The role of bile acid metabolism in bone and muscle: from analytics to mechanisms.

Osteoporosis and sarcopenia are both common age-related disorders that are associated with increased morbidity and mortality. Bone and muscle are metabolically very active tissues that require large amounts of energy. Bile acids (BAs), a group of liver-derived steroid compounds, are primarily known as emulsifiers that facilitate the resorption of dietary fat and lipids. In addition, they have pleiotropic metabolic functions in lipoprotein and glucose metabolism, inflammation, and intestinal bacterial growth. Through these effects, they are related to metabolic diseases, such as diabetes, hypertriglyceridemia, atherosclerosis, and nonalcoholic steatohepatitis. BAs mediate their metabolic effects through receptor dependent and receptor-independent mechanisms. Emerging evidence suggests that BAs are also involved in bone and muscle metabolism. Under normal circumstances, BAs support bone health by shifting the delicate equilibrium of bone turnover toward bone formation. In contrast, low or excessive amounts of BAs promote bone resorption. In cholestatic liver disease, BAs accumulate in the liver, reach toxic concentrations in the circulation, and thus may contribute to bone loss and muscle wasting. In addition, the measurement of BAs is in rapid evolution with modern mass spectrometry techniques that allow for the detection of a continuously growing number of BAs. This review provides a comprehensive overview of the biochemistry, physiology and measurement of bile acids. Furthermore, it summarizes the existing literature regarding their role in bone and muscle.

Subclinical inflammation, telomere shortening, homocysteine, vitamin B6, and mortality: the Ludwigshafen Risk and Cardiovascular Health Study.

PURPOSE: Short telomeres and B vitamin deficiencies have been proposed as risk factors for age-related diseases and mortality that interact through oxidative stress and inflammation. However, available data to support this concept are insufficient. We aimed to investigate the predictive role of B vitamins and homocysteine (HCY) for mortality in cardiovascular patients. We explored potential relationships between HCY, B vitamins, relative telomere length (RTL), and indices of inflammation. METHODS: Vitamin B6, HCY, interleukin-6 (IL-6), high-sensitive-C-reactive protein (hs-CRP), and RTL were measured in participants of the Ludwigshafen Risk and Cardiovascular Health Study. Death events were recorded over a median follow-up of 9.9 years. RESULTS: All-cause mortality increased with higher concentrations of HCY and lower vitamin B6. Patients in the 4th quartile of HCY and vitamin B6 had hazard ratios (HR) for all-cause mortality of 2.77 (95% CI 2.28-3.37) and 0.41(95% CI 0.33-0.49), respectively, and for cardiovascular mortality of 2.78 (95% CI 2.29-3.39) and 0.40 (95% CI 0.33-0.49), respectively, compared to those in the 1st quartile. Multiple adjustments for confounders did not change these results. HCY and vitamin B6 correlated with age-corrected RTL (r = - 0.086, p < 0.001; r = 0.04, p = 0.031, respectively), IL-6 (r = 0.148, p < 0.001; r = - 0.249, p < 0.001, respectively), and hs-CRP (r = 0.101, p < 0.001; r = - 0.320, p < 0.001, respectively). Subjects with the longest telomeres had a significantly higher concentration of vitamin B6, but lower concentrations of HCY, IL-6, and hs-CRP. Multiple regression analyses identified HCY as an independent negative predictor of age-corrected RTL. CONCLUSIONS: In conclusion, hyperhomocysteinemia and vitamin B6 deficiency are risk factors for death from any cause. Hyperhomocysteinemia and vitamin B6 deficiency correlate with increased mortality. This correlation might, at least partially, be explained by accelerated telomere shortening induced by oxidative stress and systemic inflammation in these circumstances.

Telomere biology and age-related diseases.

Telomeres are the protective end caps of chromosomes and shorten with every cell division. Telomere length has been proposed as a biomarker of biological age and a risk factor for age-related diseases. Epidemiologic studies show an association between leukocyte telomere length (LTL) and mortality. There is solid evidence that links LTL with cardiovascular disease. Short telomeres promote atherosclerosis and impair the repair of vascular lesions. Alzheimer's disease patients have also a reduced LTL. Telomeres measured in tumor tissue from breast, colon and prostate are shorter than in healthy tissue from the same organ and the same patient. In healthy tissue directly adjacent to these tumors, telomeres are also shorter than in cells that are more distant from the cancerous lesion. A reduced telomere length in cancer tissue from breast, colon and prostate is associated with an advanced disease state at diagnosis, faster disease progression and poorer survival. By contrast, results regarding LTL and cancer are inconsistent. Furthermore, the majority of studies did not find significant associations between LTL, bone mineral density (BMD) and osteoporosis. The present manuscript gives an overview about our current understanding of telomere biology and reviews existing knowledge regarding the relationship between telomere length and age-related diseases.

Editorial: 150 Years of the Technische Universität München: Innovation since 1868.

"… On Easter Sunday 1868, the decree founding the Polytechnische Hochschule München (today's Technische Universität München; TUM) was signed by the 23-year-old Ludwig II, King of Bavaria. Entrepreneurial spirit has been a hallmark of the TUM from the start, and it was the first German university to have a branch abroad. Interdisciplinary research is key, not least in the Department of Chemistry …" Read more in the Guest Editorial by Wolfgang A. Herrmann.

Plasma 1-carbon metabolites and academic achievement in 15-yr-old adolescents.

Academic achievement in adolescents is correlated with 1-carbon metabolism (1-CM), as folate intake is positively related and total plasma homocysteine (tHcy) negatively related to academic success. Because another 1-CM nutrient, choline is essential for fetal neurocognitive development, we hypothesized that choline and betaine could also be positively related to academic achievement in adolescents. In a sample of 15-yr-old children (n= 324), we measured plasma concentrations of homocysteine, choline, and betaine and genotyped them for 2 polymorphisms with effects on 1-CM, methylenetetrahydrofolate reductase (MTHFR) 677C>T, rs1801133, and phosphatidylethanolamineN-methyltransferase (PEMT), rs12325817 (G>C). The sum of school grades in 17 major subjects was used as an outcome measure for academic achievement. Lifestyle and family socioeconomic status (SES) data were obtained from questionnaires. Plasma choline was significantly and positively associated with academic achievement independent of SES factors (paternal education and income, maternal education and income, smoking, school) and of folate intake (P= 0.009,R(2)= 0.285). With the addition of thePEMTrs12325817 polymorphism, the association value was only marginally changed. Plasma betaine concentration, tHcy, and theMTHFR677C>T polymorphism did not affect academic achievement in any tested model involving choline. Dietary intake of choline is marginal in many adolescents and may be a public health concern.-Nilsson, T. K., Hurtig-Wennlöf, A., Sjöström, M., Herrmann, W., Obeid, R., Owen, J. R., Zeisel, S. Plasma 1-carbon metabolites and academic achievement in 15-yr-old adolescents.

One-carbon metabolites and telomere length in a prospective and randomized study of B- and/or D-vitamin supplementation.

BACKGROUND: Vitamin B deficiency is common in elderly people and has been associated with an increased risk of developing age-related diseases. B-vitamins are essential for the synthesis and stability of DNA. Telomers are the end caps of chromosomes that shorten progressively with age, and short telomers are associated with DNA instability. OBJECTIVE: In the present randomized intervention study, we investigated whether the one-carbon metabolism is related to telomere length, a surrogate marker for cellular aging. DESIGN: Sixty-five subjects (>54 years) were randomly assigned to receive either a daily combination of vitamin D3 (1200 IU), folic acid (0.5 mg), vitamin B12 (0.5 mg), vitamin B6 (50 mg) and calcium carbonate (456 mg) (group A) or vitamin D3 and calcium carbonate alone (group B). Blood testing was performed at baseline and after 1 year of supplementation. The concentrations of several metabolites of the one-carbon pathway, as well as relative telomere length (RTL) and 5,10-methylenetetrahydrofolate reductase C677T genotype, were analyzed. RESULTS: At baseline, age- and gender-adjusted RTL correlated with total folate and 5-methyltetrahydrofolate (5-methylTHF). Subjects with RTL above the median had higher concentrations of total folate and 5-methylTHF compared to subjects below the median. At study end, gender- and age-adjusted RTL correlated in group A with methylmalonic acid (MMA; r = -0.460, p = 0.0012) and choline (r = 0.434, p = 0.0021) and in group B with 5,10-methenyltetrahydrofolate (r = 0.455, p = 0.026) and dimethylglycine (DMG; r = -0.386, p = 0.047). Subjects in the group A with RTL above the median had lower MMA and higher choline compared to subjects below the median. CONCLUSIONS: The present pilot study suggests a functional relationship between one-carbon metabolism and telomere length. This conclusion is supported by several correlations that were modified by B-vitamin supplementation. In agreement with our hypothesis, the availability of nucleotides and methylation groups seems to impact telomere length. Due to the small sample size and the limitations of the study, further studies should confirm the present results in a larger cohort.

Folic acid causes higher prevalence of detectable unmetabolized folic acid in serum than B-complex: a randomized trial.

PURPOSE: Unmetabolized folic acid (UMFA) is common in serum of elderly individuals receiving folic acid (FA)-fortified foods or supplements. We studied the effect of supplementing FA or B-complex on serum concentrations of (6S)-5-methyltetrahydropteroylglutamate [(6S)-5-CH3-H4Pte] and UMFA in elderly people and explored factors associated with detectable UMFA post-supplementation. METHODS: This is a randomized single-blind non-controlled trial on 58 elderly people using daily 400 µg FA (n = 31) or 400 µg FA, 10 µg cyanocob(III)alamin and 8 mg pyridoxine (n = 27) for a median of 23 days. Main outcome includes changes in concentrations of serum (6S)-5-CH3-H4Pte and UMFA. RESULTS: Total homocysteine declined by a median of 1.6 (p = 0.074) in the FA and 1.3 µmol/L (p = 0.009) in the B-complex arms (p = 0.66 between the arms). Serum (6S)-5-CH3-H4Pte significantly (p < 0.001 vs. baseline) increased by a median of 9.2 and 6.5 nmol/L in the FA and B-complex groups, respectively (p = 0.152 between the groups). Compared to FA, B-complex reduced cystathionine and caused lower post-intervention serum UMFA, percentage of UMFA to (6S)-5-CH3-H4Pte and prevalence of UMFA ≥ 0.21 nmol/L. Higher serum cystathionine and whole-blood folate predicted higher post-intervention serum UMFA. CONCLUSIONS: FA caused higher UMFA as compared to B-complex. Pyridoxine appears to improve folate recycling. Data on serum UMFA should be interpreted in relation to other vitamins involved in folate metabolism. Serum UMFA is suggested to play a sensory role through which the cell recognizes FA available for metabolism via dihydrofolate reductase.

Prospective study of telomere length and LINE-1 methylation in peripheral blood cells: the role of B vitamins supplementation.

PURPOSE: Deficiencies of folate, vitamins B12 and D are common age-related conditions. Vitamin B12 and folate are necessary for DNA methylation. Telomeres appear to be regulated by DNA methylation. Here, we study the effect of B vitamins supplementation on telomere length and global DNA methylation in a prospective study. METHODS: In total, 60 elderly subjects were supplemented for 1 year with either vitamin B12, B6, folate, vitamin D and calcium (group A n = 31) or only vitamin D and calcium (group B n = 29). LINE-1 methylation, relative telomere length (T/S), vitamin B12, folate, homocysteine (tHcy) , 5-methyltetrahydrofolate (5-methylTHF), S-adenosylhomocysteine (SAH), S-adenosylmethionine (SAM), cystathionine and vitamin D were quantified before and after supplementation. RESULTS: At baseline, tHcy was high, vitamin D was low, and T/S did not differ between groups A and B. Vitamin supplementation increased LINE-1 methylation in group A at site 317 but reduced LINE-1 methylation in group B at site 327. There was no correlation between T/S and LINE-1 methylation at baseline. Multiple backward regression analysis revealed baseline tHcy and 5-methylTHF are significant predictors of T/S. After supplementation in group B but not in group A, LINE-1 methylation correlated inversely with T/S, and LINE-1 methylation variation was an independent predictor of T/S variation. B vitamins decreased tHcy significantly in group A. Multiple backward regression analysis showed 5-methylTHF in group A and tHcy in group B were significant predictors for LINE-1 methylation. At baseline, the lower LINE-1 methylation observed in subjects with 5-methylTHF >10 nmol/l was in agreement with a reduced methyl group transfer due to a lower SAM formation. In group B, an increase in telomere length was correlated with lower LINE-1 methylation. Subjects with hyperhomocysteinemia >12 µmol/L had compared to those with normal tHcy a reduced LINE-1 methylation accompanied by a higher SAM and SAH (that inhibits demethylation of SAM) as well as lower 5-methylTHF. Additionally, subjects with tHcy > 12 µmol/L had longer telomeres when compared with subjects having tHcy < 12 µmol/L. CONCLUSIONS: The results suggest a possible effect of B vitamins for telomere biology in blood cells. Suboptimal B vitamins status and hyperhomocysteinemia are associated with altered DNA methylation and telomere length. These data have to be confirmed in future studies.

Direct Synthesis and Bonding Properties of the First µ(2)-η(2),η(2)-Allyl-Bridged Diiridium Complex.

The direct synthesis of the first µ(2)-η(2),η(2)-allyl-bridged diiridium complex ([2](+)), bearing the uncommon counterion [IrCl2(COD)](-) ([3](-)), is described. Both bridging moieties in [2](+), namely, allyl and acetate, are introduced in a single reaction step from [{IrCl(COD)}2] (1) and allyl acetate. A combination of X-ray crystallography and density functional theory calculations reveals pronounced metal-allyl π-back-bonding.

One year B-vitamins increases serum and whole blood folate forms and lowers plasma homocysteine in older Germans.

BACKGROUND: We aimed to study the effect of long-term supplementation of B-vitamins on folate forms in serum and whole blood (WB) in elderly German subjects. METHODS: 59 participants (mean age 67 years) were randomized to daily receive either vitamin D3 (1200 IU), folic acid (500 µg), vitamin B12 (500 µg), vitamin B6 (50 mg), and calcium carbonate (456 mg) or vitamin D3 plus calcium carbonate. Serum and WB folate forms were measured before and after 6 and 12 months. RESULTS: B-vitamins supplementation for 6 months led to higher concentrations of 5-methyltetrahydrofolate (5-methylTHF) in serum (mean 49.1 vs. 19.6 nmol/L) and WB (1332 vs. 616 nmol/L). Also non-methyl-folate concentrations in serum and WB were higher after 6 months with B-vitamins supplementation. Unmetabolized folic acid (UFA) increased after supplementation. tHcy concentration was lowered after 1 year of B-vitamin supplementation (mean 13.1 vs. 9.6 µmol/L). A stronger reduction of tHcy after 1 year was found in participants who had baseline level >12.5 µmol/L (mean 17.0 vs. 11.9 µmol/L) compared to those with baseline tHcy lower than this limit (mean 9.1 vs. 7.4 µmol/L). In contrast, the increases in serum and WB 5-methylTHF were comparable between the two groups. CONCLUSIONS: One year B-vitamins supplementation increased the levels of 5-methylTHF and non-methyl-folate in serum and WB, normalized tHcy, but caused an increase in the number of cases with detectable UFA in serum. Lowering of tHcy was predicted by baseline tHcy, but not by baseline serum or WB 5-methylTHF.

The role of telomeres and vitamin D in cellular aging and age-related diseases.

Aging is a complex biological process characterized by a progressive decline of organ functions leading to an increased risk of age-associated diseases and death. Decades of intensive research have identified a range of molecular and biochemical pathways contributing to aging. However, many aspects regarding the regulation and interplay of these pathways are insufficiently understood. Telomere dysfunction and genomic instability appear to be of critical importance for aging at a cellular level. For example, age-related diseases and premature aging syndromes are frequently associated with telomere shortening. Telomeres are repetitive nucleotide sequences that together with the associated sheltrin complex protect the ends of chromosomes and maintain genomic stability. Recent studies suggest that micronutrients, such as vitamin D, folate and vitamin B12, are involved in telomere biology and cellular aging. In particular, vitamin D is important for a range of vital cellular processes including cellular differentiation, proliferation and apoptosis. As a result of the multiple functions of vitamin D it has been speculated that vitamin D might play a role in telomere biology and genomic stability. Here we review existing knowledge about the link between telomere biology and cellular aging with a focus on the role of vitamin D. We searched the literature up to November 2014 for human studies, animal models and in vitro experiments that addressed this topic.

On the concept of hemilability: insights into a donor-functionalized iridium(I) NHC motif and its impact on reactivity.

Novel iridium(I) complexes bearing N-donor-functionalized N-heterocyclic carbene ligands were synthesized. Although hemilabile coordination of the attached donor is considered beneficial in catalysis, no detailed study of this phenomenon in these systems is available to date. The present report provides insight into the hemilabile bonding properties of a N,N'-bis(pyridin-2-yl)-imidazolylidene (NCN) ligand motif on iridium(I). In most cases, the presented compounds exhibit rare fluxional hemilabile coordination of the N donor, and remarkable performance in catalytic transfer hydrogenation is observed. Further, extensive reactivity studies often led to unexpected products.

Synthesis of the first radiolabeled 188Re N-heterocyclic carbene complex and initial studies on its potential use in radiopharmaceutical applications.

A novel approach towards the synthesis of radiolabeled organometallic rhenium complexes is presented. We successfully synthesized and analyzed the first (188)Re-labeled N-heterocyclic biscarbene complex, trans-dioxobis(1,1'-methylene-bis(3,3'-diisopropylimidazolium-2-ylidene))(188)rhenium(V) hexafluorophosphate ((188)Re-4) via transmetalation using an air-stable and moisture-stable silver(I) biscarbene complex. In order to assess the viability of this complex as a potential lead structure for in vivo applications, the stability of the (188)Re-NHC complex was tested in physiologically relevant media. Ultimately, our studies illustrate that the complex we synthesized dissociates rapidly and is therefore unsuitable for use in radiopharmaceuticals. However, it is clear that the transmetalation approach we have developed is a rapid, robust, and mild method for the synthesis of new (188)Re-labeled carbene complexes.

Group 7 transition metal complexes with N-heterocyclic carbenes.

This tutorial review summarizes all works and highlights recent advances published in the growing field of group 7 N-heterocyclic carbene (NHC) complexes. It provides a valuable source for all scientists that are interested in conducting research with new compounds bearing these metals. The article provides an overview of all manganese NHC complexes that are known to date. There are a lot of examples where manganese NHC complexes show unpredicted behaviour during synthesis, very different from other transition metal NHC complexes and their higher homologues rhenium and technetium. These differences are depicted and discussed. Furthermore, the chemistry of technetium NHC compounds and their chemical behaviour are discussed. To date published work on technetium NHC chemistry has been restricted to the oxidation state +v. It was found that such compounds are very reactive but show great stability in dry air. Their radioactivity makes such compounds interesting candidates for radiochemical applications. Since most group 7 NHC chemistry was conducted on rhenium NHC complexes, this tutorial review highlights the chemical behaviour of such compounds. Rhenium(i) complexes reveal luminescent properties, making them interesting candidates for applications ranging from biological markers to organic light-emitting diodes (OLEDs). Another interesting feature is the radioactivity of some compounds, which makes them excellent candidates for radiopharmaceutical research; hence their synthesis and reactivity are discussed.

[Effect of silymarin on liver health and quality of life. Results of a non-interventional study]. / Einfluss von Silymarin auf Lebergesundheit und Lebensqualität.

BACKGROUND: Many drugs are known to have hepatotoxic side effects. The effect of silymarin on liver function and liver-injury-impaired quality of life under daily practice conditions in patients with elevated values of liver enzymes was evaluated in the present non-interventional study. METHOD: Patients with drug-induced elevated aminotransferase levels and indication for silymarin (Legalon forte) treatment for 2 to 3 months were documented prospectively over 4 months. At baseline, after 2 and 4 months, respectively, the following parameters were documented: alanine transaminase (ALT), aspartate transaminase (AST), γ-glutamyltransferase (GGT), alkaline phosphatase, total bilirubin, presence of liver-related skin symptoms and discoloured urine, severity of liver-related symptoms and quality of life. RESULTS: In total, 190 patients (53.2% male, median age 60.0 years [range 19-81]) from 48 centres participated in the non-interventional study. Among potentially hepatotoxic drugs, analgesics/anti-inflammatory drugs were used most frequently (45.8%). These drugs have been administered for a median period of 2.8 years (range 0.0-26.1). At baseline, all patients had elevated levels of ALT, AST or GGT. Fatigue, flatulence, upper abdominal discomfort, lethargy, and joint complaints were the most severe liver-related symptoms and prevalent in over 62% of patients. Quality of life was affected in 88.7% of patients. Significant reductions were achieved in all documented laboratory parameters (p < 0.001), leading to marked improvement in liver-related symptoms and increased quality of life already after 2 months. The percentage of patients with liver enzymes in the normal range increased considerably within 4 months. No adverse drug reactions were observed. CONCLUSIONS: Silymarin is a safe and efficacious treatment option for patients with elevated liver enzymes. A benefit in terms of liver-related symptoms as well as quality of life and performance was demonstrated already after 2 months of treatment.

n-3 fatty acids and the risk of atrial fibrillation, review.

Atrial fibrillation (AF) is the most frequent type of cardiac arrhythmia that affects over six million individuals in Europe. The incidence and prevalence of AF rises with age, and often occurs after cardiac surgery. Other risk factors correlated with AF comprise high blood pressure, diabetes mellitus, left atrial enlargement, ischemic heart disease, and congestive heart failure. Considering the high prevalence of AF in aging societies, strategies to prevent serious complications, such as stroke or heart failure, are important because they are correlated with high morbidity and mortality. The supplementation of sea-derived n-3 polyunsaturated fatty acids (PUFA) is widely discussed in this context, but the results of experimental and observational studies are in contrast to randomized placebo-controlled intervention trials (RCTs). Specifically, larger placebo-controlled n-3 PUFA supplementation studies with long follow-up showed a dose-dependent rise in incident AF. Daily n-3 PUFA doses of ≥1 g/d are correlated with a 50 % increase in AF risk, whereas a daily intake of <1 g/d causes AF in only 12 %. Individuals with a high cardiovascular risk (CVD) risk and high plasma-triglycerides seem particularly prone to develop AF upon n-3 PUFA supplementation. Therefore, we should exercise caution with n-3 PUFA supplementation especially in patients with higher age, CVD, hypertriglyceridemia or diabetes. In summary, existing data argue against the additive intake of n-3 PUFA for preventative purposes because of an incremental AF risk and lacking CVD benefits. However, more clinical studies are required to disentangle the discrepancy between n-3 PUFA RCTs and observational studies showing a lower CVD risk in individuals who regularly consume n-3 PUFA-rich fish.

Utility and limitations of biochemical markers of vitamin B12 deficiency.

BACKGROUND: There is an urgent need for proper utilization of laboratory markers to diagnose vitamin B12 deficiency that should reduce false negative cases. We worked out a diagnostic algorithm that provides a two-step detection of vitamin B12 deficiency using holotranscobalamin as a first line marker and methylmalonic acid (MMA) as a second line marker. MATERIALS AND METHODS: We tested 1359 serum samples sent to our laboratory for total vitamin B12 assay. Serum samples were used for the determination of holotranscobalamin, MMA and creatinine. RESULTS: Compared with total B12, holotranscobalamin showed a higher area under the receiver operating characteristic curve for detecting MMA levels > 300 nM. However, the distribution of holotranscobalamin in individuals with elevated creatinine irrespective of MMA and in individuals with elevated MMA irrespective of creatinine was shifted into the higher ranges. In the grey zone of holotranscobalamin between 23 and 75 pM (the range extending from the 90% diagnostic sensitivity to the 90% diagnostic specificity), MMA testing as a second line marker would help detecting 18% of deficient cases. Lowering MMA after vitamin B12 treatment may help setting the diagnosis of B12 deficiency in individuals with elevated creatinine. DISCUSSION: Testing for vitamin B12 deficiency should start with holotranscobalamin measurement. Holotranscobalamin between 23 and 75 pM should be followed by MMA testing that can filter substantial number of deficient cases in the grey range in individuals with normal renal function. This diagnostic strategy may significantly improve assessing vitamin B12 deficiency.

Exploring the scope of a novel ligand class: synthesis and catalytic examination of metal complexes with 'normal' 1,2,3-triazolylidene ligands.

Using new 'normal'-substituted 1,2,3-triazolylidene silver compounds as starting materials allowed for preparation of a series of molybdenum, ruthenium, rhodium, and palladium transition metal complexes bound to the new 1,2,3-triazolylidene ligand system. In this work, the first triazolylidene Mo compound is presented as well as the first structural investigation of a silver complex with a monodentate 1,2,3-triazolylidene. Furthermore, the triazolylidene Pd complex and the Mo complex were tested as precatalysts in Suzuki-Miyaura coupling and epoxidation catalysis, respectively.

Synthesis and comparison of transition metal complexes of abnormal and normal tetrazolylidenes: a neglected ligand species.

To date, only few examples of tetrazolylidene carbenes coordinated to transition metal complexes have been described. A direct comparison of "normal" tetrazolylidenes (1,4-substitution pattern) and "abnormal" tetrazolylidenes (also referred to as "mesoionic carbenes"; 1,3-substitution pattern) has not been performed at all. In this work, we describe coordination of both ligand types to a row of transition metals. For example, the first examples of tetrazolylidene Mo, Ag, or Ir complexes have been isolated and fully characterized. The mesoionic tetrazolylidene Ag compound 2d is shown to be a viable carbene transfer reagent. By means of NMR, IR, and Raman spectroscopy, as well as X-ray crystallography and computational results, the pronounced differences between the two ligand substitution patterns are highlighted. It is shown that simply changing the position of a methyl group from N3 to N4 in the heterocycle can cause enormous differences in ligand characteristics.