{"id":32123,"date":"2026-10-03T08:02:27","date_gmt":"2026-10-03T07:02:27","guid":{"rendered":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=32123"},"modified":"2026-10-03T08:45:25","modified_gmt":"2026-10-03T07:45:25","slug":"unrecognised-inorganic-metalla-aromatic-rings-the-mystery-of-cyclo-ns-titanocenes","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=32123","title":{"rendered":"Unrecognised inorganic metalla-aromatic rings? The mystery of (cyclo-N,S)-titanocenes."},"content":{"rendered":"
\n

Metalla-aromatics have been defined as metallacycles that are derived from the formal replacement of a carbon atom in the framework of an organic aromatic ring with a metal fragment.[1]<\/a><\/span>. Here I explore whether this simple definition can be broadened to sulfur-nitrogen rings which contain few[2]<\/a><\/span>,[3]<\/a><\/span> if indeed any[4]<\/a><\/span>,[5]<\/a><\/span>,[6]<\/a><\/span>,[7]<\/a><\/span> carbon atoms and which also happen to be planar aromatic molecules.<\/p>\n

Prologue<\/h4>\n

Our story starts with the authors of this article[8]<\/a><\/span> reporting the formation of the sulfur-nitrogen titanacycles 1<\/strong>[9]<\/a><\/span> and 2<\/strong>[10]<\/a><\/span> (green in the scheme 1 below) from Cp2<\/sub>Ti(CO)2<\/sub> reacting with S4<\/sub>N4<\/sub>. Isolating compound 2<\/strong> was something of a surprise, since they had been expecting\/hoping to get 3<\/strong>[11]<\/a><\/span> instead (red in scheme 3 below). This result was unexplained in the article and unremarked upon by anyone else since.<\/p>\n

\"\"<\/a>
\nScheme 1. <\/strong>Formed products 1<\/strong> – 2<\/strong>\u00a0from the reaction of Cp2<\/sub>Ti(CO)2<\/sub> with S4<\/sub>N4<\/sub>.<\/p>\n

To develop the prologue, we start with compound 7<\/strong> (Scheme 2) which was described[3]<\/a><\/span> as a (H\u00fcckel-class) 10 \u03c0-aromatic ring (6e from double bonds, 4e from S lone pairs) containing one ring carbon atom and represented in the original paper as 7a<\/strong>. In fact, NBO7 analysis[12]<\/a><\/span>,[13]<\/a><\/span>\u2021<\/sup> (as obtained with no 3-centre bonds allowed) suggests that 7b<\/strong> is a more accurate representation (4e from two \u03c0-double bonds and 6e from lone pairs = 10\u03c0 =4n+2, n=2 ). Molecule 7b<\/strong>\u00a0has a computed NICS(0) (= Nucleus Independent Chemical Shift at ring centroid) value of -9.6 ppm,[14]<\/a><\/span> NICS here being used as an approximate indicator of aromaticity\u00a0(or lack of it),\u00a0with e.g.<\/em> the archetypal 4n+2 (n=1) \u03c0-aromatic benzene giving a NICS(0) value of ~-10. For 4n\/4n+2 aromaticity selection rules, see e.g.<\/em> [15]<\/a><\/span>,[16]<\/a><\/span>
\n\"\"<\/a>
\nScheme 2. <\/strong>Compounds 7a\/b<\/strong>\u00a0and 7c<\/strong>.<\/p>\n

A replacement of the carbon and its attached ester group according to the definition above by using (Cp.CO)Ti as a transition metal results in e.g.<\/em> 7c<\/strong> – a compound not dissimilar to the compound discussed in the previous post[17]<\/a><\/span>. A MN15L\/Def2-TZVPP calculation[18]<\/a><\/span> gives the geometry shown in Figure 1. A NBO7[12]<\/a><\/span> localisation procedure\u2021<\/sup> gives the result[19]<\/a><\/span> shown as 7c<\/strong> (Scheme 2, Figure 2), with back bonding into titanium d-orbitals and the three \u03c0-bonds and two lone pairs (one N, one S) resulting in a total of ten cyclically conjugated \u03c0-electrons.<\/p>\n

\"\"
\nFigure 1.<\/strong> Calculated\u00a0MN15L\/Def2-TZVPP\/SCRF=DCM structure of compound 7c<\/strong>.<\/p>\n

\"\"<\/p>\n

Figure 2<\/strong>. Five NBO7 localised orbitals representing 7c<\/strong>.<\/p>\n

A NICS(0) NMR estimate of the aromaticity for 7c<\/strong> gives an antiaromatic value of +9.8 ppm,[20]<\/a><\/span>, and\u00a0hence tending to M\u00f6bius antiaromaticity according to the selection rules for 4n+2 (n=2) electrons.[15]<\/a><\/span>,[16]<\/a><\/span> This combination of 4n+2 (n=2) conjugated \u03c0-electrons and NMR-based antiaromaticity allows one to infer that there must be one<\/strong> orbital phase shift in the conjugated \u03c0-electrons, presumably at either the Ti=S centre or the Ti=N centre but not both. This pair of calculations provides an interesting and useful prologue to and calibration of the NICS(0) procedure as an (anti)aromaticity metric in such systems.<\/p>\n

Discussion. Molecule 1.<\/h4>\n

CASZOL10 (molecule 1<\/strong>, scheme 1, Figure 3), has an NBO7\u2021<\/sup> Lewis localised structure[21]<\/a><\/span>\u00a0shown as 1a<\/strong> rather than the literature representation 1<\/strong>[8]<\/a><\/span> (Scheme 1, Figure 4). As with 7c<\/strong>, it comprises three \u03c0-bonds and two lone pairs (one N, one S). In contrast to 7c<\/strong>, the NICS(0) = -6.7[22]<\/a><\/span> suggests moderate aromatic character, which as a 10 \u03c0-electron aromatic must have either no phase shifts or two. It suggests that, as informed by 7c<\/strong>, the Ti=N bond does not contribute any phase shifts. On the basis of these results, we suggest that molecule 1<\/strong> can be described as a H\u00fcckel type (4n+2) moderately aromatic inorganic metallacycle.<\/p>\n

\"\"
\nFigure 3<\/strong>. Calculated structure of 1<\/b>, CASZOL10. The purple point is the NICS probe.<\/p>\n

\"\"
\nFigure 4<\/strong>. Five NBO7 localised orbitals representing 1a<\/strong> (total NBO7 orbital occupancy 8.39e)<\/p>\n

Discussion. Molecule 2.<\/h4>\n

Molecule 2<\/strong> (CIWFIX10, Figure 5) has NICS(0) = -6.0 ppm[23]<\/a><\/span> for 8\u03c0 electrons (one NBO7 double bond and three NBO7 lone pairs[24]<\/a><\/span>) represented as 2a<\/strong> in scheme 1 (Figure 5).<\/p>\n

\"\"
\nFigure 5<\/strong>. Structure of 2,<\/b>\u00a0CIWFIX10. The purple point is the NICS atom.<\/p>\n

\"\"<\/p>\n

Figure 6<\/strong>. Four NBO7 localised orbitals representing 2b<\/strong> (total NBO orbital occupancy 6.92e).<\/p>\n

The moderate NMR-based aromaticity for a 4n (n=2) electron cycle implies M\u00f6bius character, [15]<\/a><\/span>,[16]<\/a><\/span> whereby significant twisting of the adjacent sulfur-nitrogen ring orbitals allows a phase shift to occur via<\/em> an unoccupied Ti d-orbital (Figure 7).<\/p>\n

\"\"
\n<\/p>\n

Discussion. Unformed Compounds (Scheme3).<\/h4>\n

\"\"<\/p>\n

Scheme\u00a03.<\/b> Unformed compounds 3<\/strong>–6<\/strong> from the reaction shown in Scheme 1.<\/p>\n

Compounds 3-6<\/strong> are all computed as higher in free energy than 2<\/strong>. Compound 3a<\/strong> (mysteriously not formed in the original report[8]<\/a><\/span>) is +13.7 kcal\/mol higher than 2<\/strong>, and clearly non-aromatic (NICS(0) = -0.29 ppm). \u00a0This may be because the ring is relatively flat and not capable of the twisting required to become M\u00f6bius aromatic, something that was possible in 2<\/strong>.[25]<\/a><\/span>,[26]<\/a><\/span>,[27]<\/a><\/span> Compound 4<\/strong> (orange in scheme) is interesting since it is only 3.9 kcal\/mol higher than 2<\/strong> and having 8\u03c0 electrons (two bonds, two lone pairs) has a NICS(0) of -8.4 ppm[28]<\/a><\/span> appropriate for a M\u00f6bius metallaaromatic and again has a ring capable of the required twisting as per 2<\/strong>. It might be synthesizable if formed by a different method.<\/p>\n

Conclusions<\/h4>\n

Both 1<\/strong> and 2<\/strong> are suggested here as examples of inorganic metallaaromatics<\/strong>, respectively of the H\u00fcckel and M\u00f6bius type,[15]<\/a><\/span>,[16]<\/a><\/span> and uniquely with no carbon atoms present in the aromatic ring – a hitherto unrecognised class of aromatic molecule<\/strong>. We here argue that the non-formation of compound 3<\/strong> as indicated in original article[8]<\/a><\/span> is because it is non-metalla-aromatic, and in its place compound 2<\/strong> is formed precisely because it IS likely to be metalla-aromatic. It is our expectation that many more such inorganic metalla-aromatics could exist.<\/p>\n

\n

\u2021<\/sup>Use of the older NBO3 procedure is deprecated here, since it is prone to converging for these types of systems to unphysical solutions. We have established that the essential character of the NBO7 orbitals does not depend on the quality of the basis set used[29]<\/a><\/span> or the DFT procedure (albeit tested only for one additional functional, \u03c9B97XD[30]<\/a><\/span>,[31]<\/a><\/span>).<\/p>\n

\n

This post has DOI: 10.59350\/dsyea-mrq85<\/a><\/p>\n

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  31. H. Rzepa, \"Cp2TiS3N4, wB97XD\/Def2-TZVPP Compound 1b, G = -2649.576493 NBO7\", 2026. https:\/\/doi.org\/10.5281\/zenodo.23039008<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    Metalla-aromatics have been defined as metallacycles that are derived from the formal replacement of a carbon atom in the framework of an organic aromatic ring with a metal fragment.. Here I explore whether this simple definition can be broadened to sulfur-nitrogen rings which contain few, if indeed any,,, carbon atoms and which also happen to […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_feature_clip_id":0,"_jetpack_memberships_contains_paid_content":false,"activitypub_content_warning":"","activitypub_content_visibility":"","activitypub_max_image_attachments":5,"activitypub_interaction_policy_quote":"anyone","activitypub_status":"federated","footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2},"jetpack_post_was_ever_published":false,"_ppma_block_editor_authors":""},"categories":[4],"tags":[],"ppma_author":[2661,2667],"class_list":["post-32123","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry"],"yoast_head":"\nUnrecognised inorganic metalla-aromatic rings? 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Here I explore whether this simple definition can be broadened to sulfur-nitrogen rings which contain few, if indeed any,,, carbon atoms and which also happen to […]","og_url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=32123","og_site_name":"Henry Rzepa's Blog","article_published_time":"2026-10-03T07:02:27+00:00","article_modified_time":"2026-10-03T07:45:25+00:00","author":"Henry Rzepa, Derek Woollins","twitter_card":"summary_large_image","twitter_misc":{"Written by":"Henry Rzepa","Estimated reading time":"7 minutes"},"schema":{"@context":"https:\/\/schema.org","@graph":[{"@type":"Article","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=32123#article","isPartOf":{"@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=32123"},"author":{"name":"Henry Rzepa","@id":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/#\/schema\/person\/2b40f7b9c872a4dc1547e040a11b6281"},"headline":"Unrecognised inorganic metalla-aromatic rings? 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Thus when the\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":24503,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=24503","url_meta":{"origin":32123,"position":2},"title":"Molecule of the year 2021: Infinitene.","author":"Henry Rzepa","date":"December 16, 2021","format":false,"excerpt":"The annual \"molecule of the year\" results for 2021 are now available ... and the winner is Infinitene., This is a benzocirculene in the form of a figure eight loop (the infinity symbol), a shape which is also called a lemniscate after the mathematical (2D) function due to Bernoulli. The\u2026","rel":"","context":"In "Chiroptics"","block_context":{"text":"Chiroptics","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=2644"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2021\/12\/infinitene.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":30548,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=30548","url_meta":{"origin":32123,"position":3},"title":"Molecules of the year 2025: Benzene-busting inverted sandwich.","author":"Henry Rzepa","date":"January 1, 2026","format":false,"excerpt":"Sandwich compounds are the colloquial term used for molecules where a metal atom such as an iron dication is \"sandwiched\" between two carbon-based rings as ligands, most commonly cyclopentadienyl anion (the \"bread\") as in e.g. Ferrocene - a molecule first discovered in 1951. An \"inverted\" sandwich is where the carbon\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":21250,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=21250","url_meta":{"origin":32123,"position":4},"title":"The Kekul\u00e9 vibration as a function of aromatic ring size. A different perspective using lemniscular rings.","author":"Henry Rzepa","date":"September 27, 2019","format":false,"excerpt":"In the previous posts, I tried to track down the onset of bond length alternation (BLA) as a function of ring size in aromatic cyclocarbons, finding the answer varied dramatically depending on the type of method used to calculate it. So here I change the system to an unusual kind\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":16987,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=16987","url_meta":{"origin":32123,"position":5},"title":"Catenated atoms and groups.","author":"Henry Rzepa","date":"October 13, 2016","format":false,"excerpt":"Chemists are as fond of records as any, although I doubt you will find many\u00a0chemical ones\u00a0in the Guinness world records list. Polytriangulanes chase how many cyclopropyl 3-rings can be joined via a vertex. Steve Bachrach on his blog reports some recent work by Peter Schreiner and colleagues and the record\u2026","rel":"","context":"In "crystal_structure_mining"","block_context":{"text":"crystal_structure_mining","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1745"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]}],"jetpack_likes_enabled":false,"authors":[{"term_id":2661,"user_id":1,"is_guest":0,"slug":"admin","display_name":"Henry Rzepa","avatar_url":"https:\/\/secure.gravatar.com\/avatar\/897b6740f7f599bca7942cdf7d7914af5988937ae0e3869ab09aebb87f26a731?s=96&d=blank&r=g","author_category":"1","first_name":"Henry","last_name":"Rzepa","user_url":"https:\/\/orcid.org\/0000-0002-8635-8390","job_title":"","description":"Emeritus Professor of Computational Chemistry at Imperial College London."},{"term_id":2667,"user_id":0,"is_guest":1,"slug":"derek-woollins","display_name":"Derek Woollins","avatar_url":"https:\/\/secure.gravatar.com\/avatar\/ead3668dd5c9efd34408a2240b9771601591363f79242ff35f432e23a9d61910?s=96&d=blank&r=g","author_category":"1","first_name":"","last_name":"","user_url":"","job_title":"","description":""}],"jetpack_featured_media_url":"","_links":{"self":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/32123","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=32123"}],"version-history":[{"count":203,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/32123\/revisions"}],"predecessor-version":[{"id":32379,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/32123\/revisions\/32379"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=32123"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=32123"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=32123"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=32123"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}