nanoparticles or molecules and NC-based metal–organic frameworks are emerging materials as they can be used as functional building blocks to make hierarchical frameworks with improved properties. Although a few assemblies have been made using monometallic NCs, the area is yet to be expanded to multimetallic alloy NCs, which can bring in several unprecedented properties.

While many of these will be pursued in the coming years, there are inherent challenges in this system. The principal one is related to the dynamics of atoms in this length scale. That would mean that stabilizing the cluster systems in the solution and solid state for extended periods would involve the use of specific ligands, which do not allow atom transfer to occur. However, in some other contexts, these dynamics may be advantageous also. The extent of dynamics may be the reason for their use in catalysis. Whatever be the case, understanding such processes will require a great amount of computational effort. As the size of the systems increases, these efforts will become increasingly demanding.