molecules Review Gold Clusters: From the Dispute on a Gold Chair to the Golden Future of Nanostructures Maria Luisa Ganadu 1,* , Francesco Demartin 2 , Angelo Panzanelli 3, Ennio Zangrando 4 , Massimiliano Peana 3 , Serenella Medici 3 and Maria Antonietta Zoroddu 3,* 1 Department of Humanities and Social Sciences, Università degli Studi di Sassari, Via Roma 151, 07100 Sassari, Italy 2 Department of Chemistry, Università degli Studi di Milano, Via Camillo Golgi, 19, 20133 Milan, Italy; [email protected] 3 Department of Chemistry and Pharmacy, Università degli Studi di Sassari, Via Vienna 2, 07100 Sassari, Italy; [email protected] (A.P.); [email protected] (M.P.); [email protected] (S.M.) 4 Department of Chemical and Pharmaceutical Sciences, Università degli Studi di Trieste, Via L. Giorgieri 1, 34127 Trieste, Italy; [email protected] * Correspondence: [email protected] (M.L.G.); [email protected] (M.A.Z.) Abstract: The present work opens with an acknowledgement to the research activity performed by Luciana Naldini while affiliated at the Università degli Studi di Sassari (Italy), in particular towards gold complexes and clusters, as a tribute to her outstanding figure in a time and a society where being a woman in science was rather difficult, hoping her achievements could be of inspiration to young female chemists in pursuing their careers against the many hurdles they may encounter. Naldini’s findings will be a key to introduce the most recent results in this field, showing how the chemistry of gold compounds has changed throughout the years, to reach levels of complexity Citation: Ganadu, M.L.; Demartin, F.; and elegance that were once unimagined. The study of gold complexes and clusters with various Panzanelli, A.; Zangrando, E.; Peana, phosphine ligands was Naldini’s main field of research because of the potential application of these M.; Medici, S.; Zoroddu, M.A. Gold species in diverse research areas including electronics, catalysis, and medicine. As the conclusion Clusters: From the Dispute on a Gold of a vital period of study, here we report Naldini’s last results on a hexanuclear cationic gold Chair to the Golden Future of 2+ cluster, [(PPh3)6Au6(OH)2] , having a chair conformation, and on the assumption, supported by Nanostructures. Molecules 2021, 26, experimental data, that it comprises two hydroxyl groups. This contribution, within the fascinating 5014. https://doi.org/10.3390/ field of inorganic chemistry, provides the intuition of how a simple electron counting may lead to molecules26165014 predictable species of yet unknown molecular architectures and formulation, nowadays suggesting Academic Editor: José A. Gascón interesting opportunities to tune the electronic structures of similar and higher nuclearity species thanks to new spectroscopic and analytical approaches and software facilities. After several decades Received: 5 April 2021 since Naldini’s exceptional work, the chemistry of the gold cluster has reached a considerable Accepted: 14 August 2021 degree of complexity, dealing with new, single-atom precise, materials possessing interesting physico- Published: 19 August 2021 chemical properties, such as luminescence, chirality, or paramagnetic behavior. Here we will describe some of the most significant contributions. Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in Keywords: gold complexes; gold clusters; transition metal complexes; phosphine ligands published maps and institutional affil- iations. 1. Introduction Luciana Naldini (Scheme1) was born in 1925 in Pisa, where she graduated with Copyright: © 2021 by the authors. full marks in general chemistry. Immediately after the end of the war, she moved to the Licensee MDPI, Basel, Switzerland. University of Milan as an assistant of professor U. Sborgi at the Faculty of General and This article is an open access article Inorganic Chemistry, where she remained even when the chair passed to professor L. distributed under the terms and Malatesta. She taught chemistry for the degree courses of biology, medicine, and geology conditions of the Creative Commons until 1980, when she was appointed full professor at the University of Sassari, where Attribution (CC BY) license (https:// she also became Dean of the Faculty of Chemistry and Director of the CNR (Consiglio creativecommons.org/licenses/by/ Nazionale delle Ricerche) Institute for the Application of Advanced Chemical Technologies 4.0/). Molecules 2021, 26, 5014. https://doi.org/10.3390/molecules26165014 https://www.mdpi.com/journal/molecules Molecules 2021, 26, x FOR PEER REVIEW 2 of 16 Molecules 2021, 26, 5014 2 of 15 delle Ricerche) Institute for the Application of Advanced Chemical Technologies to Agro- biological problems, until 1995, when she passed away following a serious illness. Her toseminal Agrobiological work is problems,testified by until many 1995, scientific when she papers passed supported away following by a sound, a serious critical illness. ap- Herproach seminal to scientific work is data. testified She attended by many national scientific and papers international supported conferences by a sound, as a criticalspeaker approachand was toalso scientific a science data. columnist She attended. She kept national reminding and international that “The Periodic conferences Table as abelongs speaker to andeveryone” was also every a science time columnist. a colleague She made kept remindingher observe that that “The someone Periodic imitated Table belongs her com- to everyone”pounds. every time a colleague made her observe that someone imitated her compounds. Scheme 1. Luciana Naldini (1960) (left); Laboratory work in Milan (right); Luciana Naldini is in the Scheme 1. Luciana Naldini (1960) (left); Laboratory work in Milan (right); Luciana Naldini is in the forefront of the picture. forefront of the picture. ThisThis contribution, contribution, in in addition addition to to being being a a tribute tribute to to Luciana Luciana Naldini Naldini as as a a researcher, researcher, wouldwould also also like like toto underlineunderline the difficulty difficulty women women encountered encountered in in reaching reaching leadership leadership po- positionssitions in in all all spheres spheres of of life, life, and science isis notnot anan exception.exception. ThisThis lack lack of of gender gender equality equality conditionsconditions in in a a scientific scientific institution, institution, which which should should be be founded founded on on the the recognition recognition of of one’s one’s merit,merit, competencies, competencies, and and creativity, creativity, regardless regardless of of other other personal personal features features and and orientations, orientations, deeplydeeply clashes clashes with with the the traditional traditional view view of of science. science. Despite Despite this this situation, situation, she she was was ap- ap- pointedpointed Full Full Professor Professor and and Director Director ofof thethe CNRCNR Institute when she moved moved to to the the island island of ofSardinia, Sardinia, where where she she was was well-liked among colleaguescolleagues andand studentsstudents and and developed developed her her independentindependent field field of of work. work. DuringDuring the the early early years years of of research research in in Milan, Milan, Luciana Luciana Naldini Naldini synthesized synthesized a a series series of of goldgold clusters clusters [1 [1,2],2] comprising comprising six six to to eleven eleven gold gold atoms atoms that that could could not not be be explained explained by by the the standardstandard rules rules of of valence valence and and only only a a few few of of these these have have been been structurally structurally characterized. characterized. AmongAmong them, them, the the unique unique Au Au6 compound6 compound exhibits exhibits a a number number of of intriguing intriguing features features with with respectrespect to to relativistic relativistic effects, effects, aurophilicity, aurophilicity, and and Au Au... …AuAu contacts contacts [ 3[3–6].–6]. These These aspects aspects of of goldgold clusters clusters were were relatively relatively new, new, since since the the first first reports reports of of the the relativistic relativistic effects effects studied studied throughthrough Dirac Dirac scattered-wave scattered-wave calculations calculations dated date backd back to theto the second second half half of theof the 1980s 1980s [7–9 [7–], when9], when also thealso relationship the relationship between between aurophilicity aurophilicity and relativistic and relativistic effects started effects to started emerge to inemerge studies in carried studies out carried on (Ph out3PAu) on 6(Phclusters3PAu)6 [ 10clusters,11] similar [10,11] to similar those prepared to those byprepared Naldini. by SuchNaldini. clusters Such were clusters first reportedwere first in reported a thesis workin a thesis dating work back dating to 1994 back [12], to of 1994 which [12], she of waswhich supervisor, she was andsupervisor, gave additional and gave light additional in this field,light butin this they field, also but concluded they also a periodconcluded of livelya period research of lively activity, research carried activity, out in carried collaboration out in collaboration with her esteemed with her coworkers, esteemed starting cowork- fromers, L.starting Malatesta, from recognized L. Malatesta, as therecognized father of goldas the clusters father chemistryof gold clusters [13], together chemistry with [13], F. Cariati,together