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Metal ClustersJoel M. SmithBaran Group Meeting
6/17/17
Definitions
Etymology of “Cluster”Derived from the Middle English “cluster” and Old English “clyster” meaning “bunch.” Also from Proto-Germanic klas meaning “to clump, lump together.” Related to icelandic “cluster; bunch of grapes.”
Rhodium (II) Acetate, $153/g
(Aldrich)
Metal Cluster History [Clustory?]
Focus of This Presentaion
Further Resources not fully in the scope of this GMBimetallic Lewis Acids (Qin GM 2015) Transition Metal Photochemistry (Farmer GM 2015)Metal Pairs (Cornella 2016) Organometallic Oddities (O’Malley 2003)
1
Co2(CO)8, $ 7.6/g (Aldrich)(exists as bridged an non-bridged isomers)
Honey Nut Clusters(crispy wheat & rice flakes with delicious honey nut
flavored clusters)
(Currently unavailable from Amazon)
The Cloisters Museum(Upper Manhattan, NYC)
Fullerene - C60$134/g (Aldrich)
12th century - Historically Metal cluster was calo- mel (Hg2Cl2). Later used to treat syphilis and Laxative for George III.1826 - First polyoxometallate discovered as PMo12O40
-4 anion by Berzelius.
1908 - Miolatti-Rosenheim Hypothesis postulated ab- out the structure of 12-molybdophosphoric acid as a polyoxometalate cluster.1928 - Linus Pauling proposes structure that accurately accounts for structure of polyoxotungstate anion1934 - J. F. Keggin obtains first crystal structure of poly- oxo-metallate. Now referred to as Keggin stucture.1953 - X-Ray Evidence for Metal-to-Metal Bonds in Cupric and Chromous Acetate (Schoening)1964 - F. A. Cotton (Wilkinson Student) discovers Re2Cl8-2 which contained metal-metal bond.Early 1960s - Cotton coins terms cluster when referring to chemical entities that contain one or more metal-metal bond.
– Homo and Heterometallic Dinuclear metal clusters– Polyoxometallate complexes– Biologically relevant metal clusters and their related congeners.– Higher order Metal clusters– Future of Metal Clusters in Synthetic Chemistry and Catalysis
Preparation of Metal Clusters
Teflon Reaction Vessel keeps sample pressurized
Autoclave used for heating (usually used
for sterilization at 121 °C)
variousmetalsalts
+heat
solventtime
(days)
metal clustercrystals
Solution synthesis– Reactions are usually carried out in flasks, filtered, and product is crystallized.– Sometimes crystallization times are on the order of weeks.– Examples of clusters generated by this method are polyoxometalates, iron-oxo clusters, etc.
various metal salts+
ligands (if needed)+ solvent
temperaturetime
metal clustercrystals
crystallization
Cluster (Chemistry) - ensemble of atoms ormolecules intermediate in size between a moleculeand a bulk solid. The can be made up of diverse nuclearities and stoichiometry. (Mingos and Wales1990, Introduction to Cluster Chemistry)
Examples: Fullerene, Diborane, Water
Metal Cluster: (1) Compounds that contain metal-metal bonds. (2) A compound that containsa group of two or more atoms where direct andsubstantial metal bonding is present. (HuheeyInorganic Chemistry Huheey, JE,Harper and Row, New York)
Examples: Co2(CO)8, Rh2(OAc)4, Stryker’s reagent.
Hydrothermal Synthesis– Crystallization of substances from high temperature conditions. Usually, the reactions are run in aqueous media.–Most reactions are run in steel pressure vessels in an autoclave. on the order of days.– The first documented growth of a crystal was by Karl Emil von Schafhäutl with the synthesis of quartz crystal. Robert Bunsen also what a major developer of the technique. – The production of quartz was done by this method after a whortage occured during WWII.
Metal ClustersJoel M. SmithBaran Group Meeting
6/17/17
Preparation of Metal Clusters (continued)
2
Solid-state synthesis– Reactions are usually carried out in alumina or silver boats at very high temperatures.– Usually reaction times are on the order of days to weeks– This method of synthesis is typically used in the preparation of unligated metal salts.– Products are typically characterized by powder x-ray diffraction analysis.
variousmetalsalts
+high temperature(500 – 1200 °C)
time
metal cluster powder
Reductive methods under CO atmosphere– Reactions usually start with an air stable oxidized form with stoichiometric metallic reductant– Some methods employ 1 atm of CO, while others require harsher pressures.– Most methods for preparation require inert atmosphere or “filtering under CO.”– Products are typically characterized by elemental analysis and x-ray diffraction.
Example 1: RhCl3 + 2Cu + 4CO + NaCl Na[Rh(CO)2Cl2] + 2Cu(CO)Cl
4Na[Rh(CO)2Cl2] + 6CO + 2H2O Rh4(CO)12 + 2CO2 + 4NaC + 4HCl– After the transformation, CO atmosphere must be present during all filtration. This is not only to prevent the complex from decomposition, but also “to prevent the precipitation of the insoluble CuCl by decomposition of Cu(CO)Cl.
Example 2: MnCl2 + 2Na + 2benzophenoneCO (200 atm)
Mn2(CO)10
– Many heterometallic metal clusters can catalyze reactions such as hydroformylation, hydrogenation, desulfurization olefin isomerization and related reactions.– For an extensive survey of these reactions, see Chem. Rev. 2015, 115, 28.– For a few examples, see below:
isomerization of 1,4-COD to 1,3-COD isomerization of 1-octene to other isomers
M = Mo, conversion of butane to ethane hydrogenation of naphthalene to cis-decalin
The Chemistry of Metal-Carbonyl ClustersExamples of metal carbonyl clusters and their geometry:
Fe3(CO)12
Fe
Fe
Fe
CO
CO
OC COCO
COOCOC
OC CO
CO
CO
Mn MnOC
OC
CO
COOC
COOCCO
OCOC
Mn2(CO)10 Co4(CO)12
CoCoCo(CO)2
Co
CO
CO
OC
OCCO
CO
CO
OCCOOC
Rh4(CO)12
RhRhRh(CO)2
Rh
CO
CO
OC
OCCO
CO
CO
OCCOOC
Ru
Ru
Ru
OC CO
CO
CO
CO
CO
OC
OCOC CO
COOC
Ru3(CO)12
Tc TcOC
OC
CO
COOC
COOCCO
OCOC
Re ReOC
OC
CO
COOC
COOCCO
OCOC
Os
Os
Os
OC CO
CO
CO
CO
CO
OC
OCOC CO
COOC
Os3(CO)12
Tc2(CO)10
Re2(CO)10Most metal carbonyl clusters can form higher order clusters (e.g. Co4(CO)12) that are useful in catalytic processes (vide infra). They are gen-erally soluble in organic solvent.
Metal ClustersJoel M. SmithBaran Group Meeting
6/17/17
3
Mo Mo
O O
iPr
iPriPr
4
Dalton Trans., 2011, 40, 9358.
RCCl4 +R
ClCl3C
cat. (3 mol %)
CDCl3
Cl3CCl
61% yield
CCl3Cl
77% yield
ClCCl3
59% yield
BuRCl4 +Bu
ClCl2RC
cat. (3 mol %)
CDCl3R = CO2Et, 69%; D, 25% COSPr, 32%
Mo Mo
N N 4
HAr Ar
CO2CH3
cat. (0.5 mol %)
CO2CH3BrPMMA (PDI = 1.4)
PhMe-d8, 50 °C, 12 h E1/2 Mo+4/Mo+5 = –0.308 Vversus FeCp2
Reactions promoted/catalyzed by metal carbonyl clustersMn2(CO)10
4Mn(CO)3Mn2(CO)10
tetramerization of acetylene: J. Am. Chem. Soc., 1960, 82, 4209.
product can be hydro-genated or oxidizedwith Mn still ligated.THF, 150 °C
Atom transfer reactions: TL, 2002, 43, 2535.
O
Mn2(CO)10
DCM, hv, BrCCl3NaOH, H2O, BTAC
(99% yield) O
Br
CCl3
Hydrogenation Reaction with (µ-H2)Os3(CO)10J. Am. Chem. Soc., 1976, 98, 1056
EtO2C CO2Et
EtO2C CO2Et
catalystH2
isolable by chromatography
PhCO2Me
CO2MeCo2(CO)8
DCM, rt
PhCO2Me
CO2Me
Co(CO)3Co
Sc(OTf)3RCHO
DCM, rt
O
RMeO2C CO2Me
Ph
Co2(CO)8
30 –95 % yield
Chem. Commun. 2009, 7339.
J. Am. Chem. Soc. 2016, 7492.
FeCl3 + MeMgBrTHF
MeMgBrPh
Br
PhMet = 40 s (5% yield)
t = 120 s (6% yield) PhMe t = 50 s
(99% yield)
– This study further validated a study done in the past by Kochi (see GM).
– This complex has been shown to be stable in solution, but the addition of ligands like TMEDA (known to enhance cross-coupling) disturbs the stability producing mononuclear species.
Metal ClustersJoel M. SmithBaran Group Meeting
6/17/17
Recent Reactions of Polyoxometalate Clusters
4
TBA4(W10O32)
H2O, 32 °C, 2–10 h
Photochemical reactions with POMs: J. Am. Chem. Soc. 1995, 117, 4704.
Me
Me
OHOOHO Me
Me
Me Me
OHO Me
Me
OH
Me
O97 % yield 90 % yield 85 % yield 81 % yield
C-H Oxidation: Nat. Chem. 2010, 2, 478.
Bu4N (!-HPV2PW10O40)H2O2 (1 equiv)
CH3CN-tBuOH, 1 –, 1–4 h
R2R1
HR2R1
OH
R3 R3
OH OH OHH
H
OH HO
HOOH
92% yield 75% yield 37% yield 93% yield 51% yield
50% yield 81% yield
Bu4N (!-HPV2PW10O40) [x-ray]
– Reactivity of this Vanadium POM cluster reminiscent of DMDO/TFDO– Selectivity Governed by catalyst size and not a radical mech!
R2R1
HR2R1
CHO
R3 R3
Polyoxometalates: Some of the First-discovered Metal Clusters
Lindquist Hexamolybdate structure Decavanadatedate structure
Dawson Structure (P2M18O62)-6 Keggin Structure (PW12O40)-3
Top View Preyssler Structure Side View (EuP5W30O110)-12
– Most POMs are synthesized from their parent oxides and structure is controlled by altering pH. – POMs are non-toxic but have interesting anti-HIV and anti-cancer properties.
Metal ClustersJoel M. SmithBaran Group Meeting
6/17/17
Reactions of Polyoxometalates
5
TBA4(W10O32)550 W Hg lamp (Pyrex filter)
CO (~ 9mM)
CH3CN, 25 °C, 16 h
carbonylation reactions with POMs: J. Am. Chem. Soc. 1995, 117, 4704.
MeCHO
54 turn. (68 % sel.)
18 turn. (69 % sel.)
12 turn. (85 % sel.) 2 turn.
C-H alkylation/alkenylation: J. Am. Chem. Soc. 1993, 115, 12212.
TBA4(W10O32), hv, C2H4 or C2H2
CH3CN, rt
R2R1
HR2R1
CHO
R3 R3
CHO CHO
MeCHO Ph
or
2 – 5% conversion
H TBA4(W10O32), hv, C2H4 or C2H2
CH3CN, rt+
85% 5%
(W10O32)-4
CO2MeCO2Me
Dehydrogenation: J. Am. Chem. Soc. 1990, 112, 6585
TBA4(W10O32), hv
CH3CN, 25 °CR2R1
H
R2R1R3R3
MeO2C CO2Me
CO2MeCO2Me
NCHO CO2Me
CO2Me
O
OCO2Me
CO2Me
C6H13
O
CO2Me
CO2Me CO2Me
CO2Me
CN
58% yield 61% yield 52% yield 90% yield 62% yield
TBA4(W10O32), sunlightMichael acceptor
CH3CN, 25 °C,
O
O
O CN
O
CNO
O
58% yield 73% yield 55% yield
R Bu4N (!-HPV2PW10O40) (0.05 mol %)H2O2 (1 equiv)
CH3CN-tBuOH, 1 –, 1–4 h
R OH
Arene C-H Oxidation: Angew. Chem., Int. Ed. 2012, 51, 7275.
OMe
OH
OMeMeO
OH MeO
OH OH
HO
OH
63% yield 93% yield 85% yield 46% yield 82% yield
R H5PV2PMo10O40 (2 equiv)
50% H2SO4rt – 90 °C
R CHO
Electrochemical Benzylic Oxidation: J. Am. Chem. Soc. 2015, 137, 5916.
CHO
I
CHO
O2N
CHO
Me
CHO
Me
CHO
F
97% (96) 45% (98) 99% (94) 99% (89) 99% (98)
H5PV2PMo10O40 (2.3 mol %)Pt electrode (1.5 V)
50% H2SO470 °C, 10 h
CHO
65% yield (95% selectivity)
– Protonated POM accepts electron from arene produc-ing radical cation.
– Radical cation under-goes proton transfer to form benzylic radical.
– Benzylic radical is further oxidized and captured by water.
– Benzylic alcohol is further oxidized by POMto aldehyde.
– Overoxidation (<1%) is not observed.
Metal ClustersJoel M. SmithBaran Group Meeting
6/17/17
Biochemical Metal Clusters
6
Iron Sulfur Clusters
Fe SFe
S
RS
RS
SR
SR
[2Fe–2S]+2 [2Fe–2S]+
Fe SFe
SRS SR
S SFeSR
[3Fe–4S]+ [3Fe–4S]0
[3Fe–4S]- [3Fe–4S]-2
Fe
SS
S
FeFe
Fe
SRS
SR
SRRS
[4Fe–4S]+3 [4Fe–4S]+2
[4Fe–4S]+ [4Fe–4S]0
Fe
SS
S
FeFe
Fe
SRS
S
RS
Fe
S
S
FeFe
Fe
SSR
S[8Fe–8S]+5 [8Fe–8S]+4
[8Fe–7S]+3 [8Fe–7S]+2
– Iron-sulfur clusters are important co-factors in metalloenzymes that participate in substrate binding, electron transfer, and regulation of gene expression.– Clusters do not form spontaneously in nature but rather require complex biosynthesis to form.– 120 distinct types of enzymes that contain Fe-S clusters.– Postulated to have an important role in the origin of life.
Fe8(µ4-O)4(µ-pz)12Br4
E1/2(1) –0.33 V E1/2(2) –0.62 V E1/2(3) –0.89 E1/2(4) –1.16
Fe
SS
S
FeFe
Fe
SRS
SRRS
Mechanism of Aconitase utilizes an Iron-Sulfur Cluster
OHOOH
O
CO2-
H-O2C
BEnz
Fe
SS
S
FeFe
Fe
SRS
SRRS
OH
OH
CO2-
CO2-+
H2OCO2
-
CO2-
-O2C
OHisocitrate
cis-Aconitate
-O2C
citrate
– Citric acid is converted to isocitrate through cis-aconitate.– The dehydration is facilitated by the iron-sulfur cluster acting as a Lewis acid. – This is one of the few mechanism where the iron cluster does not change oxidation state. Most react in electron transfer processes like the one below.
Fe SFe
S
RS
RS
N
N
N
NO
OH
R
MeOOMe
Hemee-
H BEnz O
O
R
MeOOMeUbiquinol Ubiquinone
Fe SFe
S
RS
RS
N
N
N
NH
– Iron-sulfur clusters play a role in mitochondria function through electron transport.
– This octairon oxo cluster mimics the iron sulfur clusters found in vivo. The CV trace shows that there are four reversible single electron transfer events, owing to the stability of these strutures even when placed under electronic stress.
Electron Transfer Between Metal Clusters: J. Am. Chem. Soc. 2014, 136, 15873.
– Ligated clusters are soluble in organic solvent.
– Reversible electron transfer for both complexes confirms stability over wide potential range.
– HOMO/LUMO gap promotes electron transfer between clusters.
Metal ClustersJoel M. SmithBaran Group Meeting
6/17/17
7
Metal Clusters with underdeveloped reactivity
Cu(OAc)2. + K2Sb2(tartrate)2
1. NaOAc/AcOH
2. K2CO3
Photocatalytic Water splitting by cobalt cluster (Eur. J. Inorg. Chem. 2016, 3253:
Inorg. Chem. 2016, 55, 540.
NiCl2. +N
NOH
EtOH
KOH
CoCl2. +N
NOH
EtOH
KOH
Dalton Trans. 1985, 737.
Nature, 2014, 515, 545.
[W18O54(SeO3)2]-4
– POM structure was designed for flash memory devices.
– Several reduction events for the tungsten cage (gray area)– Orange area is the oxidation of the Se dopant.