(Pentamethylcyclopentadienyl)aluminium(I)

The earliest documented synthesis and characterization of Cp*Al was by Dohmeier et al. in 1991, where four equivalents of AlCl in toluene/diethyl ether is reacted with two equivalents of 2[Mg(Cp*){{sub|2}}] to give [Cp*Al]{{sub|4}} as yellow crystals:

File:(Pentamethylcyclopentadienyl)aluminium(I)_(Pentamethylcyclopentadienyl)aluminium(I)_original_synthesis.png

Despite the above synthetic scheme successfully producing tetrameters of [Cp*Al]{{sub|4}} at reasonable yields (44%), its use of AlCl proved problematic, as AlCl synthesis requires harsh conditions and its reactive nature makes storage a challenge. As such, more facile ways of synthesising the [Cp*Al]{{sub|4}} tetramer were discovered, and required the reduction of Cp*AlX{{sub|2}} (X = Cl, Br, I) by a metal (K when X = Cl) or a metal alloy (Na/K alloys when X = Br, I):{{Cite journal|last1=Liu|first1=Yashuai|last2=Li|first2=Jia|last3=Ma|first3=Xiaoli|last4=Yang|first4=Zhi|last5=Roesky|first5=Herbert W.|date=2018|title=The chemistry of aluminum(I) with β-diketiminate ligands and pentamethylcyclopentadienyl-substituents: Synthesis, reactivity and applications|journal=Coordination Chemistry Reviews|language=en|volume=374|pages=387–415|doi=10.1016/j.ccr.2018.07.004|s2cid=105749253 |issn=0010-8545}}{{Cite journal|last1=Schulz|first1=Stephan|last2=Roesky|first2=Herbert W.|last3=Koch|first3=Hans Joachim|last4=Sheldrick|first4=George M.|last5=Stalke|first5=Dietmar|last6=Kuhn|first6=Annja|date=1993|title=A Simple Synthesis of[(Cp*Al){{sub|4}}] and Its Conversion to the Heterocubanes[(Cp*AlSe){{sub|4}}] and[(Cp*AlTe){{sub|4}}](Cp*=η{{sup|5}}-C{{sub|5}}(CH{{sub|3}}){{sub|5}})|journal=Angewandte Chemie International Edition in English|language=en|volume=32|issue=12|pages=1729–1731|doi=10.1002/anie.199317291|issn=0570-0833}}{{Cite journal|last1=Schormann|first1=Mark|last2=Klimek|first2=Klaus S.|last3=Hatop|first3=Hagen|last4=Varkey|first4=Saji P.|last5=Roesky|first5=Herbert W.|last6=Lehmann|first6=Christopher|last7=Ropken|first7=Cord|last8=Herbst-Irmer|first8=Regine|last9=Noltemeyer|first9=Mathias|date=2001|title=Sodium–Potassium Alloy for the Reduction of Monoalkyl Aluminum(III) Compounds|journal=Journal of Solid State Chemistry|language=en|volume=162|issue=2|pages=225–236|doi=10.1006/jssc.2001.9278|issn=0022-4596|bibcode=2001JSSCh.162..225S}}{{Cite journal|last1=Nagendran|first1=Selvarajan|last2=Roesky|first2=Herbert W.|date=February 2008|title=The Chemistry of Aluminum(I), Silicon(II), and Germanium(II)|journal=Organometallics|language=EN|volume=27|issue=4|pages=457–492|doi=10.1021/om7007869|issn=0276-7333}}{{Cite journal|last1=Minasian|first1=Stefan G.|last2=Arnold|first2=John|date=2008|title=Synthesis and reactivity of bis-pentamethylcyclopentadienyl diiododialane (Cp*AlI){{sub|2}}: an aluminium(ii) precursor to (Cp*Al){{sub|4}}|journal=Chemical Communications|language=en|volume=|issue=34|pages=4043–5|doi=10.1039/b806804f|pmid=18758620|issn=1359-7345}}

File:(Pentamethylcyclopentadienyl)aluminium(I)_synthesis_2.png

More exotic ways of synthesizing [Cp*Al]{{sub|4}} include the controlled disproportionation of an Al(II) dialane into constituent Al(I) and Al(III) products. For example, reacting dialane [Cp*AlBr]{{sub|2}} with a Lewis base such as pyridine the Lewis base stabilized [Cp*AlBr{{sub|2}}] and [Cp*Al]{{sub|4}}.{{Cite journal|last1=Hofmann|first1=Alexander|last2=Lamprecht|first2=Anna|last3=Jiménez-Halla|first3=J. Oscar C.|last4=Tröster|first4=Tobias|last5=Dewhurst|first5=Rian D.|last6=Lenczyk|first6=Carsten|last7=Braunschweig|first7=Holger|date=2018-08-09|title=Lewis-Base-Induced Disproportionation of a Dialane|journal=Chemistry: A European Journal|volume=24|issue=45|pages=11795–11802|doi=10.1002/chem.201802300|issn=1521-3765|pmid=29920807|s2cid=49308900 }}

Monomeric Cp*Al has also been isolated in a solid Ar matrix by heating [Cp*Al]{{sub|4}} in toluene to 133 °C and spraying the resultant vapours with Ar onto a copper block kept at 12 K.{{Cite journal|last1=Himmel|first1=Hans-Jörg|last2=Vollet|first2=Jean|date=December 2002|title=Probing the Reactivity of Aluminum(I) Compounds: The Reaction of Pentamethylcyclopentadienyl-Aluminum, Al[C{{sub|5}}(CH{{sub|3}}){{sub|5}}], Monomers with Dihydrogen in a Solid Ar Matrix to Give the New Aluminum Hydride Molecule H{{sub|2}}Al[C{{sub|5}}(CH{{sub|3}}){{sub|5}}]|journal=Organometallics|language=en|volume=21|issue=26|pages=5972–5977|doi=10.1021/om020787x|issn=0276-7333}}

X-ray crystallographic data determined Cp*Al to exist exclusively as a tetramer in its solid state. This tetramer, [Cp*Al]{{sub|4}}, consists of an Al{{sub|4}} tetrahedron, and the Cp* rings are Hapticity to the aluminium(I) cation such that the planes of the C{{sub|5}}Me{{sub|5}}{{sup|-}} rings are approximately parallel to the opposite base of the Al{{sub|4}} tetrahedron. The perpendicular distance between Al and the Cp* ring was determined through crystallography to range from 199.7 to 203.2 pm, with a mean value of 201.5 pm. The Al-Al bond in [Cp*Al]{{sub|4}} is 276.9 pm, which is slightly shorter than that of metallic aluminium, which has an Al-Al bond length of 286 pm. Additionally, the Al-Al bond in [Cp*Al]{{sub|4}} is significantly shorter than other oligomeric and polymeric Group III M(I)-η{{sup|5}}-Cp* compounds such as octahedral [InCp*]{{sub|6}} (394, 336 pm), dimeric [InCp*]{{sub|2}} (363.1 pm), and polymeric [TlCp*] (641 pm), indicating a significantly larger interaction between aluminium atoms in [Cp*Al]{{sub|4}} than monovalent Cp* compounds of In(I) and Tl(I). Additional characterization that has been performed include Raman spectroscopy, which detected a Raman active breathing vibration (A{{sub|1}}, 377 cm-1) of the Al{{sub|4}} tetrahedron in [Cp*Al]{{sub|4}}.

Natural bond orbital (NBO) analysis of [Cp*Al] and [Cp*Al]{{sub|4}} using B3LYP/6-31G(d,p) calculated the average charge transfer per Cp* fragment to an Al atom to be 0.657 and 0.641 respectively. This is slightly higher than the charge transfers calculated on [CpAl] and [Cp*Al]{{sub|4}} (0.630 and 0.591 respectively). NBO calculation of the HOMO-LUMO gap in [Cp*Al] also revealed a significant decreasing in the tetrameric [Cp*Al]{{sub|4}} complex compared to the monomeric [Cp*Al] (4.36 compared to 5.49), which is consistent with density functional theory calculations of analogous systems including superatom complexes of gold, aluminium and gallium.{{Cite journal|last1=Williams|first1=Kristen S.|last2=Hooper|first2=Joseph P.|date=2011-12-08|title=Structure, Thermodynamics, and Energy Content of Aluminum–Cyclopentadienyl Clusters|journal=The Journal of Physical Chemistry A|language=EN|volume=115|issue=48|pages=14100–14109|doi=10.1021/jp207292t|pmid=22007955|issn=1089-5639|hdl=10945/48676|bibcode=2011JPCA..11514100W|s2cid=2992445 |hdl-access=free}} Atoms in molecules (AIM) calculations calculate the Al-Al bonding to be metallic.{{Cite journal|last1=Meng|first1=Lingpeng|last2=Zeng|first2=Yanli|last3=Sun|first3=Zheng|last4=Li|first4=Xiaoyan|last5=Lu|first5=Feifei|date=2015-07-28|title=Influences of the substituents on the M–M bonding in Cp4Al4 and Cp2M2X2 (M = B, Al, Ga; Cp = C5H5, X = halogen)|journal=Dalton Transactions|language=en|volume=44|issue=31|pages=14092–14100|doi=10.1039/C5DT01901J|pmid=26171664|issn=1477-9234}} Stabilization of [Cp*Al]{{sub|4}} relative to [CpAl]{{sub|4}} is thought to arise from addition of H-H interactions on the methyl groups attached to the Cp* ligand as opposed to the increased Al-Al bonding interactions.

Despite its typically tetrameric form, the monomer Cp*Al has been isolated and studied in the gas-phase using gas-phase electron diffraction. In its gaseous monomeric form, the perpendicular distance between the Al to the Cp* ring was calculated to be 206.3(8) pm, which is slightly longer than tetrameric [Cp*Al]{{sub|4}}.

When isolated in a solid Hydrogen doped Ar matrix, monomeric Cp*Al has shown to form the hydride species H{{sub|2}}Cp*Al upon exposure to H{{sub|2}} and photolysis with a Hg lamp:

File:(Pentamethylcyclopentadienyl)aluminium(I)_reaction_with_hydrogen.png

At temperatures above 100 °C, [Cp*Al]{{sub|4}} decomposes to form pentamethylcyclopentandiene (Cp*H), metallic aluminium (Al(0)) and other non-volatile Al(III) compounds. The overall stability of [Cp*Al]{{sub|4}} is unique as there is a thermodynamic affinity for tetrameric aluminium(I) compounds ([RAl]{{sub|4}}) to disproportionate into elemental aluminium and R{{sub|3}}Al. As such, a number of different novel oligomeric structures can be synthesised when using tetrameric [Cp*Al]{{sub|4}} as a precursor. For example, treatment of [Cp*Al]{{sub|4}} with excess selenium and tellurium in mild conditions gives the unique heterocubane structures [Cp*AlSe]{{sub|4}} and [Cp*AlTe]{{sub|4}} respectively. These heterocubane structures are extremely air and moisture sensitive, leading to its decomposition and evolution of Hydrogen selenide and Hydrogen telluride respectively. Analogously, reaction of [Cp*Al]{{sub|4}} with lighter chalcogens such as Oxygen, Nitrous oxide and sulfur yield [Cp*AlX]{{sub|4}} (X = O, S).{{Cite journal|last1=Stelzer|first1=Adrian C.|last2=Hrobárik|first2=Peter|last3=Braun|first3=Thomas|last4=Kaupp|first4=Martin|last5=Braun-Cula|first5=Beatrice|date=2016-04-29|title=Completing the Heterocubane Family [Cp*AlE]{{sub|4}} (E = O, S, Se, and Te) by Selective Oxygenation and Sulfuration of [Cp*Al]{{sub|4}}: Density Functional Theory Calculations of [Cp*AlE]{{sub|4}} and Reactivity of [Cp*AlO]{{sub|4}} toward Hydrolysis|journal=Inorganic Chemistry|language=EN|volume=55|issue=10|pages=4915–4923|doi=10.1021/acs.inorgchem.6b00462|pmid=27129027|issn=0020-1669}}

File:Al(I)Cpstar_reactivity_S8_Se_O2.png

[Cp*Al]{{sub|4}} was also the used as a precursor to synthesize the first ever stable dimeric iminoalane containing an Al{{sub|2}}N{{sub|2}} heterocycle through the treatment of [Cp*Al]{{sub|4}} with Me{{sub|3}}SiN{{sub|3}} in a 1:4 molar ratio.{{Cite journal|last1=Schulz|first1=Stephan|last2=Häming|first2=Ludger|last3=Herbst-Irmer|first3=Regine|last4=Roesky|first4=Herbert W.|last5=Sheldrick|first5=George M.|date=1994-05-18|title=Synthesis and Structure of the First Dimeric Iminoalane Containing an Al{{sub|2}}N{{sub|2}} Heterocycle|journal=Angewandte Chemie International Edition in English|language=en|volume=33|issue=9|pages=969–970|doi=10.1002/anie.199409691|issn=0570-0833}} The resultant iminoalanes was characterized to contain an ideally planar Al{{sub|2}}N{{sub|2}} core ring with three coordinate aluminium and nitrogen atoms. Other dimeric iminoalanes including [Cp*AlNSi(i-Pr){{sub|3}}]{{sub|2}}, [Cp*AlNSiPh{{sub|3}}]{{sub|2}} and [Cp*AlNSi(t-Bu){{sub|3}}]{{sub|2}} have since been synthesized using [Cp*Al]{{sub|4}} as a precursor through oxidative addition of an organic azide.

File:(Pentamethylcyclopentadienyl)aluminium(I)_reaction_with_MeSiN3.png

File:(Pentamethylcyclopentadienyl)aluminium(I)_acting_as_Lewis_Acid.png

[Cp*Al] is able to act as an atypical exotic ligand in donor-acceptor type bonds. For example, mixing [Cp*Al]{{sub|4}} with the Lewis acidic B(C{{sub|6}}F{{sub|6}}){{sub|3}} forms the Al-B donor-acceptor type bond, and results in the synthesis of the adduct [Cp*Al-B(C{{sub|6}}F{{sub|6}}){{sub|3}}].{{Cite journal|last1=Gorden|first1=John D.|last2=Voigt|first2=Andreas|last3=Macdonald|first3=Charles L. B.|last4=Silverman|first4=Joel S.|last5=Cowley|first5=Alan H.|date=2000|title=A Lewis Acid Adduct of an Alanediyl: An Aluminum(I)−Boron Donor−Acceptor Bond|journal=Journal of the American Chemical Society|language=en|volume=122|issue=5|pages=950–951|doi=10.1021/ja993537p|issn=0002-7863}} Analogous main-group complexes that have been synthesised and characterised include dialane complexes [Cp*Al-Al(C{{sub|6}}F{{sub|5}}){{sub|3}}]{{Cite journal|last1=Gorden|first1=John D.|last2=Macdonald|first2=Charles L. B.|last3=Cowley|first3=Alan H.|date=2001|title=A valence isomer of a dialane|url=https://pubs.rsc.org/en/content/articlepdf/2001/cc/b007341p|journal=Chemical Communications|issue=1|language=en|pages=75–76|doi=10.1039/B007341P|issn=1359-7345|url-access=subscription}} and [Cp*Al-Al(t-Bu){{sub|3}}],{{Cite journal|last1=Schulz|first1=Stephan|last2=Kuczkowski|first2=Andreas|last3=Schuchmann|first3=Daniella|last4=Flörke|first4=Ulrich|last5=Nieger|first5=Martin|date=2006|title=Group 13−Group 13 Donor−Acceptor Complexes|journal=Organometallics|language=en|volume=25|issue=22|pages=5487–5491|doi=10.1021/om0606946|issn=0276-7333}} and group 13-group 13 complexes [Cp*Al-Ga(t-Bu){{sub|3}}].

[Cp*Al] is also able to act as a potent ligand to transition metals. For example, treatment of [Cp*Al] with [(dcpe)Pt(H)(CH{{sub|2}}t-Bu)] (dcpe = bis(dicyclohexylphosphino)ethane) yields [(dcpe)Pt(Cp*Al){{sub|2}}].{{Cite journal|last1=Weiss|first1=Dana|last2=Steinke|first2=Tobias|last3=Winter|first3=Manuela|last4=Fischer|first4=Roland A.|last5=Fröhlich|first5=Nikolaus|last6=Uddin|first6=Jamal|last7=Frenking|first7=Gernot|date=2000|title=[(dcpe)Pt(ECp*){{sub|2}}] (E = Al, Ga): Synthesis, Structure, and Bonding Situation of the First Aluminum(I) and Gallium(I) Complexes of Phosphine-Substituted Transition Metal Centers|journal=Organometallics|language=en|volume=19|issue=22|pages=4583–4588|doi=10.1021/om000310q|issn=0276-7333}} Other transition metals which use [Cp*Al] as a ligand include, but are not limited to d{{sup|10}} metal centre complexes such as [Pd(Cp*Al){{sub|4}}] and [Ni(Cp*Al){{sub|4}}],{{Cite journal|last1=Buchin|first1=Beatrice|last2=Steinke|first2=Tobias|last3=Gemel|first3=Christian|last4=Cadenbach|first4=Thomas|last5=Fischer|first5=Roland A.|date=2005|title=Synthesis and Characterization of the Novel Al{{sup|I}} Compound Al(C{{sub|5}}Me{{sub|4}}Ph): Comparison of the Coordination Chemistry of Al(C{{sub|5}}Me{{sub|5}}) and Al(C{{sub|5}}Me{{sub|4}}Ph) at d{{sup|10}} Metal Centers|journal=Zeitschrift für Anorganische und Allgemeine Chemie|language=en|volume=631|issue=13–14|pages=2756–2762|doi=10.1002/zaac.200500129|issn=0044-2313}} and lanthanide/actinide metal centre complexes such as (CpSiMe{{sub|3}}){{sub|3}}U-AlCp*, (CpSiMe{{sub|3}})3Nd-AlCp* and (CpSiMe{{sub|3}}){{sub|3}}Ce-AlCp*.{{Cite journal|last1=Minasian|first1=Stefan G.|last2=Krinsky|first2=Jamin L.|last3=Rinehart|first3=Jeffrey D.|last4=Copping|first4=Roy|last5=Tyliszczak|first5=Tolek|last6=Janousch|first6=Markus|last7=Shuh|first7=David K.|last8=Arnold|first8=John|date=2009-09-30|title=A Comparison of 4f vs 5f Metal–Metal Bonds in (CpSiMe{{sub|3}}){{sub|3}}M−ECp* (M = Nd, U; E = Al, Ga; Cp* = C{{sub|5}}Me{{sub|5}}): Synthesis, Thermodynamics, Magnetism, and Electronic Structure|journal=Journal of the American Chemical Society|language=en|volume=131|issue=38|pages=13767–13783|doi=10.1021/ja904565j|pmid=19725526|issn=0002-7863}}

File:(Pentamethylcyclopentadienyl)aluminium(I)_reacting_as_a_ligand.png

{{reflist}}

{{aluminium compounds}}

{{DEFAULTSORT:Pentamethylcyclopentadienylaluminium(I)}}

Category:Organoaluminium compounds

Category:Pentamethylcyclopentadienyl complexes

Category:Aluminium(I) compounds

Category:Tetramers (chemistry)